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and Astronomical Science.
CONTENTS.
BOOK I.
Heat.
CHAP. I.
PRELIMINARY PRINCIPLES AND DEFINITIONS.
Sect Page
1304. Heat ...... 1
1305. Sensible heat .... 16.
1306. Insensible heat .... ib.
1307. Dilatation and contraction - - ib.
1308. Liquefaction and solidification - 2
1309. Vaporization and condensation - ib.
1310. Incandescence .... ib.
1311. Combustion ..... 3
1312. Thermometers and pyrometers - ib.
1313. Conduction ..... ib.
1314. Radiation ..... 4
1315. Diathermanous media -
1316. Reflection of heat - - - ib.
1317. Refraction of heat ... 5
1318. Different senses of the terms heat
and caloric - - - - - ib.
ain thermal
- ib.
- -
1319. Hypothesis to explai
phenomena - -
CHAP. II.
THERMOMETRY.
- ib.
- 6
- ib.
- 7
- ib.
- 8
- 9
1320. Measures of temperature
1321. Thermoscopic substances
1322. Mercurial thermometer
1323. Preparation of the mercury
1324. Selection of the tube -
1325. Formation of the bulb -
13-26. Introduction of the mercury
1327. Thermometric scale arbitrary
1328. Standard points division of scale ib.
1329. Numeration of scale zero point 10
1330. No natural zero - ib.
1331. Phenomena fit to supply standard
points 16.
1332. Freezing and boiling points of
water adopted by common con-
sent ib.
1333. Determination of these points - 11
1334. Different thermometric units and
zeros Fahrenheit's scale - - ib.
1335. Centigrade scale - ... ib.
1336. Reaumur's scale - 12
1337. Methods of computing the tempe-
rature according to any one scale
when the temperature according
to any other is given ... ib.
A
Sect Page
1338. Rate of dilatation of mercury - 13
1339. Its dilatation uniform between
standard points - - - -14
1340. Use of a standard thermometer - ib.
1341. Range of the scale of thermome-
ters varies with purpose to which
they are applied - 15
1342. Qualities which render mercury a
convenient thermoscopic fluid - ib.
1343. Bulbs liable to a permanent change
of capacity, which renders cor-
rection of scale necessary - - ib.
1344. Self-registering thermometers . 16
1345. Spirit of wine thermometer - - ib.
1346. Air thermometer - - - - ib.
1347. Drebhel's air thermometer - - 17
1348. A monton's air thermometer - ib.
1349. The differential thermometer - ib.
1350. Pyrometers adapted to measure
high temperatures - - - 18
1351. Graduation of a pyrometer - - ib.
1352. Temperature of metallic standard
measures must be observed - 19
1353. Borda's pyrometric standard mea-
sure ------ 20
1354. Construction and use of a vernier 21
DILATATION OF SOLIDS.
1355. Solids least susceptible of dilata-
tion ------
1356. Homogeneous solids dilate equally
throughout their volume -
1357. Dilatation of volume and surface
computed from linear dilatation
1358. Dilatation of solids uniform be-
tween the standard thermome-
tric points -
1359. Dilatations cease to be uniform
near the point of fusion -
1360. Exceptional cases presented by
certain crystals - - - -
1361. Tabular statement of the rate of
dilatation of solids - - -
1362. Measure of the force of dilatation
and contraction of solids -
1363. Practical application of the forces
of dilatation and contraction in
drawing together the walls of a
building .....
Sect. Page
1364. Moulds for casting in metal must
be larger than the objects to be
1365. Hoops and tires tightened by the
contraction in cooling - - 27
1366. Compensators necessary in all me-
tallic buildings ... - ib.
1367. Blistering and cracking of lead
and zinc roofs * ib.
1368. Metallic inlaying liable to start - ib.
1369. Compensating pendulum - - ib.
1370. Harrison's gridiron pendulum . 28
1371. Bars of different metals mutually
attached are curved by dilata-
tion and contraction - - -29
1372. Application of this principle to
compensation pendulums - -50
1373. Its application to balance wheels - ib.
CHAP. IV.
DILATATION OF GASES.
1374. Volume of gaseous bodies depen-
dent on pressure and tempera-
ture ------ 31
1375.. Method of observing the dilatation
of gases under uniform pressure ib.
1376. Dilatation of gaseous bodies uni-
form and equal - - - . S3
1377. Amount of this dilatation ascer-
tained ib.
1373. Increment of volume correspond.
ing to 1 ib.
1379. Experiments of Gay Lussac, Du-
long and Petit, showing uni-
formity and equality of expan-
sion 34
1380. This result qualified by researches
of Rudberg and Regnault - - ib.
1381. Formula to compute the change
of volume of a gas corresponding
to a given change of tempera-
ture 35
1382. Increaseof pressure due to increase
of temperature - 37
1383. Formula? expressing the general
relation between the volume,
temperature, and pressure - ib.
1384. Examples of the effects of dilata-
tion and contraction - 38
1.385. Ventilation and warming of build-
ings ib.
1386. Effects of open fire-places and close
stoves - .... 39
1387. Methods of warming apartments - ib.
1388. Principle of an Argrind lamp - 40
1.3^9. Cau-e of atmospheric currents - 41
1390. Ex;>eriments illustrating the ex-
pansion and contraction of air - Hi.
CHAP. V.
DILATATION OP LIQUIDS.
1S91. L'quid a state of transition - - 42
1392. Rate of dilatation of liquids in ge-
neral uniform - 43
1S93. Specific gravity of liquid varies
with its temperature - - ib.
1394. Rates of dilatation of liquids - ib.
Sect. Page
1395. Exceptional phenomena mani-
fested by water approaching its
freezing point - 43
1396. Temperature of greatest density - 44
1597. Taken as the basis of the French
metrical system - - - ib.
1398. Effect of the relative densities of
different strata of the same
liquid - - - - -Hi.
1399. Process of heating a liquid - - 45
1400. Heat does not descend in a liquid ib.
1401. Experiments showing the propaga-
tion of heat through a liquid by
currents - - - - - ib.
1402. Method of warming buildings by
hot water ----- 46
; CHAP. vi.
CALORIMETRY.
1403. Quantitative analysis of heat - 47
1404. Calorimetry and thermometry - ib.
1405. Thermal unit - ib.
1406. Specific heat ib.
1407. Uniform and variable - 48
1408. Methods of solving calorimetric
problems - - - - - ib.
1409. Calorimeter of Lavoisier and La-
place t*.
1410. Application of calorimeter to de-
termine specific heat - - 50
1411. Specific heat of water uniform - ib.
1412. Method of ascertaining the specific
heat of other bodies by calori-
meter 51
1413. Method of equalization of tempe-
rature - ... ,b.
1414. Application of this method - - 52
1415. Method of cooling - - 53
1416. Results of calorimetric researches ib.
1417. Relation of specific heat to density ib.
1418. The fire syringe - - - - ib.
1419. Specific heat of gases and vapours
increases as their density is di-
minished - - - - - ib.
1420. Specific heat under constant pres-
sure and constant volume - 54
1421. Greater under a constant pressure ib.
1422. Example of the expansion of high
pressure steam - - - - ib.
1423. Low temperature of superior strata
of atmosphere - - - . ib.
1424. Line of perpetual snow - - 55
1425. Liquefaction of gases - - - ib.
1426. Development and absorption of
heat by chemical combination - 56
1427. Specific heats of simple gases equal
under the same pressure - - ib.
1428. Formula for the variation of spe-
cific heat consequent on change
of pressure - 57
1429. Relation between specific heat and
atomic weight - ib.
1430. Tables of specific heat - 58
CHAP. VII.
LIQUEFACTION AND SOLIDIFICATION.
1431. Thermal phenomena attending li-
quefaction - - - -
, CONTENTS.
vii
Sect. Page
1432. Phenomen* developed in progress
of these changes - - - 64
1433. Heat received by melting ice during
liquefaction latent - - - ib.
1434. Quantity of heat rendered latent
in liquefaction - 65
1435. When ice is liquefied quantity of
heat absorbed - ib.
1436. Latent heat of water - - - ib.
1437. Latent heat rendered sensible by
congelation - ib.
1458. Latent heat of water in liquid state
gradually disengaged in process
of congelation - 66
1439. Other methods of determining la-
tent heat of water - ib.
1440. Experimental illustration of this - 67
1441. Liquefaction and congelation must
always be gradual processes - ib.
1442. Processes of congelation and lique-
faction ----- i6.
1143. Water may continue in liquid state
below 32 ----- 68
1444. Explanation of this anomaly - ib.
1445. Useful effects produced by the heat
absorbed in liquefaction and de-
veloped in congelation of water 69
1446. Heat absorbed and developed in the
liquefaction and solidification of
other bodies - ib,
1447. Latent heat of fusion - 70
1448. Points of fusion - - - - ib.
1449. Latent heat of fusion of certain
bodies 72
1450. Facility of liquefaction propor-
tional to the quantity of latent
heat ------ ib.
1451. Other bodies besides water may
continue liquid below the point
of solidification - - - - 73
1452. Refractory bodies - ib.
1453. Alloys liquefy more easily than
their constituents - ib.
1454. Some bodies in fusing pass through
different degrees of fluidity - 74
1455. Singular effects manifested by sul-
phur ib.
1456. Points of congelation lowered by
the solution of foreign matter - ib.
1457. Points of congelation of acid solu-
tions - ib.
1458. Sudden change of volume accom-
panies congelation - ib.
1459. This expansion in the case of water
not identical with that which
lakes place below the point of
greatest density - - - 75
1160. The quantity of expansion pro-
duced in congelation is the same
for the same liquid, at whatever
temperature congelation takes
place ib.
1461. Phosphorus and oils in general
contract in congealing - - 76
1462. Some bodies expand and some con-
tract in congelation - - - ib.
14^3. Why coin is stamped and not cast ib.
1464. Contraction of mercury in cooling ib.
1465. Substances which soften before
fusion 77
14R6. Weldable metals - - - - ib.
1467. Freezing mixtures - - - ib.
1468. Apparatus for producing artificial
cold 79
Sect. Page
1469. TaWe of freezing mixtures - - 80
1470. Extraordinary degrees of artificial
cold produced by Thirolier and
Mitchell ib,
1471. Alcohol^probably congeals at about
1472. Precaution necessary in experi-
ments with freezing mixtures - ib.
1473. Greatest natural cold yet observed ib.
1474. Principle of fluxes. Example of
their application - - - ib.
1475. Infusible bodies - - - - 82
1476. Marble may be fused - 83
1477. Organic bodies are decomposed
before fusion - ib.
1478. Water separated from matter held
in solution by congelation - ib.
1479. Saturated solutions partially de-
composed by cooling - - 84
1480. Anomalous case of anhydrous sul-
phate of soda - ib.
1481. Case in which the matter held in
solution congeals with the water 85
1482. Dutch tears - - - - - ib.
1433. Use of annealing in glass manu-
facture and pottery - ib.
1484. Tempering steel - - - , 86
CHAP. VIII.
VAPORIZATION AND CONDENSATION.
1485. Evaporation of liquids in free air ib.
1486. Apparatus for observing the pro-
perties of vapour ... fo.
14S7. Vapour of liquid an elastic, trans-
parent, and invisible fluid like
1488. How its pressure is indicated and
measured - - - - - ib.
1489. When a space is saturated with
vapour ----- 89
1490. Quantity of vapour in saturated
space depends on temperature - 90
1491. Relation between pressure, tempe-
rature, and density - - - 91
1492. Pressure, temperature, and density
of the vapour of water - - ib.
1493. Vapour produced from water at all
temperatures, however low - ib.
1494. Mechanical force developed in eva-
poration 92
1495. Vapour separated from a liquid
may be dilated by heat like any
gaseous body - - - - 95
1496. Peculiar properties of superheated
vapour - - - - - ib.
1497. Vapour cannot be reduced to the
liquid state by mere compression 96
1493. Vapour which has the greatest
density due to its temperature
under any given pressure will
have the greatest density at all
other pressures, provided it do
not gain or lose heat while the
pressure is changed - - - ib.
1499. Compression facilitates the abstrac-
tion of heat by raising the tem-
perature, and" thus facilitating
condensation - 97
1500. Permanent gases are superheated
vapours - - - - - ib.
4
Till
CONTENTS.
Sect. Page
1501. Processes by which gases have been
liquefied or solidified - -97
1500. Gases which have been liquefied - 98
1503. At extreme pressures gases depart
from the common law of the
density being proportional to
pressure ----- 99
1504. State of ebullition ... - ib.
1505. Boiling point varies with the pres-
sure ------ ib.
1506. Experimental verification of this
principle 100
1507. At elevated stations water boils at
low temperatures - ib.
1508. Table of the boiling pointsof water
at various places - 101
1509. Latent heat of vapour - - - ib.
1510. Different estimates of the latent
heat of th e vapour of water - 1 03
1511. Heat absorbed in evaporation at
different temperatures - - ib.
1512. Latent heat of vapour of water as-
certained by Regnault - - 104
1513. Latent heat of other vapours as-
certained by Fabre and Silber-
mann - - - - - ib.
1514. Condensation of vapour - -105
1515. Why vessels in which liquids are
boiled are not destroyed by ex-
cessive heat - - - - ib.
1516. Uses of latent heat of steam in do-
mestic economy - 106
1517. Method of warming dwelling-
houses - - - - - ib.
1518. Effects of the temperature of dif-
ferent climates on certain liquids ib.
CHAP. IX.
CONDUCTION.
1519. Good and bad conductors - - 107
1520. Experimental illustration of con-
duction it,.
1521. Table of conducting powers - 108
1522. Liquids and gases are non-con-
ductors 109
1523. Temperature equalized in these by
circulation - - - - ib.
1524. Conducting power diminished by
subdivision and pulverization - 110
1525. Beautiful examples of this prm-
ciple in the animal economy - ib.
152fi. Uses of the plumage of birds " - ib.
1527. The wool and fur of animals - 111
1528. The bark of vegetables - - ib.
1529. Properties of the artificial clothing
of man - - - - - ib.
1530. Effects of snow on the soil in winter >b.
1.531. Matting upon exotics - - -112
1532. Method of preserving ice in hot
climates - - - - - ib.
1533. Glass and porcelain vessels why
broken by hot water - - - ib.
153*. Wine coolers .... ib.
1535. A heated globe cools inwards - ib.
1536. Example of a fluid metal cast in
spherical mould - - - 113
1537. Cooling process may be indefinitely
protracted - - - - ib,
1538. Example of the casting of the hy-
draulic press which raised the
Britannia bridge - - . ib.
Sect Page
1539. Example of streams of volcanic
1540. Example of the earth itself - - ib.
1541. Temperature increases with the
depth ib.
1542. The earth was formerly in a state
of fusion, and is still cooling - 115
CHAP. X.
RADIATION.
1543. Heat radiates like light - - ib~
1544. Thermal analysis of solar light - itf
1545. Thermal solar rays differently re-
frangible 116
15ia Physical analysis of solar light
Three spectra - ib.
1547. Relative refrangibility of the con-
stituents of solar light varies with
the refracting medium - - 117
1548. Invisible rays may be luminous
and all rays may be thermal - ib.
1549. Refraction of invisible thermal rays 118
1550. Heat radiated from each point on
the surface of a body - - - ib.
1551. Why bodies are not therefore in-
definitely cooled - - - 119
1552. Radiation is superficial or nearly so ib.
1553. Reflection of heat- - - - ib.
1554. Rate of radiation proportional to
excess of temperature of radia-
tor above surrounding medium 120
1555. Intensity inversely as square of
distance ib.
1556. Influence of surface on radiating
power - - - - - ib,
15. r >7. Reflection of heat - ... ib.
1558. Absorption of heat - - -121
1559. Tabular statement of radiating and
reflecting powers ... ib.
1560. Singular anomaly in the reflection
from metallic surfaces - - 122
1561. Thermal equilibrium maintained
by the interchange of heat by ra-
diation and absorption - - ib.
1562. Erroneous hypothesis of radiation
of cold 123
1563. Transmission of heat - - - ib.
1564. Melloni's thermoscopic apparatus ib.
1565. Results of Melloni's researches - 125
1566. Transparent media not proportion-
ally diathermanous - - - ib.
1567. Decomposition of heat by absorp-
tion ib.
1563. Absorption not superficial but li-
mited to a certain depth - - 126
1569. Physical conditions of diatherma-
nism 127
Iu70. Refraction and polarization of heat 128
1571. Application of these principles to
explain various phenomena - ib.
1572. Experiment of radiated and re-
flected heat with pair of para-
bolic reflectors - - - - ib.
1573. Materials fitted for vessels to keep
liquids warm - - - - 129
1574. Advantage of a polished stove - ib.
1575. Helmets and cuirasses should be
polished - - - - - ib.
1576. Deposition of moisture on window
panes ..... ,//.
1577. Principles which explain the phe-
nomena of dew and hoar-frost - 131
CONTENTS.
Sect. Page
1578. Dew not deposited undera clouded
sky 132
1579. Production of artificial ice by ra-
diation in hot climates - - ib.
CHAP. XL
COMBUSTION.
1580. Heat developed or absorbed in che-
mical combination - - - ib.
1581. This effect explained by specific
heat of compound being less or
greater than that of components 133
1582. Or by heat being developed or ab-
sorbed by change of state - - ib.
1583. Combustion - - - ib.
1584. Flame - - - - ib.
1585. Agency of oxygen - - ib.
1586. Combustibles - - - 131
1587. Combustion explained - - ib.
1588. Temperature necessary to produce
combustion .... ib.
1589. Light of flame only superficial -135
1590. Illuminatingpowerof combustibles ib.
1591. Constituents of combustibles xised
for illumination ... ib.
1592. Spongy platinum rendered, incan-
descent by hydrogen - - ib.
1593. Quantity of heat developed by
combustibles .... 136
1594. Table of the quantities of heat
evolved in the combustion of
various bodies .... ib.
CHAP. XII.
ANIMAL HEAT.
1595. Temperature of organized bodies
not in equilibrium with sur-
rounding medium ... 138
1596. Temperature of the blood in the
human species - - - - ib.
1597. Researches of Davy to determine
the temperature of the blood - ib.
Sect. . Page
1598. Table I. showing the temperature of
theblood of 13 individuals in dif-
ferent climates. Tablell. showing
the temperature of the blood in 6
individuals in different climates.
Table III. showing the tempe-
rature of the blood in the same
individual at different hours of
the day. Table IV. showing the
limits between which the tem-
peratures of the blood in differ-
ent races was observed to vary
in India. Table V. showing the
temperature of the blood ob-
served in different species of ani-
mals 139
1599. Deductions from these observations 141
1600. Birds have the highest and am-
phibia the lowest temperature - 142
1601. Experiments of Breschet and Bec-
querel ib.
1602. Comparative temperature of the
blood in health and sickness - ib.
1603. Other experiments by Breschet and
Becquerel - - - - - ib.
1604. Experiments to ascertain the rate
of development of animal heat ib.
1605. Total quantity of heat explained
by chemical laws without any
special vital cause ... 143
CHAP. XIII.
THE SENSATION OF HEAT.
1606. Indications of the senses fallacious
1607. Sense of touch, a fallacious mea-
sure of heat -
1608. Its indications contradictory - i't.
1609. These contradictions explained > 14S
1610. Examples of the fallacious impres-
sions produced by objects on the
touch -
1611. Feats of fire-eaters explained - ib.
BOOK II.
Magnetism.
CHAP. I.
DEFINITIONS AND PRIMARY PHENOMENA.
Sect. Page
1612. Natural magnets : loadstone - - 149
1613. Artificial magnets - - - ib.
1614. Neutral line or equator : poles - ib.
1615. Experimental illustrations of them 150
1616. Experimental illustration of the
distribution of the magnetic
force - - - - - - ib.
1617. Varying intensity of magnetic force
indicated by a pendulum - - 151
Sect. . Page
1618. Curve representing the varying
intensity 151
1619. Magnetic attraction and repulsion 152
1620. Like poles repel, unlike attract - ib.
1621. Magnets arrange themselves mu-
tually parallel with poles re-
versed 153
1622. Magnetic axis - - - - id.
1623. How ascertained experimentally - ib.
1624. Hypothesis of two fluids, boreal and
austral ..... 155
1625. Condition of the natural or un-
magnetized state ... ib.
CONTENTS.
Sect. Page
1626. Condition of the magnetized state loo
1627. Coercive force - - - - 156
1628. Coercive force insensible in soft
Sect. Page
1658. The magnetic equator - - - 17U
Its form and position not regular - ib.
Variation of the dip going north or
pered steel - ib.
1629. Magnetic substances - - - ib.
CHAP. II.
MAGNETISM BY INDUCTION.
1630. Soft iron rendered temporarily
magnetic - - - - - ib.
Lines of equal dip- - - - 171
1659. Magnetic meridians - - - ib.
1660. Method of ascertaining the decli-
nation of the needles - - ib,
Local declinations - - - 172
1661. Lines of no declination called ago-
1662. Declination in different longitudes
at equator and in lat. 45 - - ib.
1663. Isogonic lines - - - - 173
without contact - - - -157
1665. Position of magnetic poles - - ib.
1633. Magnets with poles reversed neu-
tralize each other - ib.
1631. A magnet broken at equator pro-
duces two magnets - - - 159
1667. Periodical variations of terrestrial
magnetism - ... ib.
1668. Intensity of terrestrial magnetism 175
1H69. Increases from equator to poles. - 176
not attended by its transfer be-
tween pole and pole - ib.
1636. The decomposition is molecular - ib.
1637. Coercive force of iron varies with
its molecular structure - - 160
163S. Effect of induction on hard iron or
1671. Their near coincidence with iso-
thermal lines .... ib.
1672. Equatorial and polar intensities - ib.
1673. Effect of the terrestrial magnetism
on soft iron - - - - ib.
1674. Its effects on steel bars - - 177
1639. Fcrms of magnetic needles and
bars 161
1640. Compound magnet - - - ib.
1641. Effects of heat on magnetism - 162
1642. A red heat destroys the magnetism
of iron .....<&
1643. Different magnetic bodies lose their
magnetism at different tempera-
1676. Disturbances in the magnetic in-
tensity ----- 179
1G77. Influence of the aurora borealis - ib.
CHAP. IV.
MAGNETIZATION.
1644. Heat opposed to induction - - ib.
1645. Induced magnetism rendered per-
manent by hammering and other
mechanical effects - - - ib.
1646. Compounds of iron differently sus-
ceptible of magnetism - - 163
1647. Compounds of other magnetic
bodies not susceptible - - .
1643. Magnets with consequent points - ib.
CHAP. III.
TERRESTRIAL MAGNETISM.
1644. Analogy of the earth to a magnet 164
1650. The azimuth compass - 165
1678. Effects of induction - - - ib.
1679. Their application in the produc-
tion of artificial magnets - -180
1680. Best material for artificial magnets ib.
1681. Best form for bar magnets - - ib.
1682. Horse-shoe magnets - - - ib.
1683. Methods of producing artificial
magnets by friction - - - 181
1684. Method of single touch - - ib.
1685. Method of double touch - - 182
1686. Inapplicable to compass needles
and long bars - - - - 183
1687. Magnetic saturation - - - ib.
1688. Limit of magnetic force - - ib.
1689. Influence of the temper of the bar
on the coercive force - - 184
1690. Effects of terrestrial magnetism on
16.32. The dipping-needle - - -167
1653. Analysis of magnetic phenomena
of the earth - - - - ib.
I6.H. Magnetic meridian - - - 168
1655. Declination or variation - - ib.
1656. Magnetic polarity of the earth - ib.
1G57. Change of direction ofthedipping-
1601. Means of preserving magnetic bars
from these effects by armatures
or keepers ----- 185
1692. Magnetism may lie preserved by
terrestrial induction - - 186
1693. Com]H)tind magnets - - - ib.
1694. Magnetized tracings on a steel
Complete analogy of the earth to a
1695. Influence of heat on magnetic bars ib.
Astatic needle - - - 187
BOOK III.
Electricity.
EPULSIO.NS.
Sect. Page
16ii. Electrical effects - - - - 189
1697. Origin of the name " electricity " ib.
16^8. The electric fluid - - - - 190
lt>99 Positive and negative electricity - 191
1700. Hypothesis of a single electric fluid ib.
1701. Hypothesis of two fluids - - l'J2
1702. Results of scientific research inde-
pendent of these hypotheses - ib.
1703. Hypothesis of two fluids preferred ib.
1704. Explanation of the above effects
produced by the pith balls - ib.
1705. Electricity developed by various
bodies 193
1706. Origin of the terms vitreous and
resinous fluids - - - - ib.
1707. No certain test to determine which
of the bodies submitted to fric-
tion receives positive and nega-
tive electricity - ib.
1708. Classification of positive and nega-
tive substances - - - - 194
1709. Method of producing electricity by
glass and silk with amalgams - 195
; CHAP. n.
CONDUCTION.
1710. Conducting power - - - 196
1711. Conductors and non-conductors - ib.
1712. Classification of conductors accord-
ing to the degrees of their con-
ducting power - ib.
1713. Insul-itors 197
1714. Insulating stools ... - ib.
1715. Electrics and non-electrics obso-
lete terms - - - - - ib.
1716. Two persons reciprocally charged
with contrary electricities placed
on insulating stools - - - 198
1717. Atmosphere a non-conductor - ib.
1718. Karefied air a non-conductor - ib.
1719. Use of the silk string which sus-
pends pith balls - - - 199
1720. Water a conductor - - ib.
1721. Insulators must be kept dry - ib.
1722. No certain test to distinguish con-
ductors from non-conductors - ib.
1723. Conducting power variously af-
fected by temperature - - 2CO
1724. Effects produced by touching an
electrified body by a conductor
which is not insulated - - ib.
1725. Effect produced when the touching
conductor is insulated - - 201
1726. Why the earth is called the com-
mon reservoir - ib,
1727. Electricity passes by preference on
the best conductors - ib.
CHAP. lit.
INDUCTION.
Sect. Page
1728. Action of electricity at a distance 202
1729. Induction defined - - - ib.
17JO. Experimental exhibition of its ef-
fects ...... 203
17.31. Effects of sudden inductive action 205
1732. Example in the case of a frog - ib.
1733. Inductive shock of the human
body ...... 206
1734. Development of electricity by in-
duction - - - - - ib.
CHAP. IV.
ELECTRICAL MACHINES.
1735. Parts of electrical machines - - 207
1736. The common c>lindrical machine ib.
1737. Nairne's cylinder machine - - 209
1738. Common plate machine - - ib.
17o9. Armstrong's hydro- electrical ma-
chine - - - - -210
1740. Appendages to electrical machines 212
1741. Insulating stools - ib.
1742. Discharging rods - - - 213
1743. Jointed dischargers - ib.
1744. Universal discharger ... ib.
CHAP. V.
CONDENSER AND ELECTROHIOROUS.
1745. Reciprocal inductive effects of two
conductors -
17+n. Principle of the condenser -
1747. Dissimulated or latent electricity
1748. Free electricity -
1749. Forms of condenser
1750. Collecting and condensing plates
1751. Cuthbertson's condenser
1752. The electrophorous
CHAP. VI.
ELECTROSCOPES.
1753. General principle of electroscopes 220
1754. Pith ball electroscope - - - 221
1755. Needle electroscope ... ib.
1756. Coulomb's electroscope - - 222
1757. Quadrant electrometer - - ib.
1758. Hold leal' electroscopes - - - ib.
1759. Condensing electroscope - - 2i!3
CHAP. VII.
THE LEVDEN JAR.
1760. The Leyden jar - - - - 227
1761. Effect of the metallic coating - 228
Sect. Page
1762. Water may be substituted for the
metallic coating - - -229
1763. Experimental proof that the
charge adheres to the glass and
not to the coating ... ib.
CHAP. X.
THERMAL EFFECTS OF ELECTRICITY.
Sect. Page
1799. A current of electricity passing
over a conductor raises its tem-
11(3. Charging a series of jars by cascade 231
1766. Electric battery - ... ft.
1767. Common electric battery - - 32
1768. Method of indicating and estimat-
ing amount of charge - - ib.
CHAP. VIII.
LAWS OF ELECTRICAL FORCES.
1769. Electric forces investigated by
Coulomb 234
1770. Proof-plane 235
1771. Law of electrical force similar to
that of gravitation - ib.
1772. Distribution of the electric fluid
on conductors - 236
1773. It is confined to their surfaces - ib.
1774. How the distribution varies - - 237
1775. Distribution on an ellipsoid - - ib.
1776. Effects of edges and points - -ib.
1777. Experimental illustration of the
effect of a point ... 238
1778. Rotation produced by the reaction
of points ----- 239
1779. Another experimental illustration
of this principle - - - 240
1780. Electrical orrery - - - - ib.
CHAP. JX.
1SOO. Experimental verification Wire "
heated, fused, and bunit - - 250
1801. Thermal effects are greater as the
conducting power is less - ib.
1802. Ignition of metals . - - ib.
1803. Effect on fulminating silver - 251
1804. Electric pistol - - ib.
18()5. Ether and alcohol ignited - ib.
1806. Resinous powder burnt - 252
1807. Gunpowder exploded - - ib.
1808. Electric mortar - - ib.
1809. Kinnersley's electrometer - ib.
CHAP. XI.
-LUMINOUS EFFECTS OF ELECTRICITY.
1810. Electric fluid not luminous - - 253
1811. Conditions under which light is
developed by an electric current ib.
1812. The electric spark - - - 254
1813. Electric aigrette - - - - 255
1814. The length of the spark - - ib.
1815. Discontinuous conductors produce
luminous effects - - - ib.
1816. Various experimental illustrations ib.
1817. Effect of rarefied air - - -250
1818. Experimental imitation of the
auroral light - - - - 257
1819. Phosphorescent effect of the spark ib.
1820. Leichtenberg's figures - - - ib.
18521. Ex|>eriments indicating specific
differences between the two
1781. Attractions and repulsions of elec-
1822. Electric light above the barometric
trified bodies - - - - 241
1782. Action of an electrified body on a
non-conductor not electrified - ib.
1783. Action of an electrified body on a
non-conductor charged with like
1823. Cavendish's electric barometer - 259
1824. Luminous effects produced by im-
perfect conductors - ib.
1825. Attempts to explain electric light
The thermal hypothesis - ib.
1,84. Its action on a non-conductor
charged with opposite electricity ib.
1785. Us action on a non-conductor not
182S. Hypothesis of decomposition and
recomposition - 260
1827. Cracking noise attending electric
spark 61
1786. Its action on a conductor charged
with like electricity - - - 243
1/87. Its act ion upon a conductor charged
with opposite electricity - - 244
1788. Attractions and repulsions of pith
balls explained - - - - ib.
1/89. Strong electric charges rupture
imperfect conductors Card
pierced by discharge of jar - 245
1/90. Curious fact observed by M.
CHAP. XII.
PHYSIOLOGICAL EFFECTS OF ELECTRICITY.
1828. Electric shock explained - - ih.
1829. Secondary shock - - - - 262
1830. Effect produced on the skin by
proximity to an electrified body ib.
1831. Effect of the spark taken on the
knuckle 263
1791. Wood and glass broken by dis-
1832. Methods of limiting and regulat-
ing the shock by a jar - - ib.
1792. EleWrica. belf, .' I -' I ^
1 /93. Repulsion of electrified threads - 247
1'iH- Cu ^ s em * 1 ot repulsion of pith
1834. Phenomena observed in the au-
topsis after death by the shock ib.
1795. Electrical dance - - - . J*'
1796. Curious experiments on electrified
derate discharges ... ,$.
1836. Effects upon a succession of pa-
1797. Experiment with electrified sealing *
charge - - - - - ib.
1798. Electrical see-saw . 249
1S37. RemarkableexperimentsofNollet,
Dr. Watson, and others - id.
CONTENTS.
ELECTRICITY.
Page
1838. Phenomena which supply the basis
of the electro-chemical theory - 261
Sect. ' Page
1839. Faradav's experimental illustra-
tion of this - - - -265
1S40. Effect of an electric discharge on
a magnetic needle - ib.
1841. Experimental illustration of this - 266
BOOK IV.
Voltaic Electricity.
CHAP. I.
SIMPLE VOLTAIC COMBINATION.
Sect. Page
1842. Discovery of galvanism - -267
1843. Volta's correction of galvanic
theory ----- 268
1845. Contact hypothesis of Volta - ib.
1846. Electro-motive force - - - ib.
1847. Classification of bodies according
to their electro-motive property 270
1848. Relation of electro-motive force
to susceptibility of oxydation - 271
1849. Analogy of electro-motive action
to induction - ib.
1850. Electro-motive action of gases and
liquids ..... 272
1851. Difference of opinion as to origin
of electro- motive action - -273
1852. Polar arrangement of the fluids in
all electro-motive combinations 274
1853. Positive and negative poles - - ib.
1854. Electro-motive effect of a liquid
interposed between two solid
conductors - ib.
1855. Electro-motive action of two
liquids between two solids - 276
1856. Practical examples of such combi-
nations - - - - - ib.
1857. Most powerful combinations deter-
mined ..... 277
1858. Form of electro-motive combina-
tion ...... ib.
1859. Volta's 6rst combination - - 278
1860. Vollaston's combination - - ib.
1861. Hare's spiral arrangement - - ib.
1862. Amalgamation of the zinc - - ib.
Ib63. Cylindrical combinations with one
fluid ...... 279
1864. Cylindrical combinations with two
fluids ..... 280
1865. Grove's battery - ib.
18fi6. Bunsen's battery - ... ib.
1867. Daniel's constant battery - - 281
1868. Pouillet's modification of Daniel's
battery ..... ib.
1869. Advantages and disadvantages of
these several systems - - 282
1870. Smee's battery .... ,b.
1871. Wheatstone's system - - - 283
1872. Bagration's system ... ib.
1873. Becquerel's system - ib.
1874. Schonbein's modification of Bun-
sen's battern .... 284
Sect. Page
1875. Grove's gas electro-motive appa-
ratus - - - - - -284
CHAP. II.
VOLTAIC BATTERIES.
1876. Volta's invention of the pile - -
1877. Explanation of the principle of the
pile
1878. Effect of the imperfect liquid con-
ductors - - - - -
1879. Method of developing electricity
in great quantity -
1880. Distinction between quantity and
intensity important -
1881. Volta's Brst pile - - - -
1882. The couronne des tasses
1883. Cruikshank's arrangement -
1884. Wollaston's arrangement
1885. Heliacal pile of Faculty of Sciences
at Paris
1886. Mode of forming piles -
1887. Conductors connecting the ele-
ments -'-...
1888. Pile may be placed at any distance
from place of experiment -
1889. Memorable piles Davy's pile at
Royal Institution -
1890. Napoleon's pile at Polytechnic
School -
1891. Children's great plate battery
1892. Hare's deflagrator
18!'3. Stratingh's deflagrator -
1894. Pepys' pile at London Institution
1895. Powerful batteries on Daniel's and
Grove's principles -
1896. Dry piles -
1897. Deluc's pile
ic's pile
1898. Zamboni's pile
1899. Piles of a single metal -
1900. Ritter's secondary piles
CHAP. HI.
VOLTAIC CURRENTS.
1901. The voltaic current - ib.
1902. Direction of the current - .297
190a Poles of the pile, how distinguished 298
1904. Voltaic current - ib.
1905. Case in which the earth completes
the circuit ----- 299
1906. Methods of connecting the poles
with the earth - - - - SCO
CONTENTS.
Sect. Page
1!)07. Various denominations of currents 300
1908. The electric fluid forming the cur-
rent not necessarily in motion - ib.
1909. Method of coating the conducting
wire ------ 301
1910. Supports of conducting wire- - ib.
19U. Ampere's reotrope to reverse the
1912. Pohl's reotrope - - - - 303
1913. Electrodes ib.
1914. Floating supports for conducting
wire 304
1915. Ampere's apparatus for supporting
moveable currents - ib.
1916. Mutual action of magnets and cur-
rents
1917. Electro-magnetism '-
1918. Direction of the mutual forces ex-
erted by a rectilinear current on
the poles of a magnet
1919. Circular motion of magnetic pole
round a fixed current
1920. Circular motion of a current round
a magnetic pole -
1921. Apparatus to illustrate experiment-
ally these effects -
1922. Apparatus to exhibit direction of
force impressed by a rectilinear
current on a magnetic pole
1923. Apparatus to measure the inten-
sity of this force - - -
1924. Intensity varies inversely as the
distance - - ...
1925. Case in which the current is with-
in but not at the centre of the
circle in which the pole revolves
19^6. Action of a current on a magnet,
both poles being free - - I
1927. Case in which the current is out-
side the circle described by the
1928. Case in which the current passes
through the circle - -
1929. Case in which the
within the circle ...
1930. Apparatus to illustrate eleutro-
, , magnetic rotation ...
TJjl. To cause either pole of a magnet
to revolve round a fixed voltaic
current - ....
1932. To cause a moveable current to
revolve round the fixed pole of
a magnet -
1933. Ampere's method -
1J34. To make a magnet turn on its own
axis by a current parallel to it -
urrent passes
CHAP. V.
RECIPROCAL INFLUENCE OF CIRCULATING
CURRENTS AND MAGNETS.
1935. Front and back of circulating cur-
rent - - _ . . .
1936. Axis of current - - . .
1337. Ueciprocal action of circulating
current and magnetic pole -
Sect. Page
1938. Intensity of the force vanishes
when distance of pole bears a
very great ratio to diameter of
current - 322
1939. But directive power of pole con-
tinues ib.
J940. Spiral and heliacal currents - - ib.
1941. Expedient to render circulating
currents moveable - 323
1942. Rotatory motion imparted to cir-
cular current by a magnetic pole ib.
1943. Progressive motion imparted to it 324
1944. Reciprocal action of the current on
pole ib.
1945. Action of a magnet on a circular
floating current ... ib.
1946. Reciprocal action of the current on
magnet - - - - - 325
1947. Case of instable equilibrium of the
current - - - - - ib.
1918. Case of a spiral current - - ib.
1949. Circular or spiral currents exercise
same action as a magnet - - 326
1950. Case of heliacal current - - ib.
1951. Method of neutralizing effect of
the progressive motion of such a
current - - - - -id.
1952. Right-handed and left-handed he-
lices ib.
1933. Front of current on each kind - ib.
1954. Magnetic properties of heliacal cur-
rents, their poles determined - ib.
1955. Experimental illustration of these
properties ----- 327
1956. The front of a circulating current
has the properties of a south and
the back those of a north mag-
netic pole - - - - - ib.
1957. Adaptation of an heliacal current
to Ampere's and Delarive's ap-
paratus - 323
1958. Action of an heliacal current on a
magnetic needle placed in its
axis 329
ELECTRO-MAGNETIC INDUCTION.
Inductive effect of a voltaic cur-
rent upon a magnet -
Magnetic induction of an heliacal
current - - ...
Polarity produced by the induction
of heliacal current ...
Consequent points produced -
Inductive action of common elec-
magnetized positively and nega-
tively
Results of Savary's experiments -
Magnetism imparted to the needle
affected by the non-magnetic
substance which surrounds it -
Formation of powerful electro-
force of magnet -
Electro-magnet of Faculty of Sci-
1970. Form of electro-magnets in general 33o
1971. Electro-magnetic power applied as
mechanical agent ... ib.
CONTEXTS.
Sect. Page
1972. Electro-motive power applied in
workshop of M. Frotnent - - 337
1973. Electro-motive machines con-
structed by him ... 339
1974. Applied as a sonometer- - -343
1975. Momentary current by induction - ib.
1976. Experimental illustration - - 344
1977. Momentary currents produced by
magnetic induction - - - 345
1978. Experimental illustrations - - ib.
1979. Inductive effects produced by a
permanent magnet revolving
under an electro-magnet - - 346
7!>80. Use of a contact breaker - - 348
1981. Magneto-electric machine - - ib.
1982. Effects of this machine, its medical
use 350
1983. Inductive effects of the successive
convolutions of the same helix ib.
Sect. Page
2003. The dip of a current illustrated by
Ampere's rectangle ... 362
CHAP. VIII.
RECIPROCAL INFLUENCE OF VOLTAIC
CURRENTS.
2004. Results of Ampere's researches - 363
2005. Reciprocal action of rectilinear
currents ----- 364
2006. Action of a spiral or heliacal cur-
rent on a rectilinear current - ib.
2007. Mutual action of diverging or con-
verging rectilinear currents - 365
2008. Experimental illustrations of this ib.
2009. Mutual action of rectilinear cur-
rents which are not in the same
plane 366
2010. Mutual action of different parts of
the same current ... 367
2011. Ampere's experimental verifica-
tion of this - ... ib.
2012. Action of an indefinite rectilinear
current on a finite rectilinear
current at right angles to it - 368
2013. Case in which the indefinite cur-
rent is circular - 369
2014. Experimental verification of these
principles ib.
2015. To determine in general the ac-
tion of an indefinite rectilinear
current on a finite rectilinear
current ----- 370
2016. Experimental illustration of these
principles ----- 373
2017. Effect of a straight indefinite cur-
rent on a system of diverging or
converging currents - - - ib.
2018. Experimental illustration of this
action 374
2019. Consequences deducible from this
action ib.
2020. Action of an indefinite straight
current on a circulating current 375
2021. Case in which the indefinite
straight current is perpendicular
to the plane of the circulating
current ----- 376
2022. Case in which the straight current
is oblique to the plane.of the cir- .
culating current - 377
2023. Reciprocal effects of curvilinear
currents - - - - - ib.
2024. Mutual action of curvilinear cur-
rents in general ... 373
CHAP. IX.
currents produced upon revolv-
ing metallic disks : researches of
Arago, Herschel, Babbage, and
Faraday 351
CHAP. VII.
OX VOLTAIC CURRENTS.
1985. Direction of the earth's magnetic
1986. In this part of the earth it corre-
sponds to that of the boreal pole
of an artificial magnet - -354
1987. Direction of the force impressed
by it upon a current - ib.
1988. Effect of terrestrial magnetism on
.1 vertical current - 355
19S9. Effect upon a horizontal current
directed north and south - - ib.
1990. Case of a horizontal current di-
rected east and west - - - ib.
1991. Case of a horizontal current in
any intermediate direction - ib.
1992. Effect of the earth'smagnetism on
a vertical current which turns
round a vertical axis - - - 356
1993. Effect on a current which is ca-
pable of moving in an horizontal
plane ib.
1994. Experimental illustrations of these
effects ; Pouillet's apparatus - ib.
1995. Ils application to show the effect
of terrestrial magnetism on a
horizontal current- - - - 358
1996. Its effects on vertical currents
shown by Ampere's apparatus - 359
1P97. Its effects on a circular current
1998. Its effect on a circular or spiral
current shown by Delarive's
floating apparatus - ib.
1999. Astatic currents formed by Am-
p&re's apparatus - - - 360
2000. Effect of earth'smagnetism on spi-
ral currents shown by Ampere's
apparatus ib.
2001. Effect on horizontal current shown
by Pouillet's apparatus - - 361
2002. Effect of terrestrial magnetism on
an heliacal current shown by
Ampere's apparatus - - 362
2025. Circulating currents have the mag-
netic properties - - - ib
2026. Magnetism of the earth may pro-
ceed from currents - 379
2027. Artificial magnets explained on
this hypothesis - - - - ib.
2028. Effect of the presence or absence
of coercive force - - -380
2029. This hypothesis cannot be admitted
as established until the exist-
ence of the molecular currents
shall be proved ... ib.
CONTENTS.
CHAP. X.
BEOSCOPES AND REOMETER8.
Sect. Page
2030. Instruments to ascertain the pre-
sence and to measure the inten-
sity of currents ... 380
2031. Expedient for augmenting the ef-
fect of a feeble current - - 381
032. Method of constructing a reoscope,
galvanometer, or multiplier - 382
2033. Nohili's reometer - - - 383
20o4. Differential reometer - - - 384
2035. Great sensitiveness of these instru-
ments illustrated - ib.
THERMO-ELECTRICITY.
2036. Disturbances of the thermal equi-
librium of conductors produces
a disturbance of the electric
equilibrium ... - ib.
>7. Thermo-electric current - - 385
203H. Experimental illustration - - ib.
2039. Conditions which determine the
direction ot'current - - - ib.
2040. A constant difference of tempera-
ture produces a constant current 386
20H. Different metals have different
thermo-electric energies - - ib.
012. Pouillufs thermo-electric appa-
ratus - - - - - ib.
2
4'4
151-4
32
2.v<;
896
2-4
V6-6
38
30-4
36-4
67
536
152-6
31
24-8
87-8
3-2
24-8
39
Sl-2
3S-2
5i-8
1508
30
24-
86-
4-
23-
40
32-
40-
6->
52-
149-
1338. Rate of dilatation of mercury. It has been ascertained
by experiment, that mercury, when raised from 32 to 212,
Buffers an increment of volume amounting to 2-lllths of its
volume at 32. Thus, 111 cubic inches of mercury at 32 will,
if raised to 212, become 113 cubic inches. From this may be
deduced the increment of volume which mercury receives for
U HEAT.
each degree of temperature. For, since the increase of volume
corresponding to an elevation of 180 is yfy of its volume at
32, we shall find the increment of volume corresponding to
one degree by dividing r 'j^- by 180, or, what is the same,
by dividing ^--j^- by 90, which gives -g^Vo- I* follows, there-
fore, that for each degree of temperature by which the mercury
is raised, it will receive an increment of volume amounting
to the 9990th part of its volume at 32. It follows, therefore,
that the weight of mercury which fills the portion of a ther-
mometric tube representing one degree of temperature, will
be the 9990th part of the total weight contained in the bulb
and tube.
1339. Its dilatation uniform between the standard points,
In adopting the dilatation of mercury as a measure of tem-
perature, it is assumed that equal dilatations of this fluid are
produced by equal increments of heat. Now, although it is
certain that to raise a given quantity of mercury from the
freezing to the boiling point will always require the same
quantity of heat, it does not follow that equal increments of
volume will correspond to equal increments of heat throughout
the whole extent of the thermometric scale. Thus, although
the same quantity of heat must always be imparted to the
mercury contained in the tube to raise it from 32 to 212, it
may happen that more or less heat may be required to raise it
from 32 to 42, than from 202 to 212. In other words, the
dilatation produced by equal increments of heat, in different
parts of the scale, might be variable. Experiments conducted,
however, under all the conditions necessary to ensure accurate
results, have proved that mercury is uniformly dilated between
the freezing and boiling points, or that equal increments of
heat imparted to it produce equal increments of volume. The
same uniform dilatation prevails to a considerable extent of the
scale above the boiling and below the freezing points ; but at
extreme temperatures this uniformity of expansion ceases, as
will be more fully explained hereafter.
1340. Use of a standard thermometer. A thermometer
having once been carefully graduated may be used as a standard
instrument for graduating other thermometers, just as good
chronometers once accurately set are used as regulators for
other time-pieces. To graduate a thermometer by means of
such a standard, it is only necessary to expose the two instru-
ments to the same varying temperatures, and to mark upon the
THERMOMETRY. 15
blank scale of that which is to be graduated two points corre-
sponding to any two temperatures shown by the standard ther-
mometer, and then to divide the scale accordingly.
Thus, for example, if the two instruments be immersed in
warm water and the column of the standard thermometer be
observed to indicate the temperature of 150, let the point at
which the mercury stands in the other thermometer be marked
upon its scale.
Let the two instruments be then immersed in cold water and
let us suppose that the standard thermometer indicates 50.
Let the point at which the instrument to be graduated stands
be then marked. Let the intervals of the scale between these
two points, thus corresponding to the temperatures of 50 and
150, be divided into one hundred equal parts ; each part will be
a degree in the scale, which may be continued by like divisions
above 150 and below 50.
1341. Range of the scale of thermometers varies with the
purpose to which they are applied. The range of the scale of
thermometers is determined by the purpose to which they are
to be applied. Thus, thermometers intended to indicate the
temperature of dwelling-houses need not range above or below
the extreme temperatures of the air, and the scale does not
usually extend much below the freezing point nor above 100;
and thus the sensitiveness of the instrument may be increased,
since a considerable length of the tube may represent a limited
range of the scale.
1342. Qualities which render mercury a convenient ther-
moscopic fluid. Mercury possesses several thermal qualities
which render it a convenient fluid for common thermometers.
It is highly sensitive to change of temperature, dilating with
promptitude by the same increments of heat with great regu-
larity and through a considerable range of temperature. It
will be shown hereafter that a smaller quantity of heat produces
in it a greater dilatation than in most other liquids. It freezes
at a very low and boils at a very high temperature. At the
temperatures which are not near these extreme limits, it expands
and contracts with considerable uniformity.
The freezing point of mercury being 40, or 40 below
zero, and its boiling point + 600, such a thermometer will
have correct indications through a very large range of tem-
perature.
1343. Bulbs liable to a permanent change of capacity, which
16 HEAT,
renders correction of scale necessary. It has been found that,
from some physical causes which are not satisfactorily ex-
plained, the bulbs of thermometers are liable to a change of
magnitude after the lapse of a certain time. It follows from
this that a thermometer, though accurately graduated when first
made, may become at a later period erroneous in its indications ;
since a diminution of the capacity of the bulb would cause the
standard points and all other temperatures to be raised upon the
scale. To obviate this, thermometers used for purposes re-
quiring much precision ought to be verified from time to time
by comparison with well-constructed standards, or by exposure
to the standard temperatures.
It is also found that a change of magnitude is produced in the
bulb of a thermometer by sudden changes of temperature, which
render verification necessary.
1344. Self -registering thermometers. It is sometimes needed,
in the absence of an observer, to ascertain the variations
which may have taken place in a thermometer. Instruments
called self-regulating thermometers have been contrived, which
partially serve this purpose by indicating, not the variations of
the mercurial column, but the limits of its play within a given
time. This is accomplished by floating indices placed on the
mercury within the tube, which are so adapted that one is
capable of being raised with the column, but not depressed,
and the other of being depressed but not raised. The con-
sequence is, that one of these indices will remain at the highest,
and the other at the lowebt point which the mercurial column
may have attained in the interval, and thus register the highest
point and lowest point of its range.
The self-regulating thermometers on this principle which are
the best known are Sykes and Rutherford's.
13-15. Spirit of wine thermometers. Alcohol is frequently
used as a thermoscopic liquid. It has the advantage of being
applicable to a range of temperature below the freezing point of
mercury ; no degree of cold yet observed in nature or attained
by artificial processes having frozen it. It is usually coloured
so as to render the column easily observable in the tube.
1346. Air thermometers. Atmospheric air is a good ther-
moscopic fluid. It hns the advantage over liquids in retaining
its gaseous state at all temperatures, and in the perfect uni-
formity of its dilatation and contraction. It is also highly
sensitive, indicating changes of temperature with great promp-
THERMOMETRY. 17
titude. Since, however, it is not visible, its expansion and
contraction must be rendered observable by expedients which
interfere with and render complicated its indications.
1347. DrebbeVs air thermometer. The air thermometer of
Drebbel, or according to some of Sanctorius, is represented in
j ^^ fig. 426. A glass tube, AB, open at one end, and
) & having a large thin bulb C at the other, is placed with
B its open end in a coloured liquid, so that the air
contained in the tube shall have a less pressure than
the atmosphere. A column of the liquid will there-
A fore be sustained in the tube AB, the weight of which
will represent the difference between the pressure of
\ the external air and the air inclosed in the tube.
If the bulb c be exposed to a varying temperature,
F *^ ie a ^ r included i n it will expand and contract, and
will cause the column of coloured liquid in the tube
A B to rise and fall, thereby indicating the changes of tem-
perature.
1348. AmontorCs air thermometer. Another form
of air thermometer is represented in^. 427. The
air included fills half the capacity of the bulb c, and
its expansion and contraction cause the coloured
liquid to rise or fall in the tube A B.
1 349. The differential thermometer. Of all forms
of air thermometer, that which has proved of greatest
use in physical enquiries is the differential ther-
mometer represented in fig. 428. This consists of
Fig 4^7 * wo Sl ass bulbs, A and B, connected by a rectangular
glass tube. In the horizontal part of the tube a
small quantity of coloured liquid (sulphuric acid, for example)
is placed. Atmospheric air is contained in the bulbs and tube,
10 O 10 SO
Fig. 428.
separated into two parts by the liquid. The instrument is so
adjusted that, when the drop of liquid is at the middle of the
18 HEAT.
horizontal tube, the air in the bulbs has the same pressure ; and,
having equal volumes, the quantities at each side of the liquid
are necessarily equal. If the bulbs be affected by different
temperatures, the liquid will be pressed from that side at which
the temperature is greatest, and the extent of its departure from
the zero or middle is indicated by the scale.
This thermometer is sometimes varied in its form and
arrangement, but the principle remains the same.
Its extreme sensitiveness, in virtue of which it indicates
changes of temperature too minute to be observed by common
thermometers, renders it extremely valuable as an instrument
of scientific research.
By this instrument, changes of temperature not exceeding
the 6000th part of a degree are rendered sensible.
1350. Pyrometers adapted to measure high temperatures.
The range of the mercurial thermometer being limited by the
boiling point of mercury, higher temperatures are measured by
the expansion of solids, whose points of fusion are at a very
elevated part of the thermometric scale. The solids which are
best adapted for this purpose are the metals. Being good con-
ductors, these are promptly affected by heat, and their in-
dications are immediate, constant, and regular.
Instruments adapted for the indication and measurement of
this high range of temperature are called pyrometers.
1351. Graduation of a pyrometer. To graduate a pyrometer,
let the metallic bar be immersed successively in melting ice
and boiling water, and let its lengths at these temperatures be
accurately measured. Their difference being divided by 180,
the quotient will be the increment of length corresponding to
6ne degree of temperature ; and this increment being multiplied,
the length corresponding to any proposed temperature may be
ascertained.
Let L express the length of the bar at the temperature 32.
Let L' express its length at the temperature 212.
Let i express the increase of length corresponding to 1.
We shall then have
~ 180 '
If L express in general the length of the bar at the tem-
perature expressed by T, we shall have
L = L + ZX (T-32),
THERMOMETRY.
19
which means nothing more than that the length at the tem-
perature T is found by adding to the length at the temperature
32 as many times the increment corresponding to 1 as there
are degrees in T above 32.
The instrument represented in Jig. 429. is one of the most
simple forms of pyrometer.
u
Fig. 429.
A rod of metal, t, is in contact at one end with the point of
a screw v, and at the other with a lever a, near its fulcrum.
This lever is connected with another so as to form a compound
system, such that any motion imparted by the rod to the point
on the lever a in contact with it is augmented in a high ratio,
according to the principles explained in (438). A lamp placed
under the rod t raises its temperature ; and, as it is resisted by
the point of the screw v, its dilatation must take eifect against
the lever a, which, acting on the second lever, will move the
index on the graduated arc c. The ratio of this motion to
that of the end of the bar acting on the lever being known
(438), the quantity of dilatation may be calculated.
1352. Temperature of metallic standard measures must be
observed. The standards used as measures of length for as-
certaining distances where great accuracy is required, such as
in measuring the bases in geographical surveys, are usually
rods of metal. But since these are subject to a change of
length with every change of temperature, it would follow that
the results of any measurement made by them would be at-
tended with corresponding errors.
20 HEAT.
For the common purposes of domestic and commercial
economy, such errors are too trifling to be worth the trouble
of correcting ; but this is not the case when they are applied to
scientific purposes. It is necessary in such cases to observe
the temperature of the rods at the moment each measurement
is made.
1353. Borders pyrometric standard measure. In the oper-
ation by which the great arc of the meridian in France was
measured, a very beautiful expedient was contrived by Borda,
in which the bar itself is converted into a thermometer which
indicates its own temperature. This expedient was again
rendered available for the series of experiments made by
Dulong and Petit, to ascertain the dilatation of bodies by heat.
A bar of platinum, PP', Jig. 430., was connected at one ex-
tremity with a similar bar of brass BB', of very nearly equal
length.
..a
Fig. 430.
The two bars, being screwed or rivetted together at the
extremity B, were free at every other point. Near the ex-
tremity P' of the bar of platinum, and immediately under the
extremity B' of the brass bar, a very exact scale was engraved,
the divisions of which marked the millionth part of the entire
length of the rod. The extremity B' of the brass bar carried
an index, which moved upon the divided scale. Over the point
of this was placed a microscope M, by which its position could
be ascertained, and by which the divisions of the scale could be
more exactly read off.
If the two bars, P P' and B B', were equally dilatable, it is
evident that the same change of temperature affecting both
would make no change in the position of the index ; but, brass
being more dilatable than platinum, the index pushed by the
expansion of the bar B B' would be moved towards P' through
a space greater than that by which the bar p p' would be
lengthened, and, consequently, it would be advanced upon the
scale through a space equal to the difference between the dila-
tation of the two bars.
The manner of graduating the scale upon p P' was as follows.
The compound bar being submerged in a bath of melting ice,
THERMOMETRY.
21
A
7 nr
the position of the index was observed. It was then transferred
to a bath of boiling water, when the position of the index was
again observed.
The interval between these two positions being divided into
180 equal parts, each part would represent one degree of tem-
perature ; or, if such division were too minute to be practicable,
it might be divided into a less number of equal parts, as, for
example, 36, in which case each division would correspond to
5. When the index, as most frequently happens, stands be-
tween two divisions of the scale, it is necessary to estimate or
measure its distance from one of these divisions, in order to
express its exact position. This is accomplished by a con-
trivance called a vernier, which, as it is of great use in all cases
where the observation of scales is necessary in science and the
arts, it may be useful here to describe.
1354. Construction and use of a vernier. The vernier is a
contrivance which, by a subsidiary scale, supplies the means of
estimating small fractions of the smallest
division marked on the principal scale.
Let A B, Jig. 431., represent a part of
the principal scale. Let c D be the sliding-
scale or vernier, which we will suppose
to consist of 10 divisions equal in their
total length to 11 divisions of the prin-
cipal scale. Each division of the vernier
will therefore be equal to eleven-tenths
of a division of the chief scale, and will
exceed a division of the chief scale by a
tenth of a division.
Let us suppose that the index, D, of the
vernier (which coincides with its zero),
stands, as in fig. 432., at M, between the
divisions marked 55 and 56, and that the
question is to estimate how much it is
above 55. Observe what division of the
scale coincides either exactly or most
nearly with a division of the vernier.
The number of the vernier which stands
at such division of the scale will express
the number of tenths of a division of the
chief scale between the index of the ver-
Fig. 431.
Fig. 432.
22 HEAT.
nier and the 55th division of the chief scale. In the present
case, the 4th division of the vernier coincides nearly with the
51st division of the chief scale. The point on the chief scale
indicated, therefore, by the vernier, is 55-4.
It is evident that the distance from the 55th division of the
chief scale to the point in, which coincides with the index or zero
of the vernier, is the difference between 4 divisions of the
vernier and 4 divisions of the chief scale ; and since a division
of the vernier exceeds a division of the scale by a tenth, 4
divisions of the vernier exceed 4 of the scale by four-tenths.
CHAP. III.
DILATATION OF SOLIDS.
1355. Solids least susceptible of dilatation. Of all the states
of aggregation of matter, that in which it is least susceptible of
dilatation is the solid state. This may be explained by the
energy of the cohesion of the component particles of the body,
which is the characteristic property of the solid state. It is the
nature of heat, by whatever hypothesis that agency be ex-
plained, to introduce a repulsive force among the molecules of
the body it pervades. In solid bodies -this repulsive force,
acting against the cohesive force, diminishes the tenacity of the
body. The component parts have a tendency to separate from
each other, and hence arises the phenomenon of dilatation ; but so
long as the body preserves the character of solidity, the separation
of the component molecules cannot exceed the limits of the play
of the cohesive principle ; and as these limits are very small, no
dilatation which is consistent with the character of a solid can
be considerable.
1356. Homogeneous solids dilate equally throughout their
volume. If a solid body be perfectly homogeneous, it will
dilate uniformly throughout its entire volume by an uniform
elevation of temperature. Thus, the length, breadth, and depth
will, in general, be all augmented in the same proportion.
1357. Dilatation of volume and surface computed from linear
dilatation. It is a principle of geometry, that when a solid
DILATATION OF SOLIDS. 23
body, without undergoing any change of figure, receives a small
increase of magnitude, its increase of surface will be twice, and
its increase of volume thrice, the increase of its linear dimen-
sions. That is to say, if its length be augmented by a thou-
sandth part of its primitive length, its surface will be augmented
by two thousandth parts of its primitive surface, and its volume
by three thousandth parts of its primitive volume. This is not
true in a strictly mathematical sense, but it is sufficiently near
the truth for all practical purposes.
Now, since all solid bodies of uniform structure, when affected
by heat, expand or contract without suffering any change of figure,
and since, while their change of their linear dimensions can be
easily and exactly ascertained, that of their surface or volume
would be determined with much more difficulty, the changes of
these last are deduced from the first by multiplying it by 2 for
the increment of surface, and by 3 for the increment of volume.
Thus, if it be found that a bar of zinc being raised from 32
to 212, receive an increment of length equal to the 340th part
of its length at 32, it may be inferred that its increment of
surface is two 340th parts, and that its increment of volume is
three 340th parts of its volume at 32.
1358. Dilatation of solids uniform between the standard
thermometric points. It is found that solid bodies in general
suffer an uniform rate of dilatation, through a range of tem-
perature extending from 32 to 212 ; that is to say, the incre-
ments of volume which attend each degree of temperature which
the body receives are equal. If, therefore, the entire incre-
ment of volume which such a body undergoes when it is raised
from 32 to 212 be divided by 180, the quotient will be the in-
crement of volume which it receives when its temperature is
raised one degree.
1359. Dilatation ceases to be uniform near the point of
fusion. When solids are elevated to temperatures much above
212, and more especially when they approach those tempera-
tures at which they would be fused or liquefied, the dilatations
are not uniform. As the temperature is raised, the rate of
dilatation is increased, that is to say, a greater increment
of volume attends each degree of temperature.
1360. Exceptional cases presented by certain crystals. There
are also certain exceptional cases in some crystallized bodies, in
which, notwithstanding they are homogeneous, the dilatation is
24
HEAT.
not equal in all their dimensions. Certain crystals are found
to suffer more dilatation in the direction of one axis than in the
direction of another.
1361. Tabular statement of the rates of dilatation of solids.
In the following table are given the rates of dilatation of solid
bodies according to the most recent and accredited authorities.
In the first column is given the limits of temperature between
which the dilatation has taken place; in the second is given the
increment of the linear dimensions, expressed decimally, the
linear dimension at the lower temperature being the unit.
In the third column the same is expressed as a vulgar
fraction.
Table of the linear Dilatation of Solids,
Interval
Dilatation in Fractions.
Names of Substances.
of
empera
Decimal.
Vulgar.
According to Lavoisier and Laplace.
Flint glass (English)
32 to 212
0-00081166
IfJS
Platinum (according to Broda)
0-00085655
rrW
Glass (French) with lead
0-00087199
lAi
Glass tube without lead
0-00087572
Ditto ....
0-00089694
TrV-
Ditto ....
0-00089760
TTH
Ditto ....
0-00091750
1
Glass (St. Gohain)
Steel (untempered) - r
0-00089089
0-00107880
i
Ditto ....
0-00107915
5^7
Ditto .--.
0-00107960
95B
Steel (yellow temper) at 65
Iron, soft forged -
0-00123956
0-00122045
Iron, round wire-drawn -
0-00123504
l '
Gold ....
0-00146606
SB5
Gold (French standard) annealed -
0-00151361
X
Gold (Ditto) not annealed
0-00155155
fa
Copper -
0-00171220
3^
Ditto ....
0-00171733
Ditto
0-00172240
sir
Brass -
0-00186670
333
Ditto
0-00187821
SiS
Ditto - - -
0-00188970
sfi
Silver -
0-00190868
z
Silver - -
0-00190974
it
Tin, Indian or -
0-00193765
S
Tin, Falmouth
0-00217298
X
Lead
0-00284836
DILATATION OF SOLIDS.
Interval
Dilatation in Fractions.
of
^
6
Decimal.
Vulgar.
According to Smeaton.
,fa
Steel *
^
0-00115000
JP
Steel tempered -
0-00122500
BT5
Iron -
.
0-00125833
753
Bismuth -
a
0-00139167
1\S
Copper -
0-00170000
3SS
Copper 8 parts, tin 1
0-00181667
355
Brass cast
5>
0-00187500
3*3
Brass 16 parts, tin 1
0-00190833
331
Brass wire
n
0-00193333
3T7
Telescope speculum metal
0-00193333
3T7
Solder (copper 2 pints, zinc 1)
0-00205833
6
Tin (fine)
0-00228333
T^g
Tin (grain)
0-00248333
3JS
Solder white (tin 1 part, lead 2) -
M
0-00250533
359
Zinc 8 parts, tin 1, slightly forged
0-00269167
372
Lead
n
0-00286667
315
Zinc ....
n
0-00294167
aio
Zinc lengthened -p by hammering
0-00310833
322
According to Major-General Roy.
Glass (tube) - - 32 to 217
0-00077550
T255
Glass (solid rod) -
0-00080833
T337
Glass cast (prism of)
a
0-00111000
551
Steel (rod of) -
0-00114450
571
Brass (Hamburgh)
Brass (English) rod
"
0-00185550
0-00189296
1
Brass (English), angular -
0-00189450
According to Trougliton.
Platinum -
32 to 212
0-00099180
TB5S
Steel
0-00118990
8TO
Steel wire drawn
0-00144010
si?
Copper -
0-00191880
35T
Silver -
0-00208260
1WS
According to Wollaston.
Palladium - -00 100000 ^
According to Didong and Petit.
Platinum
32 to 212
32 to 572
0-00088420
0-00275482
Sw
f
32 to 212
0-00086133
s
Glass -
32 to 392
0-00184502
I
32 to 572
0-00303252
^
Iron
Copper ... |
32 to 212
32 to 572
32 to 212
32 to 572
0-00118210
0-00440528
0-00171820
0-00564972
i
1
26 HEAT.
1362. Measure of the force of dilatation and contraction of
solids. The force with which solid bodies dilate and contract is
equal to that which would compress them through a space equal
to their dilatation, and to that which would stretch them through
a space equal to the amount of their contraction. Thus, if a
pillar of metal one hundred inches in height, being raised in
temperature, is augmented in height by a quarter of an inch, the
force with which such increase of height is produced is equal
to a weight which being placed upon the top of the pillar would
compress it so as to diminish its height by a quarter of an inch.
In the same manner, if a rod of metal, one hundred inches in
length, be contracted by diminished temperature, so as to render
its length a quarter of an inch less, the force with which this
contraction takes place is equal to that which being applied to
stretch it would cause its length to be increased by a quarter of
an inch.
1 363. Practical application of the forces of dilatation and con-
traction in drawing together the walls of buildings. This prin-
ciple is often practically applied in cases where great mechanical
force is required to be exerted through small spaces. Thus, in
cases where the walls of a building have been thrown out of the
perpendicular either by the unequal subsidence of the foundation
or by the incumbent pressure of the roof, they have been restored
to the perpendicular by the following arrangement:
A series of iron rods are carried across the building, passing
through holes in the walls, and are secured by nuts on the
outside. The alternate bars are then heated by lamps until
they expand, when the nuts, which are thus removed to some
distance from the walls by the increased length of the bars, are
screwed up so as to be in close contact with them. The lamps are
then withdrawn, and the bars allowed to cool. In cooling they
gradually contract, and the walls are drawn together by the nuts
through a space equal to their contraction. Meanwhile the in-
termediate bars have been heated and expanded, and the nuts
screwed up as before. The lamps being again withdrawn and
transferred to the first set of bars, the second set are contracted in
cooling, and the walls further drawn together. This process is
continually repeated, until at length the walls are restored to
their perpendicular position.
1364. Moulds for casting in metal must be larger thantlie^ob-
ject to be cast. In all cases where moulds are constructed for
DILATATION OF SOLIDS. 27
casting objects in metal, the moulds must be made larger than
the intended magnitude of the object, in order to allow for its con-
traction in cooling. Thus the moulds for casting cannon balls
must always be greater than the calibre of the gun, since the
magnitude of the mould will be that of the ball when the metal
is incandescent, and therefore greater than when it is cold.
1365. Hoops and tires tightened by the contraction in cooling.
Hoops surrounding water-vats, tubs and barrels, and other
vessels composed of staves, and the tires surrounding wheels, are
put on in close contact at a high temperature, and, cooling, they
contract and bind together the staves or fellies with greater
force than could be conveniently applied by any mechanical
means.
1366. Compensators necessary in all metallic structures. In
all structures composed of metal, or in which metal is used in
combination with other materials, such as roofs, .conservatories,
bridges, railings, pipes for the conveyance of gas or water, raf-
ters for flooring, &c., compensating expedients must be intro-
duced to allow the free play of the metallic bars in dilating and
contracting with the vicissitudes of temperature to which they
are exposed during the change of seasons.
These expedients vary with the way in which the metal
is applied, and with the character of the structure. Pipes
are generally so joined from place to place as to be capable of
sliding one within another, by a telescopic joint. The succes-
sive rails which compose a line of railways cannot be placed
end to end, but space must be left between their extremities
for dilatation.
1367. Blistering and cracking of lead and zinc roofs. Sheet
lead and zinc, both of which metals are very dilatable, when
used to cover roofs where they are especially exposed to vicis-
situdes of temperature, are liable to blister in hot weather by
expansion and to crack in cold weather by contraction, unless
expedients are adopted to obviate this: zinc, being much more
dilatable than lead, is more liable to these objections.
1368. Metallic inlaying liable to start. When ornamental
furniture is inlaid with metal without providing for its expansion,
the metal, being more dilatable than the wood, is liable, in a
small room, to expand and start from its seat.
1369. Compensating pe?).dulum. It has been already shown
(547) that the centre of oscillation of a pendulum ought to be
28 HEAT.
kept constantly at the same distance from its point of suspen-
sion, since otherwise the rate of the time-piece regulated by it
would not be uniform. This object has been attained by con-
necting the bob of the pendulum with the point of suspension
by rods composed of materials expansible in different degrees,
so arranged, that the dilatation of one shall augment the distance
of the centre of oscillation from the point of suspension, while
the expansion of the other diminishes it.
Let s, fig. 433., be the point of suspension, and o the centre
of oscillation, and let s be supposed to be connected with o by
means of two rods of metal, s A and A o, which are
united at A, but independent of each other at every
other point.
If such a pendulum be affected by an increase of
temperature, the rod s A will suffer an increment of
length; by which the point A and the rod A o attached
o to it will be lowered ; but, at the same time, the rod
A o being subject to the same increase of temperature,
will receive an increment of length, in consequence of
which the point o will be raised to an increased dis-
tance above the point A, at which the rods are united.
If the increment of the length of the rod A o be in this
case equal to the increment of the rod s A, then the
gen=lUO.)
Products.
Nickel, more carburetted -
0-11631
3R9-68
42-999
Cobalt, carburetted ....
0-11714
368-99
43-217
Steel (Hausmann) - . . .
0-11848
339-21
40-172
pure metal -
12728
339-21
Cast iron (white) -
0-12983
339-21
44-0 8
Carbon - -
0-24111
15288
36-873
Phosphorus .....
0-1887
196-14
37-124
Iridium (impure) ....
0-03(i83
1233-50
45-4-28
Manganese, very carburetted
0-14411
345-89
49-848
Metallic alloys.
1 Lead 1 tin - -
0-04073
1014*9
41-34
2 -
0-r4506
9*1-7
41-53
1 antimony ....
0-03880
1050-5
40-76
Bismuth. 1 tin - ...
004' 00
103-2-8
41-31
2
0-04504
933-7
4i-OS
., 2 1 antimony
0-04621
901-8
41-67
2 1 2 zinc
Lead, 2 ,, 1 bismuth ...
0-OS657
0-C4476
735-6
1 023-9
41-61
45-83
2 2 -
0-0608-2
1088-2
66-00
Mercury, 1 -
007294
1000-5
72-97
2
Of'591
912-1
60-12
Head -
0-03827
1280-1
48-90
Oxides, RO.
Protoxide of lead in powder ...
0-05118
1394-5
71-34
cast ...
05089
1394-5
70-94
Oxide of mercury -
0-05179
1365-8
70-74
Protoxide of manganese ...
0-15701
445-9
70-01
Oxide of copper -
of nickel -
0-142H1
0- 1 6234
495-7
70-39
76-21
,, calcined at the forge
15885
469-6
74-60
Mean ....
72- (!3
Magnesia .....
024394
25R-4
63-03
Oxide of zinc
0-12480
503-2
62-77
Oxides, R2 O 3 .
Peroxide of iron (iron oligist)
0-16695
978-4
163-35
slightly calcined -
0- 1 75(i9
978-4
171-90
doubly calcined -
0-17167
978-4
168-00
strongly calcined -
twice
0-1H8I4
978-4
164-44
Acid, arsenious ....
0-1-^786
1240-1
158-56
Oxide of chromium - - - -
0-17960
100.-V6
180-01
of bismuth -
0-06053
2960-7
179-22
of antimony -
0-09009
1912-9
172-34
Mean ....
m
169-73
Alumina ( Corimion) - -
V 1 9762 "
642-4
1-26-87
(sapphire) ....
0-21732
642-4
139-61
Oxides, RO 2 .
Acid, stannic -
o-ro326
935-3
87-23
titanic (artificial)
0-17164
50H-7
86-45
(rutile)
0-17032
503-7
85-79
antimouious ....
0-09535
1006-5
8C-49
95-92
Oxides, RO 3 .
Acid, tungstic
molybdic ....
0-07983
0-13240
1483-2
898-5
118-38
118-96
silicic
19132
577-5
Hi-18
boracic .....
023743
436-0
103-52
Oxides.
Oxide of magnetic iron ...
0-16780
1417-6
237-87
Sulphurels, RS.
Proto-siilphun-t of iron ...
Sulphuret of nickel -
0-13570
1*813
540-4
570-8
73-33
73-15
D 6
CO
HEAT.
Names of Substances.
Specific Heats.
Atomic Weights.
(Onj-Ken = 100.)
Products.
Sulphuret of cobalt -
of zinc ....
of lead - ...
,, of mercury ...
Proto-sulphuret of tin
0-12512
0-12303
0-050*6
0-OM17
0-08365
570-0
604-4
'.495-6
1467-0
936-5
71-34
74-35
76-00
75-06
78-34
Mean -
-
-
74-51
Sulphureti, R 2 S' 2 .
Sulphuret of antimony
., of bismuth ...
0-08403
0-06002
2210-4
3261-2
186-21
195-90
191'OG
Sulphurets, RS 2 .
Bi-sulphuret of iron -
of tin -
Sulphuret of molybdenum -
f- 13009
0-11^32
0-12334
74 1-6
1137-7
1001-0
96-45
136-66
123-46
Mean ...
-
.
129-56
Sulphurets, R 2 S.
Sulphuret of copper -
of silver ....
0-12118
0-07460
992-0
1553-0
120-21
115-86
Sulphurets.
Pyrites, magnetic - ...
C-16023
Chhrates, R^Cl 2 .
Chlorate of sodium -
of potassium ...
of mercury - - - -
,, of copper -
of silver ....
0-21401
0-17295
0-05205
0-I3S27
0-09109
733-5
5974-2
1234-0
1794-2
156-97
161-19
154-80
156-83
163-42
Mean ....
-
158-64
Chlorates, RCR
Chlorate of barium -
of calcium ....
of magnesium ...
of lead -
Pro-chlorate of mercury ...
,, of zinc - - - -
of tin -
0-08957
CI1990
164 20
0-19460
0-06641
0-06889
0-136I8
0-10:61
1299-5
989-9
698-6
601-0
1737-1
1708-4
845-8
1177-9
116-44
118-70
114-72
1 18-54
115-35
117-68
115-2!
119-59
Mean ....
Chlorate of manganese ...
0-14255
78S-5 "
117-03
112-51
Chlorides volatile, RC1 4 .
Chloride of tin -
of titanium -
0-147*9
0-19145
1620-5
1188-9
239-18
237-63
Mean "'- -
-
-
233-40
Chlorides volatile, R 2 C1 6 .
Chlorate of arsenic ....
,, of phosphorus ...
0-17604
0-20922
2267-8
1720-1
399-26
359-86
Mean. -
-
379-51
Bromates, R^Br".
Bromate of potassium -
of silver -
0-11322
0-07391
1468-2
2330-0
166-21
173-31
Mean ....
Bromate of sodium ....
0-13842
" 1269-2 "
169-76
175-63
Bromates, RBr ! .
Bromate of lead ....
0-05326
2272-8
121-00
CALOR1METRY.
Gl
Names of Substances.
Specific Heats.
\toraic Weights.
(Oxygen =- 100.)
Products.
lodates, R2R
lodate of potassium ....
of sodium -
Prot-iodate of mercury -
of silver - - - -
of copper - ...
Mean -
0-08191
0-08684
0-03949
0-06159
0-06869
2f68-2
I8<>9-2
4100-3
2929-9
2369-7
169-38
1 -2-30
162-34
180-45
167-45
lodates, RP.
lodate of lead -
of mercury -
Mean -
0-04267
0-04197
2872-8
2844-1
122-54
119-36
120-95
Ftuorates, RFR
Fluorate of calcium - - - -
0-21492
489-8
105-31
Nitrates, Az-O 5 +R-O.
Nitrate of potash - - - -
of soda -
of silver -
Mean ....
0-23875
C-27821
0-14352
1266-9
1067-9
2128-6
302-49
97-13
305-S5
301-72
Nitrates, Az 2 O 5 +RO.
Nitrate of barytes -
0-15228
1633-9
248-83
Chlorates, C1 2 O 5 + R 2 O.
Chlorate of potash -
0-20956
1532-4
321-04
Phosphates, P2Q'+2R 2 O (Pyrophosphates).
Phosphate of potash - ...
of soda -
Mean ....
0-19102
0-22833
2072-1
1674-1
395-70
382-22
389-01
Phosphate, FK)5+2 RO.
Phosphate of lead -
0-08208
3681-3
302-14
Phosphate, P-O^-fRO.
Phosphate of lime -
0-19923
1248-3
248-64
Phosphate, P2Q s +3 RO.
Phosphate of lead ....
0-07982
4985-8
397-96
Arsenites, AsO 3 +R"O.
Arsenite of potash ....
0-15631
Arsenites of lead, As-O'+3 PbO.
Arsenite of lead ....
0-07280
56235
409-37
Sulphates, SO 3 +R*O.
Sulphate of potash -
of soda - - - -
Mean -
0-19010
0-23115
1091-1
8921
207-40
206-21
206-80
Sulphates, SO 3 +RO.
Sulphate of barytes -
of strontium ...
oflead -
of lime - - - -
of magnesia -
Mean ....
0-11285
0-14279
008723
0-19656
0-22159
1458-1
1148-5
1895-7
857-2
759-5
164-54
164-01
165-39
16840
168-30
166-15
Chromates.
Chrom ate of potash -
Bi-chromate of potash -
0-18505
18937
12417
18935
229-83
358-67
Borate of potash - - ~
0-21975
1461-9
321-27
HEAT.
Names of Substances.
Specific Heats.
Uomic Weights.
(Oxy K en=100.)
Products.
Borate of soda ....
0-23323
1262-9
30 0-88
Mean -
- . -
-
Jaii-of "
Borates,WO+RO.
Borate of lead -
0-11409
2266-5
258-60
Borafes, B 2 O<:+2 R 2 O.
Borate of potash ....
of soda -
0-20478
0-25709
1025-9
826-9
219-52
212-60
Mean
216-06
Borales, BO 6 +2 RO.
Borate oflead
0-09046
1830-5
164-54
Tungstates.
Wolfram - -
0-09780
M
Silicates.
Zirconia
0-14548
,,
Carbonates, CO2+R2Q.
Carbonate of potash -
ofsoda ....
0-21623
0-27275
865-0
666-0
187-04
181-65
Mean -
-
-
181-35
Carbonates, CQ2+RO.
Carbonate of lime (Iceland spar) -
(Arragonite) -
Marble, white
0-20858
0'208f)0
0-21585
0-209^9
631-0
631-0
631-0
631-0
131-61
131-56
136-20
132-45
Chalk, white
Carbonate of barytes ....
of strontium ...
of iron ....
0-2 '585
11038
014483
0-19345
631-0
1231-9
9223
714-2
185-87
13519
133 58
138 16
Mean -
Carbonate of lead -
~ 0-08596 "
0-21743
" 1669-5 "
582-2
134-40
14355
126-59
Specific Heats of different Bodies determined by M. Rcgnault.
Names.
Srecific Heat*.
Densities.
0" 26085
Charcoal -
024150
Coke of cannel coal
0-20307
of small coal
0-20085
Welsh anthracite coal
0-20171
Phila.lelphian
0-20100
Graphite, natural
0-20187
of smelting furnaces
0-19702
of gas retorts -
Diamond -
0-20360
0-14687
Turpentine
0-4672
Camphildne
0-4656
TerJbilSne
0-45^0
Terebene -
4518
Lemon juice
Orange juice
0-4879
0-J8S6
Gin ...
0-4770
Petroleum - -
0-46R4
O'l 165
7"8f09
tempered
0-1175
7-7982
Metal of acute cymbals -
0-0858
8-57!i7
of soft cymbals, tempered
00862
8-6343
' ' annealed -
-
-
0-1937
LIQUEFACTION AND SOLIDIFICATION.
Names.
Specific Heats.
Densities.
Sulphur naturally crystallized
.
0-1776
melted for two years
,, for two months
-
0-1764
0-1803
recently -
0-1844
Water
Spii it of turpentine
Solution of chlorate of calcium
-
0-4160
0-6448
Spirit of wine, common at 36
_
0-6588
of higher degree
of still higher
-
0-8413
0-9402
Acetic acid concentrated, not crystallized
06501
Specific Heats determined by M. Eegnault.
Specific Heats.
From*0 o 15.
From 15 to 10.
From 10 to 5.
Distilled water
Spirit of turpentine -
Solution of chlorate of calcium
0-6462
0-6389
0-6423
Spirit of wine, common, No. 1.
0-6725
0-6651
0-6588
weaker, No. 2.
0-8518
08429
0-8523
still weaker, No. 3. -
0-9752
0-9682
0-9770
common
0-6774
0-6S40
0-6465
Acetic acid
0-6589
06577
0-1.609
Mercury
o-o-.-to
0-0283
0-0282
Terebene -
0-4267
0-4156
0-4154
Lemon juice - -
0-45 1
0-4424
0-4489
Petroleum -
0-4342
04325
0-4321
Benzine -
0-3932
0-3865
0-3999
Nitrobenzine -
0-3499
03478
03524
Chlorate of silicium -
0-1904
1904
1914
of titanium -
0-1828
0-1802
0-1810
Chloride of tin
01416
0-1402
0-1421
Prntochlorate of phosphorus
0-1991
0-1987
0-2017
Sulphate of carbon
0-2206
0-2183
0-2179
Ether ---
0-5157
0-5158
05207
sulphydric
04772
0-4653
0-4715
io.lhydric
0-1584
0-1584
1587
Spirit of wine - -
0-6148
0-6017
0-5987
Ether, oxalic -
0-4554
0-4. -i21
0-4629
Spirit of wood -
0-R009
0*5S68
0-5901
Ether, iodhydric
bromhydric
0-1569
0-2153
0-1556
02135
0-1574
0-2164
Chlorate of sulphur -
0-2038
0-2024
02048
Acetic acid, crvsUllizable
0-4618
04590
0-4587
CHAP. VII.
LIQUEFACTION AND SOLIDIFICATION.
1431. Thermal phenomena attending liquefaction. It has
been already explained, that when heat is imparted in sufficient
quantity to a solid body, such body will at a certain point pass
into the liquid state ; and when it is abstracted in sufficient
quantity from a liquid, the liquid at a certain point will pass
into the solid state.
64 HEAT.
1432. Certain thermal phenomena of great interest and im-
portance are developed in the progress of these changes, which
it will now be necessary to explain.
Let us suppose that a mass of ice or snow, at the temperature
of 20, is placed in a vessel and immersed in a bath of quick-
silver, under which spirit-lamps are placed. Let one thermo-
meter be immersed in the ice or snow, and another in the mer-
cury. Let the number and force of the lamps be so regulated,
that the thermometer in the mercury shall indicate the uniform
temperature of 200. The mercury imparting heat to the vessel
containing the ice, will first cause the ice to rise from 20 to
32, which will be indicated by the thermometer immersed in
the ice ; but when that thermometer has risen to 32, it will
become stationary, and the ice will begin to be liquefied. This
process of liquefaction will continue for a considerable time,
during which, the thermometer will continue to stand at 32 ;
at the moment that the last portion of ice is liquefied, it will
again begin to rise. The coincidence of this elevation with
the completion of the liquefaction may be easily observed, be-
cause ice, being lighter bulk for bulk than water, will float on
the surface, and so long as a particle of it remains unmelted it
will be visible.
Now, it is evident that during this process, the mercury
maintained at 200 constantly imparts heat to the ice : yet from
the moment the liquefaction begins until it is completed, no in-
crease of temperature is exhibited by the thermometer immersed
in the ice. If during this process no heat were received by the
ice from the mercury, the lamps would cause the temperature
of the mercury to rise above 200, which may be easily proved
by withdrawing the vessel of ice from the mercurial bath
during the process of liquefaction. The moment it is with-
drawn, the thermometer immersed in the mercury, instead of
remaining fixed at 200, would immediately begin to rise, al-
though the action of the lamps remained the same as before;
from which it is obvious that the heat, which on the removal
of the ice causes the mercury to rise above 200, was before
imparted to the melting ice.
1433. It is evident, therefore, that the heat which is received
by the melting ice during the process of liquefaction is
latent in it, being incapable of affecting the thermometer or the
senses.
LIQUEFACTION AND SOLIDIFICATION. 65
If the hand be plunged in the ice at the moment it begins to
melt, and at the moment that its liquefaction is completed, the
sense of cold will be precisely the same, notwithstanding the
large quantity of heat which must have been imparted to the
ice during the process of liquefaction.
1434. Quantity of heat rendered latent in liquefaction.
The quantity of heat which is absorbed and rendered latent in
the process of liquefaction, can be directly ascertained by the
calorimeter of Laplace and Lavoisier (1409). To ascertain this
in the case of ice, it is only necessary to place a pound of water
at any known temperature in the apparatus, and observe the
weight of ice it will dissolve in falling to any other tempera-
ture. In this way it will be found, that hi falling through 142 0> 65
it will dissolve a pound of ice ; and in general, any proposed
weight of water, in falling through this range of temperature,
will give out as much heat as will dissolve its own weight of
ice.
1435. Hence it is inferred, that when ice is liquefied, it ab-
sorbs and renders latent as much heat as would be sufficient to
raise its own weight of water from 32 to 32+142-65 =
!74-65.
1436. The latent heat of water has for the last half century
been estimated at 135, that having been the result of the ex-
perimental researches of Lavoisier and Laplace. Dr. Black's
estimate was 140, and that of Cavendish 150. A series of ex-
periments have lately been made, under conditions of greater
precision, by MM. de la Provostaye and Desains, from which
the above estimate has been inferred.
Dr. Black, who first noticed this remarkable fact, inferred
that ice is converted into water by communicating to it a
certain dose of heat, which enters into combination with it in a
manner analogous to that which takes place when bodies combine
chemically. The heat thus combined with the ice losing its
property of affecting the senses or the thermometer, the phe-
nomenon bears a resemblance to those cases of chemical combi-
nation, in which the constituent elements change their sensible
properties when they form the compound.
1437. Latent heat rendered sensible by congelation. If it be
true that water is formed by the combination of a large quantity
of heat with ice, it would necessarily follow, that, in the recon-
version of water into ice, or in the process of congelation, a cor-
66 HEAT.
responding quantity of heat must be disengaged. This fact
can be easily established, by reversing the experiment just
described.
Let us suppose that a vessel containing water at 60 is im-
mersed in a bath of mercury at the temperatui-e of 60 below
the freezing point. If one thermometer be immersed in the
mercury, and another in the water, the former will gradually
rise, and the latter fall, until the latter indicates 32. This ther-
mometer will then become stationary, and the water will begin
to freeze; meanwhile the thermometer immersed in the mercury
will still rise, proving that the water while it freezes continually
imparts heat to the mercury, although the thermometer im-
mersed in the freezing water does not fall. When the congela-
tion is completed, and the whole quantity of water is reduced
to the solid state, then, and not until then, the thermometer
immersed in the ice will again begin to fall. The thermometer
immersed in the mercury will rise without interruption, until
the two thermometers meet at some temperature below 32.
1438. It is evident from this, that the heat which was latent
in the water while in the liquid state, is gradually disengaged
in the process of congelation ; and since the temperature of the
ice remains the same as that of the water before congelation, the
heat thus disengaged must pass to some other object, which in
this case is the mercury.
When congelation takes place under ordinary circumstances,
the latent heat which is disengaged from the water which
becomes solid is in the first instance imparted to the water
which remains in the liquid state. When this water passes into
the solid state, the heat which is disengaged from it is transmit-
ted to the adjacent water which remains in the liquid state;
and so on.
1439. Other methods of determining the latent heat of water.
The latent heat of water may be further illustrated experi-
mentally as follows. Let two equal vessels, one containing a
pound of ice at 32, and the>other containing a pound of water
at 32, be both immersed in the same mercurial bath, maintained
by lamps or otherwise at the uniform temperature of 300, and
let thermometers be placed in the ice and the water. The ice
will immediately begin to melt, and the thermometer immersed
in it will remain stationary. The thermometer immersed in the
water will, however, at the same time begin to rise. When the
LIQUEFACTION AND SOLIDIFICATION. 67
liquefaction of the ice has been completed, and the thermometer
immersed in it just begins to rise, the thermometer immersed in
the water will be observed to stand at 174-65. It follows
therefore, supposing the ice and the water to receive the same
quantity of heat from the mercury which surrounds them, that
as much heat is necessary to liquefy a pound of ice as is suffi-
cient to raise a pound of water from 32 to 176 0> 65, which is
142 0- 65; a result which confirms what has been already stated.
1440. The following experiment will further illustrate this
important fact.
First let a pound of ice at 32 be placed in a vessel, and let
a pound of water at 174 '65 be poured into the same vessel.
The hot \vater will gradually dissolve the ice, and the tempe-
rature of the mixture will rapidly fall ; when the ice has been
completely dissolved, the water formed by the mixture will have
the temperature of 32. Thus although the pound of warm
water has lost 142 0> 65, the pound of ice has received no increase
whatever of temperature. It has merely been liquefied, but re-
tains the same temperature as it had in the solid state.
That it is the process of liquefaction alone which prevents the
heat received by the ice when melted from being sensible to the
thermometer, may be proved by the following experiment.
Let a pound of water at 32 be mixed with a pound of water
at 174'65, and the mixture will have the temperature of 103,
exactly intermediate between the temperatures of the compounds.
But if the pound of water at 32 had been solid instead of
liquid, then the mixture would have had, as already explained,
the temperature of 32. It is evident, therefore, that it is the
process of liquefaction, and it alone, which renders latent or in-
sensible all that heat which is sensible when the pound of water
at 32 is liquid.
1441. Liquefaction and congelation must always be gradual
processes. It was formerly supposed that water at 32 would
pass at once from the liquid to the solid state, on losing the
least portion of heat ; and that, on the other hand, a mass of ice
would pass instantly from the solid to the liquid state, on re-
ceiving the least addition of heat. What has been just explained,
however, shows that this sudden transition from the one state to
the other cannot take place.
1442. When a mass of water losing heat gradually is reduced
to 32, small portions of ice are formed, which give out their
68 HEAT.
latent heat to the surrounding liquid, and for the moment
prevent its congelation. As this liquid parts with its heat to
surrounding objects, more ice is formed, which in like manner
disengages its latent heat, and communicates it to a portion of
the water still remaining liquid, thus tending to raise its tem-
perature and keep it in the liquid state. The rapidity of the
congelation will depend on the rate at which the uncongealed
portion of the water can impart its heat to the surrounding air
and other adjacent objects.
The same principles explain the gradual process of the lique-
faction of ice. A small portion of ice first receives heat from
some external source, and having received as much heat as
would raise its own weight of water through 142'6o of the ther-
mometric scale, it becomes liquid. Then an additional portion of
ice receives the same addition of heat, and is likewise rendered
liquid ; and so the process goes on until the whole mass of ice is
liquefied.
1443. Water may continue in the liquid state below 32. It
is possible, under certain circumstances, to maintain water in the
liquid state below the freezing point. If a vessel of water be
carefully covered up, free from agitation, and exposed to a tem-
perature of 22, it will gradually fall to that temperature, still
remaining in the liquid state ; but if it be agitated, or a particle
of ice or other solid body be dropped into it, its temperature
will suddenly rise to 32, and a portion of it will be converted
into ice.
1444. Explanation of this anomaly. To explain this sin-
gular fact, it must be considered that the portion of the liquid
which is thus suddenly solidified disengages its latent heat,
which is communicated to that part of the water which still
remains liquid, and raises it from 22 to 32, and the remainder
of the heat thus disengaged becomes sensible, instead of being
latent in the ice itself, whose temperature it raises from 22
to 32.
It follows, from what has been already explained, that the
entire quantity of latent heat disengaged in this case would be
sufficient to raise as much water as is equal in weight to the
ice which has been formed through 142'6o, or, what is the
same, it would raise 14^ times this quantity of water through
10. Now, in the present case, the whole quantity of water in
the vessel, including the frozen part, has in fact been raised
LIQUEFACTION AND SOLIDIFICATION. 69
10, and it would follow, therefore, that the frozen portion
should constitute one part in 14^ of the whole mass.
This test of the quantity of latent heat of water was applied
with complete success, experimentally, by Dr. Thomson, who
showed, that when water cooled without congelation to 22 was
suddenly agitated, a portion was congealed, which bore the
proportion to the whole quantity just mentioned, that is to say,
10 parts in 142-65 of the entire mass. He found, likewise,
that the same result was obtained when the water was cooled
to any other temperature below 32 without congelation. Thus,
when water cooled to the temperature of 27 without congelation
was agitated, it was found that 28'5 part of the whole mass was
congealed. In this case, the whole mass was raised through 5 ;
and since the heat developed by the frozen portion would be
sufficient to raise 28 times this portion through 5, it follows
that the frozen portion must be the 28 - 5 part of the whole
mass.
1445. Useful effects produced by the heat absorbed in lique-
faction and developed in congelation. The great quantity of
heat absorbed by ice when it melts, and given out by water
when it freezes, subserves to the most important uses in the
economy of nature. It is from this cause that the ocean, seas,
and other large natural collections of water are most powerful
agents in equalizing the temperature of the inhabited parts of
the globe. In the colder regions, every ton of water converted
into ice gives out and diffuses in the surrounding region as
much heat as would raise a ton of liquid water from 32 to
!74-65; and, on the other hand, when a rise of temperature
takes place, the thawing of the ice absorbs a like quantity of
heat : thus, in the one case, supplying heat to the atmosphere
when the temperature falls ; and, in the other, absorbing heat
from it when the temperature rises. Hence we see why the
variations in climate are less on the sea-coast and on islands,
than in the interior of large continents.
The temperature of the air under the line does not vary
much more than 4, and that of the water varies not more
than 1.
1446. Heat absorbed and developed in the liquefaction and
solidification of other bodies. The thermal phenomena ex-
plained above with reference to water belong to a general class,
70 HEAT.
and are common, with certain modifications, to all solids which
are transformed into liquids by the addition, and to all liquids
which are transformed into solids by the abstraction, of heat.
Thus, if a mass of tin have its temperature raised by the addition
of heat until it attain the temperature of 442, it will then
become stationary, notwithstanding it receive further increments
of heat ; but the moment it becomes stationary, its fusion will
begin, and it will continue steadily at the temperature of 442
until it be completed ; but the moment the last particle of tin
has been melted, its temperature will begin to rise.
In the same manner, if lead be submitted to an in 'Tease of
temperature, it will begin to liquefy when it reaches the tem-
perature of 594; and notwithstanding the additional quantities
of heat imparted to it, its temperature will not rise above 594,
until its fusion is completed. In a word, all metals whatever,
and in general all solids which by elevation of temperature
are fused, undergo, during the process of fusion, no elevation
of temperature ; the heat imparted to them during this process
becoming latent in them, since it does not affect the ther-
mometer.
1447. Latent heat of fusion. This heat is called the latent
heat of fusion, and its quantity for each body is determined by
means similar to those already explained for water.
1448. Points of fusion. Different solids are fused at different
temperatures, but the same solid is always fused at the same
temperature, which temperature is called its point of fusion.
This point of fusion constitutes, therefore, a specific character
of the solid. The quantity of heat rendered latent during the
fusion of different metals is different, but always the same for
the same metal. This quantity is estimated or expressed by
the number of degrees which it would raise the same weight of
the same body, supposing it not to undergo the change from
the solid to the liquid state. In the same manner, all liquids
which, by the loss of heat, are converted into solids, have a
certain point, the same for each liquid, but different for different
liquids, at which they pass into the solid form. This point is
called their point of solidification, or their freezing point. It
is customary to apply the latter term only to such bodies as at
common temperatures are found in the liquid state.
The point at which a body in the liquid state solidifies, is the
LIQUEFACTION AND SOLIDIFICATION.
71
same as that at which the same body in the solid state is liquefied ;
the points, therefore, of solidification or congelation are the
same as the points of fusion or liquefaction for the same bodies.
Thus the point of fusion for ice is the same as the freezing
point for water.
Two conditions are therefore necessary to the fusion of a
solid body : first, its temperature must be reduced to the point
of fusion ; and, secondly, it must receive a certain quantity of
heat, called its heat of fusion, which will become latent in it
when the fusion has been completed.
In like manner two conditions are necessaiy to the congelation
or solidification of a liquid : first, it must be reduced to its freez-
ing point; and, secondly, it must be deprived of a certain
quantity of heat, which exists latent in it, and maintains it in
the liquid state.
In the following table are given the points of fusion of the
several bodies named in the first column.
Table showing the Point of Fusion of various Substances in
Degrees of Fahrenheit's Thermometer.
Nimes of Substances.
Fahr.
Authorities.
Plntina
Knglish wrought iron
French do
-
3082
2912
2732
Clarke.
Pouillet and Vauquelin.
Steel desist fusible) -
2A52
(most fusible)
2372
Cast manganese
22H2
brown, fusible -
2192
very fusible
whito, fusible -
2012
2012
!> Pouillet.
very fusible
Gold, very pure
1922
22x2
money
21- ^6
Copper
1922
Brass .
18.^9
Daniell.
Bronze
1652
1 Pouillet.
Antimony -
810
3
r
700
Murray.
Zinc
]
705
Cfuyton Morveau.
I
6*0
P.millet.
c
608
Pouillet.
Lead
I
590
592
Irvine.
Guy ton Morveau.
r
509
Ermann.
Bismuth ....
505
477
Pouillet.
Irvine.
I
Crichton.
c
512
Cuyton Morveau.
Tin
446
442
Pouillet.
Crichton.
L
433
Ermann.
72
HEAT.
Names of Substances.
Fahr.
Authorities.
Alloy ft parts tin, 1 part lead
381
4 -
372
3 ;; ;; -
367
,, 2 - -
385
466
If 1 3 parts lead
Alloy 3 parts tin, 1 part bismuth -
552
392
333-9
> Pouillet.
1 -
286-2
" 4 " 1 part lead, 5 parts bis-
muth -
246
Sulphur . . . -
237
226
J
Dumas.
Iodine
225
I
Alloy 2 parts lead, 3 parts tin, 5 parts bis-
muth -
212
!> Pouillet.
5 3 i> 8 ,, -
212
,, 1 i, 1 i 4
201
J
Potash . . . . T
194
162
Gay Lussac and Thenard, Pouillet.
Gay Lussac and Thenard.
136
] Pouillet.
Phosphorus i
100
Murray.
Stearicacid
158
Wax, bleached -
154
unbleached -
14-2
r-
131
Margaric acid }
to
140
120
Stearine <
to
Pouillet.
109
Spermaceti -
120
Acetic acid - - - -
113
Tallow ....
92
Ice
32 '
Oil of turpentine -
14
Mercury .....
382
1449. Latent heat of fusion of certain bodies. The latent
heat of fusion has not been so extensively investigated.
M. Person has, however, determined it for the bodies named in
the following table. The points of fusion observed by M. Person,
for the specimens tried, are given. The unit of the numbers
expressing the latent heat is, in this case, the quantity of heat
necessary to raise the same weight of water from 32 to 33.
Names of Substances.
Points
of
Fu.ion.
Latent Heat
for t'nitv
of Weight.
Names of Substances.
Points
of
Fusion.
Latent Heat!
ofWeight.
Chloride of lime
Phosphate of soda
Phosphorus -
Bees-wax (yellow)
D'Arcet s alloy
Sulphur
83-3
97-5
111-6
143-6
204-8
239-0
82-42
98-37
8-48
7832
1073
16-51
Tin -
Bismuth
Nitrate of soda
Lead -
Nitrate of potash
Zinc - -
455-0
518-0
f>90-9
629-6
642-2
793-4
25-74
22-32
1 13-36
9-27
83-12
49-43
1450. Facility of liquefaction proportional to the quantity of
latent heat. The different quantities of latent heat peculiar
to different bodies, explain the different degrees of facility with
LIQUEFACTION AXD SOLIDIFICATION. 73
which they are liquefied. Ice liquefies very slowly, because its
latent heat is considerable. Phosphorus and lead, on the other
hand, whose latent heat is small, melt very rapidly. Ice cannot
be liquefied until it lias received as much heat as would raise its
own weight of water 142-65; while lead and phosphorus are
liquefied by as much heat as would raise their own weight of
water 9. Hence it will be understood why it is that glaciers
and vast depths of snow continue on mountain ridges, such as
the Alps, in spite of the heat imparted to them during the
hottest summers ; such heat, however considerable, being only
sufficient to liquefy a portion of their superficial strata, which
descends the declivities, and feeds the streams and rivers of
which they are the sources.
1451. Other bodies besides ivater may continue liquid below
the point of solidification. The circumstance of water con-
tinuing in the liquid state below its freezing point, when kept
free from agitation, is not peculiar to that liquid. Tin fused in
a crucible was cooled by Mr. Crichton 4 below its melting
point, and yet remained liquid ; and similar phenomena have
been observed with other metals. In all such cases, the moment
solidification commences, the liquid, as in the case of water,
suddenly rises to its point of fusion ; and the same causes in all
cases favour solidification.
1452. Refractory bodies. Bodies which are difficult of
fusion are called refractory bodies. Among these, one of the
most remarkable is carbon or charcoal, one form of which is the
precious stone called the diamond. No degree of heat, as yet
attained, has reduced this substance to the liquid state ; indeed,
diamond being crystallized charcoal, it is probable that if the
fusion of charcoal could be effected, diamonds could be fabricated.
Among the most refractory bodies are the earths, such as lime,
alumina, barytes, strontia, &c. Of the metals, the most refractory
are iron and platinum, but both of these are fused by the oxy-
hydrogen blowpipe, as well as by the galvanic current.
1453. Alloys liquefy more easily than their constituents. It
is found that alloys composed of the mixture of two or more
metals, in certain proportions, frequently liquefy at a much
lower temperature than either of their constituents. Thus a
solder composed of 4 parts of lead and 6 of tin fuses at 336.
An alloy composed of 8 parts of bismuth, 5 of lead, and 3 of tin,
liquefies at a temperature below that of boiling water ; and an
II. E
74 HEAT.
alloy composed of 496 bismuth, 310 lead, 177 tin, and 26
mercury, fuses at 162-5. If a thin strip of this alloy be dipped
into water that is nearly boiling hot, it will melt like wax.
1454. Some bodies infusing pass throuyh different degrees of
fluidity. Some bodies, like water, pass from the complete solid
to the complete liquid state without passing through any in-
termediate degrees of aggregation ; while others, like wax,
tallow, and butter, become soft at temperatures considerably
below those at which they are liquefied ; and there are others,
like glass and some of the metals, which never, at any tem-
perature, attain absolute fluidity.
1455. Singular effects manifested by sulphur. Sulphur also
presents some curious exceptional circumstances in its state of
aggregation at different temperatures. If heat be gradually and
slowly imparted to it, it will be fused, and become very fluid at
302. If the supply of heat be continued, it will change its
colour and become red and viscous and considerably less fluid.
At length, heat being further supplied, and its temperature being
raised from 430 to 480, it will become altogether red, opaque,
and acquire the consistency of a thick paste.
1456. Points of congelation lowered by the solution of foreign
matter. The freezing points of liquids are generally lowered
when solids are dissolved in them. Thus, when salt is dis-
solved in water, the freezing point of the solution is always
below 32, and its distance below it depends on the quality and
quantity of salt in solution.
1457. Points of congelation of acid solutions. The strong ]
acids generally freeze at much lower temperatures than water ; j
and if they be mixed with water, the freezing point of the
mixture will hold an intermediate position between those of j
water and the pure acid. The freezing points of the acids them- ]
selves vary with their strength, but not according to any known 1
or regular law.
1458. Sudden change of volume accompanies congelation. j
When a liquid passes into the solid state by the absorption of \
heat, a sudden and considerable change of dimensions is fre- J
quently observed. This change is sometimes an increase and 1
sometimes a diminution, and in some cases no change takes
place at all. When mercury is cooled to its freezing point,
which is 39, it undergoes an instantaneous and considerable
diminution of bulk as it passes into the solid state. An
effect exactly the reverse takes place with water. When this
LIQUEFACTION AND SOLIDIFICATION. 75
liquid cools down to 32, it passes into the solid state, and in
doing so undergoes a considerable and irresistible expansion.
So great is this expansion, and so powerful is the force with
which it takes place, that large rocks are frequently burst when
water collected in their crevices freezes. It is a common oc-
currence that glass bottles containing water, left in dressing-
rooms in cold weather, in the absence of fire are broken when
the water contained in them freezes, the expansion in freezing
not being yielded to by any corresponding dilatation in the glass.
An experiment was made at Florence on a brass globe of con-
siderable strength, which was filled with water, and closed by a
screw. The water was frozen within the globe, by exposure
to a cold below 32, and in the process of freezing the water
burst the globe. It was calculated that the force necessary to
produce this effect amounted to about 28,000 Ibs.
1459. This expansion in the case of water not identical with
that which takes place below the point of greatest density.
This sudden expansion of water in freezing is a phenomenon
distinct from the expansion already noticed, which takes place
as the temperature is lowered from 38-8 to 32. The latter ex-
pansion is gradual and regular, and accompained by a gradual
and regular decrease of temperature ; but, on the other hand,
the expansion which takes place when water passes from the
state of liquid to the state of ice is sudden and even instantaneous,
and is accompanied by no change of temperature, the solid ice
having the temperature of 32, and the liquid of which it isformed
having had the same temperature just before congelation.
1460. The quantity of expansion produced in congelation is
the same for the same liquid, at ivhaterer temperature conge-
lation takes place. When water is cooled below 32 without
freezing, the expansion which took place from 38 0- 8 to 32 is
continued, and the liquid continues to dilate below 32 : when it
is afterwards solidified by agitation, or by throwing in a crystal
of ice, a sudden and considerable expansion takes place
as already described, but this expansion is always less than
would take place if it solidified at 32, by the quantity of. ex-
pansion which it suffered in cooling from 32 to the temperature
at which it was solidified. It is observed, that the expansion
which water suffers in being solidified at 32 amounts to about
one-seventh of its bulk. If it be solidified at a lower tempera-
ture, it will suffer a less expansion than this ; but the expansion
76 HEAT.
which it suffers in solidification under these circumstances, added
to the expansion which it suffers in cooling from 32 down-
wards previous to solidification, will always produce a total
amount equal to the expansion w r hich it would suffer in solidi-
fying at 32. Hence the total expansion which water under-
goes, from the temperature of greatest density (38'8) until it
becomes solid, is always the same, whatever be the temperature
at which it passes from the liquid to the solid state. The same
observations will be likewise applicable to other liquids similarly
solidified.
1461. Phosphorus and oils in general contract in congealing.
If a quantity of liquid phosphorus, at the temperature of
200, be gradually cooled, it will be observed to suffer a regular
contraction in its dimensions, according to the general laws
observed in the cooling of bodies. When it is cooled to the
temperature of about 100, it passes into the solid state, and in
doing so undergoes a sudden and considerable contraction.
Oils generally undergo this sudden contraction in the process
of freezing.
1462. Some bodies expand, and some contract, in congelation.
It may be assumed as generally true, that bodies which
crystallise in freezing undergo a sudden expansion, and that
bodies that do not crystallize in freezing, for the most part
suffer a sudden contraction. Sulphuric acid, however, is an ex-
ample of a liquid which passes from the liquid to the solid state,
and vice versa, without any discoverable expansion or contrac-
tion. Most of the metals contract in passing from the liquid
to the solid state, the exceptions being cast iron, bismuth, and
antimony, all of which undergo expansion in solidifying.
1463. Why coin is stamped, and not cast. It is evident that
a metal which contracts in solidifying cannot be made to take
the exact shape of the mould. It is for this reason that money
composed of silver, gold, or copper cannot be cast, but must be
stamped. Cast iron, on the contrary, as it dilates in solidifying,
takes the impression of a mould with great precision, as do also
certain alloys used in the arts.
1464. Contraction of mercury in cooling. The most strik- J
ing instance of sudden contraction in cooling is exhibited in the
case of mercury. This was first observed in the case of a ther- ?
mometer, which when exposed to a temperature about 40 below
zero, was observed to fall suddenly through a considerable range
LIQUEFACTION AXD SOLIDIFICATION. 77
of the scale, and in some cases the mercury was precipitated
into the bulb. It was observed that the thermometer being
exposed to a temperature lower than 40, the mercury gra-
dually falls until it arrives at about 38, and that then a great
and sudden contraction takes place at the moment the metal is
solidified.
This contraction, however, must not be understood as indi-
cating any real fall of temperature, as is the case with all the
previous and regular contractions which take place before the
solidification of the metal.
1465. Substances which soften before fusion. Substances
which soften before they melt, and which pass by degrees
from the solid to the liquid state, are mostly of organic origin,
and their point of fusion is below the temperature of boiling
water. Some of these, which are of most general utility in the
arts, are the following :
Colophany begins to melt at 275
Brown wax 110
White wax 124
Tallow 104
Pitch 91
1466. Weldable metals. The metals capable of being
welded soften before they are fused ; and the heat at which
they soften is called a welding heat. The metals which most
readily admit of being welded, are platinum and iron. At an
incipient white heat (2372) they become soft ; and, in this
state, pieces of the metal may be intimately united when
submitted to severe pressure, or when passed under the
hammer.
1467. Freezing mixtures. It may be taken as a physical
law of high generality, that a solid cannot pass into the liquid
state without absorbing and rendering latent a certain quantity
of heat. This heat may be, and often is supplied from some
other body in contact with that which is liquefied. But if no
such external supply of heat be present, and if, nevertheless, any
physical agency cause the liquefaction to take place, the body
thus liquefied will actually absorb its own sensible heat. While
it is liquefied, it will therefore fall in temperature to that extent
which is necessary to supply its latent heat of fluidity at the
expense of its sensible heat.
To render this more clear, let us imagine a pound of ice at
78 HEAT.
the temperature of 32 to be mixed with a pound of liquid
having the temperature of 103, and let this liquid be sup-
posed to have the property of dissolving the ice. When the
liquefaction is completed, the temperature of the mixture will
be 103. Now the liquid, which is here supposed to be the
solvent, neither imparts heat to the ice nor abstracts heat from
it. The ice therefore, now liquefied, contains exactly as much
heat as it contained before liquefaction, and no more. But, to
become liquid, it was necessary that 142'6o of heat should be
absorbed by it, and become latent in it. This 142 0< 65 has there-
fore been transferred from the sensible to the latent state in.
the ice itself.
This principle has been applied extensively in scientific
researches and in the arts for the production of artificial cold,
the compounds thus made being called freezing mixtures.
In all freezing mixtures, two or more substances are com-
bined, one or more of which are solid, and which have chemical
properties in virtue of which, when intimately mixed to-
gether, they enter into combination, and, in combining, liquefy.
The operation is so conducted, that no heat is supplied either by
the vessel in which the liquefaction takes place, or from any
other external source. Such being the case, it follows that
the heat absorbed in the liquefaction must be supplied by the
substances themselves which compose the mixture, and which
must therefore suffer a depression of temperature proportional
to the quantity of heat thus rendered latent.
The cold produced will be increased by reducing the tempe-
rature of the substances composing the mixture before mixing
them. Thus, let A and B be the substances mixed. Before
being combined, let them be reduced to 32 by immersing them
in snow. Let them then be mixed, and let the latent heat of
fusion be 32. The mixture will fall to zero, since the 32 3 of
sensible heat will be absorbed. But if, at the moment of mixing
them, their temperature had been 64, then the temperature of
the mixture would become 32.
The substances which may be used to produce freezing
mixtures on this principle are very various.
If equal weights of snow and common salt at 32 be mixed,
they will liquefy, and the temperature will fall to 9.
If 2 Ibs. of muriate of lime and 1 Ib. of snow be separately re-
duced to 9 in this liquid and then mixed, they will liquefy,
and the temperature will fall to 74.
LIQUEFACTION AND SOLIDIFICATION. 79
If 4 Ibs. of snow and 5 Ibs. of sulphuric acid be reduced to
74 in this last mixture, and then mixed, they will liquefy,,
and the temperature will fall to 90.
If a pound of snow be dissolved in about two quarts of
alcohol at 32, the mixture will fall nearly to 13. If
the same quantities of snow and alcohol, being reduced in
this mixture to 13, be then mixed, the temperature of the
mixture will be reduced to 58; and the same process being
repeated with like quantities in this second mixture, a further
reduction of temperature to 98 may be produced ; and so on.
1468. Apparatus for producing artificial cold. Freezing
mixtures are used for the artificial production of ice in hot
climates. The most simple apparatus for this purpose is repre-
sented \r\jig. 440., and is composed of a tin bucket B, having a
slightly conical form, in the bottom of which is a circular hole,
a little less in diameter than the bottom. In this hole is sol-
dered the mouth of another tin bucket, G E F H, also conical, but
with its smaller end upwards. A space w is thus left between
Fig. 441.
the two tin buckets, in which the water or other substance to
be cooled is placed.
The freezing mixture is placed in another vessel, IKLM,
jig. 441., similar in form to the bucket ABCD. This vessel
IKLM ought to be made of some non-conducting material.
E 4
80
HEAT.
Common glazed earthenware would answer the purpose.
When the freezing mixture is placed in it, the vessel A B c i> is
immersed in it, as represented in Jig. 440. ; so that the cold
liquid is not only in contact with the external surface of the tin
bucket AB c D, but also with the inner surface of G E F H. The
water w, or whatever other substance it is required to cool, is
therefore quickly reduced in temperature.
If it be not convenient to provide a vessel sucli as i K L M in
earthenware, a tin vessel thickly coated with woollen cloth may
be used.
1469. Table of freezing mixtures. There are a great
variety of bodies which, by combination, serve for freezing
mixtures. The following table has been collected from the
results of the researches of Walker and Lowitz. The substances
are indicated by letters as follows :
Water -
Snow, or ice
Sulphate of ammonia
,, soda
Muriate of ammonia
soda
lime
Carbonate of soda
W
I
SA
SS
MA
MS
ML
CS
Nitrate of potash
ammonia
Sulphuric acid
Nitric acid
Hydrochloric acid
Dilute -
Crystallized
- NP
NA
SA
NA
HA
d
The figures prefixed indicate the proportion by weight in
which the ingredients are mixed. Thus, 6ss + 4>iA -f- 2NP + 4dNA
signifies a mixture of 6 oz. of sulphate of soda, 4 oz. of muriate
of ammonia, 2 oz. of nitrate of potash, and 4 oz. of dilute nitric
acid.
Cold
Cold
From
to
dSced.
From
to
pro-
duced.
I5MA+5 NP+16W
+50
+ 10
40
I2I45MS+5NA
X
25
5 MA+5 NP+8 SS+
3I+2dSA
+32
23
55
16 W -
+50
+4
46
8 1+5 HA - -
+32
27
59
1 NA+1 W - -
+50
+4
4G
7 1+4 dNA
+32
30
62
1 NA+1 CS+1 W -
+ 50
7
57
4 1+5 ML -
+3-2
40
72
3 SS+2 dNA - -
+ 50
3
53
2 1+3 cML - -
50
82
6 SS+4 MA+2 NP+
3 1+4 NP -
+3i
-51
+4 dNA -
+50
-10
60
5 PS+3 NA+4 dNA
31
34
6SS+5 NA+4dNA
+50
-14
64
3I+i!dNA
46
46
9 PS+4 dNA - -
?50
12
62
8 1+3 dSA+3dNA -
10
56
46
9 PS+G NA+4 dNA
+50
21
71
11+ldSA - -
20
-60
40
8 SS+5 HA
+50
50
3 1+4 ML -
+20
-48
5 SS+4 dSA
+50
+3
47
2 1+3 ML -
15
68
53
2 1 + 1 MS -
X
5
1 !+> cML - -
66
66
51+2MS+1MA -
X
12
1 1+3 cML - -
-40
73
33
24 1+10 MS+5 MA+
8I+10dSA
68
91
23
5NP -
X
18
1470. Extraordinary degrees of artificial cold produced by
Thirolier and Mitehel. Thirolier produced a powerful freezing
LIQUEFACTION AND SOLIDIFICATION. 81
mixture, by solidifying carbonic acid, and mixing it with sul-
phuric acid or sulphuric ether. A temperature 120 below zero,
and therefore 152 below the freezing point, was thus produced.
Mi toh el, repeating the experiment, produced a still more in-
tense cold. He exposed alcohol of the specific gravity of 0'798
successively to the temperatures of 130 and 146. He
states that at the former temperature it had the consistency of
oil, and at the latter resembled melting wax.
1471. Alcohol probably congeals at about 150. If these ex-
periments can be relied on, it may be inferred that the freezing
point of alcohol, so long and hitherto so vainly sought, is pro-
bably about 150, or 182 below the freezing point of water
and 110 below that of mercury.
1472. Precautions necessary in experiments with freezing
mixtures. To ensure success in experiments on extreme
cold produced by freezing mixtures, the salts used must not
have lost their water of crystallization, because in that case they
quickly absorb water, and converting it into ice liberate caloric
and obstruct the cooling. The salts and ice used should be pul-
verized so as to dissolve quickly. When extreme cold is re-
quired, the vessel containing the freezing mixture should be
immersed in another vessel, containing also a freezing mixture,
so as to retard the mixture under experiment from receiving
heat from the vessel which contains it, and a sufficient quantity
of the ingredients forming the freezing mixture should be used.
1473. Greatest natural cold yet observed. The greatest
natural cold of which any record has been kept, was that ob-
served by Professor Hanstean between Krasnqjarsk and Nishne-
Udmiks in 55 N. lat., which he states amounted to 55
(Reaum. ?) = -91-75 F.
At Jakutsk, the mean temperature of December is 44^ F.
In 1828, from January 1 to January 10, the mean tempera-
ture of that place was 58.
In the expedition to China, in December 1839, the Russian
army experienced for several successive days a temperature of
-41-8 F.
1 474. Principle ofjluxes. Examples of their application.
The same principle which explains the effect of freezing mix-
tures, is also applicable to the phenomena attending fluxes in
metallurgy. Fluxes are certain bodies which, when mixed
with others, cause them to fuse at lower temperatures than their
82 HEAT.
proper point of fusion. It is by this means that certain metals
and metallic ores are fused, when exposed to the operation of
blast furnaces. In a certain sense, salt may be said to be a flux
for ice ; but this term flux is usually limited in its application
to bodies which are only fused at very elevated temperatures :
for example, in enamelling, and in the manufacture of glass and
of the paste by which precious stones are imitated, siliceous
sand is employed in greater or less proportion, about one-third
for enamel, and nearly three-fourths for plate glass. Now
silica is not fused at any heat attainable by commen furnaces.
M. Gaudin lately succeeded in its fusion, by means of the oxy-
hydrogen blow-pipe, and drew it into threads as fine as the
filaments of silk. When combined, however, with proper
fluxes, it fuses readily in the furnace. The fluxes used vary
according to the purposes for which the silica is applied, but
they consist generally of soda, potash, and lime, with the addi-
tion of lead for flint glass, and stannic acid for enamel. The
compound which results from the mixture of these ingredients,
by their exposure to intense heat, is reduced to a sort of pasty
fusion, but can never be said to undergo positive liquefaction.
Nevertheless, the beautiful transparency of Bohemian glass,
plate glass, flint glass, and the factitious diamonds, show that
the constituents must be combined in a very intimate manner.
Fine earthenware and porcelain are also fabricated by means
of fluxes ; for although fusion is not actually produced, nor is
there the same intimate combination of the constituents as takes
place in vitrefaction, still there is a partial combination, and an
incipient fusion. The fluxes in this case consist also of soda,
potash, lime, and sometimes magnesia, the soda and potash
however being used in their combined form of feldspar.
1475. Infusible bodies. Infusible bodies may be resolved
into two classes, those which are refractory, and which alone
can be properly said to be fusible, and those whose fusion is pre-
vented by their previous chemical decomposition or composition.
Before the invention of the oxy-hydrogen blowpipe, and other
scientific expedients for the production of intense heat, the
number of refractory substances was much more considerable
than it is at present. Scarcely any body can be said to be ab-
solutely infusible except charcoal, which under all its forms of
pure carbon, anthracite, graphite, and diamond, has resisted
fusion at the highest temperature which has yet been produced.
LIQUEFACTION AND SOLIDIFICATION. 83
The term refractory, however, is still applied to those classes
of substances which resist fusion by ordinary furnaces.
When certain compound bodies are exposed to an intense
heat, they are resolved into their constituents before they attain
the point of fusion ; and in other cases simple bodies enter into
chemical combination with others which surround them, or are
in contact with them before the fusion takes place.
The fusion, however, may in some cases of both of these classes
of bodies be effected by confining them in some envelope which
will resist the separation of their constituents if they be com-
pound, or exclude them from the contact with bodies with which
they might combine if they be simple.
1476. Marble may be fused, If marble be exposed under
ordinary circumstances to an intense heat, it will be resolved
into its constituents, lime and carbonic acid ; but if it be con-
fined in a strong gun-barrel, for example, it may be fused.
1477. Organic bodies are decomposed before fusion. Almost
all organic solids, except the resins and the fats, are infusible
before they are decomposed ; we cannot melt a piece of wood,
a leaf, a flower, or a fruit ; but after having evaporated their
liquid constituents, and dried them, the influence of heat causes
their constituents to enter into combination, and produces new
substances, which are generally volatile, and which have nothing
in common with the original substances.
1478. Water separated from matter held in solution by con-
gelation. When water holding any body in solution has its
temperature sufficiently lowered, its congelation takes place in
one or other of three ways : first, the water may congeal inde-
pendently of the body which it holds in solution ; secondly, the
body which it holds in solution may congeal, leaving the water
still liquid ; thirdly, the water and the body it holds in solution
may congeal together.
The congelation of the water independent of the substance
it holds in solution is presented in the case of the very weak
solutions. In this case, the point of congelation is always below
the freezing point. Thus, if water hold in solution a small
quantity of alcohol, acid, alkali, or salt, it will be necessary to
reduce the whole to the freezing point to produce its congelation ;
but when ice has been formed upon it, this ice will consist of
pure water, without the mixture of any proportion of the sub-
stances which the water held in solution. Thus, sea-water
84 HEAT.
freezes at 27-^, being 44 below the freezing point of pure
water; and if the ice produced upon it be withdrawn and melted,
it will produce pure water. In the same manner, if weak wine
be frozen, the ice formed upon it will be the ice of pure water,
and the wine which still remains liquid will be proportionally
stronger. This method is sometimes practised to give increased
strength to wine.
1479. Saturated solutions partially decomposed bij cooling.
Water is generally capable of holding in solution only a certain
quantity of any solid substance, and when all the substance has
been dissolved in it which it is capable of taking, the solution
is called a saturated solution. Now, it is found that the quan-
tity of solid matter of any kind which water is capable of hold-
ing in solution, increases with the temperature. Thus, water
at 212 will hold more of any given salt in solution, than would
water at 50. Let us suppose, then, that a saturated solution of
any salt is made at 200. If this solution be allowed to cool, a
part of the salt which it contains must return to the solid state,
since at lower temperatures it cannot hold in solution the
same quantity ;. and in proportion as the temperature of the so-
lution falls, the quantity of solid matter which will be formed in
it will increase. In this case, the cooling accomplishes a partial
decomposition of the solution. If the cooling be accomplished
suddenly, the salt is precipitated tumultuously and in a confused
mass, without form or cohesion ; but if the solution is allowed to
cool slowly and without agitation, the molecules of the salt col-
lect into regular crystals.
Even after the temperature of the solution has ceased to fall,
the decomposition and crystallization will continue, if the vessel
containing the solution be in a position favourable to superficial
evaporation. The water which evaporates from tiie surface
taking with it none of the salt, all that portion of salt with
which it was combined will receive the solid form, and will
collect into crystals ; and this process may be continued until, by
superficial evaporation, all the water shall have disappeared, and
nothing be left in the vessel except a collection of crystals of
the salt.
1480. Anomalous case of anhydrous sulphate of soda. The
solution of anhydrous sulphate of soda presents some remark-
able exceptional phenomena. At the temperature of 91'4 it
has a maximum of saturation, that is to say, above this point the
LIQUEFACTION AND SOLIDIFICATION. 85
proportion of salt which it contains diminishes instead of in-
creasing as the temperature is raised. However, at the boiling
point, it contains much more salt than at the common tempe-
rature. If the solution be boiled in a large tube, and when it is
well purged of air the tube be closed at the top, so as to ex-
clude the atmosphere, the cooling will take place without any
solidification ; but when the top of the tube is broken so as to
admit the air, the salt is suddenly congealed in a mass, with so
great a disengagement of heat, that the tube becomes warm to
the touch.
1481. Case in which the matter held in solution congeals with
the water. In some cases the water and the salt which it holds
in solution are solidified together. This happens when the
salts contain their water of crystallization. The phenomena are
produced in the same manner as in the case just described,
with this difference, that the molecules of salt in collecting
carry with them the molecules of the water of crystallization,
which pass also to the solid state, taking the place which be-
longs to that in the crystals. Nevertheless, the solidification of
the water disengaging in general much more latent heat than
the solidification of the salt, the crystals undergo a less rapid
increase, whether formed by mere cooling, or by evaporation
of a part of the dissolving mass.
1482. Dutch tears. When bodies liquefied by
heat are suddenly cooled, some remarkable and ex-
ceptional phenomena are often produced. Thus,
if large drops of glass in a state of fusion be let fall
into a vessel of cold water, the solidification of their
superficial parts is immediate ; that of their interior
is much more slow. There results from this a sort
of forced and unnatural arrangement of the mole-
cules of the drop, which explain the singular pheno-
menon produced by Dutch Tears, so called from
the form they assume, as represented in Jig. 442.
' If the extremity of the tail of one of these be broken,
in an instant the entire mass cracks, and is reduced to powder.
This arises from the fact that, the glass not being cooled slowly
and gradually, the molecules in solidifying have not had time
to assume their natural position, and, being in a forced position,
on the least disturbance separate.
1483. Use of annealing in glass manufacture and pottery.
86 HEAT.
To prevent this, articles manufactured of glass are submitted to
the process called annealing after their fabrication, a process
in which, being again raised to a certain temperature, they are
allowed to cool very'slowly. Pottery iu general is submitted
to the same process.
1484. Tempering steel. The temper of steel is a quality
analogous to this. Being heated almost to the point of fusion,
and being plunged in water, it becomes as brittle as glass. In
this state, it is said to have the highest temper. If it is tem-
pered only at a cherry red, it is less hard and less brittle. This
is what is called the ordinary temper. In short, it may be
annealed in an infinite variety of degrees over a fire of small
charcoal, according to the temper which it is desired to impart
to it. The oxidation which it suffers at the surface indicates
by the colour which it gives to it the degree of annealing which
it has received : thus it sometimes acquires a blue colour and
sometimes a straw colour, the latter colour indicating a harder
and less elastic quality.
CHAP. VIII.
VAPORIZATION AND CONDENSATION.
1485. Evaporation of liquids in free air. If a liquid be
exposed in an open vessel, it will be gradually converted into
vapour, which mixing with the atmosphere will be dissipated,
and after a certain time the liquid will disappear. This pheno-
menon, called evaporation, was formerly explained by the sup-
position that the air had a certain affinity for the liquid in
virtue of which the air dissolved it, just as water dissolves
sugar or salt.
A conclusive proof of the falsehood of this hypothesis was
presented by the fact, that the vaporization of the liquid takes
place in a vacuum, and that the presence of air not only does
not cause more of the liquid to be evaporated than would have
been evaporated in its absence, but actually retards and obstructs
the evaporation.
1486. Apparatus for observing the properties of vapour.
VAPORIZATION AND CONDENSATION.
87
A
6
Fig. 443.
To be enabled to examine and observe with clear-
ness and precision the mechanical properties of the
vapour of any liquid, it is necessary to provide
means by which such vapour can be separated from
air and all other gases and vapours, since, being
mixed with these, its properties would be modified,
so that it would be difficult to determine what ef-
fects are due to the vapour, and what to the gases
with which it is combined.
This object has been attained by apparatus, the
principle of which we shall now explain.
Let A B, fig. 443., be a glass bulb and tube, the
bore of the tube being very small compared with
the capacity of the bulb. Let the tube be widened
into a sort of bell-shaped mouth at the end B, and
let a graduated scale be engraved upon it, the zero
being near the bulb.
Let the tube, held with the open end B upwards,
be filled with pure mercury well
purged of air, as described in (714)
et seq. Placing the finger on B
to prevent the escape of the mer-
cury or the entrance of air, let the
tube be inverted, and the end B
immersed in a trough of mercury,
as represented in fig. 444. If it be
immersed to such a depth that the
height of the top of the bulb A
above the level L L of the mercury
in the trough is less than the height
of the barometric column, the mer-
cury will not fall from the bulb,
being sustained there by the atmo-
spheric pressure.
But if the bulb be raised to a
greater height A above L i/, the
column of mercury will not rise with
it, but will stand at the height of
the barometric column.
Let the bulb be raised to such a
Fig. 444. height A, that the zero of the scale
88 HEAT.
engraved on the tube shall be at a height above L i/ equal to
the barometric column. In that case the level of the column
of mercury in the tube will coincide with the zero of the scale,
and the space in the bulb and tube above this level will be a
vacuum. Let this space be s A', and let s M represent the
column of mercury which corresponds in height with the baro-
meter.
Let C v,Jig- 445., be a small iron cylinder containing mercury,
above which is a piston by which it can be pressed downwards.
This piston is urged by a screw, so as to
be capable of being moved with accu-
racy through any proposed space, however
small. Attached to the bottom of the
cylinder c D is a very fine tube D P, bent
into a rectangular form so as to present its
mouth upwards. This capillary tube is
filled with the liquid, the vapour of which
' Fio . 445 it is desired to submit to observation. By
means of the screw acting on the piston,
any proposed quantity of this liquid can be expelled from the
mouth P of the tube.
This instrument being immersed in the trough L \!,fig. 444.,
and the mouth of the tube P being directed into the bell-shaped
end of the tube B, a certain small quantity of the liquid is
expelled by means of the screw, and issues from p. It rises by
its relative levity through the mercury, and arrives at the top s
of the column. There it instantly disappears, and at the same
time the mercury falls to a lower level.
1487. Vapour of a liquid an elastic, transparent, and invisible
fluid like air. The cause of this will be easily understood.
The minute drop of liquid which rises to the surface is con-
verted into vapour on arriving there, and is diffused in that
state throughout the entire capacity of the tube and bulb. It
is transparent and invisible like air ; and therefore, notwith-
standing its pressure, the bulb and tube appear to be empty,
as they would if they were filled with air.
1488. How its pressure is indicated and measured. But
this vapour being, like air, an elastic fluid, exercises a certain
pressure upon the mercurial column s M, which pressure is
manifested and measured by the fall of that column. The
summit, which before stood at the zero of the scale, now stands
VAPORIZATION AND CONDENSATION. 89
at a lower point, and the number of the scale indicating it3
position, expresses the pressure of the vapour in inches of
mercury. Thus, if the summit s of the column stand at half
an inch below zero, the pressure of the vapour in the bulb is
such as would support a column of mercury half an inch in
height.
Now let us suppose another small drop of the liquid to be in-
jected by the apparatus^. 444. Like effects will ensue, and
the summit s of the column will fall still lower, showing that
the pressure of the vapour is augmented.
1489. When a space is saturated with vapour. By repeat-
ing this process, it will be found, that when a certain quantity
of the liquid has been injected, no more vapour will be produced,
and the liquid will float on the summit s of the mercurial
column without being vaporized. The summit of the column will
not be further depressed.
It appears, therefore, that the space in the bulb and tube is
then saturated with vapour. It has received all that it is capable
of containing. That this is the case will be rendered manifest
by elevating the tube. The summit s of the column still main-
taining its height above L i/, a greater space will be obtained
above s, and it will be accordingly found, that a portion of the
liquid which previously floated on s will be vaporized, and if
the tube be still more elevated, the whole will disappear.
Since during this process the height s M of the mercurial
column in the tube remains unaltered, it follows that the pressure
of the vapour remains the same.
By comparing the volume of the liquid ejected from ?,fig. 444.,
with the volume of the tube and bulb tilled by the vapour into
which it is converted, the density of the vapour, or, what is the
same, the column of vapour into which one unit of volume of the
liquid is converted, may be ascertained.
There are, however, other circumstances connected with this
process, which are not rendered apparent, and which it is im-
portant to observe and comprehend.
When the liquid rises to the surface of the mercurial column
and expands into vapour, it absorbs a certain quantity of heat
which becomes latent in it. This heat must be supplied by the
tube, the bulb, and the mercury ; and as the temperature of
these does not permanently fall, this heat is replaced, and
their temperature restored by the surrounding air. The
90 HEAT.
quantity of heat absorbed in the evaporation of the liquid will
be presently shown. Meanwhile it must be observed, that the
supply of the latent heat is essential to the evaporation of the
liquid. If the mercury on which the liquid floats, and the glass
by which it is inclosed, were absolute non-conductors, and could
impart no heat whatever to the liquid, then the evaporation
could not take place.
It appears from what has been explained, that when the space
above the mercury has been charged with a certain quantity of
liquid in the state of vapour, or, what is the same, when the
vapour it contains has attained a certain density, all further
evaporation ceases ; and any liquid which may be injected will
remain in the liquid state, floating on the mercury. So long as
the temperature of the surrounding medium, and consequently
that of the bulb and its contents, remain unaltered, and so long
as any liquid remains floating on the mercury, the pressure and
the density of the vapour in the bulb will be unaltered. If the
bulb be raised, so as to give more space for the vapour, a pro-
portionally increased quantity of the liquid will be vaporized ;
and if by depressing the tube the volume of the vapour be
diminished, a corresponding part of it will return to the liquid
state. In the one case, heat will be absorbed by the liquid
evaporated ; and in the other, heat will be developed by the
vapour condensed. This heat is borrowed from the surrounding
atmosphere in the one case, and imparted to it in the other ;
since, otherwise, the bulb and its contents must undergo a change
of temperature, contrary to what was supposed.
1490. Quantity of vapour in saturated space depends on tem-
perature But let us now consider what will be the effect of
raising or lowering the temperature of the bulb and its contents.
The bulb being charged with vapour, and a stratum of uneva-
porated liquid floating on the mercury, let the temperature of the
medium surrounding the bulb be raised through any proposed
number of degrees of the thermometric scale. This will be im-
mediately followed by the evaporation of a part of the liquid
floating on the mercury, and a depression of the column. An
increased volume of vapour is therefore now contained in thebulb
and tube ; but if this increase of volume be compared with the
increased quantity of liquid evaporated, it will be found to be less
in proportion ; and it consequently follows, that the density of
VAPORIZATION AND COMPENSATION. 91
the vapour is augmented ; and since the column of mercury has
been more depressed, and since this depression measures the
pressure of the vapour, it follows that this pressure has been
also augmented.
1491. Relation between pressure, temperature, and density.
Thus it appears that the pressure and density of the vapour
produced from the liquid floating on the mercury are augmented
as the temperature of the liquid is augmented, and consequently
diminished as that temperature is diminished.
In short, a certain relation subsists between the temperature,
pressure, and density, such that when any one of these are
known, the other two can always be found. If this general re-
lation were known, and could be expressed by an arithmetical
formula, the pressure and density of the vapour corresponding
to any proposed temperature, or the temperature corresponding
to any proposed density or pressure, could always be ascer-
tained by calculation. But the theory of heat has not supplied
the means of determining this relation by any general principles ;
and, consequently, the pressures and densities of the vapour of
liquids at various temperatures have been determined only by
experiment and observation.
1492. Pressure, temperature, and density of the vapour of
water. Different liquids at the same temperature produce
vapours having different densities and pressures. But of all
liquids, that of which the vaporization is of the greatest
physical importance, and consequently that which has been
the subject of the most extensive system of observations, is
water.
If water be introduced above the mercurial column in the
apparatus above described, and be exposed successively to
various temperatures, the pressures and densities of the vapour
it produces can be observed and ascertained.
It is thus found that, in all cases, water passing into the
vaporous state undergoes an enormous enlargement of volume,
and that this enlargement increases as the temperature at which
the evaporation takes place is diminished. Thus, if the tem-
perature be 212, a cubic inch of water swells into 1696 cubic
inches ; and if the temperature be 77, it swells into 23090 cubic
inches of vapour.
1493. Vapour produced from water at all temperatures, how-
ever low. There is no temperature, however low, at which
92 HEAT.
water will not evaporate. If the bulb and tube be exposed to
the temperature of 32, the mercurial column in the tube will
be lower than the barometric column by two-tenths of an inch,
a small, but still observable quantity ; and even if the tem-
perature be reduced still lower, so that the liquid floating on
the mercury shall become solid ice, there will still be a vapour
in the bulb, of appreciable pressure and density. Thus, a piece
of ice at the temperature of 4, (that is, 36 below the freezing
point) produces a vapour whose pressure is represented by a
column of mercury of a twentieth of an inch.
The relation between the temperature, pressure, and density
of the vapour of water, from the lowest temperatures and pres-
sures to temperatures corresponding to a pressure twenty-four
times greater than that of the atmosphere, has been ascertained
by direct observation ; and by the comparison of these observ-
ations, an empirical formula has been found, which expresses
the general relation of the temperature and pressure with such
precision, within the range of the temperatures and pressures
observed, that it may be applied without risk of any important
error to the computation of the pressures, temperatures, and
densities, through a certain range of the scale, beyond the limits
to which observation and experiment have extended. In this
way, the temperatures and densities of the vapour, corresponding
to all pressures up to fifty times the pressure of the atmosphere,
have been computed and tabulated.
1494. Mechanical force developed in evaporation. When a
liquid expands into vapour, it exerts a certain mechanical force,
the amount of which depends on the pressure of the vapour,
and the increased volume which the liquid undergoes in evapo-
ration. Thus, if a cubic inch of a liquid swells by evaporation
into 2000 cubic inches of vapour having a pressure of 10 Ibs.
per square inch, it is easy to show that a mechanical force is
developed in such evaporation which is equivalent to 20,000 Ibs.
raised through one inch. For, if we imagine a cubic inch of
the liquid confined in a tube, the bore of which measures a
square inch, it will, when evaporated, fill 2000 inches of such
tube, and in swelling into that volume will exert a pressure of
10 Ibs., so that it would in fact raise a weight of 10 Ibs. through
that height. Now 10 Ibs. raised through the height of 2000
.inches, is equivalent to 20,000 Ibs. raised through the height of
one inch.
VAPORIZATION AND CONDENSATION. 93
Since, however, it is customary to express the mechanical
effect by the number of pounds raised through one foot, the
mechanical effect produced in the evaporation of each cubic
inch of a liquid will be found by multiplying the number
which expresses the volume of vapour produced by the unit of
volume of the liquid by the number expressing the pressure of
the vapour in pounds per square inch, and dividing the product
by 12.
In the following tables, the relation between the temperature,
pressure, density, volume, and mechanical effect of the vapour
of water are given as determined by observation so far as the
pressure of twenty-four atmospheres, and by analogy from that
to the pressure of fifty atmospheres.
TABLE I.
Showing the Pressure, Volume, and Density of the Vapour of
JVater produced at the Temperatures expressed in the first
Column, as well as the mechanical Effect developed in the
Process of Evaporation.
Temperature,
FarVnheit.
Pressure.
Volume of
v TnTuro a r
V6lume of
Densitv of Vapour
(Density of
Water =1).
Mechanical
Effect in Lbs.
raised 1 Foot.
Inches of
Mercury.
Lbs. per
Square Inch.
4
0-0^2
003
650588
0-00000154
1395
5
0-074
0-04
470898
212
1423
14
0-104
0-05
342984
292
1451
23
0-144
0-07
251358
398
1480
32
0-1 69
o-io
182323
540
1483
33-8
0-212
010
174495
673
1514
35-6
0-226
o-n
164332
609
1519
37-4
0-241
12
154*42
646
1525
392
0-257
0-13
145886
6S6
1531
41
0-274
0-13
137488
727
153G
42-8
0-291
0-14
129587
772
1542
44-6
0-310
0-15
122241
818
1549
46-4
0-330
0-16
115305
867
1555
48-2
0-351
0-17
108790
919
1559
50
O'o73
0-18
102670
974
1565
51-8
0-397
0-19
99202
0-00001032
1607
53-6
0-422
0-21
91564
1097
1577
55-4
0-448
0-22
86426
1157
1582
57-2
0-476
0-23
81686
1224
1588
59
0-505
025
77(08
1299
1590
60-8
0537
0-26
72913
1372
1598
62-6
0-570
0-28
1451
1604
64-4
0'6 n 4
0-30
65201
1534
1610
6C-2
0-641
0-31
61654
1622
1615
68
0-682
033
58224
1718
16-21
69-8
0-721
0-35
55206
1811
1626
71-6
0-764
0-37
52260
1914
1632
73-4
0-810
0-40
49487
2021
1638
75-2
0-858
0-42
46^77
2133
1(144
77
0-909
0-45
44411
2252
1649
78-8
0-963
0-47
42084
2376
1655
80-6
1-019
0-50
39895
2507
1661
82-4
1-078
0-53
37*38
S643
1667
84-2
1-143
0-56
35796
2794
1672
94
HEAT.
sssssr-
Pressure.
Volume of
Vapour contain-
ing Unit of
Volume of
Water.
TSST
Effect in'lJbs.
raised 1 Foot.
Inches of
Mercury.
Square Kch.
86
1-206
0-59
34041
0-00t029:t8
1678
87-8
1 276
063
32-291
3097
1634
89-6
1'349
0-G6
30650
3-J63
1689
91-4
1-425
0-70
29112
343>
1694
932
1-506
0-74
27636
3619
1700
95
1-591
0-74
26253
3809
1706
96-8
1-683
0-82
24*97
4017
1712
98-6
1-773
0-87
23704
4219
17i7
100-4
1-873
092
22513
4442
1722
102-2
1-974
0-97
21429
4666
1728
104
2-087
1-02
20343
4916
1734
10V8
2-196
1-08
19396
5156
1740
107-6
2-315
1 13
18469
5418
1746
109-4
2-439
1-20
17572
5691
1751
111-2
2-584
1-27
16805
6023
1774
113
2-707
1-33
15938
6274
17ti2
1148
2-850
1-40
15185
6585
1768
1166
3-OOfl
147
14472
6910
1774
118-4
3-158
1-55
13809
7242
1781
1202
3-322
1-63
13154
7602
1785
122
3-494
1-71
12546
7970
1791
123-8
3-673
1-80
11971
8354
1796
125-6
3861
1-89
11424
8753
1802
127-4
4-058
1-99
10901
9174
1807
129-2
4-263
2-09
10410
9606
1813
131
4-477
2-19
9946
0-0001 0054
1819
132-8
4-700
2-30
9501
10525
1824
131-6
4-934
2-42
9082
11011
1830
136-4
5-177
2-54
8680
11523
1836
138-2
5-431
2-66
8303
12044
1842
140
5-695
2-79
7937
12599
1847
141-8
5-973
2-93
7594
13179
1853
143-6
6-258
3-07
7-267
13760
IBM
145-4
6-558
3-21
b957
14374
1864
147-2
6-869
3-37
6662
15010
1869
149
7-193
3-53
65
3-984
2(105
192-2
19-897
W75
2474
40417
2011
194
20-680
10-14
2387
41891
2017
195-8
21-488
10-53
2304
43405
2023
197-G
22-321
10-94
2224
4 1956
2028
l!i9-4
23-179
1136
2148
46556
2034
201-2
24-062
11-80
2075
48201
2040
203
24-971
12-24
2015
49886
2045
204-8
25-!)08
12-70
1938
51613
2051
206-6
26874
13-17
1873
53388
5056
208-4
27-860
13-66
1812
55191
2062
210-2
28-877
14 16
1751
57(155
20C6
212
29-921
14-67
1696
58955
2073
VAPORIZATION AND CONDENSATION.
95
TABLE II.
Showing the Temperature, Volume, and Density of Vapour
of Water, corresponding to Pressures of from 1 to 50
Atmospheres.
From 1 to 24 Atmospheres obtained by Observation.
24 to 50 Analogy.
Pressure,
sphra.
Tempe-
raiure,
Fahrenheit.
Volume of
H&
V K-
W
tfKft
Pressure,
sphSe7.
Tempe-
Fahrenh'eit
Volume of
Vapour
produced by
Unit of
Volume of
Water.
Density of
Vapour
(Density of
Water =1).
!
212
1696
0-0005895
13
380-66
16374
0-006107
l|
233-96
1167-8
8563
14
386-96
153-10
6527
2
250-52
897-09
0-0011147
15
392-90
144-00
6944
N
263-84
731 -39
13673
16
398-48
135-90
7359
3*
27-V18
619 19
16150
17
403-88
12871
7769
3J
2*5-08
537-96
18589
18
408-92
12228
8178
4
293-72
476-26
20997
19
41378
116-51
8583
a
300-38
427-18
23410
20
418-46
111-28
8986
5
3(17 58
388-16
25763
21
422-96
106-53
9387
N
314-24
35.V99
209I
22
427-28
102 19
9785
6
3v('-:i6
328-93
30402
23
431-42
98-21
0-010182
65
326-30
305-98
3i683
24
435-56
9456
10575
r
331 70
286-12
3491 1
25
439-34
91-17
10!) 68
"i
336-92
26882
37217
30
457-16
77-50
12903
8
341-78
253-59
39434
35
472-64
68-20
14(63
9
3SO-78
227-98
43865
40
486-50
60f8
16644
10
35S-88
207-36
48220
45
499-10
5406
18497
11
366-80
190-27
52557
50
51062
49-31
20306
12
371-00
175-96
56834
1495. Vapour separated from a liquid may be dilated by heat
like any gaseous body. In these tables the vapour is con-
sidered as being in the state of the greatest density which is
compatible with its temperature. It must be remembered that
vapour separated from the liquid may, by receiving heat from
any external source, be raised like so much air, or other gaseous
fluid, to any temperature whatever, and that the elevation of
its temperature under such circumstances is attended with the
same effects as atmospheric air. If it be so confined as to be
incapable of expansion, its pressure will be augmented a ^^th
part by each degree of temperature it receives ; and if it be
capable of expanding under an uniform pressure, then its volume
will be augmented in the same ratio.
1496. Peculiar properties of superheated vapour. Vapour
which receives a supply of heat after it has been separated
from the liquid, and which may therefore be denominated
superheated vapour, has some important properties which dis-
tinguish it from the vapour which proceeds directly from the
liquid.
The vapour which proceeds directly from a liquid by the
96 HEAT.
process of evaporation, contains no more heat than is essential
to its maintenance in the vaporous form. If it lose any portion
of this heat, a part of it will become liquid ; and the more it
loses, the more will return to the liquid state, until, being de-
prived of all the heat which it had received in the process of
evaporation, the whole of the vapour will become liquid.
But, in the case of superheated vapour, the effects are
different. Such vapour may lose a part of its heat and still
continue to be vapour. In fact, no part of it can be reduced to
the liquid state until it lose all the heat which had been im-
parted to it after evaporation.
1497. Vapour cannot be reduced to the liquid state by mere
compression. It is sometimes affirmed that vapour may, by
mere mechanical compression, be reduced to the liquid state.
This is an error. It is true neither in relation to vapour raised
directly from liquids, nor of superheated vapour.
1498. Vapour which has the greatest density due to its tem-
perature under any given pressure, will have the greatest
density at all other pressures, provided it do not gain or lose
heat while the pressure is changed. If vapour raised directly
from a liquid, at any proposed pressure, be, after separation
from the liquid, either compressed into a diminished volume
or allowed to expand into an increased volume, its temperature
will be raised in the one case and lowered in the other ;
and, at the same time, its pressure will be augmented by the
diminution and diminished by the augmentation of volume. It
will be found, however, that the temperature, pressure, and
volume will in every case be exactly those which the vapour
would have had if it had been directly raised from the liquid
at that temperature and pressure.
Thus, the vapour raised from water at the temperature of 68
has a volume 58224 times greater than the water that produced
it (see Table I. p. 93.). Now let this vapour, being separated
from the water, be compressed until it be reduced to a volume
which is only 1696 times that of the water which produced it,
and its temperature will rise to 212, exactly that which it
would have had if it had been directly raised from the water
under the increased pressure to which it has been subjected.
In the same manner, whatever other pressure the vapour
may be submitted to, it will still, after compression, continue to
be vapour, and will be identical in temperature and volume
VAPORIZATION AND CONDENSATION. 97
with the vapour which would be raised from the same liquid
directly if evaporated under the increased pressure.
1499. Compression facilitates the abstraction of heat by
raising the temperature, and thus facilitates condensation. ,
Although mere compression cannot reduce any part of a volume
of vapour to the liquid state, it will facilitate such a change by
raising the temperature of the vapour without augmenting the
quantity of heat it contains, and thereby rendering it possible
to abstract heat from it. Thus, for example, if a volume of vapour
at the temperature of 32 be given, it may be difficult to convert
any portion of it into a liquid, because heat cannot be easily
abstracted from that which has already a temperature so low.
But if this vapour, by compression, and without receiving any
accession of heat, be raised to the temperature of 212, it can
easily be deprived of a part of its heat by placing it in contact
with any conducting body at a lower temperature; and the
moment it loses any part of its heat, however small, a portion
of it will be reduced to the liquid state.
1 500. Permanent gases are superheated vapours. It may be
considered as certain, that all that class of bodies which are
denominated permanent gases are the superheated vapours of
bodies which, under other thermal conditions, would be found
in the liquid or solid state. It is easy to conceive a thermal
condition of the globe, which would render it impossible that
water should exist save in the state of vapour. This would be
the case, for example, if the temperature of the atmosphere were
212 with its present pressure. A lower temperature, with the
same pressure, would convert alcohol and ether into permanent
gases.
1501. Processes by which gases have been liquefied and
solidified. The numerous experiments by which many of the
gases hitherto regarded as permanent have been condensed and
reduced to the liquid, and, in some cases, to the solid state,
have further confirmed the inferences based on these physical
analogies. The principle on which such experiments have in
general been founded is, that if, by any means, the heat which a
superheated vapour has received after having assumed the form
of vapour can be taken from it, the condensation of a part of
it must necessarily attend any further loss of heat, since, by
what has been explained, it will be apparent that no heat will
II. F
98
HEAT.
remain in it except what is essential to its maintenance in the
vaporous state.
The gas which it is desired to condense is first submitted to
severe compression, by which its temperature is raised either
by diminishing its specific heat or by developing heat that was
previously latent in it. The compressed gas is at the same
time surrounded by some medium of the most extreme cold ; so
that, as fast as heat is developed by compression, it is absorbed
by the surrounding medium.
When, by such means, all the heat by which the gas has been
surcharged has been abstracted, and when no heat remains save
what is essential to the maintenance of the elastic state, the gas
is in a thermal condition analogous to that of vapour which has
been directly raised by heat from a liquid, and which has not
received any further supply of heat from any other source. It
follows, therefore, that any further abstraction of heat must
cause the condensation of a corresponding portion of the gas.
1502. Gases which have been liquefied. The following
gases, being kept at the constant temperature of 32 by de-
priving them of heat as fast as their temperature was raised
by compression, have been reduced to the liquid state. The
pressures necessary to accomplish this are here indicated :
Names
of Oases
rondens
d.
Pressure under which
Condensation took place.
Sulphurous acid -
Adhere,
Cyanogen gas
Hydriodic acid -
Amraoniacal gas -
Hydrochloric add
Protoxide' of azote
2-3
4-0
4-4
8-0
37-0
Carbonic aci.i
39-0
If these substances be regarded as liquids, the above pressures
would be those under which they would vaporize at 32. If
they be regarded as vapours, they are the pressures under
which they would be condensed at 32.
M. Pouillet succeeded in condensing some of these gases
at the following higher temperatures and greater pressures :
Temperature,
Pressure,
Fahrenheit.
Atmospheres.
Sulphurous acid --.-..
Ammoniacal gas ------
Protoxide of azote - -
4G'4
50
51-8
25
5
43
Carbonic acid -----.--
50
45
VAPORIZATION AND CONDENSATION. 99
Hydrochloric acid has been reduced to a liquid at 50 under
a pressure of 40 atmospheres.
1 503. Under extreme pressures, gases depart from the common
Jaw of the density being proportional to the pressure. In these
experimental researches, it has been found that when the gases
are submitted to extreme compression, and deprived of a large
portion of the surcharged heat, they begin to depart from the
general law in virtue of which the density of gaseous bodies at
the same temperature is proportional to the compressing force,
and they are found to acquire a density greater than that
which they would have under this general law. This would
appear, therefore, as a departure from the law, preliminary to
the final change from the gaseous to the liquid state ; and in
this point of view, analogies have been observed which render
it probable that the point of condensation of several of the
gases not yet liquefied has been very nearly approached.
Thus, it has been found that the density of several of them,
among which may be mentioned light carburetted hydrogen
and defiant gas (heavy carburetted hydrogen), has been sen-
sibly greater than that due to the compressing force under
extreme degrees of compression.
1504. State of ebullition, boiling point. If heat be continu-
ally imparted to a liquid, its temperature will be augmented,
but will only rise to a certain point on the thermometric scale.
At that point it will remain stationary, until the whole of the
liquid shall be converted into vapour. During this process,
vapour will be formed in greater or less quantity throughout
the entire volume of the liquid, but more abundantly at those
parts to which the heat is applied. Thus if, as usually happens,
the heat be applied at the bottom of the vessel containing the
liquid, the vapour will be formed there in large bubbles, and
will rise to the surface, producing that agitation of the liquid
which has been called boiling or ebullition.
This limiting temperature is called the boiling point of the
liquid.
Different liquids boil at different temperatures. The boiling
point of a liquid is therefore one of its specific characters.
1 505. Boiling point varies with the pressure. Liquids in ge-
neral being boiled in open vessels, are subject to the pressure
of the atmosphere. If this pressure vary, as it does at dif-
100 HEAT.
ferent times and places, or if it be increased or diminished by
artificial means, the boiling point will undergo a corresponding
change. It will rise on the thermonietric scale as the pressure
to which the liquid is subject is increased, and will fall as that
pressure is diminished.
The boiling point of water is 212, when subject to a pres-
sure expressed by a column of 30 inches of mercury. It is
185, when subject to a pressure expressed by 17 inches of
mercury.
In general, the temperatures at which water would boil
under the pressures expressed in the second column of the table
(1494) are expressed in the first column.
1506. Experimental verification of this principle. Let
water at the temperature of 200, for example, be placed in a
glass vessel, under the receiver of an air-pump, and let the air
be gradually withdrawn. After a few strokes of the pump the
water will boil; and if the gauge of the pump be observed, it
will be found that its altitude will be about 23^ inches. Thus,
the pressure to which the water is submitted has been reduced
from the ordinary pressure of the atmosphere to a diminished
pressure expressed by 23^ inches, and we find that the tempe*
rature at which the water boils has been lowered from 212 to
200. Let the same experiment be repeated with water at the
temperature of 180, and it will be found that a further rare-
faction of the air is necessary, but the water will at length boil.
If the gauge of the pump be now observed, it will be found to
stand at 15 inches, showing that at the temperature of 180
water will boil under half the ordinary pressure of the atmo-
sphere. This experiment may be varied and repeated, and it
will always be found that water will boil at that temperature
which corresponds to the pressure given in the table.
1507. At elevated stations water boils at low temperatures.
It is well known, that as we ascend in the atmosphere, the pres-
sure is diminished in consequence of the quantity of air we leave
below us, and that, consequently, the barometer falls. It follows,
therefore, that at stations at different heights in the atmosphere,
water will boil at different temperatures ; and that the boiling
point at any given place must therefore depend on the eleva-
tion of that place above the surface of the sea. Hence the
boiling point of water becomes an indication of the height of
the station, or, in other words, an indication of the atmospheric
VAPORIZATION AND CONDENSATION.
101
pressure, and thus the thermometer serves in some degree the
purposes of a barometer.
1508. Table of the boiling points of water at various places.
In the following table the various temperatures are shown at
which water boils in the different places therein indicated.
Table of the boiling Points of Water at different Elevations
above the Level of the Sea.
Names of Places.
Above Level
of Sea.
Mean Height
of Barometer.
Thermometer.
Feet.
Incha.
D
Farm of Antisana -
13455
17-87
1874'
Town of Micuipampa (Peru)
11870
9541
19-02
2075
190-2
194-2
Town of Caxamarca (Peru)
9384
20-91
191-5
Santa Fe de Bogota
8731
21-42
195-6
Cuenca (Quito) -
21-50
195-8
Mexico ...
7471
22-52
198-1
Hospice of St. Gothard -
6808
23-07
199-2
St. Veron (Maritime Alps)
6693
23-15
199-4
Breuil ( Valley of Mont Cervin)
Maurin (Lower Alps)
St.Remi ... -
6585
6240
5265
2327
23-58
24-45
199-6
200-3
202-1
Heas (Pyrenees) -
48(7
24-88
202-8
Gavanne (Pyrenees)
4738
2-T96
203-0
Briancon -
4285
2539
203-9
Barege ( Pyrenees)
Palace of San Ildefonso (Spain)
4164
3790
25-51
25-87
204-1
204-8
Baths of Mont d'Or ( Auvergne)
3412
26-26
205-7
Pontarlier -
2717
26-97
206-8
Madrid
1995
2772
208'0
Innspruck -
1857
27-87
208-4
Munich -
1765
27-95
208-6
Lausanne -
1663
28-08
208-9
Augsburg - -
1558
28 19
209 1
Salzburg - -
1483
28-27
209-1
Neufchatel -
1437
2831
2093
Plombieres
1381
28-39
209-3
Clermont-Ferrand (Prefecture)
Geneva and Fribu g -
1348
1221
28-54
209-3
209-5
Ulm
1211
28-58
209-7
Ratisbon -
1188
28-58
209-7
Moscow -
984
210-2
Gotha -
935
2886
210-2
Turin - -
755
2906
210-4
Dijon
712
29-!4
210-6
Prague -
587
29-25
210-7
Macon (S:ione)
551
29-29
210-9
Lyons (Rhone)
532
29-33
210-9
Cassel
518
29-33
210-9
Gottingen - -
440
29-41
211-1
Vienna (Danube)
436
29-41
211-1
Milan (Botanic Garden) -
420
29-45
211-1
Bologna - -
397
29-49
211-1
Parma -
305
29-57
211-3
Dresden - -
295
29-61
211-3
Paris (Royal Obse vatory, first floor)
Rome (Capitol)
213
151
29-69
29-76
211-5
211-6
Berlin
131
29-76
211-6
1509. Latent heat of vapour. When a liquid is converted
into vapour, a certain quantity of heat is absorbed and rendered
latent in the vapour.
102
HEAT.
The vapour which proceeds from the
liquid has the same temperature as
the liquid. It can be shown, how-
ever, experimentally, that, weight fur
weight, it contains much more heat.
To render this manifest, let B, Jig.
446., be a vessel containing water,
which is kept in the state of ebulli-
tion and at the temperature of 212
by means of a lamp, or any other source of heat. Let the steam
be conducted by a pipe c to a vessel A, which contains a quantity
of water at the temperature of 32. The steam issuing from the
pipe is condensed by the cold water, and mixing with it, gra-
dually raises its temperature until it attains the temperature of
212, after which the steam ceases to be condensed, and escapes
in bubbles at the surface, as common air would if driven into
the water from the pipe.
If the quantity of water in A be weighed before and after this
process, its weight will be found to be increased in the ratio of
11 to 13. Thus 11 Ibs. of water at 32, mixed with 2 Ibs. of
water in the form of steam at 212, have produced 13 Ibs. of
water at 212, so that the 2 Ibs. of water which were introduced
in the form of steam at 21 2 have been changed from the va-
porous to the liquid state, retaining however their temperature
of 212, and have given to 11 Ibs. of water which were pre-
viously in A at 32 as much heat as has been sufficient to raise
that quantity to 212.
It follows, therefore, that any given weight of water in the
form of steam at 212 contains as much heat latent in it as is
sufficient to raise 5^ times its own weight of water from 32 to
212, that is, through 180 of the thermometric scale.
If it be assumed that to raise a pound of water through 180
requires 180 times as much heat as to raise it one degree, it
will follow that the quantity of latent heat contained in a pound
of Avater in the form of steam at 212 is 5 x 180990 times as
much as would raise a pound of water through one degree.
This fact is usually expressed by stating that steam at 212
contains 990 degrees of latent heat.
The same important fact can also be made manifest in the
following manner. Let a lamp, or any source of heat which acts
VAPORIZATION AND CONDENSATION. 103
in a regular and uniform manner, be applied to a vessel con-
taining any given quantity of water which is at 32 when the
process commences, and let the time be observed which the
lamp takes to raise the water to 212. Let the lamp continue
to act in the same uniform manner until all the water has been
converted into steam, and it will be found that the time neces-
sary for such complete evaporation will be exactly 5^- times
that which was necessary to raise the water from the freezing
to the boiling point. In a word, it will require 5^ times as
long an interval to convert any given quantity of water into
steam as it will take to raise the same quantity of water, by the
same source of heat, from the freezing to the boiling point ; and
consequently it follows, that 5 times as much heat is absorbed
in the evaporation of water, as is necessary to raise it without
evaporation through 180 of temperature.
1510. Different estimates of the latent heat of the vapour of
water. Different experimental inquirers have estimated the
heat rendered latent by Avater in the process of evaporation at
2 12 as follows:
Watt
Southern
Lavoisier
Rumford
Desprez
- 950
- 945
- 1000
- 1004'8
- 955'8
Kegnault ------ 967 -5
Fabre and Silbermann - - - 964-8
In round numbers, it may therefore be stated that as much
heat is absorbed in converting a given quantity of water at
212 into steam as would be sufficient to raise the same quantity
of water to the temperature of 1200 when not vaporized.
1511. Heat absorbed in evaporation at different temperatures.
It was observed at an early epoch in the progress of dis-
covery, that the heat absorbed in vaporization was less as the
temperature of the vaporizing liquid was higher. Thus a given
weight of water vaporized at 212, absorbs less heat than would
the same quantity vaporized at 180. It was generally assumed
that the increase of latent heat, for lower as compared with
higher temperatures, was equal to the difference of the sensible
heats, and consequently, that the latent heat added to the sensible
104
HEAT.
heat, for the same liquid, must always produce the same sum.
Thus, if water at 212 absorb in vaporization 950 of heat,
water at 262 would only absorb 900, and water at 162 would
absorb 1000.
The simplicity of this result rendered it attractive, and, as
the general result of experiments appeared to be in accordance
with it, it was generally adopted. M. Regnault has, however,
lately submitted the question, not only of the latent heat of
steam, but also its pressure, temperature, and density, to a
rigorous experimental investigation, and has obtained results
entitled to more confidence, and which show that the sum of
the latent and sensible heats is not rigorously constant.
1512. Latent heat of vapour of water ascertained by Regnault.
The pressures and densities obtained by M. Regnault are
in accordance with those given in (1494). The latent heats
are given in the following table, where I have given their
sums, and shown what does not seem to have been hitherto
noticed, that they increase by a constant difference.
Sum of
Sum of
Temp.
'ifeaT'
Latent Heat
and
Temp.
ifeat 1 *
Latent Heat
Sensible Heat.
Sensible Heat.
320
1092-6
1124-6
248
939-6
1187-6
50
1080-0
1130-0
2G6
927-0
1193-0
68
1067-4
1135-4
284
914-4
1 198-4
86
1054-8
1140-8
302
901-8
1203-8
104
1042-2-
1146-2
320
889-2
12(9-2
122
1029-6
1151-6
338
874-8
1212-8
140
1017-0
1157-0
356
802-2
1218-2
158
1004-4
1162-4
374
849-6
1223-6
176
991-8
1167-8
392
835-2
1227-2
194
979-2
1173-2
410
822-6
1232-6
212
966-6
1178-6
428
808-2
1236-2
230
952-2
11822
446
793-8
1239-8
It appears, therefore, that the sum of the latent and sensible
heats is not constant, but increases by a constant difference, a
difference however which, compared with the sum itself, is very
small, and for limited ranges of the thermometric scale, when
extreme accuracy is not required, may be disregarded.
1513. Latent heat of other vapours ascertained by Fabre
and Silbermann. The latent heat of the vapours of other
liquids have been ascertained by MM. Fabre and Silbermann,
and are given, as well as the specific heats, in the following
table :
VAPORIZATION AND CONDENSATION.
105
Names of Substances.
Temperature.
W
Latent
Heat.
Water
212
1
964-8
Carburetted hydrogen
Ditto
392
482
0-49
0-50
108
108
Pyroligneous acid
Alcohol, absolute
151-7
172-4
0-67
0-64
475-2
3744
Valerian
172-4
0-59
217-8
ethalic
172-4
0-51
104-4
Ether, sulphuric
100-4
0-50
163-8
Valerianic
Acid, formic
236-3
212
0-52
065
124-2
3042
acetic
248
0'5I
183-6
butyric
327-2
0-41
207
Valerianic
347
48
187-2
Ether, acetic
165-2
0-48
190-8
Butyrate of Metylfi
ne
199-4
049
156-6
Essence of turpeni
ne
312-8
0-47
124-2
Terebene -
Oil of lemons
312-8
329
0-52
050
1^0-6
126
1514. Condensation of vapour. Since by continually impart-
ing heat to any body in the liquid state it at length passes into
the form of vapour, analogy suggests that by continually with-
drawing heat from a body in the vaporous state, it must ne-
cessarily return to the liquid state ; and this is accordingly
generally true. The vapour being exposed to cold is deprived
of a part of that heat which is necessary to sustain it in the
aeriform state, and a part of it is accordingly restored to the
liquid form, and this continues until by the continual abstrac-
tion of heat the whole of the vapour becomes liquid; and as a
liquid, in passing to the vaporous form, undergoes an immense
expansion or increase of bulk, so a vapour in returning to the
liquid form undergoes a corresponding and equal diminution of
bulk. A cubic inch of water, transformed into steam at 212,
enlarges in magnitude to nearly 1700 cubic inches. The same
steam being reconverted into water, by abstracting from it the
heat communicated in its vaporization, will be restored to its
former bulk, and Avill form one cubic inch of water at 212.
Vapours arising from other liquids will undergo a like change,
differing only in the degree of diminution of volume which they
suffer respectively. The diminished space into which vapour
is contracted when it passes into the liquid form, has caused this
process to be called condensation.
1515. Why vessels in which liquids are boiled are not de-
stroyed by extreme heat. The absorption of heat in the process
by which liquids are converted into vapour will explain why a
vessel containing a liquid that is constantly exposed to the action
106 HEAT.
of fire can never receive such a degree of heat as would destroy
it. A tin kettle containing water may be exposed to the action of
the most fierce furnace, and I'emain uninjured ; but if it be ex-
posed without containing water to the most moderate fire, it
will soon be destroyed. The heat which the fire imparts to the
kettle containing water, is immediately absorbed by the steam
into which the water is converted. So long as water is contained
in the vessel, this absorption of heat will continue ; but if any
part of the vessel not containing water be exposed to the fire,
the metal will be fused, and the vessel destroyed.
1516. Uses of latent heat of steam in domestic economy.
The latent heat of steam may be used with convenience for
many domestic purposes. In cookery, if the steam raised from
boiling water be allowed to pass through meat or vegetables,
it will be condensed upon their surface, imparting to them the
latent heat which it contained before its condensation, and thus
they will be as effectually boiled as if they were immersed in
boiling water.
1517. Method of warming dwelling -houses. In dwelling-
houses where pipes convey cold water to different parts of
the building, steam pipes carried through the building will
enable hot water to be procured in every part of it with speed
and facility. The cock of the steam pipe being immersed in a
vessel containing cold water, the steam which escapes from it
will be condensed by the water, which receiving the latent heat
will soon be raised to any required temperature below the
boiling point. "Warm baths may thus be prepared in a few
minutes, the water of which would require a long period to
boil.
1518. Effects of the temperatures of different climates on
certain liquids. The variations of temperature incident to any
part of the globe are included within narrow limits, and these
limits determine the bodies which are found to exist there most
commonly in the solid, liquid, or gaseous state.
A body whose boiling point is below the lowest temperature
of the climate must always exist in the state of vapour or gas ;
and one whose point of fusion is above the highest tem-
perature must always be solid. Bodies whose point of fusion is
below the lowest temperature, while their boiling point is above
the highest temperature, will be permanent liquids. A body
whose point of fusion is a little above the lowest limit of the
CONDUCTION. 107
temperature, will exist generally as a liquid, but occasionally as
a solid. Water in these climates is an example of this. A
liquid, on the other hand, whose boiling point is a little below
the highest limit of temperature, will generally exist in the
liquid, but occasionally in the gaseous form. Ether in hot
climates is an example of this, its boiling point being 98.
Some bodies are only permanently retained in the liquid
state by the atmospheric pressure. Ether and alcohol are
examples of these. If these liquids be placed under the re-
ceiver of an air-pump, and the pressure of the air be par-
tially removed, they will boil at the common temperature of
the air.
CHAP. IX.
CONDUCTION.
1519. Good and bad conductors. When heat is imparted to
one part of any mass of matter, the temperature of that part
is raised above that of the other parts. This inequality,
however, is only temporary. The heat gradually diffuses
itself from particle to particle throughout the volume of the
body, until a perfect equilibrium of temperature has been
established. Different bodies exhibit a different facility in
this gradual transmission of heat. In some it passes more
rapidly from the hotter to the colder parts than in others.
Those bodies in which it passes easily and rapidly, are good
conductors. Those in which the temperature is equalized
slowly, are bad conductors.
1520. Experimental illustration of conduction. Let AB,
Fig. 447.
Jig. 447., be a bar of metal having a large cavity formed at its
extremity A, and having a series of small cavities formed at
108 HEAT.
equal distances throughout its length at TJ, T 2 , T 3 , &c. Let
the bulbs of a series of thermometers be immersed in mercury
in these cavities severally. These thermometers will all
indicate the same temperature, being that of the bar AB.
Let the large cavity A, at the end of the bar, be filled with
mercury at a high temperature, 400 for example.
After the lapse of some minutes the thermometer T will
begin to rise ; after another interval the thermometer T 2 will
begin to be affected ; and the others, T 3 , T 4 , &c., will be suc-
cessively affected in the same way ; but the thermometer T I} by
continuing to rise, will indicate a higher temperature than T 2 ,
and T 2 a higher temperature than T 3 , and so on. After the
lapse of a considerable time, however, the thermometer TJ will
become stationary. Soon afterwards T 2 , having risen to the
same point, will also become stationary; and, in the same
manner, all the others having successively risen to the same
point, will become stationary.
If several bars of different substances of equal dimensions be
submitted to the same process,
the thermometers will be more or
less rapidly affected according as
they are good or bad conductors.
An apparatus by which this is ex-
Fig. 448. hibited in a striking manner is
represented in Jig. 448. A series
of rods of equal length and thickness are inserted at the same
depth in the side of a rectangular vessel, passing across the inte-
rior of the vessel to the opposite side. The rods, which are silver,
copper, iron, glass, porcelain, wood, &c., are previously covered
with a thin coating of wax, or any other substance which will
melt at a low temperature. Boiling water or heated mercury
is poured into the vessel, and imparts heat to those parts of the
rods which extend across it. It is found that the heat as it
passes by conduction along the rods, melts the wax from their
surface. Those which are composed of the best conductors
silver, for example will melt off the wax most rapidly; the less
perfect conductors less rapidly ; and on the rods composed of the
most imperfectly conducting materials, such as glass or porce-
lain, the wax will not be melted beyond a very small distance
from the point where the rod enters the vessel.
1521. Table of conducting powers. By experiments con-
CONDUCTION. 109
ducted on this principle, it has been found that the conducting
powers of the subjoined substances are in the ratio here ex-
pressed, that of gold being 100.
Gold - - - 100-00
Platinum - - 98'10
Silver - - - 97-30
Copper ... 89-82
Iron ... 37-41
Zinc ... 36-37
Tin ... 30-38
Lead - - - 17"96
Marble - - - 2-34
Porcelain - - 1'22
Brick earth - - M3
It is evident, therefore, that metals are the best conductors of
heat, and in general the metals which have the greatest specific
gravity are the best conductors, as will appear by comparing
the preceding numbers with those given in the table of specific
gravity (782). It is also found that among woods, with some
exceptions, the conducting power increases with the density.
The conducting power of nut wood, however, is greater than
that of oak.
Bodies of a porous, soft, or spongy texture, and more espe-
cially those of a fibrous nature, such as wool, feathers, fur, hair,
&c., are the worst conductors of heat.
1522. Liquids and gases are non-conductors. Liquids are
almost absolute non-conductors. Let a tall narrow glass vessel
having a cake of ice at the bottom be filled with strong alcohol
at 32. Let two thermometers be immersed in it, one near the
surface, and the other at half the depth. If the alcohol be in-
flamed at the surface, the thermometer near the surface will
rise, but that which is at the middle of the depth will be
scarcely aiFected, and the ice at the bottom will not be dis-
solved.
Bodies in the gaseous state are probably still more imperfect
conductors than liquids.
1523. Temperature, equalized in these by circulation The
equilibrium of temperature is, however, maintained in liquid and
gaseous bodies by other principles, which are more prompt in
their action than the conductibility even of the solids which
possess that quality in the highest degree. When the strata of
fluids, whether liquid or gaseous, are heated, they become by
expansion relatively lighter than those around them. If they
have any strata above them, which generally happens, they
rise by their buoyancy, and the superior strata descend. There
are thus two systems of currents established, one ascending and
110 HEAT.
the other descending, by which the heat imparted to the fluid is
transfused through the mass, and the temperature is equalized.
1524. Conducting power diminished by subdivision and pul-
verization. The conducting power of all bodies is diminished
by pulverizing them, or dividing them into fine filaments.
Thus sawdust, when not too much compressed, is one of the
most perfect non-conductors of heat. A casing of sawdust is
found to be the most effectual method of preventing the escape
of heat from the surface of steam boilers and steam pipes.
If, however, the sawdust be either much compressed on the
one hand, or too loosely applied on the other, it is not so perfect
a non-conductor. In the one case, the particles being brought
into closer contact, transmit heat from one to another ; and in
the other case, the air circulating too freely among them, the
currents are established by which the heat is transfused through
the mass.
To produce, therefore, the most perfect non-conductor, the
particles of the body must have naturally little conductibility,
and they must be sufficiently compressed to prevent the circu-
lation of currents of air among them, and not sufficiently com-
pressed to give them a facility of transmitting heat from par-
ticle to particle by contact.
1525. Beautiful examples of this principle in the animal
economy. The animal economy presents numerous and beau-
tiful examples of the fulfilment of these conditions. It is
generally necessary to the well-being of the animal to have a
temperature higher than that of the medium which it in-
habits. In the animal organization, there is a principle by
which heat is generated. This heat has a tendency to escape
and to be dissipated at the surface of the body, and the rate
at which it is dissipated depends on the difference between the
temperature of the surface of the body and the temperature of
the surrounding medium. If this difference were too great, the
heat would be dissipated faster than it is generated, and a loss
of heat would take place, which, being continued to a certain
extreme, would destroy the animal.
Nature has provided an expedient to prevent this, which
varies in its efficiency according to the circumstances of the
climate and the habits of the animal.
1526. Uses of the plumage of birds. The plumage of birds
is composed of materials which are bad conductors of heat, and
CONDUCTION. Ill
are so disposed as to contain in their interstices a great quantity
of air without leaving it space to circulate. For those species
which inhabit the colder climates a still more effectual provision
is made, for, under the ordinary plumage, which is adapted to
resist the wind and rain, a still more fine and delicate down is
found, which intercepts the heat which would otherwise escape
through the coarser plumage. Perhaps the most perfect insu-
lator of heat is swansdown.
1527. The wool and fur of animals The wool and fur of
animals are provisions obviously adapted to the same uses.
They vary not only with the climate which the species inhabits,
but in the same individual they change with the season. In
warm climates the furs are in general coarse and sparse, while
in cold countries they are fine, close, light, and of uniform tex-
ture, so as to be almost impermeable to heat.
1528. The bark of vegetables. The vegetable, not less than
the animal kingdom, supplies striking illustrations of this prin-
ciple. The bark, instead of being hard and compact, like the
wood which it clothes, is porous, and in general formed of dis-
continuous laminas and fibres, and for the reasons already ex-
plained is a bad conductor of heat, and thus prevents such a
loss of heat from the surface of the wood under it as would be
injurious to the tree.
A tree stripped of its bark perishes as an animal would if
stripped of its fleece, or a bird of its plumage.
1529. Properties of the artificial clothing of man. Man is
endowed with faculties which enable him to fabricate for him-
self covering similar to that which nature has provided for
other animals; and "where his social condition is not sufficiently
advanced for the accomplishment of this, his object is attained
by the conquest of inferior animals whose clothing he appro-
priates.
Clothes are generally composed of some light non-conducting
substances which protect the body from the inclement heat or
cold of the external air. In summer clothing keeps the body
cool ; in winter, warm. Woollen substances are worse con-
ductors of heat than cotton, cotton than silk, and silk than
linen. A flannel shirt more effectually intercepts heat than
cotton, and a cotton than a linen one.
1530. Effects of snow on the soil in winter. What the
plumage does for the bird, wool for the animal, and clothing for
112 HEAT.
the man, snow does in winter for the soil. The farmer and the
gardener look with dismay at a hard and continued frost which
is not preceded by a fall of snow. The snow is nearly a non-
conductor, and, when sufficiently deep, may be considered as
absolutely so. The surface may therefore fall to a temperature
greatly below 32, but the bottom in contact with the vegetation
of the soil does not share in this fall of temperature, remaining
at 32, a temperature at that season not incompatible with the
vegetable organization. Thus the roots and young shoots are
protected from a destructive cold.
1531. Matting upon exotics. The gardener who rears exotic
vegetables and fruit-trees, protects them from the extreme cold
of winter by coating them with straw, matting, moss, and other
fibrous materials which are non-conductors.
1532. Method of preserving ice in hot climates. If we would
preserve ice from dissolving, the most effectual means would be
to wrap it in blankets. Ice-houses may be advantageously sur-
rounded with sawdust, which keeps them cold by excluding
the heat, by the same property in virtue of which it keeps steam
boilers warm by including the heat.
Air being a bad conductor of heat, ice-houses are sometimes
constructed with double walls, having a space between them.
This expedient is still more effectual, if the space be filled with
loose sawdust.
1 533. Glass and porcelain vessels, why broken by hot water.
Glass and porcelain are slow conductors of heat, which explains
the fact that vessels of this material are so often broken by sud-
denly pouring hot water into them. If it be poured into a glass
tumbler, the bottom, with which the water first comes in contact,
expands, but the heat not passing freely to the upper part, this
expansion is limited to the bottom, which is thus forced from the
upper part and a crack is produced.
1534. Wine-coolers. When wine-coolers have a double
casing, the external space is filled with a non-conductor.
1535. A heated globe cools inwards. When a solid body, a
globe for example, is heated at the surface, the heat passes
gradually from the surface to the centre. The temperature of
the superficial stratum is greater, and the temperature of the
centre less than those of the intermediate parts, and the tem-
perature of the successive strata are gradually less proceeding
from the surface to the centre.
CONDUCTION. 113
But if the globe be previously heated, so as to have an uniform
temperature from the centre to the surface, and be allowed to
cool gradually, the superficial stratum will first fall some
degrees below the stratum within it. This latter will fall
below the next stratum proceeding inwards ; and in the same
way each successive stratum proceeding from the surface to the
centre will attain a temperature a little lower than the stratum
under it, the temperatures augmenting from the surface to the
centre.
After an interval, of greater or less duration according to the
magnitude of the globe, the conductability and specific heat of
the matter of which it is composed, the temperature to which it
had been raised, and the temperature of the medium, it will be
reduced to an uniform temperature, which will be that of the
surrounding medium.
1536. Example of fluid metal cast in spherical mould. If a
mass of fluid metal be cast in a spherical mould, the surface only
will be solidified in the first instance. It will become a sphe-
rical shell, filled with liquid metal. As the cooling proceeds, the
shell will thicken, and after an interval of time, the length of
which will depend on the circumstances above mentioned, the
ball will become solid to its very centre, the last portion soli-
dified being that part of the metal which is at and immediately
around the centre.
It is evident that the superficial stratum will first cease to be
incandescent ; and in the same way each successive stratum pro-
ceeding from the surface to the centre will cease to be incan-
descent before the stratum within it.
If in the process of cooling, and after the globe ceases to be
red hot, it were cut through the centre, it would be found that
the central parts would be still incandescent ; and if its magni-
tude were sufficiently considerable, it would be found that even
after the superficial stratum had been reduced to a moderate
temperature, strata nearer the centre would be red hot, and the
central part still fluid.
1537. Cooling process may be indefinitely protracted. The
interval which must elapse before the thermal equilibrium would
be established, might be hours, days, weeks, months, years, or
even a long succession of ages, according to the magnitude and
physical qualities of the material composing the globe.
1538. Example of the castings of the hydraulic press, which
114 HEAT.
raised the Britannia Bridge The cylinder of the hydraulic
press by which the tubes of the Britannia bridge were elevated
was formed of a mass of fluid iron weighing 22 tons. This
enormous casting, after being left in the mould for three days
and nights, was still red hot at the surface. After standing to
cool in the open air for ten days, it was still so hot that it could
only be approached by men well inured to heat.
1539. Example of streams of volcanic lava. The torrents
of liquid lava which flow from volcanos become solid on their
external surface only to a certain thickness. The lava in the
interior of this shell still continues fluid. The stream of lava
thus forms a vast tube, within which that portion of the lava
still liquid flows for a long period of time. Months and even
years sometimes elapse, before the thermal equilibrium of these
volcanic masses is established.
1540. Example of the earth itself. The globe of the earth
itself presents a stupendous example of the play of these prin-
ciples. The vicissitudes of temperature incidental to the surface
extend to an inconsiderable depth. At the depth of an hundred
feet in our climates, they are completely effaced. At this depth,
the thermometer no longer varies with the seasons. In the
rigour of winter, and the ardour of summer, it stands at the
same point. This stratum, which is called the first stratum
of invariable temperature, is found to be at Paris at the depth
of 88-6 feet. The thermometer in the vaults under the Ob-
servatory at that depth has continued without variation at 1 1 - 82
cent. = 53-50 Fahr. for nearly two centuries.
1541. Temperature increases with the depth. At greater
depths the temperature increases, but is always invariable for
the same depth. An increase of temperature takes place in
descending, at the rate of one degree for every 54-^- feet of
depth. Thus the water which issues from the artesian wells
at Grenelle near Paris, and which rises from a depth of 1800
feet, has a constant temperature of 27'7 cent. = 82 Fahr.
It is apparent that the earth is a globe undergoing the
gradual process of cooling, and that each stratum proceeding
inwards towards the centre augments in temperature. It follows,
therefore, that a part at least of the superficial heat of the earth
proceeds from within. It is certain, nevertheless, by taking into
account all the conditions of the question, that the cooling goes
on so slowly, as to have no sensible influence on the tempera-
RADIATION. 115
ture at the surface, which is therefore governed by the solar
heat, and the heat of the medium or space in which our globe,
in common with the other planets, moves. It has been com-
puted that the quantity of central heat which reaches the
surface in a year would not suffice to dissolve a cake of ice a
quarter of an inch think.
1542. The earth was formerly in a state of fusion, and it
is still cooling. The globe of the earth therefore manifesting
the effects of a mass which, having been at some antecedent
period at an elevated temperature, is undergoing the process of
gradually cooling from the surface inwards, it is probable that
its central parts may be still in a state of incandescence or
fusion.
CHAP. X.
RADIATION.
1543. Heat radiates like light Heat, like light, is propagated
through space by radiation in straight lines, and its rays, like
those of light, are subject to transmission, reflection, and ab-
sorption by such bodies as they encounter in various degrees.
All that has been explained (896. et seq.} respecting the re-
flection of light from unpolished, perfectly and imperfectly
polished surfaces, its refraction by transparent media, its inter-
ference, inflexion, and polarization, may, with little modification,
be applied to the rays of heat submitted to like conditions.
1544. Thermal analysis of solar light. It has been already
shown that solar light is a compound principle, consisting of
rays differing one from another, not only in their luminous
qualities of colour and brightness, but also in their thermal and
chemical properties (1076. et seq.*).
Let ss,y%r. 449., represent a pencil of solar light transmitted
through a prism ABC, so as to be resolved into a divergent fan
of rays, and to form a spectrum as described in (1053.) et seq.
Let L and i/ be the limits of the luminous spectrum. If the
bulb of a thermometer be placed at L. it will not indicate any
elevation of temperature ; and if it be gradually moved down-
116
HEAT.
wards along the spectrum, it will not begin to be sensibly
affected until it arrives at the boundary of the violet and blue
Fig. 449.
spaces, where it will show an increased temperature. As it is
moved downwards from this point, the temperature will con-
tinue to increase until it is brought to the lower extremity I/ of
the luminous spectrum. If it be then removed below this point,
instead of falling to the temperature of the medium around the
spectrum, as might be expected, and as would in fact happen
if no rays of heat transmitted through the prism passed below
i/, it will descend slowly and gradually, and will in some cases
even show an increased temperature to a certain small distance
below i/. In fine, it will be found that the thermometer will
not fall to the temperature of the surrounding medium until
it arrives at a certain distance H' below i/, the extremity of the
luminous spectrum.
1545. Thermal solar rays differently refrangible. From
this and other similar experiments, it is inferred that thermal
rays which are not luminous, or at least not sensibly so, enter into
the composition of solar light, and that these rays are differently
refrangible, their mean refrangibility being less than the mean
refrangibility of the luminous rays.
It has been explained (1077) that the chemical rays which
enter into the composition of solar light are also differently
refrangible, and have a mean refrangibility greater than that
of the luminous rays.
1546. Physical analysis of solar light Three spectra.
According to this view of the constitution of solar light, the
prism ABC must be regarded as producing three spectra, a
chemical spectrum cc', a luminous spectrum LI/, and a thermal
spectrum HH'. The luminous or chromatic spectrum, the only
one visible, lies between, and is partly overlaid by, the other
RADIATION. 117
two, the chemical spectrum extending a little above, and the
thermal a little below it. If we imagine a screen MN placed
before the prism, composed of a material pervious to the lumi-
nous, but impervious to the chemical and thermal rays, then the
luminous spectrum LI/ alone will remain, and neither a ther-
mometer nor the chloride of silver, nor any other chemical sub-
stance, will be affected when exposed in it. If the screen MN
be pervious only to the thermal rays, then the luminous and
chemical rays will be intercepted, and the thermal spectrum HH'
alone will be manifested. The thermometer exposed in it will
indicate the variations of calorific influence already explained,
showing the greatest thermal intensity at or near that point at
which the red extremity of the luminous spectrum would have
been found, had the luminous rays not been intercepted.
1547. Relative refrangibility of the constituents of solar light
vary with the refracting medium. If prisms composed of
different materials be used, it will be found that the mean re-
frangibility of the thermal rays will vary according to the
material of the prism and heat ; consequently, the position of the
point of greatest thermal intensity will be subject to a like
variation.
If a hollow prism be filled with water or alcohol, the point of
greatest thermal intensity will be about the middle of the yellow
space of the luminous spectrum. If a prism of sulphuric acid,
or a solution of corrosive sublimate, be used, it will be in the
orange space. With a crown glass prism it will be in the red
space ; and with one of flint glass, a little below that space.
1548. Invisible rays may be luminous, and all rays may be
thermal. In the preceding explanation, the solar light is re-
garded as consisting of three distinct species of rays, the
chemical, the luminous, and the thermal. It is not necessary,
however, for the explanation of these phenomena, to adopt this
hypothesis. The light may be considered as consisting of rays
which, differing in refrangibility, possess the other physical
qualities also in different degrees. So far as the sensibility of
thermometers enables us to detect the thermal property, it ceases
to exist at a certain point, H G, near the boundary of the blue
and violet spaces ; but the diminution of thermal intensity, in
approaching this point, as indicated by the thermometer, is very
gradual; and it cannot be denied, that a thermal influence may
exist above that point, which is, nevertheless, too feeble to affect
the thermoscopic tests which are used. In the same manner it
118 HEAT.
may be maintained, that a chemical influence may exist below
the point c', but too feeble to affect any of the tests which have
been applied to it.
But it may be asked, if all the component rays possess all
the properties in different degrees, how happens it that the
chemical rays above L, and the thermal rays below i/, are not
visible? To this it may be answered, that the presence of the
luminous quality is determined by its effects on the eye ; and the
discovery of its presence must therefore depend on the sen-
sibility of that organ. To pronounce that there are no luminous
rays beyond the limits of the chromatic spectrum, would be
equivalent to declaring the sensibility of the eye to be unlimited.
Now, it is notorious that the sensibility of sight, in different
persons, is different ; and, even in the same individual, varies at
different times. Circumstances render it highly probable that
many inferior animals have a sensation of light, and a perception
of visible objects, where the human eye has none; and it is
therefore consistent with analogy to admit the possibility, if
not the probability, that the invisible thermal rays below i/, and
the invisible chemical rays above L, may be of the same nature
as the other rays of the spectrum, all enjoying the luminous,
thermal, and chemical properties in common, the apparent
absence of these properties in the extreme rays being ascribable
solely to the want of sufficient sensibility in the only tests of
their presence which we possess.
Fortunately, however, the deductions of physical science,
though they may be facilitated by these and other hypotheses,
are not dependent on them, but on observed facts and phe-
nomena, and cannot, consequently, be shaken by the failure of
such theories.
1549. Refraction of invisible thermal rays. If a hole be
made in the screen upon which the prismatic spectrum is thrown,
in the space L' H' below the red extremity of the spectrum
upon which the invisible thermal rays fall, these rays will pass
through it, and may be submitted to all the experiments on
reflection, refraction, inflexion, interference, and polarization,
which have been explained in relation to light. This has been
done, and they have been found to manifest effects similar to
those exhibited by luminous rays.
1550. Heat radiated from each point on the surface of a body.
It has been shown (902. et seq.) that when a body is either
luminous, like the sun, or illuminated, like the moon, each point
RADIATION. 119
upon its surface is an independent centre of radiation or focus,
from which rays of light diverge or radiate in all directions!
It is the same with regard to heat. All bodies, whatever be
the state or condition, contain more or less of this physical
principle ; and rays of heat accordingly issue from every point
upon their surface, as from a focus, and diverge or radiate in
all directions through the surrounding space.
1551. Why bodies are not therefore indefinitely cooled.
This being the case, it would follow that by such continual and
unlimited radiation, bodies -would gradually lose their heat, and
indefinitely fall in temperature. It must be considered, however,
that such radiation being universal, each body, while it thus
radiates heat, receives upon its surface the rays of heat which
proceed from other bodies around it. So many of these rays
as it absorbs tend to increase its temperature, and to replace
the heat dispersed by its own radiation. There is thus between
body and body a continual interchange of heat by radiation ;
and according as this interchange is equal or unequal, the tem-
perature of the radiating body will rise or fall. If it radiate
more than it absorbs, it will fall ; if less, it will rise. If it absorb
as much exactly as it radiates, its temperature will be maintained
stationary.
1552. Radiation is superficial or nearly so. Radiation takes
place altogether from points either on the surface or at a very
small depth below it. The circumstances which affect it have
been made manifest by a beautiful series of experiments made
by the late Sir John Leslie. The principles on which his mode
of experimenting was founded, are easily explained.
1553. Reflection of heat.~Let a cubical canister of tin,
fig. 450., be placed in the axis of
a parabolic metallic reflector M,
in the focus /of which is placed
the bulb of a sensitive differential
thermometer. If the canister
be placed with one of its sides at
right angles to the axis of the re-
flector, and be filled with boiling
water, the thermometer will in-
stantly show an increase of tem-
perature caused by the heat radiated from the surface of the
canister, and collected into a focus upon the ball by the
reflector.
120 HEAT.
The experiment may be varied by filling the canister with
liquids at all temperatures, with snow and with freezing mix-
tures having various degrees of artificial cold. The surface of
the canister may be varied in material by attaching to it
different substances, such as paper, metallic foil, glass, por-
celain, &c. It may be varied in texture by rendering it rough
or smooth, and in colour by any colouring matter.
In this way the influence of all these physical conditions
upon the radiation from the surface may be, and has been,
ascertained.
The results of such experimental researches have been
briefly as follows :
1554. Rate of radiation proportional to excess of temperature
of radiator above surrounding medium. The rate at which
the radiating body loses or gains temperature, other things being
the same, is proportional to the difference between its own tem-
perature and that of the surrounding medium, where this dif-
ference is not of very extreme amount.
1555. Intensity inversely as square of distance. The in-
tensity of the heat radiated is, like that of light, other things
being the same, inversely as the square of the distance from the
centre of radiation (907).
1556. Influence of surface on radiating power. The ra-
diating power varies with the nature of the surface, and its
degree of polish or roughness.
In general, the more polished a surface is, the less will be its
radiation. Whatever tarnishes or roughens the surface of metal,
increases its radiation.
Metallic are in general less powerful radiators than non-
metallic surfaces.
1557. Rejection of heat. When the rays of heat encounter
any surface, they are more or less reflected from it. Surfaces,
therefore, in relation to heat, are perfect or imperfect, good or
bad reflectors.
In the experiments above described, the reflecting powers
of different surfaces were ascertained by constructing the
concave reflector M of different materials, or by coating its
surface variously, or, in fine, by submitting its surface to any
desired physical conditions- Thus, when a reflector of glass is
substituted for one of metal, the radiating surface of the canister
remaining the same, it is found that the effect on the thermo-
EADIATION. 121
meter is diminished. Glass is therefore a less perfect re-
flector than metal. If the surface of the reflector be coated
with lampblack, no effect whatever is produced on the ther-
mometer. Such a surface does not, therefore, reflect the
thermal rays.
1558. Absorption of heat. To determine the physical con-
ditions which affect the absorbing power of a surface, it is only
necessary, in the experiment above described, to vary the surface
of the ball^of the thermometer, which is placed in the focus
of the reflector, for, as the heat is radiated by c and reflected
by M, it is absorbed by t.
By coating the ball of the thermometer, therefore, with
metallic foil, paper, lampblack, and other substances, and by
rendering it in various degrees rough and smooth, the effects
of these modifications on the thermometer are rendered manifest,
and the comparative absorbing powers are ascertained.
In this way it has been ascertained that the same physical
conditions which increase the radiation and diminish the reflec-
tion, increase the absorption. The best radiators are the most
powerful absorbers and the most imperfect reflectors.
1559. Tabular statement of radiating and reflecting
poivers. The relative radiating, absorbing, and reflecting
powers of various surfaces have been submitted to a still more
rigorous analysis by M. Melloni, whose researches were greatly
favoured by the fine climate of Naples, where they were prin-
cipally made. The results are given in the following table,
in the first column of which the numbers express the radiating
and absorbing powers, that of a surface covered with the smoke
of a lamp being expressed by 100. The absorbing power of this
surface is complete. The reflecting power is, as will be ob-
served, the complement of the absorbing power.
122
HEAT.
Table showing the absorbing and reflecting Powers of various
Surfaces according to the Experiments of Melloni.
li
-
i!
**
=;
Is
Names.
1?
Kama.
o
J!
If
S*
3j_
Smoke-blackened surface
100
Metallic mirrors a little tar-
Carbonate of lead -
100
nished - - - -
17
83
Writing paper
98
90
2
10
nearly polished
Brass cast, imperfectly po-
14
86
China ink
85
15
lished - ...
11
89
Gumlac -
72
hammered,
9
91
Silver foil on glass
Cast iron polished -
27
25
73
75
highly polished
,, cast, ,,
7
7
93
93
Mercury (nearly) -
Wrought iron polished
Zinc polished
Steel,
23
23
19
17
77
77
81
83
Copper coated on iron -
varnished -
hammered or cast
Gold plating -
7
14
7
5
93
86
93
95
Platinum, thick coat, imper
fectly polish d
24
76
Gold deposited on polished
steel
3
97
plate on coppe
leaves
17
17
83
Silver, hammered and well
polished - - - -
3
97
Tin- - - -
14
86
Silver, cast, and well polished
3
97
1560. Singular anomaly in the reflection from metallic
surfaces. The numbers given in this table, which will be ob-
served to differ considerably from those determined by Leslie
and others, have been obtained by the recent elaborate experi-
mental researches of MM. de la Provostaye and Desains. In
these experiments an anomalous circumstance was observed on
varying the angle of incidence of the thermal rays. It was
found that, in the case of glass, the proportion of rays reflected
increased with the angle of incidence, as happens with luminous
rays, but that with polished metallic surfaces, the same propor-
tion was reflected at all incidences up to 70, and beyond this
limit the proportion reflected, instead of increasing, as would
have been expected, was greatly diminished.
1561. Thermal equilibrium maintained by the interchange of
heat by radiation and absorption. From all that has been
here explained it will be apparent that the state of thermal
equilibrium is maintained among any system of bodies by a
continual interchange of heat by radiation and absorption. The
heat which each body receives from others in its presence, it
partly absorbs and partly reflects. Those rays which it absorbs
tend to raise its temperature ; and this temperature would soon
rise above that which the thermal equilibrium requires, but
that the body radiates heat from all points of its surface ; and
RADIATION. 12,3
the total quantity thus radiated is equal to the total quantity
absorbed. If either of these quantites were permanently greater
or less than the other, the temperature of the body would either
indefinitely rise, or indefinitely fall, according as the heat
absorbed or radiated might be in excess.
If a body, at any given temperature, be placed among other
bodies, it will immediately affect them thermally, just as a
candle brought into a room illuminates all bodies in its pre-
sence, with this difference, however, that if the candle be extin-
guished, no more light is diffused by it ; but no body can be
thermally extinguished. All bodies, however low be their tem-
perature, contain heat, and therefore radiate it.
1562. Erroneous hypothesis of radiation of cold. If a ball
of ice be brought into the presence of a thermometer, the ther-
mometer will fall ; and hence it was erroneously inferred that
the ice emitted rays of cold. The effect, however, is otherwise
explained. The ice and the ball of the thermometer both
radiate heat, and each absorbs more or less of what the other
radiates towards it. But the ice being at a lower temperature
than the thermometer, radiates less than the thermometer, and
therefore the thermometer absorbs less than the ice, and con-
sequently falls.
If the thermometer placed in presence of the ice had been at
a lower temperature than the ice, it would, for like reasons,
have risen. The ice in that case would have warmed the ther-
mometer.
1563. Transmission of heat. When rays of heat are inci-
dent on the surfaces of certain media, they penetrate them in
greater or less quantity, according to the nature and properties
of the medium, just as rays of light pass through bodies which
are more or less transparent or diaphanous.
Media which are pervious to heat are said to be diatherma-
nous, and those which are impervious are called athermanous.
Bodies are diathermanous in different degrees, or altogether
athermanous, according to their various physical characters,
their thickness, the state of their surface, the nature of the heat
which is incident upon them, and other conditions.
1564. Mellon? s thermoscopic apparatus. Nearly all the
knowledge we possess in this branch of the physics of heat is
the result oft the recent researches of M. Melloni. The ther-
moscopic apparatus contrived and applied with singular felicity
124
HEAT.
and success by him, consisted of a thermo-galvanic pile acting
upon a highly sensitive galvanometer. It will be explained
hereafter that if the thermal equilibrium be disturbed in certain
metallic combinations, an electric current will be produced, the
intensity of which will be proportional to the difference of tem-
perature produced, and that the force of such a current can be
measured by the deviation it produces in a magnetic needle,
round which it is conducted spirally upon a coil of metallic wire
coated with a non-conducting substance.
The general form and arrangement of this apparatus, and the
manner of applying it to
thermal researches, are re-
presented \nfigs. 451, 452,
453, and 454.
Upon the stand s is placed
the source of heat which is
submitted to experiment.
Those which M. Mellon i
selected were a lamp L,
with a concave reflector t ;
a spiral wire of platinum
Ti, fig. 452., rendered incan-
descent by the flame of a
spirit-lamp; a plate of copper
i, fig. 453., blackened with
smoke, and raised to the tem-
perature of 700 by a spirit-
lamp ; and, in fine, a cubical
Fig. 451.
canister K, fig. 454., similar
to those used by Leslie.
J-ig. 452.
Fig. 453.
Fig. 454.
RADIATION. 125
On the stand T was placed the body a*, through which the
rays of heat were to be transmitted, and which was formed
into a thin plate. An athermanous screen / was interposed,
having in it an aperture to limit the pencil of rays transmitted
to x. Another athermanous screen was placed at c, movable
upon a joint by which the pencil proceeding from the lamp
could be intercepted or transmitted at pleasure.
The thermo-voltaic pile was placed at p, having one end
presented to the thermal pencil, and movable in a case fitting it,
in which it was capable of sliding. Its poles p and n were con-
nected by conducting wires with the galvanometer, the needle
of which indicated by its deflection the intensity of the heat by
which the pile p was affected.
1565. Results of Melloms researches. The series of expe-
riments made with this apparatus gave the remarkable, and in
many respects unexpected, results which we shall now briefly
state.
The only substance found to be perfectly diathermanous was
rocksalt. Plates of this crystal transmit nearly all the heat
which enters them, no matter from what source. Of the inci-
dent rays 7*7 per cent, are reflected from both surfaces of the
plate, and the whole of the remaining 92*3 per cent, are trans-
mitted. There is no absorption.
Bodies in general are less athermanous the higher the tempe-
rature of the radiator.
1566. Transparent media not proportionally diathermanous.
Media are not diathermanous in proportion as they are trans-
parent. On the contrary, certain media which are nearly opaque
are highly diathermanous, while others which are highly trans-
parent are nearly athermanous. Thus, black glass and plates of
smoked quartz so opaque that the disk of the sun in the meri-
dian is barely visible through them, are much more diatherma-
nous than plates of alum, which are very transparent ; and
plates of quartz smoked to opacity are more diathermanous
than when clean and transparent. In like manner, black glass
is more diathermanous than colourless glass.
1567- Decomposition of heat by absorption. The thermal
pencil is composed of rays, some of which are absorbed, and
others transmitted by certain media. This effect is altogether
analogous to that which is produced by coloured media on
light. If a pencil of solar light be incident upon red glass, the
126 HEAT.
red rays alone will be transmitted, those of the other colours
being absorbed ; but if the red light transmitted through such a
plate be received upon a second red plate, there will be no
further absorption, at least so far as depends on the colour of
the light. In like manner, when a thermal pencil enters cer-
tain diathermanous media, a part of its rays are intercepted,
others being transmitted. If these last be received upon
another plate of the same diathermanous substance, they will
pass freely through it without further absorption.
It is therefore inferred that such a medium decomposes by
absorption the thermal pencil in the same manner as a coloured
transparent medium decomposes by absorption a pencil of white
light. This inference is confirmed by the fact that different
partially diathermanous media absorb different constituents of
the thermal pencil. Thus we may cause its entire absorption
by causing it successively to pass through two media, each of
which absorbs the rays transmissible by the other.
This is also analogous to the effects of coloured transparent
media upon luminous pencils. If a pencil of solar light be
successively incident upon two plates, one of red and the other
of the complementary tint of bluish-green, it will be wholly
absorbed, the second plate absorbing all the rays transmitted by
the first.
1568. Absorption not superficial, but limited to a certain
depth. The partial absorption produced by such imperfectly
diathermanous media is not effected at the surface. The rays
are absorbed gradually as they pass through the medium. This,
however, is not continual. All absorption ceases after they
have passed through a certain thickness, and the rays trans-
mitted by a plate of that thickness would, in passing through a
second plate of the same substance, undergo no further absorp-
tion.
Glass and rock crystal are each partially diathermanous, the
thermal rays transmitted and absorbed however being different.
If a thermal pencil pass through a plate of glass of a certain
thickness, a part of the rays composing it will be absorbed. If
the rays transmitted be received on another similar plate of
glass, they will be all or nearly all transmitted, no further ab-
sorption taking place. But if these rays thus transmitted by the
glass be received upon a plate of rock crystal of sufficient thick-
ness, a portion of them will be absorbed. Now if the glass and
RADIATION.
127
the rock crystal had each the power of absorbing the rays
transmitted by the other, their combination would be absolutely
athermanous, just as two plates of coloured glass would be
opaque, if each transmitted only the colours complementary to
those transmitted by the other.
1569. Physical conditions of diathermanism. It appears
from the researches of Melloni, that the physical conditions
which render bodies more or less diathermanous have no con-
nection with those which affect their transparency. Water is
one of the least diathermanous substances, although its trans-
parency is so nearly perfect. If, therefore, it be desired to
transmit light without heat, or with greatly diminished heat,
it is only necessary to let the rays pass through water, by
which they will be strained of a great part of their heat.
If the quantity of radiant heat transmitted through air be
expressed by 100, the following numbers will express the
quantity transmitted through an equal thickness of the sub-
stances named below.
- 30
- 27
- 21
- 20
- 17
- 15
- 15
- 12
- 11
It appears, therefore, that of all solid bodies rock salt is the
most diathermanous, and alum the least so. Of all liquids,
bisulphuret of carbon is the most, and water the least, dia-
thermanous.
It is evident from this table, that bodies are not diather-
manous and transparent in the same degree. Rocksalt is less
transparent but more diathermanous than glass.
It has been found that the power of thermal rays to penetrate
an imperfectly diathermanous body is augmented by raising
the temperature of the radiator. This is rendered very appa-
rent in the case of glass, which is much more diathermanous to
heat radiated by a body at a very high than by one at a mode-
rate temperature. This may explain the fact that bodies in
general are more diathermanous to solar light than to light
proceeding from artificial sources.
G 4
Air
Rocksalt (transparent)
Flint glass
Bisulphuret of carbon
Calcareous spar (transp
Rock crystal -
Topaz, brown -
Crown glass -
Oil of turpentine
- 100
- 92
- 67
- 63
arent) - 62
- 62
- 57
- 49
- 31
Rape oil
Tourmaline (green)
Sulphuric ether
Gypsum
Sulphuric acid
Nitric acid
Alcohol
Alum, crystals
Water
128 HEAT.
It is found that heat radiated by bodies which are in a state
of ignition or incandescence penetrate diathermanous media
more freely than those radiated by bodies which are not lumi-
nous. This is in accordance with the general principle already
stated, that thermal rays penetrate diathermanous bodies more
easily the higher is the temperature of the radiator.
Experiments on the thermal analysis of solar light were made
by transmitting a pencil of solar light, either obtained directly
or by reflection, through the aperture in the screen f,Jiff- 451.
1570. Refraction, reflection, and polarization of heat. Ex-
periments on the refraction, reflection, and polarization of heat
were made, by placing on the stand t,fig. 451., prisms of various
materials, reflecting surfaces, polariscopes, or double refracting
crystals. The thermoscopic apparatus was in each case placed
in such a position as to receive the deflected thermal pencil.
In this manner pencils of heat proceeding from various
sources were submitted to the same effects of refraction, re-
flection, and polarization as have been already described in
Book IX. with respect to light, and analogous results were
obtained ; the thermal rays being subject to the same general
laws of reflection and refraction as prevail in relation to lumi-
nous rays.
1571. Application of these principles to explain various phe-
nomena. The general principles regulating the radiation,
absorption, reflection, and transmission of heat, which have
been here stated, serve to explain and illustrate various experi-
mental facts and natural phenomena, as will appear from what
follows :
If two concave parabolic reflectors be placed as described in
(946), any radiator of heat placed in the focus of either will
produce a corresponding effect upon a thermometer placed in
the focus of the other, the rays of heat issuing from the radi-
ating body being twice reflected and collected into the focus of
the second reflector, upon the principle explained in (946).
1572. Experiment of radiated and reflected heat with pair
of parabolic reflectors. Let E and K',fig. 455., be two such
reflectors. If lighted charcoal be placed in the focus F of one,
it will ignite amadou or any other easily inflammable substance
in the other, even though the distance between the reflectors
be twenty or thirty feet.
If a sensitive thermometer, such as the differential thermo-
RADIATION.
129
meter (1349), be placed at F', it will show an increase or dimi-
nution of temperature, according as a hot or cold body is placed
Fig. 454.
at F. If a small globe filled with hot water be placed there,
an increase will be indicated ; and if the globe be filled with
snow or with a freezing mixture, a decrease will be mani-
fested.
1573. Materials fitted for vessels to keep liquids warm.
Vessels intended to hold liquids at a higher temperature than
that of the surrounding medium, should be constructed of ma-
terials which are bad radiators. Thus tea-urns, tea-pots, &c.,
are best adapted for their purpose when made of polished metal,
and worst when of black porcelain. A tea-kettle keeps water
hotter more effectually if clean and polished, than if covered
with the black of soot and smoke. Polished fire-irons remain
longer before a hot fire without being heated than rough un-
polished ones.
1574. Advantage of an unpolished stove. A polished stove
is a bad radiator ; one with a rough and blackened surface a
good radiator. The latter is therefore better adapted for warm-
ing an apartment than the former.
1575. Helmets and cuirasses should be polished. The helmet
and cuirass worn by cavalry is a cooler dress than might be
imagined, the polished metal being nearly a good radiator of
heat, and throwing off the solar rays,
1576. Deposition of moisture on window panes. When the
external air, which generally happens, is at a lower tempera-
ture than the air included in the room, it will be observed that
a deposition of moisture will be formed upon the inner surface
of the panes of glass in the windows. This is produced by the
G 5
130 HEAT.
vapor suspended in the atmosphere of the room being con-
densed by the cold surface of the glass. If the external air in
this case be at a temperature below 32, the deposition on the
inner surface of the glass will be congealed, and a rough coat-
ing of ice will be exhibited upon it.
Let two small pieces of tinfoil be fixed, one upon a part of
the external surface of one of the panes, and the other upon
the internal surface of another pane, in the evening ; it will be
found in the morning that that part of the internal surface of
the pane upon which is placed the external foil will be nearly
free from ice, while the surface of the internal foil will be more
thickly covered with ice than the parts of the inner surface of
the glass which are not covered with foil: these effects are
easily explained by radiation. When the tinfoil is placed on
the external surface, it reflects the heat which strikes on that
surface, and protects that part of the surface which is covered
from its action. The heat radiated from the objects in the
room striking on the inner surface of the glass penetrates it,
and encountering the foil attached to the exterior surface, is
reflected by it through the glass, and its escape into the external
atmosphere is intercepted ; the portion of glass, therefore, op-
posite to the tinfoil, is subject to the action of the heat radi-
ated from the chamber, but protected from the action of the
external heat. The temperature of that part of the glass is
therefore less depressed by the external atmosphere than the
temperature of those parts which are not covered by tinfoil.
Now glass being a bad conductor of heat, the temperature of
that part opposite to the external foil does not immediately
affect the remainder of the pane, and consequently we find
that, while the remainder of the interior surface of the pane is
thickly covered with ice, the portion opposite the tinfoil is
comparatively free from it. On the contrary, when the tin-
foil is applied on the internal surface, it reflects perfectly
the heat radiated from the objects in the room, while it admits
through the dimensions of the glass the heat proceeding from
the external atmosphere. The portion of the glass, therefore,
covered by the tinfoil, becomes colder than any other part
of the pane, and the tinfoil itself partakes of this tempera-
ture, which is not raised by the effect of the radiation of ob-
jects in the room, because the tinfoil itself is a good reflector
and a bad absorber. Hence the tinfoil presents a colder sur-
RADIATION. 131
face to the atmosphere in the room, than any other part of the
surface of the pane, and consequently receives a more abundant
deposition of ice.
1577. Principles which explain the phenomena of dew and
hoarfrost. A clear unclouded sky in the absence of the sun
radiates but little heat towards the earth ; consequently, if good
radiators be exposed to such an aspect, they must suffer a fall
of temperature, since they lose more by radiation than they
receive.
Let a glass cup, for example, be placed in a silver basin, and
exposed during a cold night to a clear sky ; it will be found in
the morning that a copious deposition of moisture will have
been made on the glass, from which the silver vessel is per-
fectly free. Reversing the experiment, let a silver cup be
placed in a glass basin, and similar results will ensue, the basin
being perfectly covered with moisture, from which the cup
is free. This is easily explained: the metal, being a bad
radiator of heat, preserves its temperature ; the glass, being a
good radiator of heat, loses by radiation much more than it re-
ceives, and, consequently, its temperature falls, and it condenses
the vapour in the air around it.
The result of experiments of this kind supplied Dr. Wells
with his celebrated theory, by which he explained the pheno-
menon of dew.
According to what has been explained, it appears that the
objects which are good radiators, exposed to a clear sky at night,
will become colder than the surrounding atmosphere, and will
consequently condense the water suspended in the air around
them; while objects which are bad radiators will not do this.
Grass, foliage, and other products of vegetation are in general
good radiators. The vegetation, therefore, which covers the
surface of the ground in an open country on a clear night will
receive a deposition of moisture from the atmosphere ; while the
objects which are less perfect radiators, such as earth, stones,
See., do not in general receive such depositions. In the close
and sheltered streets of cities, the deposition of dew is rarely
observed, because there the objects are exposed to reciprocal
radiation, and an interchange of heat takes place which maintains
their temperature.
The effect of the radiation of foliage is strikingly manifested
by the following example. Of two thermometers, one laid
132 HEAT.
among leaves and grass, and the other suspended at some height
above them, the latter will be observed to fall at night many
degrees below the former.
1578. Dew not deposited under a clouded sky. In a cloudy
night, dew is not deposited, because in this case, although vege-
tation radiates as perfectly as before, the clouds also radiate,
and an interchange of heat takes place between them and the
surface of the earth, by which the fall of temperature pro-
ducing dew is prevented.
1579. Production of artificial ice by radiation in hot climates.
Artificial ice is sometimes produced in hot climates by the
following process. A position is selected, not exposed to the
radiation of surrounding objects, and a quantity of dry straw is
spread on the ground, on which pans of porous earthenware are
disposed in which the water to be cooled is placed. The water
radiates heat to the firmament, and receives no heat in return.
The straw upon which the vessels are placed, being a bad con-
ductor, intercepts the heat, which would otherwise be imparted
to the water in the vessels from the earth. The porous nature
of the pans also allowing a portion of the water to penetrate
them, produces a rapid evaporation, by which a considerable
quantity of the heat of the water is carried off in a latent state
by the vapour. Heat is thus dismissed at once by evaporation
and radiation, and the temperature of the water in the pans is
diminished until it attains the freezing point. In the morning
the water is found frozen, and is collected and placed in cellars
surrounded with straw or other bad conductor, which prevents
its liquefaction.
CHAP. XL
COMBUSTION.
1580. Heat developed or absorbed in chemical combination.
It has been already explained, that when two substances enter
into chemical combination, so as to form a new compound,
heat is generally either developed or absorbed, so that although
the components before their union have the same temperature,
COMBUSTION. 133
the temperature of the compound which results will be gene-
rally above or below this common temperature, and sometimes
considerably so.
1581. This effect explained by specific heat of compound
being less or greater than that of components. If no change
in the state of aggregation of the constituents is produced by
their union, this phenomenon is explained by the specific heat
of the compound being less or greater than that of the com-
ponents, according as the temperature of the compound is
greater or less than that of the components. If greater, it is
because, the specific heat being less, the actual quantity of heat
contained in the compound gives it a higher temperature ; if
less, because it gives it a lower temperature.
1 582. Or by heat being developed or absorbed by change of
state. If the state of aggregation of either or both of the
components be changed, heat which was latent becomes sensible,
and raises the temperature of the compound ; or heat which was
sensible becomes latent, and lowers it. Thus when a solid
mixed with a liquid is dissolved in it, the solid in liquefying
absorbs and renders latent the same quantity of heat which
would have been necessary to melt it. This heat being ab-
stracted from the sensible heat of the compound lowers the tem-
perature. This phenomenon has been already noticed in the
case of freezing mixtures.
1583. Combustion. But of all the cases in which heat is
developed by chemical combination, the most important are those
in which combustion is produced.
When the quantity of heat suddenly developed by the chemical
combination of two bodies renders the compound luminous, the
bodies are said to burn, and the phenomenon is called combustion.
If the product of the combination be solid it is called fire ; if
gaseous, flame.
1584. Flame Flame, therefore, is gas rendered white hot
by the excessive heat developed in the combination which pro-
duces it.
1585. Agency of oxygen. It happens that, among the in-
finite variety of substances whose combination is productive of
this class of phenomena, one of the two combining bodies is
almost invariably oxygen gas. A few other substances, such as
chlorine, bromine, and iodine, produce similar effects ; but in all
ordinary cases of combustion, and universally where that effect
134 HEAT.
is resorted to as a source of artificial heat, one of the combining
substances is oxygen gas.
On this account this gas has been called a supporter of com-
bustion.
1586. Combustibles The substances which combining with
it produce the phenomenon of combustion are called combus-
tibles.
1587. Combustion explained. One of the circumstances
which render combustion so ordinary a phenomenon, is the fact
that the oxygen which forms one of the constituents of the at-
mosphere is either mechanically mixed in it, or, if chemically
united, is held in combination by the weakest possible affinity.
It therefore floats in the air in a state of almost complete freedom,
ready to combine with any body for which it has the least
affinity. When the temperature of a combustible, therefore, is
so elevated as to weaken sufficiently its cohesion, its molecules
enter into combination with the oxygen of the air, and heat and
light, and all the effects of combustion, are manifested.
1588. Temperature necessary to produce combustion. The
temperature necessary to produce this combination is different
for different substances ; phosphorus combines with oxygen, and
burns in the atmosphere if raised to 148. Hydrogen gas will
not burn till raised to incandescence. According to Sir H. Davy,
the temperatures necessary to the combustion of the several
combustibles here named are in the following order :
1. Phosphorus.
2. Phosphoretted hydrogen.
3. Hvdrogen and chlorine.
4. Sulphur.
5. Hydrogen and oxygen.
6. Olefiantgas.
7. Sulphuretted hydrogen.
8. Alcohol.
9. Wax.
10. Carbonic oxide.
1 1 . Carburetted hydrogen.
The heat developed iif the process of combustion is itself the
means of sustaining and rendering continuous the combustion.
If any source of heat of sufficient intensity be applied to the
wick of a candle, the matter of the wick will combine with the
oxygen of the air and will burn. The heat evolved in this com.
bustion will dissolve the wax or tallow, which ascending through
the meshes of the wick is converted into vapour, and being thus
raised to the necessary temperature, enters into combustion ;
and so the process is continued so long as a supply of tallow or
wax is conveyed to the wick.
COMBUSTION. 135
1589. Light of flame only superficial. It is evident the
light of the flame is only superficial, that part alone being in
combustion which is in contact with the air. The flame of a
candle or lamp is therefore, so far as regards light, hollow. It
is a column of gas with a luminous surface. As the gas within
the surface rises, it gets into contact with the air and becomes
luminous, and this continues until the column is brought to a
point. Thus the flame of a candle or lamp gradually tapers
until all the combustible vapour proceeding from the oil, wax,
or tallow receives its due complement of oxygen from the air,
and passes off. It speedily loses that high temperature which
renders it luminous, and the flame terminates.
1590. Illuminating power of combustibles. The light
afforded by lamps or candles formed of different substances has
different illuminating powers, according to the constituents of
these substances and the heat developed in their combustion.
The light, however, is not proportional to the heat. Hy-
drogen gas, which developes in its combustion a very intense
heat, produces but a feeble light.
1591. Constituents of combustibles used for illumination.
The chief constituents of the combustibles which are used for
the purposes of illumination are carbon and hydrogen, and the
whiteness of the flame is determined in a great degree by the
proportion of carbon.
The combination in this case produces carbonic acid and
water, the carbon combining with the oxygen to produce the
former, and the hydrogen to produce the latter.
1592. Spongy platinum rendered incandescent by hydrogen.
If a jet of hydrogen gas be directed upon a small mass of
spongy platinum, the metal will become incandescent, and will
continue so as long as the gas acts upon it, without, however,
suffering any permanent change.
An apparatus for producing an instantaneous light has been
contrived on this principle. By turning a stop-cock commu-
nicating with a small bottle in which the gas is generated in the
usual way, the jet of gas is thrown upon a small cup contain-
ing the spongy metal, which immediately becoming incan-
descent, is capable of lighting a match.
Some other metals, palladium, iridium, and rhodium, are
susceptible of the same effect.
This effect has not been yet explained in a clear or satis-
136 HEAT.
factory manner. See Turner's Chemistry, by Liebig and
Gregory, 8th edit. p. 542.
1593. Quantity of heat developed by combustibles. The
determination of the quantity of heat evolved by different com-
bustibles, is a question not only of great scientific interest, but
of considerable importance in the arts and manufactures. The
mutual relation between the quantities of the combustible, the
oxygen, and the heat developed, if accurately ascertained, could
not fail to throw light, not only on the theory of combustion,
but on the physics of heat in general. In the arts and manu-
factures, the due selection of combustible matter depends in a
great degree upon the quantity of heat developed by a given
weight in the process of combustion.
Nevertheless, there is no part of experimental physics in
which less real progress has been made, and in which the
process of investigation is attended with greater difficulties.
Experiments were made on certain combustibles by Lavoisier
and Laplace, by burning them in their calorimeter, and observing
the quantity of ice dissolved by the heat which they evolved.
Drs. Dalton and Crawford, Count Rumford and Despretz, as
well as Sir H. Davy, made various experiments with a like
object. It was not, however, until the subject was taken up by
Dulong that any considerable progress in discovery was made.
Unhappily, that eminent experimental inquirer died before his
researches were completed. Much valuable information has
been collected from his unfinished memoranda. The inquiry
has since been resumed by MM. Favre and Silbermann, and
has been prosecuted with much zeal and success. The estimates
which they have obtained of the quantities of heat developed in
the combustion of various substances, are found to be in general
accordance with those which appear to have been obtained by
Dulong, in the cases wftere they have operated on the same
combustible. Thus, in the case of hydrogen, the most important
of the substances under inquiry, Dulong found the heat de-
veloped to be expressed by 34601, while MM. Favre and
Silbermann estimated it at 34462, with relation to the same
thermal unit.
1594. Table of the quantities of heat evolved in the combus-
tion of various bodies. In the following table is given the heat
developed in the combustions of the substances named in the
first column ; the thermal unit being the heat necessary to raise
COMBUSTION.
137
a weight of water equal to that of the combustible one degree
of the scale of Fahrenheit's thermometer.
Quantity of Heat
Names of Substances.
Formula;.
givenbylof
Combustion.
Hydrogen, at 15
Carbon, from C to CO 2
62,031-6
14,544-7
from sugar, from C to CO 2 -
14,471-6
Graphite, natural. No. 1.
14',C60'7
from high mines, No. 1. -
14,013-5
natural, No. 2.
14,006-7
Diamond
13,986-2
Graphite, from high mines, No. 2. -
Diamond, heated - - - - -
13,926-8
14,181-7
Oxide, from carbon, at CO 2 -
4,324-9
Gas, marsh ------
C 2 H 4
23,513-4
olefiant
C 4 H 4
21,344-0
Paramylene ------
C 10 H'o
20,683-8
Amylene -------
C 22 H "*2
20,346-3
Celine - - - - -
C32 H32
20,278*8
19,941-3
Metamyline ------
C 40 H 4 o
19.671-3
Ether, sulphuric -
H02-|-C8 H8
HO 2 +C2o H 20
16,248*6
Spirit of wood
Alcohol
HO2+C 4 H 4
9>12-7
12,931*2
valeric
H02+C10 H'o
16,125-5
ethalic ------
H02+C32 H32
19,132-6
Acetone -------
C6 H+O 2
13,149-0
Aldhyde, ethalic
C32 H 3 2 Q2
18,616*0
Formiate of methylene - - - - -
Acelale
C 4 H 4 0<
C H6 0"
18,892-8
7,555-3
9,615-6
Formiate of alcohol
C6 H O 4
9,502-2
Ether, acetic ------
C8 H 8 O 4
11,320-9
Butyrate of methylene - - - - -
CM H'o O 4
12,237-3
Ether, butyric ------
C'2 H 12 O 4
12,763-6
alcohol
Acetate of alcohol, valeric - - - -
13,276-1
14,102-8
14,348-2
Ether, valaramilic
C20 H2 O 1
15,378-5
Acid, formic
O 4 -^-C 2 H 2
3,600-0
acetic ------
6,309-4
butyric ------
01+C8 R8
10,121-4
valeric
4 +C'o H'o
11,590-2
ethalic
O^-fC 38 H 38
16,956-0
phunic ------
C' 2 H6 08
17,676-0
14,116-1
Terebene- ------
19 193*4
Essence of turpentine - - - - -
C-20 H"
19,533*6
citron - - - - -
C20 H'6
19,726-2
Sulphur, native melted - - - - -
3,998-0
at instant of crystallization -
4,065-1
of carbon -
6,1 20-9
Carbon burnt with peroxide of azote at 10 -
Decomposition of peroxide of azote - - -
20,084-2
19,962-9
water oxygenated, 1 oxygen
2,345-4
Decomposition of oxide of silver absorbs
Iceland spar for CO2 and C to O, absorbs
Aragonite combined gives - - - -
- 39-8
-554-6
+ 68-9
,, separated absorbs - - - -
554*6
separated after combination absorbs
-485*6
138 HEAT.
CHAP. XII.
ANIMAL HEAT.
1595. Temperature of organized bodies not in equilibrium
with surrounding medium. Organized bodies in general
present a striking exception to the law of equalization of tem-
perature, since, with some rare exceptions, these bodies are
never at the temperature of the medium which surrounds them.
The human body, as is well known, has a permanent and
invariable temperature much more elevated than that of the
atmosphere. The animals of the polar regions are much
warmer than the ice upon which they rest, and those which
inhabit tropical climates colder in general than the air they
respire. The temperature of the bodies of birds is not that of
the atmosphere, nor of fishes that of the sea.
There is therefore, in organized bodies, some proper source
of heat, or rather some provision by which heat and cold can be
produced at need ; for the ponderable matter which composes
the bodies of these creatures must, like all ponderable matter,
be subject to the general law of equilibrium of temperature.
It is therefore necessary to ascertain what is the temperature
of organized creatures ; what are the quantities of heat which
they evolve in a given time to maintain this temperature ; and
what is the physical apparatus by which that heat is elaborated.
1596- Temperature of the blood in the human species. The
temperature of the blood in the human species is found to be
the same throughout the whole extent of the body, and is that
which is indicated by a thermometer, whose bulb is placed
under the tongue and held there until the mercurial column
becomes stationary. This temperature is 98'6, subject to
extremely small variations, depending on health, age, and
climate.
1597. Researches of Davy to determine the temperature of
the blood. Dr. John Davy, Inspector of Army Hospitals,
availed himself of the opportunities presented by his professional
appointment, and of a voyage made by him to the East, to make
an extensive and valuable series of observations on the tem-
perature of the blood in man, in different climates, at different
ANIMAL HEAT.
139
ages, and among different races, as well as upon the inferior
animals. These observations were made between 1816 and
1820.
The first series of observations were made during a voyage
from England to Ceylon, and, therefore, under exposure to
very various climates and temperatures. The temperature of
the blood was observed by means of a sensitive thermometer
applied under the tongue near its root, with every precaution
necessary to ensure accuracy. The principal results obtained
are collected and arranged in the following tables:
TABLE I.
1598. Showing the Temperatures of the Blood of 13 Indi-
viduals in different Climates.
Age.
Air, 60.
Air, 78.
Air, 79-50.
Air, 80.
24
98-5
99
100
99-5
28
99-5
99-5
99-5
25
9JF25
9875
98-5
99-75
17
99
99
100
20
98-75
98
99-5
100
98-25
98-75
99
99-5
25
98
101
40
9975
43
99
40
99-5
13
,
100
4
99-5
TABLE II.
Showing the Temperatures of the Blood of 6 Individuals in
different Climates.
Age.
Air, 69.
Air, 83.
Air, 82.
Air, 843.
35
20
40
35
20
24
98
98
99
98
93
98
99
99
99
99-75
995
99-5
102
101
98-5
99
99
100
98-5
98
98
98
TABLE III.
Showing the Temperatures of the Blood in the same Individual
at different Hours of the Day.
Hour.
Air.
Blood.
Sensation.
A. M.
60-5
98
Cool
9
G6
97-5
Cold.
1 P. M.
78
98-5
Cool.
4
79
98-5
Warm.
6
71
99
Warm.
11
09
98
Cool.
140
HEAT.
TABLE IV.
Shoioing the Limits between which the Temperature of the Blood
in different Races was observed to vary in India. Air, 75
to 81.
Races.
Temperature.
Races.
Temperature.
Cape Hottentots
Sinhalese -
Albinoes -
Half caste
White Children
Kandians -
96-5 to 995
100 101-5
101 * 101-75
100 102
101 102
97-5 99
Vaidas
African Negroes -
Malays -
Sepoys -
English -
98 to 98-5
98-5 99-5
98-5 99-5
98 100
98 101
TABLE V.
Showing the Temperature of the Blood observed in different
Species of Animals.
Name.
Air.
Temperature.
Place of Observation.
Mammalia.
Monkey
86
IMft
Colombo.
Pangolin
80
90*
Bat -
82
100
Vampyre
70
100
Squirrel
81
102
_
Rat - -
80
102
Guinea-pig
102
Chatham.
Hare
80
100
Colombo.
Ichneumon -
81
103
Jungle cat
Curdog
Jackal
80
84
99
103
101
KanTy.
Colombo.
Cat -
60
79
101
102
London.
Kandy
Felix pardus -
Horse
81
80
102
99-5
Colombo.
Kandy.
Sheep
101 to 104
Scotland.
67
78
103 to 104
104 to 105
Colombo.
Goat
78
103 to 104
Colombo.
Ox
Summer.
100
Edinburgh.
Elk
80
78
102
103
Kandy.
Mount Lavinia.
Hog
75
105
Doombera.
80
105
Mount Lavinia.
E'lephant
80
99-5
Colombo.
Porpoise
72
100
Lat. N. 8 23' at sea.
Birds
Falcon
77-5
99
Colombo.
Screech-owl -
60
106
London.
Jackdaw
85
107
Kandia.
Thrush
60
K.9
London.
Sparrow
80
108
Kandia.
Pigeon
60
108
London.
78
109-5
Mount Lavinia.
Jungle "fowl -
78
83
1075
108-5
Ceylon.
Common fowl
40
108-5
Edinburgh.
78
110
Mount Lavinia.
.
108
Guinea'fowl -
110
Turkey
Procellarea equinoxiale
79
109
103-5 to 105-5
Lat. i3'.
ANIMAL HEAT.
141
Name.
Air.
Temperature.
Place of Observation.
P. capensis
Common hen -
59
77
105-5
110
Lat. S. 34 1' at sea.
Mount Lavinia.
cock
77
111
Chicken
77
111
Malay cock
Goose -
110
106 to 107
Duck - ...
110111
Teal - ...
Snipe - ...
83
108-109*
Colombo.
Plover ....
Peacock ....
83
105
105-108
Ceylon.
Kornegalle.
Amphibia.
Testudo midas ...
79-5
80
84
88-5
Lat. N. 2 7'.
T. geometrica -
86
61
80
S5
62-5
87
Colombo.
Colombo.
Rana ventricosa ...
Common frog ...
Iguana ....
80
60
82
77
64
82J
Kandy.
Edinburgh.
Colombo.
Serpents -
U|
n|
88|
84}
Fishes.
Shark
71-
77
Lat. S. 8 23'.
Bonito ....
78*
82*
Lat. S. 1 14'.
Trout - - - - -
56
58
Edinburgh.
,
56
58
L. Katrine.
Eel
51
51
Chatham.
Flying-fish ....
77
78
Lat. N. 6 57'.
MoUusca.
Oyster ....
Snail
82
76*
82
76 to 76|
Mount Lavinia.
Kandy.
Crustacea.
Crayfish -
Crab
80
72
79
72
Colombo.
Kandy.
Insects.
Scarabaeus pilularius
76
77
Kandy.
73
74
Blatta orientalis
83
74-75
_
Gryllus hoematopus ?
Apis ichneumonia? -
Papillio agamemnon
62
75
78
80
Cape.
Kandy.
Scorpio afer -
79
80
a
~
1599. Deductions from these observations. The conclusions
deduced from these observations and experiments are, that the
temperature of man, although nearly constant, is not exactly so ;
that it is slightly augmented with the increased temperature of
the climate to which the individual is exposed ; that the tem-
perature of the inhabitants of a warm climate is higher than
those of a mild ; and that the temperature of the different races
of mankind is, cceteris paribus, nearly the same. This is the
more remarkable, inasmuch as among those whose tem-
peratures thus agree, there is scarcely any condition in common
* This was the temperature of the heart, which lies near the surface. In
the deeply-seated muscles the temperature was 99.
142 HEAT.
except the air they breathe. Some, such as the Vaida, live
almost exclusively on animal food ; others, as the priests of
Boodho, exclusively on vegetables ; and others, as Europeans and
Africans, on both.
1600. Birds have the highest, and amphibia the lowest tem-
perature. Of all animals birds have the highest temperature ;
mammalia come next; then amphibia, fishes, and certain
insects. Mollusca, Crustacea, and worms stand lowest in the
scale of temperature.
1601. Experiments of Breschet and Becquerel. Experi-
ments were made by MM. Breschet and Becquerel to ascertain
the variation of the temperature of the human body in a state
of health and sickness. They employed for this purpose com-
pound thermoscopic needles, composed of two different metals,
which, being exposed to a change of temperature, indicated with
great sensitiveness the sensible heat by which they were affected,
by means of a galvanometer on a principle similar to the
electroscopic apparatus used by M. Melloni, already described,
(1564). The needles were adapted for use by the method of
acupuncture.
1602. Comparative temperature of blood in health and sick-
ness. It was found that in a state of fever, the general tempe-
rature of the body sometimes rose from 1 0% 8 to 3'6.
It was also ascertained in several cases of local chronic and
accidental inflammation, that the temperature of the inflamed
part was a little higher than the general temperature of the
body, the excess however never amounting to more than from
l-8 to 3-6.
1603. Other experiments by Breschet and Becquerel. It
resulted from these researches that, in the dog, the arterial
blood exceeds in temperature the veinous by about l - 8. It
was also found that the temperature of the bodies of the
inhabitants of the valley of the Rhone and those of the Great
St. Bernard, both men and inferior animals, were the same.
1604. Experiments to ascertain the rate of development of
animal heat. A series of experiments was made by Lavoisier
and Laplace to determine, by means of their calorimeter already
described, the quantity of heat developed in a given time by
various animals ; but more recently much more extensive re-
searches in this department were made by Dulong, which have
produced important results. In these experiments the animal
ANIMAL HEAT. 143
under examination was shut up in a copper cage sufficiently
capacious to be left at ease, and being submerged in a glass
vessel of water, the air necessary for respiration was supplied
and measured by a gasometer, while the products of respiration
were carried away through the water, to which they imparted
their heat, and were afterwards collected and analyzed. Each
experiment was continued for two hours. After the proper
corrections had been applied, the heat developed by the animal
was calculated by the heat imparted to the water.
Dulong determined these thermal quantities with great pre-
cision for numerous animals of different species, young and
adult, carnivorous and frugivorous. The animals during the
experiment being subject neither to inconvenience nor fatigue,
it might be assumed the heat they lost was equal to that which
they reproduced. On analyzing the products of respiration it
was found that they were changed as air is which has under-
gone combustion. The oxygen of the atmospheric air which
was introduced into the cage was in fact combined with carbon
and formed carbonic acid. So far, therefore, as concerned this
point, a real combustion may be considered as having taken
place in the lungs. Thus much was inferred in general as to
the source of animal heat from the discoveries of Lavoisier.
1605. Total quantity of heat explained by chemical laics
without any especial vital cause. It remained, however, to
verify this discovery by showing that the exact quantity of heat
evolved in the animal system could be accounted for by the
chemical phenomena manifested in respiration ; and this Dulong
accomplished.
After having determined the quantity of heat lost by the
animal, he calculated the quantity of heat produced by respir-
ation. The air which was furnished to the animal was mea-
sured by the gasometer, and the changes which it suffered were
taken into account by analyzing the products of combustion
discharged through the water from the cage. These products
were as follows :
1. The vapour of water.
2. Carbonic acid.
3. Azote.
The vapour of water analyzed gave a certain quantity of
oxygen and hydrogen, the carbonic acid a certain quantity of
144 HEAT.
carbon and oxygen, and the azote was sensibly equal to the
quantity of that gas contained in the atmospheric air supplied
to the animal. .It followed that the oxygen of the atmospheric
air which had been supplied combined in the lungs partly with
carbon and partly with hydrogen, producing by respiration
carbonic acid and the vapour of the water, being exactly the
products resulting from the combustion of a lamp or candle.
Now the quantity of heat produced by the combustion of given
quantities of carbon and hydrogen being taken and compared
with the quantity of animal heat developed, as given by the
heat imparted to the water, was found exactly to correspond ;
and thus it followed that the source of animal heat is the same
as the source of heat in the common process of combustion.
When these researches were first made, it appeared that the
quantity of heat actually developed in the animal system ex-
ceeded the quantity computed to result from the chemical change
which the air suffered in respiration, and it was consequently
inferred that the balance was due to a certain nervous energy
or original source of heat existing in the animal organization
independently of the common laws of physics. Dulong, how-
ever, had the sagacity to perceive that the phenomenon admitted
of a more satisfactory and simple explanation, and succeeded at
length in showing that the difference which had appeared
between the quantity of heat developed in respiration, and the
quantity due to the chemical changes which the air suffered in
this process, was accounted for by the fact that the quantity of
heat developed in the combustion of hydrogen and oxygen had
been under-estimated, and that when the correct coefficient
was applied, the quantity of heat due to chemical changes suf-
fered by the air in respiration was exactly equal to the quantity
of heat developed in the animal system.
CHAP. XIII.
THE SENSATION OF HEAT.
1606. Indications of the senses fallacious. The senses, though
appealed to by the whole world as the most unerring witnesses
of the physical qualities of bodies, are found, when submitted
THE SENSATION OF HEAT. 145
to the severe scrutiny of the understanding, not only not the
best sources of exact information as to the qualities or degrees
of the physical principles by which they are severally affected,
but the most fallible guides that can be selected, often in-
forming us of a quality which is absent, and of the absence of
one which is present.
Nor should this be any matter of surprise. Our Maker in
giving us organs of sense did not design to supply us with
philosophical instruments. The eye, the ear, and the touch,
though admirably adapted to serve our purposes, are not severally
a telescope, a monochord, and a thermometer. An eye which
would enable us to see the inhabitants of a planet, would ill
requite its owner for that ruder power which guides him through
the town he inhabits, and enables him to recognize the friends
who surround him. The comparison of the instruments which
are adapted for the uses of commerce and domestic economy
with those destined for scientific purposes supply an appropriate
illustration of these views. The delicate balance used by the
chemist in determining the analysis of the bodies upon which
he is engaged would, by reason of its very perfection and
sensibility, be utterly useless in the hands of the merchant or
the housewife. Each class of instruments has, however, its
peculiar use, and is adapted to give indications with that degree
of accuracy which is necessary, and required for the purposes
to which it is applied.
1607. Sense of touch a fallacious measure of heat. The
touch is the sense by which we acquire a perception of heat".
It is evident, nevertheless, that it cannot inform us of the quan-
tity of heat which a body contains, much less of the relative
quantities contained in any two bodies. In the first place, the
touch is not aifected by heat which exists in the latent state.
Ice-cold water and ice itself have the same degree of cold to
the touch, and yet it has been proved that the former contains
140 of heat more than the latter.
1608. Its indications contradictory. But it may be said
that even the thermometer does not in this case indicate the
presence of the excess of heat in the liquid. The sense of feel-
ing will however be found almost as fallacious as regards the
temperature of bodies ; for it is easy to show that the sense of
warmth depends as much upon the condition of the part of the
146 HEAT.
body which touches or is surrounded by the warm or cold
medium, as on the temperature of that medium itself.
If the two hands be plunged, one in water at the temperature
of 200 and the other in snow, and being held there for a
certain time are transferred to water of the intermediate tem-
perature of 100, this water will appear warm to one hand and
cold to the other ; warm to the hand which had been plunged
in the snow, and cold to the hand which had been plunged in
the water at 200.
If on a hot day in summer we descend into a deep cave, it
will feel cold ; if we descend into the same deep cave on a frosty
day in winter, it will feel warm; yet a thermometer in this
case will prove that in the winter and in the summer it has
exactly the same temperature.
1609. These contradictions explained. These apparent ano-
malies are easily explained. The sensation of heat is relative.
When the body has been exposed to a high temperature, a
medium which has a lower temperature will feel cold, and
when it has been exposed to a low temperature it will feel
warm.
If in a room raised to a high temperature, as in a vapour or
hot-air bath, we touch with the hand different objects, they will
appear to have very different temperatures ; a woollen carpet
will feel cold, marble slabs warm, and metal objects very hot.
If, on the other hand, we are in a room at a very low temperature,
all these properties will be reversed ; the carpet will feel warm,
the marble slabs cold, and the metallic objects colder still.
These effects are easily explained. A woollen carpet is a
non-conductor of heat. When surrounding objects are at a
more elevated temperature than that of the body, the woollen
carpet partaking in this temperature will when touched feel
cool, because, being a non-conductor of heat, the heat which
pervades it does not pass freely to the part of the body which
touches it. A marble slab being a better conductor, and a
metallic object a still better, the heat will pass from them more
freely to the part of the body which touches them, and they
accordingly appear hotter.
But if the room be at a temperature much lower than the
body, then when we touch the woollen carpet the heat does not
pass from our body to the carpet because it is a non-conductor,
and as we do not lose heat the carpet feels warm ; but when
THE SENSATION OF HEAT. 147
we touch the marble, and still more a metallic object, the heat
passes more and more freely from our body to these objects,
and being sensible of a loss of heat more or less rapid, we feel
cold.
1610. Examples of the fallacious impressions produced by
objects on the touch. When we plunge in a cold bath, we are
accustomed to imagine that the water is colder than the air and
surrounding objects ; but if a thermometer be immersed in the
water, and another suspended in the air, they will indicate the
same temperature. The apparent cold of the water arises from
the fact that it abstracts from our bodies heat more rapidly
than air does, being a denser fluid and a greater number of
particles of it coming into contact at once with the surface of
the body. A linen feels colder than a cotton, and a cotton
colder than a flannel shirt, yet all the three are at exactly the
same temperature. Linen is a better conductor of heat than
cotton, and cotton than flannel, and, consequently, the heat
passes more freely through the first than the second, and through
the second than the third.
The sheets of a bed feel cold, and the blankets warm, and yet
they are of the same temperature, a fact which is explained
in the same manner.
The air which is impelled against a lady's face by her fan
feels cold, while the same air at rest around her feels warm ;
yet it is certain that the temperature of the air is not lowered
by being put in motion. The apparent coolness is explained
in this case by a slight evaporation, which is effected upon the
skin by the motion given to the air by the fan.
1611. Feats of fire-eaters explained. Some of the feats
performed by quacks and fire-eaters in exposing their bodies to
fierce temperatures may be easily explained upon this principle.
When a man goes into an oven raised to a very high tem-
perature., he takes care to place under his feet a cloth or mat
made of wool or other non-conducting substance upon which
he may stand with impunity at the proposed temperature. His
body is surrounded with air raised it is true to a very high
temperature, but the extreme tenuity of this fluid causes all
that portion of it in contact with the body at any given time to
produce but a slight effect in communicating heat. The exhi-
bitor always takes care to be out of contact with any good con-
ducting substance, and when he exhibits the effect produced by
H 2
148 HEAT.
the oven in which he is enclosed upon other objects, he takes as
much care to place them in a situation very different from that
which he himself has occupied. He exposes them to the effect
of metal or other good conductors.
Meat has been exhibited dressed in the apartment with the
exhibitor. A metal surface is in this case provided, and
probably raised to a much higher temperature than the atmo-
sphere in which the exhibitor is placed.
149
BOOK THE SECOND.
MAGNETISM.
CHAP. I.
DEFINITIONS AND PRIMARY PHENOMENA.
1612. Natural magnets Loadstone. Certain ferruginous
mineral ores are found in various countries, which being brought
into proximity with iron manifest an attraction for it. These
are called NATURAL MAGNETS or LOADSTONES ; the former term
being derived from MAGNESIA, a city of Lydia, in Asia Minor,
where the Greeks first discovered and observed the properties
of these minerals.
1613. Artificial magnets. The same property may be im-
parted to any mass of iron having any desired magnitude or
form, by processes which will be explained hereafter. Such
pieces of iron having thus acquired these properties are called
ARTIFICIAL MAGNETS ; and it is with these chiefly that scientific
experiments are made, since they can be produced in unlimited
quantity of any desired form and magnitude, and having the
magnetic virtue within practical limits in any desired degree.
1614. Neutral line or equator Poles. This attractive
power, which constitutes the peculiar character of the magnet,
whether natural or artificial, is not diffused uniformly over
every part of its surface. It is found to exist in some parts
with much greater force than in others, and on a magnet a
certain line is found where it disappears. This line divides the
magnet into two parts or regions, in which the attractive power
prevails in varying degrees, its energy augmenting with the
distance from the neutral line just mentioned.
This neutral line thus dividing the magnet into two different
regions of attraction may be called the EQUATOR of the magnet.
H 3
150 MAGNETISM.
The two regions of attraction separated by the equator are
called the poles of the magnet.
Sometimes this term pole is applied, not generally to the two
parts into which the magnet is divided by the equator, but to
two points upon or within them, which are the centres of all
the magnetic attractions exercised by the surface, in the same
manner as the centre of gravity is the centre of all the gravi-
tating forces which act upon the particles of a body.
1615. Experimental illustration of them. The neutral line
and the varying attraction of the parts of the surface of the
magnet which it separates may be manifested experimentally
as follows. Let a magnet, whether natural or artificial, be
rolled in a mass of fine iron filings. They will adhere to it,
and will collect in two tufts on its surface, separated by a space
upon which no filings will appear. The thickness with which
the filings are collected will increase as the distance from the
space which is free from them is augmented.
This effect, as exhibited by a na-
tural magnet of rough and irregular
form, is represented in^. 456. ; and
as exhibited by an artificial magnet
in the form of a regular rod or
cylinder whose length is considerable
as compared with its thickness, is
Fig. 456. represented in fig. 457. ; the equator
being represented by EQ, and the poles by A and B.
Fig. 457.
1616. Experimental illustration of the distribution of the mag-
netic force. The variation of the attraction of different parts
of the magnet may also be illustrated as follows. Let a magnet,
whether natural or artificial, be placed under a plate of glass or
a sheet of paper, and let iron filings be scattered on the paper or
glass over the magnet by means of a sieve, the paper or glass
being gently agitated so as to give free motion to the particles.
DEFINITIONS AND PRIMARY PHENOMENA. 151
They will be observed to affect a peculiar arrangement corre-
sponding with and indicating the neutral line or equator and the
poles of the magnet as represented in fig. 458., where E Q is the
equator, and A and B the poles of the magnet.
Fig. 458.
1617. Varying intensity of magnetic force indicated by a
pendulum. The varying intensity of the attraction of different
parts of the surface of the magnet may be ascertained by pre.
senting such surface to a small ball of iron suspended by a
fibre of silk so as to form a pendulum. The attraction of the
surface will draw this ball out of the perpendicular to an extent
greater or less, according to the energy of the attraction. If
the equator of the magnet be presented to it, no attraction will
be manifested, and the force of the attraction indicated will be
augmented according as the point presented to the pendulum
is more distant from the equator and nearer to the pole.
1618. Curve representing the varying intensity. This vary-
ing distribution of the attractive force over the surface of a
magnet may be represented by a curve whose distance from the
magnet varies proportionally to the intensity of this force.
Thus if, in fig. 459., EQ be the equator and A and B the poles
of the magnet, the curve ECDF may be imagined to be drawn
in such a manner that the distance of its several parts from the
bar EB shall be everywhere proportional to the intensity of the
152 MAGNETISM.
attractive force of the one pole, and a similar curve EC'D'F'
will in like manner be proportional to the varying attractions
Fig. 459.
of the several parts of the other pole. These curves necessarily
touch the magnet at the equator E Q, where the attraction is
nothing, and they recede from it more and more as their distance
from the equator increases.
1619. Magnetic attraction and repulsion. If two magnets,
being so placed as to have free motion, be presented to each
other, they will exhibit either mutual attraction or mutual
repulsion, according to the parts of their surfaces which are
brought into proximity. Let E and ^',fig. 460., be two magnets,
A E B
Fig. 460.
their poles being respectively A B and A' B'. Let the two poles
of each of these be successively presented to the same pole of a
third magnet. It will be found that one will be attracted and
the other repelled. Thus, the poles A and A' will be both
attracted, and the poles B and B' will be both repelled by the
pole of the third magnet, to which they are successively pre-
sented.
1620. Like poles repel, unlike attract. The poles A and A',
which are both attracted, and the poles B B', which are both
repelled by the same pole of a third magnet, are said to be like
poles ; and the poles A and B', and B and A', one of which is
attracted and the other repelled by the same pole of a third
magnet, are said to be unlike poles.
Thus the two poles of the same magnet are always unlike
poles, since one is always attracted, and the other repelled by
the same pole of any magnet to which they are successively
presented.
DEFINITIONS AND PRIMARY PHENOMENA. 153
If two like poles of two magnets, such as A and A' or B and B',
be presented to each other, they will be mutually repelled ; and
if two unlike poles, as A and B' or B and A', be presented to each
other, they will be mutually attracted.
Thus it is a general law of magnetic force, that like poles
mutually repel and unlike poles mutually attract.
1621. Magnets arrange themselves mutually parallel with
poles reversed. If a magnet A B be placed in a fixed position
on a horizontal plane, and another magnet be suspended freely
at its equator E' by a fibre of untwisted silk, the point of
suspension being brought so as to be vertical over the equator
E of the fixed magnet, the magnet suspended being thus
free to revolve round its equator E' in a horizontal plane, it
will so revolve, and will oscillate until at length it comes to
rest in a position parallel to the fixed magnet AB; the like
poles, however, being in contrary directions, that is to say, the
pole A' which is similar to A being over B, and the pole B' which
is similar to B being over A. This phenomenon follows
obviously from what has been just explained ; for if the magnet
A' B' be turned to any other direction, the arm E B attracting the
unlike arm E' A', and at the same time the arm E A attracting
the unlike arm E' B', the suspended magnet A' B' will be under the
operation of forces which have been already described (160),
and which are called a couple, consisting of two equal and con-
trary forces whose combined effect is to turn the magnet round
E' as a centre. When, however, the magnet A' B' ranges itself
parallel to A B, the like poles being in contrary directions, the
forces exerted balance each other, since the pole A attracts B' as
much as the pole B attracts A'.
1622. Magnetic axis. It has been already stated that certain
points within the two parts into which a magnet is divided by
the equator, which are the centres of magnetic force, are the
magnetic poles. A straight line joining these two points is
called the magnetic axis.
1623. How ascertained experimentally. If a magnet have
a symmetrical form, and the magnetic force be uniformly diffused
through it, its magnetic axis will coincide with the geometrical
axis of its figure. Thus, for example, if a cylindrical rod be
uniformly magnetized, its magnetic axis will be the axis of the
cylinder ; but this regular position of the magnetic axis does
not always prevail, and as its direction is of considerable iin-
H 5
154
MAGNETISM.
portance, it is necessary that its position may in all cases be
determined. This may be done by the following expedient :
Let the magnet, the direction of whose axis it is required to
ascertain, be suspended as already described, with its equator
exactly over that of a fixed magnet resting upon a horizontal
plane. The suspended magnet will then settle itself into such
a position that its magnetic axis will be parallel to the magnetic
axis of the fixed magnet which is under it. Its position when
thus in equilibrium being observed, let it be reversed in the
stirrup, so that without changing the position of its poles, its
under side shall be turned upwards, and vice versa. If after
this change the direction of the bar remains unaltered, its
magnetic axis will coincide with its geometrical axis ; but if, as
will generally happen, it take a different direction after being
reversed, then the true direction of the magnetic axis will be
intermediate between its directions before and after reversion.
To render this more clear, let A ~R,fig. 461., be the geometrical
axis of a regularly shaped prismatic magnet, and let it be
M required to discover the direction of its
^ * A magnetic axis. Let a b be the poles, and
JL- -J*^^/ the line JIN passing through them there-
\ j~ 7 fore its magnetic axis.
*\l I If this magnet be reversed in the man-
y / ner already described over a fixed magnet,
A / its magnetic axis in the new position will
/ \ / coincide with its direction in the first posi-
/ \ / tion, and the magnet when reversed will
/ \ / take the position represented by the dotted
\ / \l \l line, the geometrical axis being in the
A direction A' B', intersecting its former
i / \ direction A B at o. The poles a b will
/ \ coincide with their former position, as
\ I \ will also the magnetic axis M N. It is
,\ / J evident that the geometric axis o A will
-~-~r~ f orm with the magnetic axis oa the same
angle as it forms with that axis in the
second position, that is to say, the angle
Fig. 461. AOM will be equal to the angle A'OM;
and, consequently, the magnetic axis M N will bisect the angle
A o A', formed by the geometric axis of the magnet in its second
position.
DEFINITIONS AND PRIMARY PHENOMENA. 155
1624. Hypothesis of two fluids, boreal and austral. These
various phenomena of attraction and repulsion, with others
which will presently be stated, have been explained by different
suppositions, one of which assumes that all bodies susceptible of
magnetism are pervaded by a subtle imponderable fluid, which is
compound, consisting of two constituents called, for reasons
which will hereafter appear, the austral fluid and the boreal
fluid. Each of these is self-repulsive ; but they are reciprocally
attractive, that is to say, the austral fluid repels the austral, and
the boreal the boreal ; but the austral and boreal fluids recipro-
cally attract.
1625. Condition of the natural or unmagnetized state.
When a body pervaded by the compound fluid is in its natural
state and not magnetic, the two fluids are in a state of com-
bination, each molecule of the one being combined by attract-
tion with a molecule of the other ; consequently, in such state,
neither attraction nor repulsion is exercised, inasmuch as what-
ever is attracted by a molecule of the one is repelled by a
molecule of the other which is combined with it.
1626. Condition of the magnetized state. When a body
is magnetic, and manifests the powers of attraction and repul-
sion such as have been described, the magnetic fluid which
pervades it is decomposed, the austral fluid being directed on
one side of the equator, and the boreal fluid on the other. That
side of the equator towards which the austral fluid is directed is
the austral, and that towards which the boreal fluid is directed
is the boreal pole of the magnet.
If the austral poles of two magnets be presented to each other,
they will mutually repel, in consequence of the mutual repulsion
of the fluids which predominate in them ; and the same effect
will take place if the boreal poles be presented to each other.
If the austral pole of the one magnet be presented to the boreal
pole of another, mutual attraction will take place, because the
austral and boreal fluids, though separately self-repulsive, are
reciprocally attractive.
It is in this manner that the hypothesis of two self-repulsive
and mutually attractive fluids supplies an explanation of the
general magnetic law, that like poles repel and unlike poles
attract.
It must be observed that the attraction and repulsion in this
hypothesis are imputed not to the matter composing the mag-
H 6
156 MAGNETISM.
netic body, but to the hypothetical fluids by which this matter
is supposed to be pervaded.
1627. Coercive force The force with which the particles
of the austral and boreal fluids are combined, varies in different
bodies, in some being so slight that their decomposition is readily
effected, in others being so energetic that it is only accomplished
with considerable difficulty. It is found that in bodies where
the decomposition of the magnetic fluids is resisted, its recom-
position is also resisted, and that where the fluids are separated
with difficulty, when once separated they are recombined with
difficulty.
This force, by which the decomposition and recomposition of
the constituents of the magetic fluid are resisted, is called the
coercive force.
A different and more probable hypothesis for the explanation
of the phenomena will be explained hereafter.
1628. Coercive force insensible in soft iron most active in
highly tempered steel. Of the magnetic bodies, that in which
the coercive force is most feeble is soft iron, and that in which
it is manifested with greatest energy is highly tempered steel.
It might indeed be assumed hypothetically that the magnetic
fluid pervades all bodies whatsoever, but that its coercive force
in bodies which are said to be unsusceptible of magnetism is
such as to yield to no method of decomposition yet discovered.
1629. Magnetic substances. The only substances in which
the magnetic fluid has been decomposed, and which are there-
fore susceptible of magnetism, are iron, nickel, cobalt, chro-
mium, and manganese, the first being that in which the mag-
netic property is manifested by the most striking phenomena.
CHAP. II.
MAGNETISM BY INDUCTION.
1630. Soft iron rendered temporarily magnetic. If the ex-
tremity of a bar of soft iron be presented to one of the poles of
a magnet, this bar will itself become immediately magnetic. It
will manifest a neutral line and two poles, that pole which is in
contact with the magnet being of a contrary name to the pole
MAGNETISM BY INDUCTION. 157
which it touches. Thus, if AB,^. 462., be the bar of soft iron
which is brought in contact with the boreal pole b of the
Fig. 46'2.
magnet a b, then A will be the austral and B the boreal pole of
the bar of soft iron thus rendered magnetic by contact, and E
will be its equator, which however will not be in the middle of
the bar, but nearer to the point of contact.
These effects are thus explained by the hypothesis of two
fluids.
The attraction of the boreal pole of the magnet a b acting
upon the magnetic fluid which pervades the bar AB, decomposes
it, attracting the austral fluid towards the point of contact A,
and repelling the boreal fluid towards B. The austral fluid
accordingly predominates at the end A, and the boreal at the
end B, a neutral line or equator E separating them.
This state of the bar AB can be rendered experimentally
manifest by any of the tests already explained. If it be rolled
in iron filings, they will attach themselves in two tufts sepa-
rated by an intermediate point which is free from them ; and if
the test pendulum (1617) be successively presented to different
points of the bar, the varying intensity of the attraction will be
indicated.
If the bar AB be detached from the magnet, it will instantly
lose its magnetic virtue, the fluids which were decomposed and
separated will spontaneously recombine, and the bar will be re-
duced to its natural state, as may be proved by subjecting it
after separation to any of the tests already explained.
Thus is manifested the fact that the magnetism of soft iron
has no perceptible coercive force. The magnetic fluid is de-
composed by the contact of the pole of any magnet however
feeble, and when detached it is recomposed spontaneously and
immediately.
1631. This may be effected by proximity without contact.
It is not necessary, to produce these effects, that the bar of soft
iron should be brought into actual contact with the pole of a
magnet. It will be manifested, only in a less degree, if it be
brought into proximity with the pole without contact. If the
bar AB be presented at a small distance from the pole b, it will
manifest magnetism in the same manner ; and if it be gradually
158 MAGNETISM.
removed from the pole, the magnetism it manifests will dimi-
nish in degree, until at length it wholly disappears.
If the end B instead of A be presented to b, the poles of the
temporary magnet will be reversed, B becoming the austral,
and A the boreal.
If a series of bars of soft iron AB, A'B', A"B", be brought into
successive contiguity so as to form a series without absolute
a, e A B A. B ..A a""
Fig. 463.
contact, as represented in fig. 463., the extremity A of the first
being presented to the boreal pole b of the fixed magnet, then
each bar of the series will be rendered magnetic. The attrac-
tion of the boreal fluid at b will decompose the magnetic fluid
of the bar AB, attracting the austral fluid towards A, and re-
pelling the boreal fluid towards B. The boreal fluid thus driven
towards B will produce a like decomposition of the fluid in the
second bar A'B', the austral fluid being attracted towards A'
and the boreal repelled towards B' ; and like effects will be
produced upon the next bar A"B", and so on.
If the bars be brought gradually closer together, the intensity
of the magnetism thus developed will be increased, and will
continue to be increased until the bars are brought into contact.
1632. Induction. This process, by which magnetism is
developed by magnetic action at a distance, is called induction ;
and the bars AB, A'B', &c. are said to be magnetized by induction.
1633. Magnets with poles reversed neutralize each other.
If a second magnet of equal intensity with the first be laid upon
a b with its poles reversed, so that its austral pole will coincide
with b and its boreal with a, the bars AB, A'B', A"B" magnetized by
induction will instantly be reduced to their natural state, and
deprived of the magnetic influence. This is easily explained.
The attraction of the pole b, which draws towards it the austral
and repels the boreal fluids of the bar AB, is neutralized by the
attraction and repulsion of the austral pole of the second magnet
laid upon it, which repels the austral fluid of the bar AB with a
force equal to that with which the boreal fluid of the pole b
attracts it, and attracts the boreal fluid with as much force as
that with which the pole b repels it. Thus the attraction and
repulsion of the two poles of the combined magnets neutralize
MAGNETISM BY INDUCTION. 159
each other, and the fluids which were decomposed in the bar
AB spontaneously recombine ; and the same effects take place in
the other bars.
All these effects may be rendered experimentally manifest by
submitting the bars AB,A'B',A"B" to any of the tests already
explained.
1634. A magnet broken at its equator produces two magnets.
It might be supposed, from what has been stated, that if a
magnetic bar were divided at its equator, two magnets would
be produced, one having austral and the other boreal magnetism,
so that one of them would attract an austral and repel a boreal
pole, while the other would produce the contrary attraction and
repulsion. This, however, is not found to be the case. If a
magnet be broken in two at its equator, two complete magnets
will result, having each an equator at or near its centre, and
two poles, austral and boreal ; and if these be again broken, other
magnets will be formed, each having an equator and two poles
as before ; and in the same manner, whatever be the number of
parts, and however minute they be, into which a magnet is
divided, each part will still be a complete magnet, with an
equator and two poles.
1635. Decomposition of magnetic fluid not attended by its
transfer between pole and pole. It follows from this, that it
cannot be supposed that the decomposition of the magnetic fluid
which is produced when a body is magnetized, is attended with
an actual transfer of the constituent fluids towards those regions
of the magnet which are separated by its equator. It cannot,
in a word, be assumed that the boreal fluid passes to one, and
the austral fluid to the other side of the equator ; for if this were
the case, the fracture of the magnet at the equator would leave
the two parts, one surcharged with austral and the other with
boreal fluid, whereas by what has been just stated it is apparent
that after such division both parts will possess both fluids.
1636. The decomposition is molecular. The decomposition
which takes place is therefore inferred to be accomplished spon-
taneously in each molecule which composes the magnet ; each
molecule is invested by an atmosphere composed of the two fluids,
and the decomposition takes place in these atmospheres, the
boreal fluid passing to one side of the molecule, and the austral
fluid to the other. When a bar is magnetized, therefore, the
material molecules which form it are invested with the magnetic
J60 MAGNETISM.
fluids, but the austral fluids are all presented towards the austral
pole, and the boreal fluids towards the boreal pole. When the
bar is not magnetic, but in its natural state, the two fluids sur-
rounding each molecule are diffused through each other and com-
bined, neither prevailing more at one side than the other.
1637. Coercive force of iron varies with its molecular struc-
ture. Iron in different states of aggregation possesses different
degrees of coercive force to resist the decomposition and re-
composition of the magnetic fluid. Soft iron, when pure, is
considered to be divested altogether of coercive force, or at least
it possesses it in an insensible degree. In a more impure state,
or when modified in its molecular structure by pressure, per-
cussion, torsion, or other mechanical effects, it acquires more or
less coercive power, and accordingly resists the reception of
magnetism, and when magnetism has been imparted to it, retains
it with a proportional force. Steel has still more coercive force
than iron, and steel of different tempers manifests the coercive
force in different degrees, that which possesses it in the highest
degree being the steel which is of the highest temper, and
which possesses in the greatest degree the qualities of hardness
and brittleness.
1638. Effect of induction on hard iron or steel. If a bar of
hard iron or steel be placed with its end in contact with a
magnet in the same manner as has been already described with
respect to soft iron, it will exhibit no magnetism ; but if it be kept
in contact with the magnet for a considerable length of time, it
will gradually acquire the same magnetic properties as have been
described in respect to bars of soft iron, with this difference,
however, that having thus acquired them, it does not lose them
when detached from the magnet, as is the case with soft iron.
Thus it would appear, that it is not literally true that a bar of
steel when brought into contact with the pole of a magnet
receives no magnetism, but rather that it receives magnetism
in an insensible degree ; for if continued contact impart sensible
magnetism, it must be admitted that contact for shorter intervals
must impart more or less magnetism, since it is by the accumu-
lation of the effects produced from moment to moment that the
sensible magnetism manifested by continued contact is pro-
duced.
It appears, therefore, that the coercive energy of the bar of
steel resists the action of the magnet, so that while the pole of
MAGNETISM BY INDUCTION. 161
the magnet accomplishes the decomposition of the magnetic
fluid in a bar of soft iron instantaneously, or at least in an in-
definitely small interval of time, it accomplishes in a bar of steel
the same decomposition, but only after a long protracted interval,
the decomposition proceeding by little and little, from moment
to moment during such interral.
Various expedients, as will appear hereafter, have been con-
trived, by which the decomposition in the case of steel bars
having a great coercive force is expedited. These consist gene-
rally in moving the pole of the magnet successively over the
various points of the steel bar, upon which it is desired to pro-
duce the decomposition, the motion being always made with the
contact of the same pole, and in the same direction. The pole
is thus made to act successively upon every part of the surface
of the bar to be magnetized, and being brought into closer con-
tact with it acts more energetically ; whereas when applied to
only one point, the energy of its action upon other points is en-
feebled by distance, the intensity of the magnetic attraction
diminishing, like that of gravity, in the same proportion as the
square of the distance increases.
Since steel bars having once received the magnetic virtue in
this manner retain it for an indefinite time, artificial magnets
can be produced by these means of any required form and
magnitude.
1639. Forms of magnetic needles and bars. Thus a mag-
netic needle generally receives the form of a lozenge, as repre-
sented in fig. 464., having a co-
nical cap of agate at its centre,
which is supported upon a pivot
in such a manner as that the
needle is free to turn in a hori-
zontal plane, round the pivot as
a centre. In this case the weight
of the needle must be so regu-
lated as to be in equilibrium on
Figl 464> the pivot.
Bar magnets are pieces of steel in the form of a cylinder or
prism whose length is considerable compared with their depth
or thickness. In producing such magnets certain processes are
necessary, which will be explained hereafter.
1640. Compound magnet. Several bar magnets, equal and
J f
I
162 MAGNETISM.
similar in magnitude, being placed one upon the other with
their corresponding poles together, form a compound magnet.
1641. Effects of heat on magnetism. It is evident from
what has been stated respecting the various degrees of coercive
force manifested by the same metal in different states of aggre-
gation, that the magnetic qualities depend upon molecular
arrangement, and that the same body in different molecular
states will exhibit different magnetic properties.
Since the elevation or depression of temperature by pro-
ducing dilatation and contraction affects the molecular state of
a body, it might be expected to modify also its magnetic pro-
perties, and this is accordingly found to be the case.
1642. A red heat destroys the magnetism of iron. If a
magnet, no matter how powerful, natural or artificial, be raised
to a red heat, it will lose altogether its magnetic virtue. The
elevation of temperature and the molecular dilatation conse-
quent upon it destroys the coercive force and allows the
recombination of the magnetic fluid. When after such change
the magnet is allowed to cool, it will continue divested of its
magnetic qualities. These effects may, however, be again
imparted to it by the process already mentioned.
1643. Different magnetic bodies lose their magnetism at dif-
ferent temperatures. M. Pouillet found that this phenomenon
is produced at different temperatures for the different bodies
which are susceptible of magnetism. Thus the magnetism of
nickel is effaced when it is raised to the temperature of 660,
iron at a cherry red, and cobalt at a temperature much more
elevated.
1644. Heat opposed to induction. But not only does in-
creased temperature depi'ive permanent magnets of their mag-
netism, but it renders even soft iron insusceptible of magnetism
by induction, for it is found that soft iron rendered incandescent
does not become magnetic when brought into contact or con-
tiguity with the pole of a magnet.
1645. Induced magnetism rendered permanent by hammering
and other mechanical effects. If a bar of soft iron when
rendered magnetic by induction be hammered, rolled, or
twisted, it will retain its magnetism. It would follow, there-
fore, that the change of molecular arrangement produced by
these processes confers upon it a coercive force which it had not
previously.
MAGNETISM BY INDUCTION. 163
1646. Compounds of iron differently susceptible of magnet-
ism. Compounds of iron are in general more or less sus-
ceptible of magnetism, according to the proportion of iron they
contain.
Exceptions, however, to this are presented in the peroxide,
the persulphate, and some other compounds containing iron in
small proportion, in which the magnetic virtue is not at all
present.
1647. Compounds of other magnetic bodies not susceptible.
Nickel, cobalt, chromium, and manganese are the only simple
bodies which, in common with iron, enjoy the magnetic pro-
perty, and this property completely disappears in most of the
chemical compounds of which they form a part.
Magnetism, however, has been rendered manifest under a
great variety of circumstances connected with the development
of electricity which will be fully explained in a subsequent
Book.
1648. Magnets with consequent points. In the production
of artificial magnets it frequently happens that a magnetic bar
has more than one equator, and consequently more than two
poles. This fact may be experimentally ascertained by ex-
posing successively the length of a bar to any of the tests already
explained. Thus, if presented to the test pendulum, it will
be attracted with a continually decreasing force as it approaches
each equator, and with an increasing force as it recedes from it.
If the bar be rolled in iron filings, they will be attached to it in
a succession of tufts separated by spaces where none are attached,
indicating the equators.
If it be placed under a glass plate or sheet of paper on which
fine iron filings are sprinkled, they will arrange themselves
according to a series of concentric curves, as represented in
fig. 465.
164
MAGNETISM.
It is evident that the magnetic bar in this case is equivalent
to a succession of independent magnets placed pole to pole.
The equators in these cases are called consequent points.
CHAP. III.
TERRESTRIAL MAGNETISM.
1649. Analogy of the earth to a magnet. If a small and
sensitive magnetic needle, suspended by a fibre of silk so as
to be free to assume any position which the attractions that act
upon it may have a tendency to give to it, be carried over a
magnetic bar from end to end, it will assume in different positions
different directions, depending on the effect produced by the
attractions and repulsions exercised by the bar upon it.
Let ab,Jig. 466., be such a needle, the thread of suspension oe
being first placed vertically over the equator E of the magnetic
1
E
Fig. 466.
bar AB. The austral magnetism of AE will attract the boreal
magnetism of be and will repel the austral magnetism of ae;
and in like manner the boreal magnetism of BE will attract the
austral magnetism of ae and will repel the boreal magnetism of
be. These attractions and repulsions will moreover be re-
spectively equal, since the distances of ae and be from BA and
BE are equal. The needle a b will therefore settle itself
parallel to the bar AB, the pole a being directed to B, and the
pole b being directed to A.
If the suspending thread oe be removed towards A to PC,
the attraction of A upon b will become greater than the at-
TERRESTRIAL MAGNETISM. 165
traction of B upon a, because the distance of A from b will be
less than the distance of B from a ; and, for a like reason, the
repulsion of A upon a will be greater than the repulsion of B
upon b. The needle a b will therefore be affected as if the end
b were heavier than a, and it will throw itself into the inclined
position represented in the figure, the pole a inclining down-
wards.
If it be carried still further towards A, the inequality of the
attractions and repulsions increasing in consequence of the
greater inequality of the distances of a and b from A and B,
the inclination of b downwards will be proportionally aug-
mented, as represented at p'.
In fine, when the thread of suspension is moved to a point
p" over the pole A, the needle will become vertical, the pole b
attracted by A pointing downwards.
If the needle be carried in like manner from E to B, like
effects will be manifested, as represented in the figure, the pole
a inclining downwards arising from the same causes.
A magnetic needle similarly suspended, carried over the surface
of the earth in the directions north and south, undergoes changes
of direction such as would be produced, on the principles ex-
plained above, if the globe were a magnet having its poles at
certain points, not far distant from its poles of rotation.
To render this experimentally evident, it will be necessary to
be provided with two magnetic instruments, one mounted so that
the needle shall have a motion in a horizontal plane round a
vertical axis, and the other so that it shall have a motion in a
vertical plane round a horizontal axis.
1650. The azimuth compass. The instrument called the
azimuth compass consists of a magnetic bar or needle balanced
on a vertical pivot, so as to be capable of turning freely in a
horizontal plane, the point of the needle playing in a circle, of
which its pivot is the centre.
The instrument is variously mounted and variously desig-
nated, according to the circumstances and purpose of its appli-
cation.
When used to indicate the relative bearings or horizontal
directions of distant objects, whether terrestrial or celestial, a
graduated circle is placed under the needle and concentric with
it. The divisions of this circle indicate the bearings of any
distant object in relation to the direction of the needle.
166
MAGNETISM.
The pivot in this form of compass is rendered vertical by
means of a plumb-line or spirit-level.
1651. The mariner's compass. When the azimuth compass
is used for the purpose of navigation, the pivot supporting the
needle is fixed in the bottom of a cylindrical box, which is
closed at the top by a plate of glass, so as to protect it from the
air. The magnetic bar is attached to the under side of a cir-
cular card, upon which is engraved a radiating diagram, which
divides the circle into thirty-two parts called points. The
compass box is suspended so as to preserve its horizontal posi-
tion undisturbed by the motion of the vessel, by means of two
concentric hoops called gimbals, one a little less than and in-
cluded within the other. It is supported at two points upon
the lesser hoop, which are diametrically opposite, and this lesser
hoop itself is supported by two points upon the greater hoop,
which are also diametrically opposite, but at right angles to the
former. By these means the box, being at liberty to swing in
two planes at right angles to each other, will maintain itself
horizontal, and will therefore keep the pivot supporting the
needle vertical, whatever be the changes of position of the
vessel.
This arrangement is represented in fig. 467., a vertical sec-
tion of the compass box being given in fig. 468.
Fig. 467. Fig. 468.
The sides of the cylindrical box are bb', its bottom//', and
the glass which covers it v. The magnetic bar or needle is sup-
ported on a vertical pivot by means of a conical cup, and can
be raised and lowered at pleasure by means of a screw w. The
compass card is represented in section at rr f , fig. 467., and the
divisions upon it marked by radiating lines called the rose are
represented in fig. 468.
TERRESTRIAL MAGNETISM.
167
Two narrow plates, p and p f , are attached to the sides of the
box so as to be diametrically opposed. In p there is a narrow
vertical slit. In p' there is wider vertical slit, along which is
stretched vertically a thin wire. The eye placed at o looks
through the two slits, and turns the instrument round its sup-
port until the object of observation is intersected by the ver-
tical wire, extended along the slit p'. Provisions are made
in the instrument by which the direction thus observed can
be ascertained relatively to that of the needle. The angle
included between the direction of the observed object, and
that of the needle, is the bearing of the object relatively to the
needle.
The compass box is suspended within the hoop e e, at two
points z z' diametrically opposed, and the hoop e e 1 is itself sus-
pended within the fixed hoop c c', at two points x a/, also diame-
trically opposed, but at right angles to z z 1 .
1652. The dipping needle. The apparatus represented in
fig. 469., called the dipping needle,
consists of a magnetic needle e e, sup-
ported and balanced on a horizontal
axis, and playing therefore in a ver-
tical plane. The angles through
which it turns are indicated by a
graduated circle / 1', the centre of
which coincides with the axis of the
needle, and the frame which sup-
ports it has an azimuth motion round
a vertical axis, which is indicated
and measured by the graduated
horizontal circle z z'. The instru-
ment is adjusted by means of a
spirit-level, and regulating screws
inserted in the feet.
Fig- 469. 1653. Analysis of magnetic phe-
nomena of the earth. Supplied with these instruments, it will
be easy to submit to observation the magnetic phenomena
manifested at different parts of the earth.
If the azimuth compass be placed anywhere in the northern
hemisphere, at London for example, the needle will take a certain
position, forming an angle with the terrestrial meridian, and
directing one pole to a point a certain number of degrees west
168 MAGNETISM.
of the north, and the other to a point a like number of degrees
east of the south. If it be turned aside from this direction, it
will when liberated oscillate on the one side and the other of
this direction, and soon come to rest in it.
Since an unmagnetized needle would rest indifferently in any
direction, this preference of the magnetized needle to one parti-
cular direction must be ascribed to a magnetic force exerted by
the earth attracting one of the poles of the needle in one direc-
tion, and the other pole in the opposite direction.
That this is not the casual attraction of unmagnetic ferrugi-
nous matter contained within the earth, is proved by the fact that,
if the direction of the needle be reversed, it will, when liberated,
make a pirouette upon its pivot, and after some oscillations re-
sume its former direction.
This remarkable property is reproduced in all parts of the
earth, on land and water, and equally on the summits of lofty
mountains, in the lowest valleys, and in the deepest mines.
1654. Magnetic meridian. The direction thus assumed by
the horizontal needle, in any given place, is called the MAGNETIC
MERIDIAN of that place.
The direction of a needle which would point due north and
south is the TRUE MERIDIAN, or the TERRESTRIAL MERIDIAN of
the place.
1655. Declination or variation. The angle formed by the
MAGNETIC MERIDIAN and the TERRESTRIAL MERIDIAN is Called
sometimes the VARIATION, and sometimes the DECLINATION of
the needle. We shall adopt by preference the latter term.
The declination is said to be EASTERN or WESTERN, according as
the pole of the needle, which is directed northwards, deviates to
the east or to the west of the terrestrial meridian.
1656. Magnetic polarity of the earth. To explain these
phenomena, therefore, the globe of the earth itself is considered
as a magnet, whose poles attract and repel the poles of the
horizontal needle, each pole of the earth attracting that of an
unlike name, and repelling that of a like name.
If, therefore, the northern pole of the earth be considered as
that which is pervaded by boreal magnetism, and the southern
pole by austral magnetism, the former will attract the austral
and repel the boreal pole, and the latter will attract the boreal
and repel the austral pole of the needle. Hence it will follow
that the pole of the needle which is directed northwards is the
TERRESTRIAL MAGNETISM. 169
austral, and that which is directed southwards is the boreal
pole.
16<>7. Change of direction of the dipping-needle. It was
shown in (1649) that when a needle which is free to play in
a vertical plane was carried over a magnet, it rested in the
horizontal position only when suspended vertically over the
equator of the magnet, and its austral and boreal poles were in-
clined downwards, according as the needle was suspended at the
boreal or austral side of the equator, and that this inclination
was augmented as the distance from the equator at which the
needle was suspended was increased. Now it remains to be
seen whether any phenomenon analogous to this is presented
by the earth.
For this purpose, let the dipping-needle,^. 469., be arranged
with its axis at right angles to the direction of the needle of
the azimuth compass. It will then be found, that in general
the dipping-needle will not rest in a horizontal position, but will
assume a direction inclined to the vertical line, as represented
in the figure, one pole being presented downwards, and the other
upwards.
The angle which the lower arm of the needle makes with the
horizontal line is called the dip.
If this apparatus be carried in this hemisphere northwards,
in the direction in which a horizontal needle would point, the
austral pole will be inclined downwai'ds, and the dip will con-
tinually increase ; but if it be carried southwards, the dip will
continually diminish. By continuing to transport it southwards,
the dip continually diminishing, a station will at length be found
where the needle will rest in the horizontal position. If it be
carried further southwards, the boreal pole will begin to turn
downwards ; in other words, the dip will be south instead of
north, and as it is carried further southwards, this dip will con-
tinue to increase.
If the needle be carried northwards, in this hemisphere the
dip continually augmenting, a station will at length be attained
where the needle will become vertical, the austral pole being
presented downwards, and the boreal pole upwards.
In the same manner, in the southern hemisphere, if the needle
be carried southwards, a station will at length be attained
where it will become vertical, the boreal pole being presented
downwards, and the austral pole pointing to the zenith.
170 MAGNETISM.
Complete analogy of the earth to a magnet. By com-
paring these results with those which have been already de-
cribed in the case where the needle was carried successively over
a magnetic bar, the complete identity of the phenomena will be
apparent, and it will be evident that the earth and the needle
comport themselves in relation to each other exactly as do a
small and a great magnet, over which it might be carried, the
point where the needle is horizontal being over the magnetic
equator, and those two points where it is vertical being the
magnetic poles.
1658. The magnetic equator. The needle being brought
to that point where it rests horizontal, the magnetic equator
will be at right angles to its direction. By transporting it suc-
cessively in the one or the other direction thus indicated, the
successive points upon the earth's surface where the needle rests
horizontal, and where the dip is nothing, will be ascertained.
The line upon the earth traced by this point is the magnetic
equator.
Its form and position not regular. This line is not, as
might be expected, a great circle of the earth. It follows
a course crossing the terrestrial equator from south to north,
on the west coast of Africa, near the island of St. Thomas,
at about 7 or 8 long. E., in a direction intersecting the
equator at an angle of about 12 or 13. It then passes
across Africa towards Ceylon, and intersects that island near
the point of the Indian promontory. It keeps a course
from this of from 8 to 9 of N. lat. through the Indian Archi-
pelago, and then gradually declining towards, the line again
intersects it at a point in the Pacific Ocean in long. 170 TV.,
the angle at which it intersects the line being more acute than
at the other point of intersection. It then follows a course a
few degrees south of the line, and striking the west coast of
South America near Lima, it crosses the South American con-
tinent, attaining the greatest south latitude near Bahia ; and
then again ascending towards the line, traverses the Atlantic and
strikes the coast of Africa, as already stated, "near the island of
St. Thomas.
The magnetic equator, unlike the ecliptic, is not any regular
curve, but follows the course we have just indicated in a
direction slightly sinuous.
Variation of the dip, going north or south It has been
TERRESTRIAL MAGNETISM. 171
explained, that proceeding from nortli or south, from the
magnetic equator, the needle dips on the one side or on the
other, the dip increasing with the distance from the magnetic
equator to which the needle is transported north or south.
Lines of equal dip. The lines of equal dip, therefore,
may be considered as bearing the same relation to the magnetic
equator, which parallels of latitude hear to the terrestrial equator,
being arranged nearly parallel to the former, though not in a
manner so regular as in the case of parallels of latitude.
1659. Magnetic meridians. If the horizontal needle be
transported north or south, following a course indicated by its
direction, it will be carried over a magnetic meridian. These
magnetic meridians, therefore, bear to the magnetic equator a
relation analogous to those which terrestrial meridians bear to
the terrestrial equator, but, like the lines of equal dip, they are
much more irregular.
1660. Method of ascertaining the declination of the needles.
Astronomy supplies various methods of determining in a
given place the declination of the needle. It may be generally
stated that this problem may be solved by observing any object
whose angular distance from the true north is otherwise known,
and comparing the direction of such object with the direction
of the needle. Let p, Jig. 470., be the place of observation ; let
p N be the direction of the true north, or, what is the same, the
direction of the terrestrial meridian ; and let
p N' be the direction of the magnetic needle,
or, what is the same, the magnetic meridian.
The angle N p N' will then be the declination
of the needle, being the angle formed by the
terrestrial and magnetic meridians (1655).
Let O be any object seen on the horizon in
the direction P o ; the angle o P N is called the
true azimuth of this object, and the angle OPN'
is called its magnetic azimuth.
This magnetic azimuth may always be ob-
served by means of an azimuth compass.
If, then, an object be selected whose true azimuth is otherwise
known, the declination of the needle may be determined by
taking the difference between the true and magnetic azimuths
of the object.
i 2
172 MAGXETISM.
There are numerous celestial objects of which the azimuths
are either given in tables, or may be calculated by rules and
formulae supplied by astronomy ; such, for example, as the sun
and moon at the moments they rise or set, or when they are at
any proposed or observed altitudes. By the aid of such objects,
which are visible occasionally at all places, the declination of
the needle may be found.
Local declinations. At different places upon the earth's
surface the needle has different declinations. In Europe its
mean declination is about 17, increasing in going westward.
1661. Lines of no declination called agonic lines. There
are two lines on the earth's surface which have been called
AGONIC LINES, upon which there is no declination ; and where>
therefore, the needle is directed along the terrestrial meridian.
One of these passes over the American and the other over
the Asiatic continent, and the former has consequently been
called the AMERICAN and the latter the ASIATIC AGONIC. These
lines run north and south, but do not follow the course of
meridians.
It has been ascertained that their position is not fixed, but is
liable to sensible changes in considerable intervals of time.
1662. Declination in different longitudes, at equator, and in
lat. 45. In proceeding in either direction, east or west from
these lines, the declination of the needle gradually increases,
and becomes a maximum at a certain intermediate point between
them. On the west of the Asiatic agonic the declination is
west, on the east it is east.
At present the declination in England is about 24 TV. ; in
Boston in the U. States it is 5^ W. Its mean value in Europe
is 17 W. At Bonn it is 20, at Edinburgh 26, Iceland 38,
Greenland 50, Konigsberg 13, and St. Petersburg 6.
The following table, however, will exhibit more distinctly
the variation of the declination in different parts of the globe.
The longitudes expressed in the first column are measured west-
ward from the meridian of Paris, and the declinations given in
the second column are those which are observed on the terres-
trial equator, those in the third column corresponding to the
mean latitude of 45.
TERRESTRIAL MAGNETISM.
173
Table of the Declinations of the magnetic Needle in different
Longitudes, and in Lat. Q and Lot. =45.
I.ongitu les West
of the Meridian
Declin
itions.
Longitudes West
Declin
itions.
of Paris.
Lat. = 0.
Lat. = 45.
of Paris." 1 "
Lat. 0.
Lat. = 450.
19 W
22 W
190
9 E
flE
10
19 W
25 W
200
8 E
E
20
16 W
26 W
210
5 E
E
30
11 W
25 W
220
3 E
E
40
4 W
24 W
230
2 E
E
50
3 E
24 W
240
W
6:>
5 E
20 W
250
70
8 E
11 W
260
1 E
; E
10 E
3 W
270
3 E
E
90
10 E
4 E
280
E
100
8 E
11 E
290
E
110
6 E
17 E
300
2 W
E
120
5 E
18 E
310
7 W
W
130
5 E
19 E
320
11 W
W
140
6 K
19 E
330
13 W
10 W
InO
6 E
19 E
340
17 W
14 W
160
7"E
19 E
350
18 W
17 W
170
9 K
17 E
360
19 W
22 W
180
10 E
14 E
1663. Isogonic lines. Lines traced upon the globe at a point
at which the magnetic needle has the same declinations, are
called ISOGONIC LINES. These, as well as the ISOCLINIC LINES, or
lines of equal dip, are irregular in their arrangement, and not
very exactly ascertained.
1664. Local dip. The local variations of the dip are also im-
perfectly known. In Europe it ranges from 60 to 70. In 1836
the dip observed at the undermentioned places was as follows :
Pekin
Home
Brussels
St. Petersburg
St. Helena
Rio de Janeiro
- 54 C
- 61 C
- 68 C
- 71 C
- 14 C
- 13 C
50'
30'
1665. Position of magnetic poles. The determination of the
precise position of the magnetic poles, or the points where the
dip is 90, is attended with considerable difficulty, inasmuch as
for a considerable distance round that point the dip is nearly 90.
Hansteen considered that there were grounds for supposing
that there were two magnetic poles in each hemisphere. One
of these in the northern hemisphere he supposed to be west of
Hudson's Bay in 80 lat. N., and 96 long. W. ; and the other in
Northern Asia in 81 lat. N., and 116 long. E. The two
I 3
174 MAGNETISM.
southern magnetic poles he supposed to be situate near the
southern pole. This supposition, however, appears to be at
present abandoned, and the observations of GAUSS lead to the
'conclusion that there is but one magnetic pole in each hemisphere.
In the northei-n voyages made between 1829 and 1833,
Sir James Ross found the dipping-needle to stand vertical in
the neighbourhood of Hudson's Bay at 70 5' 1 7" lat. N., and
114 55' 18" long. W. The dipping-needle, according to the
observations of Sir James Ross, was nowhere absolutely vertical,
departing from the vertical in all cases by a small angle, amount-
ing generally to one minute of a degree. This, however, might
be ascribed to the error of observation, or the imperfection of
instruments exposed to such a climate.
The existence of the magnetic pole, however, at or near the
point indicated, was proved by carrying roun,d it at a certain
distance a horizontal needle, which always pointed to the spot
in whatever direction it was carried. Gauss has fixed the
position of the magnetic pole in the southern hemisphere by
theory at 72 35' lat. S., and 152 30' long. E.
1666. Magnetic poles not antipodal. It will be perceived,
therefore, that the magnetic poles, unlike the terrestrial poles,
are not antipodal to each other ; or, in other words, they do not
form the extremities of the same diameter of the globe : they
are not even on the same meridian. If Gauss's statement be
assumed to be correct, the southern magnetic pole is on a
meridian 152 30' E. of the meridian of Greenwich, and there-
fore 207 30' W. of that meridian, whereas the northern
magnetic pole is on a meridian 114 55' 18" W. The angle,
therefore, under the two meridians passing through the two
poles will be about 921. It would follow, therefore, that these
points lie upon terrestrial meridians nearly at right angles to
each other, and that upon these they are at nearly equal distances
from the terrestrial poles ; the distance of the northern mag-
netic pole from the northern terrestrial pole being nearly 20,
and the distance of the southern magnetic pole from the southern
terrestrial pole being about 17^.
1667. Periodical variations of terrestrial magnetism. It
appears, from observations made at intervals of time more or
less distant for about two centuries back, that the magnetic
condition of the earth is subject to a periodical change; but
neither the quantity nor the law of this change is exactly
known. It was not until recently that magnetic observations
TERRESTRIAL MAGNETISM.
175
were conducted in such a manner as to supply the data
necessary for the development of the laws of magnetic varia-
tion, and they have not been yet continued a sufficient length
of time to render these laws manifest.
Independently of observation, theory affords no means of
ascertaining these laws, since it is not certainly known what
are the physical causes to which the magnetism of the earth
must be ascribed.
In the following table are given the declinations of the needle
observed at Paris between the years 1580 and 1835, and the
dip between the years 1671 and 1835.
Table of Declinations observed at Paris.
Year.
Declination.
Year.
Declination.
1580
11 30' E
1816
22 25' W
1618
8
1817
22 19
1663
1823
22 23
1678
1 30 W
1824
22 23
1700
8 10
1825
22 22
1780
19 55
J827
22 20
1785
22
1828
22 5
1805
22 5
1829
22 12
1813
22 28
1832
22 3
1814
22 34
1835
22 4
Table of the Dip observed at Paris.
Year.
Dip.
Year.
Dip.
K,71
73
1819
68 2V
1754
72 15'
1820
68 20
1776
72 25
1821
68 14
1780
71 48
1822
68 11
1791
70 52
1823
63 8
1798
69 51
1825
68
1806
69 12
1826
68
1810
68 50
1829
67 41
1814
68 36
mi
67 40
1816
68 40
1835
67 24J
1818
68 35
1668. Intensity of terrestrial magnetism. The intensity of
terrestrial magnetism, like that of a common magnet, may be
estimated by the rate of vibration which it produces in a
magnetic needle submitted to its attraction. This method of
determining the intensity of magnetic force is in all respects
analogous to those by which the intensity of the earth's attrac-
tion is determined by a common pendulum (549). The same
needle being exposed to a varying attraction will vary its rate
I 4
176 MAGNETISM.
of vibration, the force which attracts it being proportional to
the square of the number of vibrations which it makes in a
given time. Thus, if at one place it makes ten vibrations per
minute, and in another only eight, the magnetic force which
produces the first will be to that which produces the second
rate of vibration, as 100 to 64.
1669. Increases from equator to poles. In this manner it
has been found that the intensity of terrestrial magnetism is
least at the magnetic equator, and that it increases gradually in
approaching the poles.
1670. Isodynamic lines. Those parts of the earth where
the magnetic intensities are equal are called isodynamic lines,
and resemble in their general arrangement, without however
coinciding with them, the isoclinic curves or magnetic parallels
of equal dip.
1671. Their near coincidence with isothermal lines. It
has been found that there is so near a coincidence between
the isodynamic and the isothermal lines, that a strong pre-
sumption is raised that terrestrial magnetism either arises from
terrestrial heat, or that these phenomena have at least a common
origin.
1672. Equatorial and polar intensities. It appears to
follow from the general result of observations made on the
intensity of terrestrial magnetism, that its intensity at the
poles is to its intensity at the equator nearly in the ratio
of 3 to 2.
1673. Effect of the terrestrial magnetism on soft iron> If
anything were wanted to complete the demonstration that the
globe of the earth is a true magnet, it would be supplied by
the effects produced by it upon substances susceptible of mag-
netism, but which are not yet magnetized. It has been already
shown that when a bar of soft iron is presented to the pole of
a magnet, its natural magnetism is decomposed, the austral fluid
being attracted to one extremity, and the boreal fluid repelled
to the other, so that the bar of soft iron becomes magnetized,
and continues so as long as it is exposed to the influence of the
magnet. Now, if a bar of soft iron be presented to the earth in
the same manner, precisely the same effects will ensue. Thus, if
it be held in the direction of the dipping-needle, so that one of its
ends shall be presented in the direction of the magnetic attraction
of the earth, it will become magnetic, as may be proved by any
TERRESTRIAL MAGNETISM. 177
of the tests of magnetism already explained. Thus, if a sensi-
tive needle be presented to that end of the bar which in the
northern hemisphere is directed downwards, austral magnetism
will be manifested, the boreal pole of the needle being attracted,
and the austral pole repelled. If the needle be presented to
the upper end of the bar, contrary effects will be manifested ;
and if it be presented to the middle of the bar, the neutral line
or equator will be indicated. If the bar be now inverted, the
upper end being presented downwards, and vice versa, still
parallel to the dipping-needle, its poles will also be inverted,
the lower, which previously was boreal, being austral, and vice
versa.
If the bar be held in any other direction, inclined obliquely
to the dipping-needle, the same effects will be manifested, but
in a less degree, just as would be the case if similarly presented
to an artificial magnet ; and, in fine, if it be held at right angles
to the direction of the dipping-needle, no magnetism whatever
will be developed in it.
1674. Its effects on steel bars. If the same experiments be
made with bars of hard iron or steel, no sensible magnetism
will at first be developed ; but if they be held for a consider-
able time in the same position, they will at length become
magnetic, as would happen under like conditions with an arti-
ficial magnet. Iron and steel tools which are hung up in work-
shops in a vertical position are found to become magnetic, an
effect explained by this cause.
1675. Diurnal variation of the needle. Besides the changes
in the magnetic state of the earth, the periods of which are
measured by long intervals of time, there are more minute and
rapid changes depending apparently upon the vicissitudes of the
seasons and the diurnal changes.
The magnitude of the diurnal variation depends upon the
situation of the place, the day, and the season, but is obviously
connected with the function of solar heat. At Paris it is ob-
served that during the night the needle is nearly stationary ;
at sunrise it begins to move, its north pole turning westwards,
as if it were repelled by the influence of the sun. About noon,
or more generally between noon and three o'clock, its western
variation attains a maximum, and then it begins to move east-
ward, which movement continues until some time between nine
and eleven o'clock at night, when the needle resumes the
i 5
178 MAGNETISM.
position it had when it commenced its western motion in the
morning.
The amplitude of this diurnal range of the needle is, accord-
ing to Cassini's observations, greatest during summer and least
during winter. Its mean amount for the months of April,
May, June, July, August, and September is stated at from
13 to 15 minutes; and for the months of October. November,
December, January, and March, at from 8 to 10 minutes.
There are, however, occasionally, days upon which its range
amounts to 25 minutes, and others when it does not surpass
5 or 6 minutes. Cassini repeated his magnetic observations in
the cellars constructed under the Paris observatory at a depth
of about a hundred feet below the surface, and therefore
removed from the immediate influence of the light and heat of
the day. The amplitude of the variations, and all the pecu-
liarities of the movement of the needle here, were found to be
precisely the same as at the surface.
In more northern latitudes, as, for example, in Denmark,
Iceland, and North America, the diurnal variations of the
needle are in general more considerable and less regular. It
appears, also, that in these places the needle is not stationary
during the night, as in Paris, and that it is towards evening
that it attains its maximum westward deviation. On the
contrary, on going from the north towards the magnetic equator
the diurnal variations diminish, and cease altogether on ar-
riving at this line. It appears, however, according to the ob-
servations of Captain Duperrey, that the position of the sun
north or south of the terrestrial equator has a perceptible
influence on the oscillation of the needle.
On the south of the magnetic equator the diurnal variations
are produced, as might be expected, in a contrary manner ; the
northern pole of the magnet turns to the east at the same hours
that, in the northern hemisphere, it turns to the west.
It has not yet been certainly ascertained whether in each
hemisphere these diurnal variations of the needle correspond in
the places where the eastern and western declinations also
correspond.
The dip is also subject to certain diurnal variations, but
much smaller in their range than in the case of the horizontal
needle.
As a general result of these observations it may be inferred,
MAGNETIZATION. 179
that if a magnetic needle were suspended in such a manner
as to be free to move in any direction Avhatever, it would,
during twenty-four hours, move round its centre of suspension
in such a manner as to describe a small cone, whose base would
be an ellipse or some other curve more or less elongated, and
whose axis is the mean direction of the dipping-needle.
1676. Disturbances in the magnetic intensity. The intensity
as well as the direction of the magnetic attraction of the earth
at a given place are subject to continual disturbances, independ-
ently of those more regular variations just mentioned.
These disturbances are in general connected with the elec-
trical state of the atmosphere, and are observed to accompany
the phenomena of the aurora borealis, earthquakes, volcanic
eruptions, sudden vicissitudes of temperature, storms, and other
atmospheric disturbances.
1677. Influence of aurora borealis. During the appearance
of the aurora borealis in high latitudes, a considerable deflection
of the needle is generally manifested, amounting often to several
degrees. So closely and necessarily is magnetic disturbance
connected with this atmospheric phenomenon, that practised
observers can ascertain the existence of an aurora borealis by
the indications of the needle, when the phenomenon itself is not
visible.
CHAP. IV.
MAGNETIZATION.
1678. Effects of induction. The process by which artificial
magnets are produced are all founded upon the property of in-
duction (Ch. II.). When one of the poles of a magnet is presented
to any body which is susceptible of magnetism, it will have a
tendency to decompose the magnetic fluid in the body to which
it is presented, attracting one of its constituents and repelling
the other. If the coercive force by which the fluids are com-
bined be greater than the energy of the attraction of the mag-
net, no decomposition will take place, and the body to which
180 MAGNETISM.
the magnet is presented will not be magnetized, but the coercive
force with which the fluids are united will be rendered more
feeble, and the body will be more susceptible of being magnetized
than before.
If, however, the energy of the magnetic force of the magnet
presented to it be greater than the coercive force with which
the fluids are united, a decomposition will take place, which will
be more or less in proportion as the force of the magnet exceeds
in a greater or less degree the coercive force which unites the
magnetic fluids.
1679. Their application in the production of artificial mag-
nets. These principles being well understood, the methods of
producing artificial magnets will be easily rendered intelligible.
It has been already explained, that pure soft iron is almost,
if not altogether, divested of coercive force, so that a bar of this
substance is converted into a magnet instantaneously when the
pole of a magnet is presented to it; but the absence of coercive
force, which renders this conversion so prompt, is equally effica-
cious in depriving the bar of its magnetism the moment the
magnet which produces this magnetism is removed.
1680. Best material for artificial magnets. Soft iron, there-
fore, is inapplicable when the object is to produce permanent
magnetism. The material best suited for this purpose is steel,
especially that which has a fine grain, a uniform structure, and
is free from flaws. It is necessary that it should have a certain
degree of hardness, and that this should be uniform through its
entire mass. If the hardness be too great, it is difficult to im-
part to it the magnetic virtue; if not great enough, it loses its
magnetism for want of sufficient coercive force. To render
steel bars best fitted for artificial magnets, it has been found
advantageous to confer upon them in the first instance the
highest degree of temper, and thus to render them as hard and
brittle as glass, and then to anneal them until they are brought
to a straw or violet colour.
1681. Best form for bar magnets. The intensity of artificial
magnets depends also, to some extent, upon their form and
magnitude. It has been ascertained, that a bar magnet has the
best proportion when its thickness is about one-fourth and its
length twenty times its breadth.
1682. Horse-shoe magnets. Bar magnets are sometimes
MAGNETIZATION.
181
shaped in the form of a horse-shoe, and are
hence called HORSE-SHOE MAGKETS, as repre-
sented in fig. 471. When magnets are con-
structed in this form, the distance between the
two poles ought not to be greater than the
thickness of the bar of which the magnet
consists. The surface of the steel forming
both bars, in horse-shoe magnets, should be
rendered as even and as well polished as pos-
sible.
1683. Methods of producing artificial mag-
nets by friction. Two methods of imparting
magnetism by friction are known as those of
DUHAMEL and JEriNUS. The former is sometimes called the
method of single touch, and the latter the method of double touch.
1684. Method of single touch. The method of Duhamel, or
of single touch, is practised as follows. The bar A' v',jig. 472.,
B L, A.
Fig. 472.
which is to be magnetized, is laid upon a block 'of wood L pro-
jecting at each end a couple of inches. Under the ends are
placed the opposite poles A and B of two powerful magnets, so
as to be in close contact with the bar to be magnetized. The
influence of the pole A will be to attract the boreal fluid of the
bar towards the end B', and to repel the austral fluid towards
the end A' ; and the effect of the pole B will be similar, that
is to say, to repel the boreal fluid towards the end B', and to
attract the austral towards the end A'. It is evident, therefore,
that if the coercive force of the magnetism of the bar A' B' be
not greater than the force of the magnets A and B, a decom-
position will take place by simple contact, and the bar A' B' will
be converted into a magnet, having its austral pole at A' and its
boreal pole at B' ; and, indeed, this will be accomplished even
though the coercive force of the bar A' B" be considerable, if it
be left a sufficient length of time under the influence of the
magnets A and B.
182 MAGNETISM.
But without waiting for this, its magnetization may be accom-
plished immediately by the following process. Let two bar
magnets a and b be placed in contact with the bar A' B', to be mag-
netized near its middle point, but without touching each other,
and let them be inclined in opposite directions to the bar A! B',
at angles of about 30, as represented in the figure. Let the bar
which is applied on the side B' have its austral pule, and that
which is applied on the side A' its boreal pole in contact with the
bar A' B', and to prevent the contact of the two bars a and b, let
a small piece of wood, lead, copper, or other substance not sus-
ceptible of magnetism, be placed between them. Taking the
two bars a and b, one in the right and the other in left hand, let
them now be drawn in contrary directions, slowly and uniformly
along the bar A'B', from its middle to its extremities, and being
then raised from it, let them be again placed as before, near its
middle point, and drawn again uniformly and slowly to its ex-
tremities ; and let this process be repeated until the bar A' B'
has been magnetized.
It is evident that the action of the two magnetic poles a and
b will be to decompose the magnetic fluid of the bar A' B', and
that in this they are aided by the influence of the magnets A
and B, which enfeeble, as has been already shown, the coercive
force.
This method is applicable with advantage to magnetize, in the
most complete and regular manner, compass needles, and bars
whose thickness does not exceed a quarter of an inch.
1685. Method of double touch. When the bars exceed this
thickness, this method is insufficient, and that of uEpinus, or the
method of double touch, is found more effectual. This method
is practised as follows.
The bars a and b are placed as before, but instead of being
held in the two hands are attached to a triangle, by which they
are maintained permanently in their position, and held together.
Being placed at the centre of the bar A'B', they are moved
together first to one extremity B', and then back along the length
of the entire bar to the other extremity A'. They are then
again drawn over the bar to B', and so backwards and for-
wards continuously until the bar is magnetized. The operation
is always terminated when the bars have passed over that half
of the bar A'B' opposite to that upon which the motion com-
menced. Thus if the operation commenced by moving the
MAGNETIZATION. 183
united bars a b from the centre to the end B', it will be termi-
nated when they are moved from the extremity A' to the middle.
1686. Inapplicable to compass needles and long bars.
By this method a greater quantity of magnetism is developed
than in that of Duhamel, but it should never be employed for
magnetizing compass needles or bars intended for delicate ex-
periments, since it almost always produces magnets with poles
of unequal force, and frequently gives them consequent points
(1648), especially when the bars have considerable length.
1687. Magnetic saturation. Since the coercive force proper
to each body resists the recomposition of the magnetic fluids, it
follows that the quantity of magnetism which a bar or needle is
capable of retaining permanently, will be proportional to this
coercive force. If, by the continuance of the process of magne-
tization and the influence of very powerful magnets, a greater
development of magnetism be produced than corresponds with
the coercive force, the fluids will be recomposed by the mutual
attraction until the coercive force resists any further recom-
position. The tendency of the magnetic fluids to unite being
then in equilibrium with the coercive force, no further recom-
position will take place, and the bar will retain its magnetism
undiminished.
When the bar is in this state, it is said to be magnetized to
saturation.
It has been generally supposed that when bars are surcharged
with magnetism they lose their surplus and fall suddenly to the
point of saturation, the recomposition of the fluids being in-
stantaneous.
M. Pouillet, however, has shown that this recomposition is
gradual, and after magnetization there is even in some cases a
reaction of the fluids which is attended with an increase instead
of a diminution of magnetism. He observes that it happens not
unfrequently that the magnetism is not brought to permanent
equilibrium with the coercive force for several months.
1688. Limit of magnetic force. It must not be supposed
that by the continuance of the processes of magnetization which
have been described above, an indefinite development of mag-
netism can be produced. "When the resistance produced by the
coercive force to the decomposition of the fluids becomes equal
to the decomposing power of the magnetizing bars, all further
increase of magnetism will cease.
184 MAGNETISM.
It is remarkable that if a bar which has been magnetized to
saturation by magnets of a certain power be afterwards sub-
mitted to the process of magnetization by magnets of inferior
power, it will lose the excess of its magnetism and fall to the
point of saturation corresponding to the magnets of inferior
power.
1689. Influence of the temper of the bar on the coercive force.
Let a bar of steel tempered at a bright red heat be magnetized
to saturation, and let its magnetic intensity be ascertained by
the vibration of a needle submitted to its attraction. Let its
temper be then brought by annealing to that of a straw colour,
and being again magnetized to saturation, let its magnetic
intensity be ascertained. In like manner, let its magnetic in-
tensities at each temper from the highest to the lowest be
observed. It will be found that the bars which have the highest
temper have the greatest coercive force, and therefore admit of
the greatest development of magnetism, but even at the lowest
tempers they are still, when magnetized to saturation, susceptible
of a considerable magnetic force.
- Although highly tempered steel has this advantage of re-
ceiving magnetism of great intensity, it is, on the other hand,
subject to the inconvenience of extreme brittleness, and conse-
quent liability to fracture. A slight reduction of temper causes
but a small diminution in its charge of magnetism, and renders
it much less liable to fracture.
1690. Effects of terrestrial magnetism on bars. It has been
already shown that the inductive power of terrestrial magnetism
is capable of developing magnetism in iron bars, and, under
certain conditions, of either augmenting, diminishing, or even
obliterating the magnetic force of bars already magnetized. In
the preservation of artificial magnets, therefore, this influence
must be taken into account.
According to what has been explained, it appears that if
a magnetic bar be placed in the direction of the dipping-needle
in this hemisphere, the earth's magnetism will have a tendency
to attract the austral magnetism downwards, and to repel the
boreal upwards. If, therefore, the austral pole of the bar be
presented downwards, this tendency will preserve or even
augment the magnetic intensity of the bar. But if the magnet
be in the inverted position, having the boreal pole downwards,
opposite effects will ensue. The austral fluid being attracted
MAGNETIZATION. 185
downwards, and the boreal driven upwards, a recombination of
the fluids will take place, which will be partial or complete ac-
cording to the coercive force of the bar. If the coercive force
of the bar exceed the influence of terrestrial magnetism, the
effect will be only to diminish the magnetic intensity of the
bar ; but if not, the effect will be the recomposition of the
magnetic force and the reduction of the bar to its natural
state ; but if the bar be still held in the same position, the
continued effect of the terrestrial magnet will be again to
decompose the natural magnetism of the bar, driving the austral
fluid downwards and repelling the boreal upwards, and thus
reproducing the magnetism of the bar with reversed polarity.
1691. Means of preserving magnetic bars from these effects
by armatures or keepers. It is evident, therefore, that when
it is desired to preserve magnetized bars unaltered, they must
be protected from these effects of the terrestrial magnet, and
the manner of accomplishing this is by means of ARMATURES or
KEEPERS.
When the magnetic bars to be preserved are straight bars of
equal length, they are laid parallel to each other, their ends
. corresponding, but with poles reversed,
^^| * so that the austral pole of each shall be
_|[ in juxtaposition with the boreal pole of
A the other, as represented \njig. 473.
A bar of soft iron called the keeper
is applied as represented at K, in contact with the two opposite
poles A and B', and another similar bar E 7 in contact with A' and
B, so as to complete the parallelogram. In this arrangement the
action of the poles A and B' upon the keeper K is to decompose
its magnetism, driving the austral fluid towards B' and the
boreal fluid towards A'. The boreal fluid of K exercises a
reciprocal attraction upon the austral fluid of A, and the austral
fluid of K exercises a corresponding attraction upon the boreal
fluid of B'. Like effects are produced by the keeper K' at the
opposite poles A' and B.
In this manner the decomposition of the fluids in the two
bars A B and A' B' is maintained by the action of the keepers
K and K'.
If the magnet have the horse-shoe form, this object is attained
by a single keeper, as represented \njig. 471.
The keeper K is usually formed with a round edge, so as to
186 MAGNETISM.
touch the magnet only in a line, and not in a surface, as it
would do if its edge were flat. It results from experience that
a keeper kept in contact in this manner for a certain length of
time with a magnet, augments the attractive force, and appears
to feed, as it were, the magnetism.
1692. Magnetism may be preserved by terrestrial induction.
Magnetic needles, suspended freely so as to obey the attrac-
tion of terrestrial magnetism, do not admit of being thus pro-
tected by keepers ; but neither do they require it, for the austral
pole of the needle being always directed towards the boreal pole
of the earth, and the boreal pole of the needle towards the
austral pole of the earth, the terrestrial magnet itself plays the
part of the keeper, continually attracting each fluid towards its
proper pole of the magnet, and thus maintaining its magnetic in-
tensity.
1693. Compound magnets. Compound magnets are formed
by the combination of several bar magnets of similar form and
equal magnitude, laid one upon another, their corresponding
poles being placed in juxtaposition.
A compound horse-shoe magnet, such as that represented in
Jig. 471., is formed in like manner of magnetized bars, superposed
on each other and similar in form, their corresponding poles
being placed in juxtaposition. These bars, whether straight or
in the horse-shoe form, are separately magnetized before being
combined by the methods already explained.
In the case of the horse-shoe magnet a ring is attached to the
keeper, and another to the top of the horse-shoe, /#. 471., so that
the magnet being suspended from a fixed point, weights may be
attached to the keeper tending to separate it from the magnet.
In this way horse-shoe magnets often support from ten to
twenty times their own weight.
1694. Magnetized tracings on a steel plate. If the pole of a
magnet be applied to a plate of steel of about one-tenth of an
inch thick and of any superficial magnitude, such as a square
foot, and be moved slowly upon it, tracing any proposed figure,
the line traced upon the steel plate will be rendered magnetic,
as will be indicated by sprinkling steel filings upon the plate.
They will adhere to those points over which the magnet has
been passed, and will assume the form of the figure traced upon
the plate.
169.5. Influence of heat on magnetic bars. The influence of
MAGNETIZATION. 187
heat upon magnetism, which was noticed at a very early period
in the progress of magnetic discovery, has lately been the subject
of a series of experimental researches by M. Kupffer, from which
it appears that a magnetic bar when raised to a red heat does
not lose its magnetism suddenly at that temperature, but parts
with it by slow degrees as its temperature is raised. This
curious fact was ascertained by testing the magnetism of the bar,
by the means explained in (1668), at different temperatures,
when it was found that at different degrees of heat it produced
different rates of oscillation of the test needle.
It was also ascertained that, in order to deprive a magnetic
bar of all its magnetism when raised to a given temperature, a
certain length of time was necessary.
Thus a magnetic bar plunged in boiling water, and retained
there for ten minutes, lost only a portion of its magnetism, and
after being withdrawn and again plunged in the water for some
length of time, it lost an additional portion of its attractive force ;
and by continuing in the same manner its immersion for the
same interval, its magnetic force was gradually diminished, a
part still, however, remaining after seven or eight such im-
mersions.
A magnetic bar, when raised to a red heat, not only loses its
magnetism, but it becomes as incapable of receiving magnetism
from any of the usual processes of magnetization, as would be
any substance the most incapable of magnetism.
Astatic needle. All magnets freely suspended being subject to
the influence of terrestrial magnetism, the effects produced upon
them by other causes are necessarily compounded with those of
the earth. Thus, if a magnetic needle be exposed to the influence
of any physical agent, which, acting independently upon it, would
cause its north pole to be directed to the east, the pole, being at
the same time affected by the magnetism of the earth, which
acting alone upon it would cause it to be directed to the north,
will take the intermediate direction of the north-east. When,
in such cases, the exact effect of the earth's magnetism on the
direction of the needle is known, and the compound effect is
observed, the effect of the physical agent by which the needle is
disturbed may generally be eliminated and ascertained. It is,
nevertheless, often necessary to submit a magnetic needle to
experiments, which require that it should be rendered indepen-
dent of the directive influence of the earth's magnetism, and
188 MAGNETISM.
expedients have accordingly been invented for accomplishing
this.
A needle which is not affected by the earth's magnetism is
called an ASTATIC NEEDLE.
A magnetic needle freely suspended over a fixed bar magnet
will have a tendency, as already explained, to take such a
position that its magnetic axis shall be parallel to that of the
fixed magnet, the poles being reversed. Now if the fixed
magnet be placed with its magnetic axis coinciding with the
magnetic meridian, the poles being reversed with relation to
those of the earth, its directive influence on the needle will be
exactly contrary to that of the earth. "While the earth has a
tendency to turn the austral pole of the needle to the north, the
magnet has a tendency to turn it to the south. If these ten-
dencies be exactly equal, the needle will totally lose its polarity,
and will rest indifferently in any direction in which it may be
placed.
As the influence of the bar magnet on the needle increases
as its distance from it is diminished, and vice versa, it is evident
that it may always be placed at such a distance from it, that its
directive force shall be exactly equal to that of the earth.
In this case, the needle will be rendered astatic.
A needle may also be rendered astatic by connecting with it
a second needle, having its magnetic axis parallel and its poles
reversed, both needles having equal magnetic forces. The com-
pound needle thus formed being freely suspended, the directive
power of the earth on the one will be equal and contrary to its
directive power on the other, and it will consequently rest in-
definitely in any direction.
It is in general, however, almost impracticable to ensure the
exact equality of the magnetism of two needles thus combined.
If one exceed the other, as is generally the case, the compound
will obey a feeble directive force equal to the difference of their
magnetism.
189
BOOK THE THIRD.
ELECTRICITY.
CHAP. I.
ELECTRICAL ATTRACTIONS AND REPULSIONS.
1696. Electrical effects. If a glass tube being well dried be
briskly rubbed with a dry woollen cloth, the following effects may
be produced.
The tube, being presented to certain light substances, such as
feathers, metallic leaf, bits of light paper, filings of cork or pith
of elder, will attract them.
If the friction take place in the dark, a bluish light will be
seen to follow the motions of the cloth.
If the glass be presented to a metallic body, or to the knuckle
of the finger, a luminous spark accompanied by a sharp cracking
sound, will pass between the glass and the finger.
On bringing the glass near the skin, a sensation will be pro-
duced like that which is felt when we touch a cobweb.
The same effects will be produced by the cloth with which
the glass is rubbed as by the glass itself.
An extensive class of bodies submitted to the same kind of
mutual friction produce similar effects.
1697. Origin of the name Electricity. The physical agency
from which these and like phenomena arise has been called
ELECTRICITY, from the Greek word j/Xe/crpov (electron), signifying
amber, that substance having been the first in which the pro-
perty was observed by the ancients.
To study the laws which govern electrical forces, let an ap-
paratus be provided, called an electric pendulum, consisting of
a small ball B', fig. 474., about the tenth of an inch in diameter,
turned from the pith of elder, and suspended, as represented in
the figure, by a fine silken thread attached to a convenient
stand.
190
ELECTRICITY.
If the glass tube, after being rubbed as above described, be
brought into contact successively with two pith balls thus
suspended, and then separated from them, a property will be
imparted to the balls in virtue of which they will be repelled
by the glass tube when it is brought nearer them, and they will
in like manner repel each other when brought into proximity.
Thus, if the glass tube s, fig. 474., be brought nearer the ball
B', the ball will depart from its vertical position, and will incline
itself from the tube in the position B.
Fig. 474.
Fig. 475.
If the two balls, being previously brought into contact with
the tube, be placed near each other, as \\\fig. 475., they will in-
cline from each other, departing from the vertical positions B
and B', and taking the positions b and b\
1698. The electric fluid. These effects are explained by
the supposition that a subtle and imponderable fluid has been
developed upon the glass tube which is self-repulsive ; that by
touching the balls, a portion of this fluid has been imparted to
them, which is diffused over their surface, and which, for reasons
that will hereafter appear, cannot escape by the thread of sus-
pension ; that the fluid remaining on the glass tube repels this
fluid diffused on the balls, and therefore repels the balls them-
selves which are invested by the fluid; and, in fine, that the fluid
diffused on the one ball repels and is repelled by the fluid dif-
fused on the other ball, and that the balls being covered by the
fluid are reciprocally repelled.
A vast body of phenomena, the most important of which
will be described in the following chapters, have converted this
supposition into a certainty, accepted by all scientific authorities.
ELECTRICAL ATTRACTIONS AXD REPULSIONS. 191
The fluid producing these effects is called the ELECTRIC
FLUID.
1699. Positive and negative electricity. If the hand which
holds the cloth be covered with a dry silk glove, the cloth, after
the friction with the glass, will exhibit the same effects as above
described. If it be brought into contact with the balls and se-
parated from them, it will repel them, and the balls themselves
will repel each other.
It appears, therefore, that by the friction the electric fluid is at
the same time developed on the glass and on the cloth.
If after friction the glass be brought into contact with one
ball Bjjtfy. 475., and the cloth with the other B', other effects
will be observed. The glass, when presented to the ball B', will
attract it, and the cloth presented to the ball B will attract it.
The balls, when brought near each other, will now exhibit
mutual attraction instead of repulsion.
It follows, therefore, that the electric fluid developed by friction
on the cloth differs from that developed on the glass, inasmuch
as instead of being characterized by reciprocal repulsion they
are mutually attractive.
The supposition, therefore, which has been briefly stated above
will require modification.
1700. Hypothesis of a single electric fluid. According to
some, the effect of the friction is to deprive the cloth of a portion
of its natural charge of electricity, and to surcharge the glass
with what the cloth loses ; and accordingly the glass is said to be
positively, and the cloth negatively electrified.
On this supposition, bodies in their natural state have always
a certain charge or dose of the electric fluid, the repulsive effect
of which is neutralized by the attraction exercised by the body
upon it. The electric equilibrium which constitutes this natural
state may be deranged, either by overcharging the body with
the electric fluid, or by withdrawing from it a part of what it
naturally possesses. In the former case, the repulsion of the
surplus charge not being neutralized by the attraction of the
body takes effect. In the latter case, the attraction of the body
being more than equal to the repulsion of the charge of electri-
city upon it, will take effect upon any electricity which may
come within the sphere of its action.
This, which is called the SINGLE FLUID THEORY, was the
hypothesis adopted by FRANKLIN, and after him by most English
192 ELECTRICITY.
electricians until recently, when phenomena were developed
in experimental researches, of which it failed to afford a satis-
factory explanation ; and, accordingly, the hypothesis of two
fluids, which was generally received on the Continent, has
found more favour also in England.
1701. Hypothesis of two fluids. According to the THEOUY
OF TWO FLUIDS, hodies in their natural or unelectrified state
are charged with a compound electric fluid consisting of two
constituent, called by some the VITREOUS and RESINOUS, and hy
others the POSITIVE and NEGATIVE, fluids. These fluids are each
self-repulsive, but are mutually attractive. When they pervade
a body in equal quantity, their mutual attractions, neutralizing
each other, keep them in repose, like equal weights suspended
from the arms of a balance. When either is in excess, the
body is positively or negatively electrified, as the case may be,
the attraction or repulsion of the surplus of the redundant fluid
being effective.
1702. Results of scientific research independent of these hy-
potheses. Since the language in which the phenomena of
electricity are described and explained must necessarily have
relation to these hypotheses, it has been necessary in the first
instance thus briefly to state them. It must, however, be re-
membered, that the question of the validity of these theories
does not affect the conclusions which will be deduced from ob-
servation, the proper use of hypotheses being limited to their
convenience in supplying a nomenclature to the science and
in grouping and classifying the phenomena.
1703. Hypothesis of two fluids preferred. The hypothesis
of two fluids supplying, on the whole, the most complete and
satisfactory explanation of the phenomena, is that which we
shall here generally adopt; but we shall retain the terms POSI-
TIVE and NEGATIVE electricity, which, though they are derived
originally from the theory of a single fluid, are generally adopted
by scientific writers who adhere to the other hypothesis.
1704. Explanation of the above effects produced by the pith
balls. We are then to consider that when the glass tube and
woollen cloth are submitted to mutual friction, their natural
electricities are decomposed, the positive fluid passing to the
glass, and the negative to the cloth. The glass thus becomes
surcharged with positive, and the cloth with negative, electricity.
The pith ball K,fig. 475., touched by the glass, receives the
ELECTRICAL ATTRACTIONS AND REPULSIONS. 193
positive fluid from it, and the pith ball B' touched by the cloth
receives the negative fluid from it. The ball B therefore be-
comes positively, and the ball B' negatively, electrified by
contact.
Since the contrary electricities are mutually attractive, the
balls B and B' in this case attract each other ; and, since like elec-
tricities are mutually repulsive, the glass rod repels the ball B,
and the cloth repels the ball B'.
17C5. Electricity developed by various bodies. A numerous
class of bodies, when submitted to friction, produce effects
similar to those described in the case of glass and woollen cloth.
If a stick of resin or sealing-wax be rubbed by a woollen
cloth, like effects will follow : but, in this case, the electricity
of the wax or resin will be contrary to that of the glass, as may
be rendered manifest by the pith balls. If B be electrified by
contact with the glass, and B' by contact with the resin or wax,
they will attract each other, exactly as they did when B' was
electrified by contact with the cloth rubbed upon the glass.
It appears, therefore, that while glass is positively, resin is
negatively, electrified by the friction of woollen cloth.
1706. Origin of the terms vitreous and resinous fluids. It
was this circumstance which gave the name of VITREOUS elec-
tricity to the POSITIVE, and RESINOUS electricity to the NEGATIVE
fluid. This nomenclature is, however, faulty ; inasmuch as
there are certain substances by the friction of which glass will
be negatively electrified, and others by which resin will be
positively electrified.
When a woollen cloth is rubbed on resin or wax which, as
has been stated, it electrifies negatively, it is itself electrified
positively ; since the natural fluid being decomposed by the
friction, and the negative element going to the resin, the posi-
tive element must be developed on the cloth.
Thus it appears that the woollen cloth may be electrified by
friction either positively or negatively, according as it is rubbed
upon resin or upon glass.
1707. No certain test to determine which of the bodies sub-
mitted to friction receives positive, and which negative, elec-
tricity. In general, when any two bodies are rubbed together,
electricity is developed, one of them being charged with the
positive, and the other with the negative, fluid. A great
number of experimental researches have from time to time been
n. K
194 ELECTRICITY.
undertaken, with a view to the discovery of the physical law
which determines the distribution of the constituent electric
fluids in such cases between the two bodies, so that it might in
all cases be certainly known which of the two would be posi-
tively, and which negatively, electrified. These inquiries,
however, have hitherto been attended with no clear or certain
general consequences.
It has been observed, that hardness of structure is generally
attended with a predisposition to receive positive electricity.
Thus, the diamond, submitted to friction with other stones or
with glass, becomes positively electrified. Sulphur, when
rubbed with amber, becomes negatively electrified, the amber
being consequently positive ; but if the amber be rubbed upon
glass or diamond, it will be negative.
It is also observed that when heat is developed by the friction
of two bodies, that which takes most heat is negatively, and
the other positively, electrified.
In short, the decomposition of the electricity and its distri-
bution between the rubbing bodies is governed by conditions
infinitely various and complicated.
An elevation of temperature will frequently predispose a
body to take negative, which would otherwise take positive,
electricity. An increase of polish of the surface produces a
predisposition for the positive fluid. The colour, the molecular
arrangement, the direction of the fibres in a textile substance,
the direction in which the friction takes place, the greater or
less pressure used in producing it, all affect more or less in par-
ticular cases the interchange of the fluids and the relative elec-
tricities of the bodies. Thus, a black silk ribbon rubbed on
one of white silk takes negative electricity. If two pieces of
the same ribbon be rubbed transversely, one being stationary
and the other moved upon it, the former takes positive, the
latter negative, electricity. ./Epinus found that "copper and
sulphur rubbed together, and two similar plates of glass, evolved
electricity, but that the interchange of the fluids was not always
the same. There are substances, disthene, for example, which,
when submitted to friction, develope positive electricity at some
parts, and negative at other parts of their surface, although
their structure and the state of the surface be perfectly uniform.
1 708. Classification of positive and negative substances.
Of all known substances, a cat's fur is the most susceptible of
ELECTRICAL ATTRACTIONS AND REPULSIONS. 195
positive, and probably sulphur of negative electricity. Between
these extreme substances others might be so arranged, that any
substance in the list being rubbed upon any other, that which
holds the higher place will be positively, and that which holds
the lower place negatively, electrified. Various lists of this
kind have been proposed, one of which is as follows :
1. Fur of a cat.
2. Polished glass.
3. WooUen cloth.
4. Feathers.
5. Wood.
Pfaff gives the following :
1. Fur of a cat.
2. Diamond.
3. Fur of a dog.
4. Tourmaline.
5. Glass.
6. Wool. '
7. Paper.
Ritter proposes the following :
1. Diamond.
2. Zinc.
3. Glass.
4. Copper.
5. Wool.
6. P
6. aper.
7. Silk.
8. Gum lac.
9. Eough glass.
8. White silk.
9. Black silk.
10. Sealing-wax.
11. Colophon.
12. Amber.
13. Sulphur.
6. Silver.
7. Black silk.
8. Grey silk.
9. Grey manganeseous earth.
10. Sulphur.
1 709. Method of producing electricity by glass and silk with
amalgam. Experience has proved that the most efficient
means of developing electricity in great quantity and intensity
is by the friction of glass upon a surface of silk or leather
smeared with an amalgam composed of tin, zinc, and mercury
mixed with some unctuous matter. Two parts of tin, three of
zinc, and four of mercury, answer very well. Let some fine
chalk be sprinkled on the surface of a wooden cup, into which
the mercury should be poured hot. Let the zinc and tin melted
together be then poured in, and the box being closed and well
shaken, the amalgam may be allowed to cool. It is then finely
pulverized in a mortar, and being mixed with unctuous matter
may be applied to the rubber.
196 ELECTRICITY.
CHAP. II.
CONDUCTION.
1710. Conducting power. Bodies differ from each other in a
striking manner in the freedom with which the electric fluid
moves upon them. If the electric fluid be imparted to a certain
portion of the surface of glass or wax, it will be confined strictly
to that portion of the surface which originally receives it, by
contact with the source of electricity ; but if it be in like manner
imparted to a portion of the surface of a metallic body, it will
instantaneously diffuse itself uniformly over the entire extent
of such metallic surface, exactly as water would spread itself
uniformly over a level surface on which it is poured.
1711. Conductors and non-conductors. The former class
of bodies, which do not give free motion to the electric fluid on
their surface, are called NON-CONDUCTORS; and the latter, on
which unlimited freedom of motion prevails, are called CON-
DUCTORS.
1712. Classification of conductors according to the degrees of
their conducting power. Of all bodies the most perfect conductors
are the metals. These bodies transmit electricity instantaneously,
and without any sensible obstruction, provided their dimensions
are not too small in relation to the quantity of electricity im-
parted to them.
The bodies named in the following series possess the con-
ducting power in different degrees in the order in which they
stand, the most perfect conductor being first, and the most
perfect non-conductor last in the list. The black line divides
the most imperfect conductors from the most imperfect non-
conductors, but it must be observed that the position of this line
is arbitrary, the exact relative position of many of the bodies
composing the series not being certainly ascertained. The
series, however, will be useful as indicating generally the bodies
which have the conducting and non-conducting property in a
greater or less degree.
All the metals.
Well-burnt charcoal.
Plumbago.
Concentrated acids.
Powdered charcoal.
Dilute acids.
Saline solutions.
Metallic ores.
CONDUCTION.
197
Animal fluids.
Sea-water.
Spring-water.
Rain-water.
Ice above 13 Fahrenheit.
Snow.
Living vegetables.
Living animals.
Flame.
Smoke,
Steam.
Salts soluble in water.
Rarefied air.
Vapoiir of alcohol.
Vapour of ether.
Moist earth and stones.
Powdered glass.
Flowers of sulphur.
Caoutchouc.
Camphor.
Some siliceous and argillaceous
stones.
Dry marble.
Porcelain.
Dry vegetable bodies.
Baked wood.
Dry gases and air.
Leather.
Parchment.
Dry paper.
Feathers.
Hair.
Wool.
Dyed silk.
Bleached silk,
Eaw silk.
Transparent gems.
Diamond.
Mica.
All vitrifactions.
Glass.
Jet.
Wax.
Sulphur.
Resins.
Amber.
Gum-lac.
Dry metallic oxides.
Oils, the heaviest the best.
Ashes of vegetable bodies.
Ashes of animal bodies.
Many transparent crystals, dry.
Ice below 13 Fahrenheit.
Phosphorus.
Lime.
Dry chalk.
Native carbonate of barytes.
Lycopodium.
1713. Insulators. Good non-conductors are also called
INSULATORS, because when any body suspended by a non-con-
ducting thread, or supported on a non-conducting pillar, is
charged with electricity, such charge will be retained, since it
cannot escape by the thread or pillar, which refuses a passage
to it in virtue of its non-conducting quality.
Thus, a globe of metal supported on a glass pillar or suspended
by a silken cord being charged with electricity, will retain the
charge ; whereas, if it were supported on a metallic pillar, or
suspended by a metallic wire, the electricity would pass away
by its free motion over the surface of the pillar or the wire.
1714. Insulating stools. Stools formed with glass legs are
called INSULATING STOOLS, because any body charged with
electricity and placed upon them will retain its electric charge.
1715. Electrics and non-electrics obsolete terms. Conducting
bodies were formerly called NON-ELECTRICS, and non-conducting
bodies were called ELECTRICS, from the supposition that the latter
were capable of being electrified by friction, but the former not.
198 ELECTRICITY.
The incapability of conductors to be electrified by friction
was, however, afterwards shown to be only apparent, and
accordingly the use of these terms has been discontinued. .
If a rod of metal be submitted to friction, the electricity
evolved is first diffused over its entire surface in consequence
of its conducting property, and thence it escapes by the hand
of the operator which holds it, and which, though not as per-
fect a conductor as the metal, is sufficiently so to carry off
the electricity, so as to leave no sensible trace of it on the
metal.
But if the metal rod be suspended by a dry silken thread
(which is a good non-conductor), or be supported on a pillar
of glass, and then be struck several times with the fur of a cat,
it will be found to be negatively electrified, the fur which
strikes it being positively electrified.
1716. Two persons reciprocally charged with contrary elec-
tricity when placed on insulating stools. In like manner, two
persons standing on insulating stools, if one strike the other two
or three times with the fur of a cat, he that strikes will have
his body positively, and he that is struck negatively, electrified,
as may be ascertained by the method already explained, of pre-
senting to them successively the pith ball B, fig. 474., previously
charged with positive electricity. It will be repelled by the
body of him that strikes, and attracted by that of him who is
struck.
But if the same experiment be made without placing the two
persons on insulating stools, the same effects will not ensue,
because, although the electricities are developed as before by
the action of the fur, it immediately escapes through the feet
to the ground.
1717. The atmosphere a non-conductor. Atmospheric air
must manifestly belong to the class of non-conductors, for if it
gave a free passage to electricity, the electrical effects excited
on the surface of any body surrounded with it would soon pass
away; and no electrical phenomena of a permanent nature
could be produced, unless the bodies were removed from the
contact of the air. It is found, however, that resin and glass,
when excited by friction, retain their electricity for a considerable
time.
1718. Rarefied air a conductor. Air, however, when rarefied,
loses in a great degree its non-conducting property; and an
CONDUCTION. 199
electrified body soon loses its electricity if placed in the
exhausted receiver of an air-pump.
The electric fluid may therefore be considered as forming
a coating upon the surface of electrified bodies, and as being
held upon them by the tension or pressure of the surrounding
air.
1719. Use of the silk string which suspends pith balls. In
the experiments described in (1697) et seq. with the pith balls,
the silken string by which they are suspended acts as an in-
sulator. The pith of elder being a conductor, the electric fluid
is diffused over the ball ; but the silk being a non-conductor, it
cannot escape. If the ball were suspended by a metallic wire
attached to a stand composed of any conducting matter, the
electricity would escape, and the effects described would not
ensue. But if the metallic wire were attached to a glass rod or
other non-conductor, the same effects would be produced. In
that case the electricity would be diffused over the wire as well
as over the ball.
1720. Water a conductor. Water, whether in the liquid
or vaporous form, is a conductor, though of an order greatly
inferior to the metals. This fact is of great importance in
electrical phenomena. The atmosphere contains suspended in
it always more or less aqueous vapour, the presence of which
impairs its non-conducting property. Hence, electrical experi-
ments always succeed best in cold and dry weather.
Hence it appears why the most perfect non-conductors lose
their virtue if their surface be moist, the electricity passing
by the conducting power of the moisture.
1721. Insulators must be kept dry. This circumstance also
shows why it is necessary to dry previously the bodies on which
it is desired to develope electricity by friction.
It will be apparent from what has been explained, that it
would be more correct to designate bodies as good and bad
conductors in various degrees, than as conductors and non-con-
ductors. There exists no body which, strictly speaking, is
either an absolute conductor or absolute non-conductor.
1722. No certain test to distinguish conductors from non-
conductors. No relation has been discovered between the
physical conditions which determine the conduction of light and
heat, and those which determine the conduction of electricity.
Electricity is transmitted, not like light and heat, through the
200 ELECTRICITY.
interior dimensions of bodies, but only on their surfaces. Glass,
which is an almost perfect conductor of light, is a non-conductor
of heat and electricity. Sealing-wax, which is a non-conductor
of electricity, is also a non-conductor of light and heat. The
metals, on the other hand, are conductors of heat and electricity,
but are non-conductors of light. Water is a conductor of elec-
tricity and light, but a non-conductor of heat.
1723. Conducting power variously affected by temperature.
The conducting power of bodies is affected in different ways
by their temperature. In the metals it is diminished by eleva-
tion of temperature ; but in all other bodies, and especially in
liquids, it is augmented. Some substances, which are non-con-
ductors in the solid state, become conductors when fused.
Sir H. Davy found that glass raised to a red heat became a
conductor ; and that sealing-wax, pitch, amber, shell-lac, sul-
phur, and wax became conductors when liquefied by heat.
The manner in which electricity is communicated from one
body to another depends on the conducting property of the
body imparting and the body receiving it.
1724. Effects produced by touching an electrified body by a
conductor which is not insulated. If the surface of a non-
conducting body, glass, for example, be charged with electricity,
and be touched over a certain space, as a square inch, by a con-
ducting body which is not insulated, the electricity which is
diffused on the surface of contact will pass away by the con-
ductor, but no other part of the electricity with which the body
is charged will escape. A patch of the surface corresponding
with the magnitude of the conductor will alone be stripped of
its electricity.
The non-conducting property of the body will prevent the
electricity which is diffused over the remainder of its surface
from flowing into the space thus drained of the fluid by the
conductor.
But if the body thus charged with electricity, and touched
by a conductor not insulated, be a conductor, the effects pro-
duced will be very different. In that case, the electricity which
covers the surface of contact will first pass off; but the moment
the surface of contact is thus drained of the fluid which
covered it, the fluid diffused on the surrounding surface will
flow in and likewise pass off, and thus all the fluid diffused over
the entire surface of the body will rush to the surface of con-
CONDUCTION. 210
tact and escape. These effects, though, strictly speaking, suc-
cessive, will be practically instantaneous ; the time which
elapses between the escape of the fluid which originally covered
the surface of contact, and that which rushes from the most
remote parts to the surface of contact, being inappreciable.
1725. Effect produced when the touching conductor is in-
sulated. If a conducting body which is insulated and charged
with electricity, be brought into contact with another conduct-
ing body, which is also insulated and in its natural state, the
electricity will diffuse itself over the surfaces of both conductors
in proportion to their relative magnitudes.
If s express the superficial magnitude of an insulated con-
ducting body, E the quantity of electricity with which it is
charged, and s' the superficial magnitude of the other insulated
conductor with which it is brought into contact, the charge E
will, after contact, be shared between the two conductors in the
ratio of s to s' ; so that
E x ,= the charge retained by s,
s -f- s
s'
E x ; >= the charge received by s'.
s-f-s
1726. Why the earth is called the common reservoir. If
the second conductor s' be the globe of the earth, s' will bear
a proportion to s which, practically speaking, is infinite ; and
consequently the quantity of electricity remaining on s, ex-
pressed by
EX
s + s"
will be nothing. Hence the body s loses its entire charge
when put in conducting communication with the ground.
An electrified body being a conductor, is therefore reduced
to its natural state when put into electric communication with
the ground, and the earth has been therefore called the common
reservoir, to which all electricity has a tendency to escape, and
to which it does in fact always escape, unless its passage is in-
tercepted by non-conductors.
1727. Electricity passes by preference on the best conductors.
If several different conductors be simultaneously placed in
contact with an insulated electrified conductor so as to form a
K 5
202 ELECTRICITY.
communication between it and the ground, the electricity will
always escape by the best conductor. Thus, if a metallic
chain or wire be held in the hand, one end touching the ground
and the other being brought into contact with the conductor,
no part of the electricity will pass into the hand, the chain
being a better conductor than the flesh of the hand. But if,
while one end of the chain touch the conductor, the other be
separated from the ground, then the electricity will pass into
the hand, and will be rendered sensible by a convulsive shock.
CHAP. III.
INDUCTION.
1728. Action of electricity at a distance. If a body A
charged with electricity of either kind be brought into prox-
imity with another body B in its natural state, the fluid with
which A is surcharged will act by attraction and repulsion on
the two constituents of the natural electricity of B, attracting
that of the contrary, and repelling that of the same kind. This
effect is precisely similar to that produced on the natural
magnetic fluid in a piece of iron when the pole of a magnet is
presented to it.
If the body B in this case be a non-conductor, the electric fluid
having no free mobility upon its surface, its decomposition will
be resisted, and the body B will continue in its natural state not-
withstanding the attraction and repulsion exercised by A on the
constituents of its natural electricity. But if B be a conductor,
the fluids having freedom of motion on its surface, the fluid
similar to that with which B is charged will be repelled to the
side most distant from B, and the contrary fluid will be attracted
to the side next to B. Between these regions a neutral line will
separate those parts of the body B over which the two opposite
fluids are respectively diffused.
1729. Induction defined. This action of an electrified body
exerted at a distance upon the electricity of another body is called
INDUCTION, and is evidently analogous to that which produces
similar phenomena in the magnetic bodies (1630). j
INDUCTION. 203
1730. Experimental exhibition of its effects. To render
it experimentally manifest, let s and s'ifig. 476., be two metallic
Fig. 476.
balls supported on glass pillars ; and let A A." be a metallic cylin-
der similarly mounted, whose length is ten or twelve times
its diameter, and whose ends are rounded into hemispheres.
Let s be strongly charged with positive, and s' with negative
electricity, the cylinder A A' being in its natural state.
Let the balls s and s' be placed near the ends of the cylinder
A A', their centres being in line with its axis, as represented in
the figure. The positive electricity of s will now attract the
negative, and repel the positive constituent of the natural elec-
tricity of A A', so as to separate them, drawing the negative fluid
towards the end A, and repelling the positive fluid towards the
end A'. The negative electricity of s' will produce a like effect,
repelling the negative electricity of A A' towards A, and drawing
the positive towards A'.
Since the cylinder A A' is a conductor, and therefore the fluids
have freedom of motion on its surface, this decomposition will
take effect, and the half o A of the cylinder next to s will be
charged with negative, and the half o A' next to s' with positive
electricity.
That such is in fact the condition of A A' may be proved by
presenting a pith ball (1697) pendulum charged with positive
electricity to either half of the cylinder. When presented to
O A' it will be repelled, and when presented to o A it will be
attracted.
If the two balls s s' be gradually removed to increased but
equal distances from the ends A and A', the recomposition of the
fluids will gradually take place ; and when the balls are altogether
K 6
Jo
204 ELECTRICITY.
removed the cylinder A A' will recover its natural state, the
fluids which had been separated by the action of the balls being
completely recombined by their mutual attraction.
Let a metallic ring n',fig. 477., be supported on a rod or hook
of glass n, and let two pith balls b b' be'jSus-
C J pended from it by fine wires, so that when hanging
vertically they shall be in contact. Let a ball of
'/j^ 3 / " metal r, strongly charged with positive electricity,
be placed over the ring n' at a distance of eight
or ten inches above it. The presence of this ball
will immediately cause the pith balls to repel
v each other, and they will diverge to increased
\ distances the nearer the ball r is brought to the
Fig7477 . Tm S n '- If tn e ball r be gradually raised to
greater distances from the ring, the balls b b'
will gradually approach each other, and will fall to their position
of rest vertically under the ring when the ball r is altogether
removed.
If the charge of electricity of the balls s and s', fig. 476, or of
the ball r,fig. 477., be gradually diminished, the same effect will
be produced as when the distance is gradually increased ; and,
in like manner, the gradual increase of the charge of electricity
will have the same effect as the gradual diminution of the dis-
tance from the conductor on which the action takes place.
If the ring n', the balls b b', and the connecting wire be first
feebly charged with negative electricity, and then submitted to
the inductive action of the ball r charged with positive elec-
tricity, placed, as before, above the ring, the following effects will
ensue. When the ball r approaches the ring, the balls b b', which
previously diverged, will gradually collapse until they come into
contact. As the ball r is brought still nearer to n', they will
again diverge, and will diverge more and more, the nearer the
ball r is brought to the ring.
These various effects are easily and simply explicable by the
action of the electricity of the ball r on that of the ring. When
it approaches the ring, the positive electricity with which it is
charged decomposes the natural electricities of the ring, repel-
ling the positive fluid towards the balls. This fluid combining
with the negative fluid with which the balls are charged, neu-
tralizes it, and reduces them to their natural state: while this
effect is gradually produced, the balls b b' lose their divergence
INDUCTION. 205
and collapse. But when the ball r is brought still nearer to the
ring, a more abundant decomposition of the natural fluid is pro-
duced, and the positive fluid repelled towards the balls is more
than enough to neutralize the negative fluid with which they are
charged ; and the positive fluid prevailing, the balls again diverge
with positive electricity.
These effects are aided by the attraction exerted by the posi-
tive electricity of the ball r on the negative fluid with which
the balls b b' are previously charged.
If the electrified ball, instead of being placed above the ring,
be placed at an equal distance below the balls b b', a series of
effects will be produced in the contrary order, which the
student will find no difficulty in analysing and explaining.
If the ball r be charged with negative electricity, it will
produce the same effects when presented above the ring as
when, being charged with positive electricity, it is presented
below it.
In all cases whatever, the conductor whose electrical state
has been changed by the proximity of an electrified body returns
to its primitive electrical condition when the disturbing action
of such body is removed ; and this return is either instantaneous
or gradual, according as the removal of the disturbing body is
instantaneous or gradual.
1731. Effects of sudden inductive action. It appears, there-
fore, that sudden and violent changes in the electrical condition
of a conducting body may take place, without either imparting
to or abstracting from such body any portion of electricity.
The electricity with which it is invested before the inductive
action commences, and after such action ceases, is exactly the
same ; nevertheless, the decomposition and recomposition of the
constituent fluids, and their motion more or less sudden over it
and through its dimensions, are productive often of mechanical
effects of a very remarkable kind. This is especially the case
with imperfect conductors, which offer more or less resistance
to the reunion of the fluids.
1732. Example in the case of a frog. Let a frog be sus-
pended by a metallic wire which is connected with an insulated
conductor, and let a metallic ball, strongly charged with positive
electricity, be brought under without, however, touching it.
The effects of induction already described will ensue. The
positive fluid will be repelled from the frog towards the insulated
206 ELECTRICITY.
conductor, and the negative fluid will be attracted towards it,
so that the body of the frog will be negatively electrified ; but
this taking place gradually as the electrified ball approaches, is
attended with no sensible mechanical effect.
If the electrified ball, however, be suddenly discharged, by
connecting it with the ground by a conductor, an instantaneous
revulsion of the electric fluids will take place between the body
of the frog and the insulated conductor with which it is con-
nected ; the positive fluid rushing from the conductor, and the
negative fluid from the frog, to recombine in virtue of their
mutual attraction. This sudden movement of the fluids will
be attended by a convulsive motion of the limbs of the frog.
1733. Inductive shock ofjhe human body. If a person stand
close to a large conductor strongly charged with electricity, he
will be sensible of a shock when this conductor is suddenly
discharged. This shock is in like manner produced by the
sudden recomposition of the fluids in the body of the patient, by
the previous inductive action of the conductor.
1734. Development of electricity by induction. A conductor
may be charged with electricity by an electrified body, though
the latter shall not lose any of its own electricity or impart any
to the conductor so electrified. For this purpose, let the con-
ductor to be electrified be supported on a glass pillar so as to
insulate it, and let it then be connected with the ground by a
metallic chain or wire. If it be desired to charge it with
positive electricity, let a body strongly charged with negative
electricity be brought close to it without touching it. On the
principles already explained, the negative electricity of the con-
ductor will be repelled to the ground through the chain or wire ;
and the positive electricity will, on the other hand, be attracted
from the ground to the conductor. Let the chain or wire be
then removed, and, afterwards, let the electrified body by whose
inductive action the effect is produced be removed. The con-
ductor will remain charged with positive electricity.
It may in like manner be charged with negative electricity,
by the inductive action of a body charged with positive elec-
tricity.
ELECTRICAL MACHINES.
207
CHAP. IV.
ELECTRICAL MACHINES.
1735. Parts of electrical machines. An electrical machine is
an apparatus by means of which electricity is developed and
accumulated in a convenient manner for the purposes of experi-
ment.
All electrical machines consist of three principal parts, the
rubber, the body on whose surface the electric fluid is evolved,
and one or more insulated conductors, to which this electricity
is transferred, and on which it is accumulated.
The rubber is a cushion stuffed with hair, bearing on its
surface some substance, which by friction will evolve electricity.
The body on which this friction is produced is glass, so shaped
and mounted as to be easily and rapidly moved against the
rubber with a continuous motion. This object is attained by
giving the glass the form either of a cylinder revolving on its
geometrical axis, or of a circular plate revolving in its own
plane on its centre.
The conductors are bodies having a metallic surface and a
great variety of shapes, and always mounted on insulating
pillars, or suspended by insulating cords.
1736. The common cylindrical machine. A hollow cylinder
of glass A B, Jig. 478., is supported in bearings at c, and made to
Fig. 478.
Fig. 479.
revolve by means of the wheels c and D connected by a band, a
handle R being attached to the greater wheel. The cushion H,
represented separately in fig. 479., is mounted on a glass pillar,
208 ELECTRICITY.
and pressed with a regulated force against the cylinder by means
of springs fixed behind it. A chain K i,, fig. 478., connects the
cushion with the ground. A flap of black silk equal in width
to the cushion covers it, and is carried over the cylinder, ter-
minating above the middle of the cylinder on the opposite side.
The conductor is a cylinder of thin brass M N, the ends of
which are parts of spheres greater than hemispheres. It is
supported by a glass pillar o P. To the end of the conductor
next the cylinder is attached a row of points represented
separately in fig. 480 , which are presented close to the surface
of the cylinder, but without touching it.
The extent of this row of points corre-
sponds with that of the rubber.
As the efficient performance of the
machine depends in a great degree on the
good insulation of the several parts, and as glass is peculiarly
liable to collect moisture on its surface which would impair its
insulating virtue, it is usual to cover the insulating pillars of the
rubber and conductor, and all that part of the cylinder which
lies outside the cushion and silk flap, with a coating of resinous
varnish, which, while its insulating property is more perfect
than that of glass, offers less attraction to moisture.
To explain the operation of the machine, let us suppose that
the cylinder is made to revolve by the handle K. Positive
electricity is developed upon the cylinder, and negative elec-
tricity on the cushion. The latter passes by the conducting
chain to the ground. The former is carried round under the
flap, on the surface of the glass, until it arrives at the points
projecting from the conductor. There it acts by induction
(1729) on the natural electricity of the conductor, attracting
the negative electricity to the points and repelling the positive
fluid. The negative electricity issuing from the points com-
bines with and neutralizes the positive fluid diffused on the
cylinder, the surface of which, after it passes the points, is
therefore restored to its natural state, so that when it arrives
again at the cushion it is prepared to receive by friction a fresh
charge of the positive fluid.
It is apparent, therefore, that the effect produced by the
operation of this machine is a continuous decomposition of the
natural electricity of the conductors, and an abstraction from it
of just so much negative fluid as compensates for that which
ELECTRICAL MACHINES.
209
escapes by the cushion and chain KL to the earth. The con-
ductor is thus as it were drained of its negative electricity by a
stream of that fluid, which flowing constantly from the points
passes to the cylinder, and thence by the cushion and chain to
the earth. The conductor is therefore left surcharged with
positive electricity.
1737. Naime's cylinder machine. This apparatus, which is
adapted to produce at pleasure either positive or negative elec-
tricny, is similar to the last, but has a second conductor MF,
fig. 481., in connection with the cushion. When it is desired
to collect positive electricity, the conductor MF is put in con-
nection with the ground, and the machine acts as that described
above. When it is desired to collect negative electricity, the
conductor M" B is put in connection with the ground, and the
conductor MF is insulated. In this case a stream of positive
electricity flows continually from MF through the cushion to
the cylinder, and thence by the conductor JI'B to the 'ground,
leaving the conductor MF charged with negative electricity.
Fig. 481.
1738. Common plate machine. This apparatus consists of
a circular plate of glass AB, fig. 482., mounted as represented in
the figure. It is embraced between two pair of cushions at E
and E', a corresponding width of the glass being covered by
a silk sheathing extending to F', where the points of the
conductors are presented. The handle being turned in the
direction of the arrow, and the cushions being connected by
conducting chains with the ground, positive electricity is de-
veloped on the glass, and neutralized as in the cylinder machine,
by the negative electricity received by induction from the con-
210 ELECTRICITY.
ductors, which consist of a long narrow cylinder, bent into a
form to adapt it to the plate. It is represented at MN, a branch
MO being carried parallel to the plate and bent into the form
MOPQ, so that the part PQ shall be presented close to the plate
under the edge of the silk flap. A similar branch of the con-
ductor extends on the other side, terminating just above the
edge of the lower silk flap.
The principle of this machine is similar in all respects to
that of the common cylinder machine. With the same weight
and bulk, the extent of rubbing surface, and consequently the
evolution of electricity, is much greater than in the cylinder
machines.
1739. Armstrong's hydro-electrical machine. A new species
of electric machine has resulted from the accidental observation
of an electric shock produced by the contact of a jet of high
pressure steam issuing from a boiler at Newcastle-on-Tyne in
1840. Mr. Armstrong of that place took up the inquiry, and
succeeded in contriving a machine for the production and
accumulation of the electricity by the agency of steam. Pro-
fessor Faraday investigated the theory of the apparatus, and
showed that the origin of the electrical development was the
friction of minute aqueous particles produced by the partial
condensation of the steam against the surface of the jet from
which the steam issued.
The hydro-electric machine has since been constructed in
various forms and dimensions.
Let a cylindrical boiler a, fig. 483., whose length is about
twice its diameter, be mounted on glass legs v, so as to be in a
state of insulation.
f is the furnace door, the furnace being a tube within the
boiler.
s is the safety-valve.
h is the water-gauge, a glass tube indicating the level of the
water in the boiler.
r a regulating valve, by which the escape of steam from the
boiler may be controlled.
t a tube into which the steam rushes as it escapes from r.
e three or more jet pipes, through which the steam passes
from t, and from the extremities of which it issues in a
series of parallel jets.
d a condensing box, the lower half of Avhich contains water
at the common temperature.
ELECTRICAL MACHINES.
211
g the chimney.
g' an escape pipe for the vapour generated in the condensing
box d.
b the conductor which takes from the steam the electricity
which issues with it from the jet pipes e.
k the knob of the conductor from which the electricity may
be received and collected for the purpose of experiment.
Fig. 483.
The jet pipes e traverse the middle of the condensing box d,
above the surface of the water contained in it. Meshes of
cotton thread surround these tubes within the box, the ends of
which are immersed in the water. The water is drawn up by
the capillary action of these threads, so as to surround the tubes
with a moist coating, which by its low temperature produces a
slight condensation of the steam as it passes through that part
of the tube.
The fine aqueous particles thus produced within the tube are
carried forward with the steam, and on issuing through the jet
pipe rub against its sides. This friction decomposes the natural
212 ELECTRICITY.
electricity, the negative fluid remaining on the jet, and the
positive being carried out with the particles of water, and im-
parted by them to the conductor b.
It will be apparent that in this arrangement the interior
surface of the jet plays the part of the rubber of the ordinary
machine, and the particles of water that of the glass cylinder
or plate, the steam being the moving power which maintains
the friction.
In order to ensure the efficiency of the friction, the conduit
provided for the escape of the steam is not straight but angular.
A section of the jet pipe near its extremity
is represented in fig, 483 a. The steam
issuing from the box b encounters a plate
of metal m which intercepts its direct pas-
sage to the mouth of the jet. It is corn-
Fig. 483 a " P e M e d to turn downwards, pass under the
edge of this plate, and, rising behind it,
turn again into the escape pipe, which is a tube formed of par-
tidge wood enclosed with in the metal pipe n.
It is found that an apparatus thus constructed, the length of
the boiler being 32 inches and its diameter 16 inches, will
develope as much electricity in a given time as three common
plate machines, whose plates have a diameter of 40 inches, and
are worked at the rate of 60 revolutions per minute.
A machine on this principle, and on a great scale of magni-
tude, was erected by the Royal Polytechnic Institution of
London, the boiler of which was 78 inches long, and 42 inches
diameter. The maximum pressure of the steam at the com-
mencement of the operation was sometimes 90 Ibs. per sq. inch.
This, however, fell to 40 Ibs. or less. Sparks have been ob-
tained from the conductor at the distance of 22 inches.
1740. Appendages to electrical machines. To facilitate the
performance of experiments, various accessories are usually
provided with these machines.
1741. Insulating stools. Insulating stools, constructed of
strong, hard 'wood, well baked and dried, and supported on legs
of glass coated with resinous varnish, are useful when it is re-
quired to keep for any time any conducting body charged with
electricity. The body is placed on one of these stools while it
is being electrified.
Thus, two persons standing on two such stools, may be
ELECTRICAL MACHINES.
213
charged, one with positive, and the other with negative, elec-
tricity. If, when so charged, they touch each other, the con-
trary electricities will combine, and they will sustain a nervous
shock proportionate to the quantity of electricity with which
they were charged.
1742. Discharging rods. Since it is frequently necessary
to observe the effects of points and spheres, pieces such as
Jigs. 484, 485. are provided, to be inserted in holes in the con-
ductors ; also metallic balls, Jigs. 486, 487., attached to glass
handles for cases in which it is desired to apply a conductor to
an electrified body without allowing the electricity to pass to
the hand of the operator. With these rods the electricity may
be taken from a conductor gradually by small portions, the
ball taking by each contact only such a fraction of the whole
charge as corresponds to the ratio of the surface of the ball to
the surface of the conductor.
1743. Jointed dischargers. To establish a temporary con-
nection between two conductors, or between a conductor and
the ground, the jointed dischargers, Jigs. 488, 489., are useful.
The distance between the balls can be regulated at pleasure by
means of the joint or hinge by which the rods are united.
Fig. 484. Fig. 485. Fig. 486. Fig. 487.
Fig. 489.
1744. Universal discharger. The universal discharger, an
instrument of considerable convenience and utility in experi-
mental researches, is represented in Jig. 490. It consists of a
wooden table to which two glass pillars A and A' are attached.
At the summit of these pillars are fixed two brass joints capable
of revolving in a horizontal plane. To these joints are at-
214 ELECTRICITY.
tached brass rods cc', terminated by balls DD', and having glass
handles EE'. These rods play on joints at BB', by which they
can be moved in vertical planes.
Fig. 490.
The balls DD' are applied to a wooden table sustained on a
pillar capable of having its height adjusted by a screw T. On
the table is inlaid a long narrow strip of ivory extending in
the direction of the balls DD'. These balls DD' can be un-
screwed, and one or both may be replaced by forceps, by which
may be held any substance through which it is desired to
transmit the electric charge. One of the brass rods c is con-
nected by chain or a wire with the source of electricity, and
the other with the ground.
The electricity is transmitted by bringing the balls DD' with
the substance to be operated on between them, within such a
distance of each other as will cause the charge to pass from one
to the other through the introduced substance.
CHAP. V.
CONDENSER AND ELECTROPHOROUS.
1745. Reciprocal inductive effects of two conductors. If a
conductor A communicating with the ground be placed near
another conductor B insulated and charged with a certain
quantity of electricity E, a series of effects will ensue by the
reciprocal inductive power of the two conductors, the result of
which will be that the quantity of electricity with which B is
charged will be augmented in a certain proportion, depending
on the distance between the two conductors through which the
CONDENSER AND ELECTROPHOROUS. 215
inductive force acts. The less Ibis distance is the more energetic
the induction will be, and the greater the augmentation of the
charge of the conductor B.
To explain this, we are to consider that the electricity E,
acting on the natural electricity of A, repels a certain quantity
of the fluid of the same name to the earth, retaining on the
side of A next to B the fluid of the contrary name. This fluid
of a contrary name thus developed in A reacts upon the natural
electricity o,f B, and produces a decomposition in the same
manner, augmenting the charge E by the fluid of the same
name decomposed, and expelling the other fluid to the more
remote side of B. This increased fluid in B again acts upon
the natural electricity of A, producing a further decomposition ;
and this series of reciprocal inductive actions producing a suc-
cession of decompositions in the two conductors, and accumu-
lating a tide of contrary electricities on the sides of the con-
ductors which are presented towards each other, goes on through
an indefinite series of reciprocal actions, which, nevertheless,
are accomplished in an inappreciable interval of time ; so that,
although the phenomenon in a strict sense is physically pro-
gressive, it is practically instantaneous.
To obtain an arithmetical measure of the amount of the
augmentation of the electrical charge produced in this way, let
us suppose that a quantity of electricity on B, which we shall
take as the unit, is capable of decomposing on A a quantity
which we shall express by m, and which is necessarily less than
the unit, because nothing short of actual contact would enable
the electricity of B to decompose an equal quantity of the elec-
tricity of A.
If, then, the unit of positive electricity act from B upon A, it
will decompose the natural electricity, expelling a quantity of
the positive fluid expressed by m, and retaining on the side
next to B an equal quantity of the negative fluid. Now, this
negative fluid m acting on the natural electricity of B at the
same distance will produce a proportionate decomposition, and
will develope on the side of B next to A an additional quantity
of the positive fluid, just so much less than m as m is less than
1. This quantity will therefore be m x m, or m*.
This quantity m 2 of positive fluid again acting by induc-
tion on A, will develope, as before, a quantity of negative
fluid expressed by m 2 x m, or m 3 . And in the same manner
216 ELECTRICITY.
this will develope on B an additional quantity of positive fluid
expressed by m 3 x m, or m 4 . These inductive reactions being
indefinitely repeated, let the total quantity of positive electricity
developed on B be expressed by P, and the total quantity of
negative electricity developed on A by N, we shall have
&c. ad inf.
&c. ad inf.
Each of these is a geometrical series ; and, since m is less than
1, they are decreasing series. Now, it is proved in arithmetic,
that although the number of terms in such series be unlimited,
their sum is finite, and that the sum of the unlimited number
of terms composing the first series is ^ ^ an( ^ tnat f tne
second -. We shall therefore have
In this case we have supposed the original charge of the
conductor B to be the unit. If it consist of the number of
units expressed by E, we shall have
It follows, therefore, that the original charge E of the con-
ductor B has been augmented in the ratio of 1 to 1 m' 2 by the
proximity of the conductor A.
The less is the distance between the conductors A and B, the
more nearly m will be equal to 1, and therefore the greater
will be the ratio of 1 to 1 m 2 , and consequently the greater
will be the augmentation of the electrical charge of B produced
by the presence of A.
For example, suppose that A be brought so near B, that the
positive fluid on B will develope nine-tenths of its own quantity
of negative fluid on A. In that case m=^=0'9. Hence it
appears, that 1 m 2 =l 0-81=0-19 ; and, consequently, the
charge of B will be augmented in the ratio of 0' 19 to 1, or of
19 to 100.
1746. Principle of the condenser. In such cases the elec-
tricity is said to be CONDENSED on the conductor B by the in-
COXDEXSER AND ELECTROPHOROUS. 217
ductive action of the conductor A, and apparatus constructed for
producing this effect are called CONDENSERS.
1747. Dissimulated or latent electricity. The electricity
developed in such cases on the conductor A is subject to the
anomalous condition of being incapable of passing away, though
a conductor be applied to it. In fact, the conductor A in the
preceding experiment is supposed to be connected with the
earth by conducting matter, such as a chain, metallic column, or
wire. Yet the charge of electricity N does not pass to the earth
as it would immediately do if the conductor B were removed.
In like manner, all that portion of the positive fluid p which
is developed on B by the inductive action of A, is held there by
the influence of A, and cannot escape even if the conductors be
applied in contact with it.
Electricity thus developed upon conductors and retained there
by the inductive action of other conductors, is said to be latent
or dissimulated. It can always be set free by the removal of
the conductors by whose induction it is dissimulated.
1748. Free electricity. Electricity, therefore, which is
developed independently of induction, or which, being first
developed by induction, is afterwards liberated from the in-
ductive action, is distinguished as free electricity.
In the process above described, that part of the charge P of
the conductor B which is expressed by E, and which was im-
parted to B before the approach of the conductor A, is free, and
continues to be free after the approach of E. If a conductor
connected with the earth be brought into contact with B, this
electricity E will escape by it ; but all the remaining charge of
B will remain, so long as the conductor A is maintained in its
position.
If, however, E be discharged from B, the charge which
remains will not be capable of retaining in the dissimulated
state so great a quantity of negative fluid on A as before. A
part will be accordingly set free, and if A be maintained in con-
nection with the ground it will escape. If A be insulated, it
will be charged with it still, but in a free state.
If this free electricity be discharged from A, the remaining
charge will not be capable of retaining in the latent state so
large a quantity of positive fluid on B as previously, and a part
of what was dissimulated will accordingly be set free, and may
be discharged.
218 ELECTRICITY.
In this manner, by alternate discharges from the one and the
other conductor, the dissimulated charges may be gradually
liberated and dismissed, without removing the conductors from
one another or suspending their inductive action.
1749. Forms of condensers. Condensers are constructed in
various forms, according to the strength of the electric charges
they are intended to receive. Those which are designed for
strong charges require to have the two conductors separated
by a non-conducting medium of some considerable thickness,
since, otherwise, the attraction of the opposite fluids diffused on
A and B would take effect ; and they would rush to each other
across the separating space, breaking their way through the
insulating medium which divides them. In this case, the
distance between A and B being considerable, the condensing
power will not be great, nor is it necessary to be so, since the
charges of electricity are by the supposition not small or feeble.
In case of feeble charges, the space separating the conductors
may be proportionally small, and, consequently, the condensing
power will be greater.
Condensers are usually constructed with two equal circular
plates, either of solid metal or having a metallic coating.
1750. Collecting and condensing plates. The plate corre-
sponding to the conductor A in the preceding paragraphs is
called the CONDENSING PLATE, and that which corresponds to B
the COLLECTING PLATE. The collecting plate is put in com-
munication with the body whose electrical state it is required
to examine by the agency of the condenser, and the condensing
plate is put in communication with the ground.
1751. Cuthberfsons condenser. A form of condenser con-
trived by Cuthbertson is represented \nfig. 491. The collecting
plate B is supported on a glass pillar, and communicates by a
chain attached to the hook D with the source
of electricity under examination. The con-
// densing plate A is supported on a brass pillar,
movable on a hinge, and communicating
with the ground. By means of the hinge the
disk A may be moved to or from B. The
space between the plates in this case may be
merely air, or, if strong charges are used, a
plate of glass may be interposed.
Fig. 491. When used for feeble charges, it is usual to
CONDENSER AND ELECTROPHOROUS. 219
cover the condensing plate with a thin coating of varnished
silk, or simply with a coating of resinous varnish. An instru-
ment thus arranged is represented in fig. 492., where b b f , the
condensing plate, is a disk of wood coated
with varnished silk tt'. The collecting
plate c c' has a glass handle m, by which
it may be raised, and a rod of metal a b
by which it may be put in communication
with the source of electricity under ex-
amination.
The condensing plate in this case has generally sufficient
conducting power when formed of wood, but may be also made
of metal, and, instead of varnished silk, it may be coated with
gum-lac, resin, or any other insulator.
When the plate cc' has received its accumulated charge, its
connection with the source of electricity is broken by removing
the rod ab ; and the plate cc' being raised from the condensing
plate, the entire charge upon it becomes free, and may be sub-
mitted to any electroscopic test.
1752. The electrophorous. A small charge of free elec-
tricity may by the agency of induction be made to produce a
charge of indefinite amount, which may be imparted to any in-
sulated conductor. This is effected by the electrophorous, an
instrument consisting of a circular cake, composed of a mixture
of shell-lac, resin, and Venice turpentine,
cast in a tin mould AB, fig. 493. Upon
this is laid a circular metallic disk c, rather
less in diameter than AB, having a glass
handle.
As* 3B Before applying the disk c, the resinous
Fig. 493. surface is electrified negatively by striking
it several times with the fur of a cat. The disk c being then
applied to the cake AB, and the finger being at the same time
pressed upon the disk c to establish a communication with the
ground through the body of the operator, a decomposition takes
place by the inductive action of the negative fluid on the resin.
The negative fluid escapes from the disk c through the body
of the operator to the ground, and a positive charge remains,
which is prevented from passing to the resin partly by the thin
film of air which will always remain between them even when
220 ELECTRICITY.
the plate C rests upon the resin, and partly by the non-conduct-
ing virtue of the resin.
When the disk c is thus charged with positive electricity kept
latent on it by the influence of the negative fluid on AB, the
finger being previously removed from the disk c, let it be raised
from the resin and the electricity upon it, before dissimulated,
will become free, and may be imparted to any insulated con-
ductor adapted to receive it.
The charge of negative electricity remaining undiminished on
the resin AB, the operation may be indefinitely repeated ; so that
an insulated conductor may then be charged to any extent, by
giving to it the electric fluid drop by drop thus evolved on the
disk c by the inductive action of A B.
This is the origin of the name of the apparatus.
CHAP. VI.
ELECTROSCOPES.
1753. General principle of electroscopes. Electroscopes in
general consist of two light conducting bodies freely suspended,
which hang vertically and in contact, in their natural state.
When electricity is imparted to them they repel each other,, the
angle of their divergence being greater or less according to the
intensity of the electricity diffused on them. These electroscopic
substances may be charged with electricity either by direct com-
munication with the electrified body, in which case their elec-
tricity will be similar to that of the body ; or they may be acted
upon inductively by the body under examination, in which case
their electricity may be either similar or different from that of
the body, according to the position in which the body is pre-
sented to them. In some cases, the electroscope consists of a
single light conductor to which electricity of a known species is
first imparted, and which will be attracted or repelled by the
body under examination when presented to it, according as
the electricities are like or unlike.
These instruments vary infinitely in form, arrangement, mode
ELECTROSCOPES. 221
of application, and sensitiveness, according to the circumstances
under which they are placed, and the intensities of the electri-
cities of which they are expected to detect the presence, measure
the intensity, or indicate the quality. In electroscopes, as in all
other instruments of physical inquiry, the most delicate and
sensitive is only the most advantageous in those cases in which
much delicacy and precision are required. A razor would be
an ineffectual instrument for felling timber.
1754. Pith ball electroscope. One of the most simple and
generally useful electroscopic instruments is the pendulous pith
ball already mentioned (1697), the action of which may now be
more fully explained. When an electrified body is presented
to such a ball suspended by a silken thread, it acts by induc-
tion upon it, decomposing its natural fluid, attracting the consti-
tuent of the contrary name to the side of the ball nearest to it,
and repelling the fluid of the same name to the side most remote
from it. The body will thus act at once by attraction and re-
pulsion upon the two fluids ; but since that of a contrary name
which it attracts is nearer to it than that of the same name
which it repels, and equal in quantity, the attraction will prevail
over the repulsion, and the ball will move towards the electri-
fied body. "When it touches it, the fluid of a contrary name, which
is diffused round the point of contact, combining with the fluid
diffused upon the body, will be neutralized, and the ball will re-
main charged with the fluid of the same name as that with
which the body is electrified, and will consequently be repelled
by it. Hence it will be understood why, as already mentioned,
the pith ball in its neutral state is first attracted to an electrified
body, and after contact with it repelled by it.
1755. The needle electroscope. The electric needle is an
electroscopic apparatus, somewhat less simple, but more sensitive
than the pendulum. It consists of a rod of copper terminated
^ by two metallic balls B and B', jig. 494.,
B/ which are formed hollow in order to render
them more light and sensitive. At the
middle point of the rod which connects them
is a conical cup, formed of steel or agate,
suspended upon a fine point, so that the
Fig. 494. needle is exactly balanced, and capable of
turning freely round the point of support in a horizontal plane,
222
ELECTRICITY.
like a magnetic needle. A very feeble electrical action ex-
erted upon either of the balls B or B' will be sufficient to put
the needle in motion.
1 756. Coulomb's electroscope. The electroscope of Coulomb,
better known as the balance of torsion, is an apparatus still
more sensitive and delicate for indicating the existence and in-
tensity of electrical force. A needle gg', fig. 495., formed of
gum-lac, is suspended by a fibre of raw silk/ At one extremity
it carries a small disk e, coated with metallic foil, and is so
balanced at the point of suspension, that the needle resting
horizontally is free to turn in either direction
round the point of suspension. When it turns,
it produces a degree of torsion or twist of
the fibre which suspends it, the reaction of
which measures the force which turns the
needle. The thread is fixed at the top to a
small windlass t, by which the needle can be
raised or lowered, and the whole is included
in a glass cage, to preserve the apparatus
from the disturbance of the air. Upon this
glass cage, which is cylindrical, is a graduated
circle d d', which measures the angle through
which the needle is deflected. In the cover of
the cage an aperture o is made, through which may be intro-
duced the electrified body whose force it is desired to indicate
and measure by the apparatus.
1757. Quadrant electrometer. This instrument, which is
generally used as an indicator on the conductors
of electrical machines, consists of a pillar AB,
fig. 496., of any conducting substance, termi-
nated at the lower extremity by a ball B. A rod,
also a conductor, of about half the length, termi-
nated by a small pith ball D, plays on a centre C
in a vertical plane, having behind it an ivory
semicircle graduated. When the ball B is
charged with electricity, it repels the pith ball D,
Fig. 496. and the angle of repulsion measured on the gra-
duated arc supplies a rough estimate of the intensity of the
electricity.
1758. Gold leaf electroscope. A glass cylinder ABC D, fig.
Fig. 495.
ELECTROSCOPES.
223
Fig. 497.
497., is connected to a brass stand E, and closed at
the top by a circular plate AB. The brass top G is
connected by a metallic rod with two slips of gold
leafy, two or three inches in length, and half an inch
in breadth. In their natural state they hang in
contact, but when electricity is imparted to the plate
G, the leaves becoming charged with it indicate its
presence, and in some degree its intensity, by their
divergence. On the sides of the glass cylinder
opposite the gold leaves are attached strips of tin-
foil, communicating with the ground. When the
leaves diverge so much as to touch the sides of the
cylinder, they give up their electricity to the tinfoil, and are
discharged. This instrument may also be affected inductively.
If the electrified body be brought near to the plate G, its natural
electricity will be decomposed ; the fluid of the same name as
that with which the body is charged will be repelled, will
accumulate in the gold leaves, and will cause them to diverge.
1759. Condensing electroscope. The condenser applied to
the electroscope supplies an instrument which has the same
fl analogy to the common electroscope as the com-
pound has to the simple microscope. An electro-
scope with such an appendage is represented in
fig. 498. The condenser is screwed on the top,
the condensing plate communicating with the
electroscope, and the collecting plate being laid
over it. When the collecting plate is put into
communication with the source of electricity to be
examined, a charge is produced by induction in
Fig. 498. ^0 con( j ens j n g plate under it, and a charge of
a contrary name is collected in the electroscope, the leaves of
which will diverge in this case with an electricity similar in
name to that of the body under examination.
In the use of instruments of such extreme sensitiveness, many
precautions are necessary to guard against disturbances, which
would interfere with their indications, and expose the observer to
errors. The plates of the condenser in some experiments may
be exposed to chemical action, which, as will hereafter appear,
is always combined with the development of electricity. In such
eases, the condenser of the electroscope should be composed of
224 ELECTRICITY.
gilt plates. The apparatus is sometimes included in a glass
case, to protect it from atmospheric vicissitudes; and to preserve
it from hygrometric effects, a cup of quicklime is placed in the
case to absorb the humidity. The plates of the condenser at-
tached to electroscopes vary from four to ten inches in diameter.
When greater dimensions are given to them, it is difficult to
make them with such precision as to ensure the exact contact of
their surfaces.
Becquerel used plates of glass twenty inches in diameter, accu-
rately ground together with emery, and coated with thin tin-
foil. This apparatus had great sensibility, but as the metal was
very oxidable, the results were disturbed by chemical effects not
easily avoided. A coating of platinum or gold would have been
more free from disturbing action.
CHAP. VII.
THE LEYDEN JAR.
THE inductive principle which has supplied the means in
the case of the condenser of detecting and examining quan-
tities of electricity so minute and so feeble as to escape all
common tests, has placed, in the Leyden jar, an instrument
at the disposal of the electrician by which artificial electricity
may be accumulated in quantities so unlimited as to enable him
to copy in some of its most conspicuous effects the lightning of
the clouds.
To understand the principle of the Leyden jar, which at one
time excited the astonishment of all Europe, it is only neces-
sary to investigate the effect of a condenser of considerable mag-
nitude placed in connection, not with feeble, but with energetic
sources of electricity, such as the prime conductor of an electrical
machine. In such case it would be evidently necessary that the
collecting and condensing plates should be separated by a non-
conducting medium of sufficient resistance to prevent the union
of the powerful charges with which they would be invested.
Let A B, fig. 499., represent the collecting plate of such a con-
THE LEYDEN JAR. 225
denser, connected by a chain K
with the conductor E of an electric
machine ; and let A' B' be the con-
densing plate connected by a chain
K' with the ground. Let c D be a
plate of glass interposed between
A' B' and A B.
Let e express the quantity of
electricity with which a superficial
unit of the conductor E is charged.
It follows that e will also express
the free electricity on every super-
Fi-.~499 ficial unit of the collecting plate
A B ; and if the total charge on each
superficial unit of A B, free and dissimulated, be expressed by a,
we shall, according to what has been already explained, have
The charge on the superficial unit of the condensing plate
A' B' being expressed by a', we shall have
1-w 2
which will be wholly dissimulated.
If s express the common magnitude of the two plates A' B'
and A B, and E express the entire quantity of electricity accu-
mulated on A B, and E' that accumulated on A 7 B', we shall have
1-m 2
It is evident, therefore, that the quantity of electricity with
which the plates A B and A' B' will be charged, will be aug-
mented, firstly, with the magnitude (s) of the plates ; secondly,
with the intensity (e) of the electricity produced by the machine
upon the conductor E ; and thirdly, with the thinness of the glass
plate c D which separates the plates A' B' and A B. The thinner
this plate is, the more nearly equal to 1 will be the number m,
and consequently the less will be 1 m 2 , and the greater the
quantity E.
226 ELECTRICITY.
When the machine has been worked until e ceases to increase,
the charge of the plates will have attained its maximum. Let
the chains K and K' be then removed, so that the plates A B and
A' B' shall be insulated, being charged with the quantities of
electricity of contrary names expressed by E and E'.
If a metallic wire w, or any other conductor, be now placed
so as to connect the plate A B with the plate A' B', the free elec-
tricity on the former passing along the conductor w will flow to
the plate A' B', where it will combine with or neutralize a part
of the dissimulated fluid. This last being thus diminished in
quantity, will retain by its attraction a less quantity of the fluid
on A B, a corresponding quantity of which will be liberated, and
will therefore pass along the conductor w to the plate A' B',
where it will neutralize another portion of the dissimulated fluid;
and this process of reciprocal neutralization, liberation, and con-
duction will go on until the entire charge E' upon the plate A' B'
has been neutralized by a corresponding part of the fluid E ori-
ginally diffused on the plate AB.
Although these effects are strictly progressive, they are prac-
tically instantaneous. The current of free electricity flows
through the conductor w, neutralizes the charge E', and liberates
all the dissimulated part of E in an interval so small as to be
quite inappreciable. In whatever point of view the power of
conduction may be regarded, a sudden and violent change in
the electrical condition of the conductor w must attend the phe-
nomenon. If the conductor w be regarded merely as a channel
of communication, a sort of pipe or conduit through which the
electric fluid passes from A B to A' B', as some consider it, so
large an afflux of electricity may be expected to be attended
with some violent effects. If, on the other hand, the opposite
fluids are reduced to their natural state, by decomposing suc-
cessively the natural electricity of the parts of the conductor w,
and taking from the elements of the decomposed fluid the elec-
tricities necessary to satisfy their respective attractions, a still
more powerful effect may be anticipated from so great and
sudden a change.
Such phenomena are accordingly found to be attended with
some of the most remarkable effects presented in the whole
domain of physical research. If the charge E be sufficiently
strong, and the intermediate conductor w be thin metallic wire,
it will be instantly rendered incandescent, and may even be
THE LEYDEN JAR. 227
fused. If the human body be made the conducting medium,
however inconsiderable the charge may be, an effect is produced
on the nerves which is to most persons extremely disagreeable,
and if the charge be considerable, it may even have the effect
of destroying animal life.
In order to divest these principles of whatever is adventitious,
and to bring their general character more clearly into view, we
have here presented them in a form somewhat different from
that in which they are commonly exhibited in electrical experi-
ments. The phenomenon which has just been explained, consist-
ing merely in the communication of powerful charges of elec-
tricity of contrary kinds, on the opposite faces of glass or other
non-conductor, by means of metal maintained in contact with
the glass, it is evident that the form of the glass and of the
metal in contact with it have no influence on the effects.
Neither has the thickness or volume of the metal any relation
to the results. Thus the glass, whose opposite faces are charged,
may have the form of a hollow cylinder or sphere, or of a
common flask or bottle, and the metal in contact with it need
not be massive or solid plates, but merely a coating of metallic
foil.
1760. The Leyden jar. In experimental researches, there-
fore, the form which is commonly given to the glass, with a
view to develope the above effects, is that of a cylinder or jar,
A u, fig. 500., having a wide mouth and a flat bottom. The
shaded part terminating at C is a coating of tinfoil placed on
the bottom and sides of the jar, a similar coating
being attached to the corresponding parts of the
interior surface. To improve the insulating power
of the glass it is coated above the edge of the tin-
foil with a varnish of gum-lac, which also renders
it more proof against the deposition of moisture.
A metallic rod, terminated in a ball D, descends into
the jar, and is jointed in contact with the inner
coating.
This apparatus is generally known by the name
of the Leyden phial, the experiments produced with
it having been first exhibited at the city of Leyden,
in Holland.
To understand the action of this apparatus it is only
necessary to consider the inner coating and the metallic rod as
L 6
228 ELECTRICITY.
representing the metallic surface A B, Jig. 499., and the outer
surface A' B', the jar itself playing the part of the inter-
vening non-conducting medium. If the ball D be put in com-
munication by a metallic chain with the conductor of the
electric machine, and the external coating c B with the ground,
the jar will become charged with electricity, in the same manner
and on the same principles exactly as has been explained in
the case of the metallic surfaces A' B' and A B, Jig. 499.
If, when a charge of electricity is thus communicated to the
jar, the communication between D and the conductor be removed,
the charge will remain accumulated on the inner coating of the
jar. If in this case a metallic communication be made between
the ball D and the outer coating, the two opposite electri-
cities on the inside and outside of the jar will rush towards
each other and will suddenly combine. In this case there is no
essential distinction between the functions of the outer and
inner coating of the jar, as may be shown by connecting the
inner coating with the ground and the outer coating with the
conductor. For this purpose, it is only necessary to place the
jar upon an insulating stool, surrounding it by a metallic chain
in contact with its outer coating, which should be carried to
the conductor of the machine; while the ball r>, which com-
municates with the inner coating, is connected by another chain
to the ground. In this case the electricity will flow from the
conductor to the outer coating, and will be accumulated there
by the inductive action of the inner coating, and all the effects
will take place as before.
If, after the jar is thus charged, the communication between
the outer coating and the conductor be removed, and a metallic
communication be made between the inner and outer coating,
the electricities will, as before, rush towards each other and
combine, and the jar will be restored to its natural state.
To charge the jar internally, it will be sufficient to hold it
with the hand in contact with the external coating, presenting
the ball D to the conductor of the machine. The electricity
will flow from the conductor to the inner coating, and the
external coating will act inductively, being connected through
the hand and body of the operator with the earth.
1761. Effect of the metallic coating. The metallic coatings
of the jar have no other effect than to conduct the electricity
to the surface of the glass, and when there, to afford it a free
THE LEYDEN JAR. 229
passage from point to point. Any other conductor would,
abstractedly considered, serve the same purpose ; and metallic
foil is selected only for the facility and convenience with which
it may be adapted to the form of the glass, and permanently
attached to it. That like effects would attend the use of any
other conductor may be easily shown.
1762. Water may be substituted for the metallic coating.
Let a glass jar be partially filled with water, and hold it in the
hand by its external surface. Let a chain or rod connected
with the conductor of an electric machine be immersed in
the water. A stream of electricity will flow from the machine
to the water, exactly as it did from the machine to the inner
coating of the jar ; and the inductive action of the hand
communicating through the body of the operator with the
ground, will produce a charge of electricity in the water upon
exactly the same principle as the inductive action of the
external coating of the jar communicated the charge of electri-
city on the internal coating. If, after the charge has been
communicated in the water, the operator plunge his other
hand in the water, so as to form a communication between the
water within the jar and the hand applied to its external
surface, the opposite electricities will rush towards each other
through the hand of the operator, and their motion will be
rendered sensible by a strong nervous convulsion.
1763. Experimental proof that the charge adheres to the
glass and not to the coating. The electricity with which the
jar is charged in this case resides, therefore, on the glass, or on
the conductor by which it passes to the glass, or is shared by
these.
To determine where it resides, it is only necessary to provide
means of separating the jar from the coating after it has been
charged, and examining the electrical state of the one and the
other. For this purpose let a glass jar be provided, having a
loose cylinder of metal fitted to its interior, which can be.
placed in it or withdrawn from it at pleasure, and a similar loose
cylinder fitted to its exterior. The jar being placed in the
external cylinder, and the internal cylinder being inserted in it,
let it be charged with electricity by the machine in the manner
already described. Let the internal cylinder be then removed,
and let the jar be raised out of the external cylinder. The two
cylinders being then tested by an electroscopic apparatus, will
230
ELECTRICITY.
be found to be in their natural state. But if an electroscope
be brought within the influence of the internal or external
surface of the glass jar, it will betray the presence of the one
or the other species of electricity. If the glass jar be then in-
serted in another metallic cylinder made to fit it externally,
and a similar metallic cylinder made to fit it internally be
inserted in it, it will be found to be charged as if no change had
taken place. On connecting by metallic communication the
interior with the exterior, the opposite electricities will rush
towards each other and combine. It is evident, therefore, that
the seat of the electricity, when a jar is charged, is not the
metallic coating, but the surface of the glass under it.
1764. Improved form of Ley den jar. An improved form
of the Leyden jar is represented mjfig. 501. Besides the pro-
visions which have been already explained,
there is attached to this jar a hollow brass cup
C cemented into a glass tube. This tube passes
through the wooden disk which forms the cone
of the jar, and is fastened to it. It reaches to
the bottom of the jar. A communication is
formed between c and the internal coating by
a brass wire terminating in the knob D. This
wire, passing loosely through a small hole in
the top, may be removed at pleasure for the
purpose of cutting off the communication
between the cup and the interior coating. This
wire does not extend quite to the bottom of the
jar, but the lower part of the tube is coated
with tinfoil, which is in contact with the wire,
and extends to the inner coating of the jar.
At the bottom of the jar a hook is provided,
by which a chain may be suspended so as to form a communi-
cation between the external coating and other bodies. When
ajar of this kind is once charged, the wire may be removed or
allowed to fall out by inverting the jar, in which case the jar
will remain charged, since no communication exists between
its internal and external coating ; and as the internal coating is
protected from the contact of the external air, the absorption of
electricity in this case is prevented. An electric charge may
thus be transferred from place to place, and preserved for any
length of time.
Fig. 501.
THE LEYDEN JAR. 231
In the construction of cylindrical jars it is not always possible
to obtain glass of uniform thickness, for which reason jars are
sometimes provided of a spherical form.
1765. Charging a series of jars by cascade. In charging a
single jar, an unlimited number of jars, connected together by
conductors, may be charged with very nearly the same quantity
of electricity. For this purpose let the series of jars be placed
on insulating stools, as represented in fig. 502., and let c be
Fig. 502.
metallic chains connecting the external coating of each jar with
the internal coating of the succeeding one. Let D be a chain
connecting the first jar with the conductor of the chain, and D'
another chain connecting the last jar with the ground. The
electricity conveyed to the inner coating of the first jar A acts
by induction on the external coating of the first jar, attracting
the negative electricity to the surface, and repelling the
positive electricity through the chain c to the inner coating
of the second jar. This charge of positive electricity in the
second jar acts in like manner inductively on the external coat-
ing of this jar, attracting the negative electricity there, and
repelling the positive electricity through the chain c to the
internal coating of the third jar ; and in the same manner the
internal coating of every succeeding jar in the series will be
charged with positive electricity, and its internal coating with
negative electricity. If, while the series is insulated, a dis-
charger be made to connect the inner coating of the first with
the outer coating of the last jar, the opposite electricities will
rush towards each other, and the series of jars will be restored
to their natural state.
1766. Electric battery When several jars are thus com-
bined to obtain a more energetic discharge than could be formed
by a single jar, the system is called an electric battery, and
the method of charging it, explained above, is called charging
by cascade.
232 ELECTRICITY.
After the jars have been thus charged, the chains connecting
the outer coating of each jar with the inner coating of the suc-
ceeding one are removed, and the knobs are all connected one
with another by chains or metallic rods, so as to place all the
internal coatings in electric connection, and the outer coatings
are similarly connected. By this expedient the system of jars
is rendered equivalent to a single jar, the magnitude of whose
coated surface would be equal to the sum of all the surfaces of
the series of jars. The battery would then be discharged by
placing a conductor between the outer coating of any of the
jars and one of the knobs.
If s express the total magnitude of the coating of the series
of jars, the total charge of the battery will be expressed ap-
proximately by
1767. Common electric battery. It is not always convenient,
however, to practise this method. The jars composing the bat-
tery are commonly placed in a box, as represented \\ijig. 503.,
coated on the inside with tinfoil, so as to form a metallic commu-
nication between the external coating of all the jars. The knobs,
which communicate with their internal coating, are connected
by a series of metallic rods in the manner represented in the
figure ; so that there is a con-
tinuous metallic communica-
tion between all the internal
coatings. If the metallic rods
which thus communicate with
the inner coating be placed
in communication with the
Fig. 5C3. conductor of a machine, while
the box containing the jars
is placed in metallic communication with the earth, the battery
will be charged according to the principles already explained
in the case of a single jar, and the force of its charge will be
equal to the force of the charge of a single jar, the magnitude
of whose external and internal coating would be equal to the
sum of the internal and external coating of all the jars com-
posing the battery.
1768. Method of indicating and estimating the amount of
THE LEYDEX JAR. 233
the charge. In charging a jar or a battery there is no ob-
vious means by which the amount of the charge imparted
to the jar can be indicated. It is to be considered that the
internal coating is, in effect, a continuation of the conductor ;
and if the jars had no external coating, the communication of
the internal coating with the conductor would be attended
with no other effect than the distribution of the electricity over
the conductor and the internal coating, according to the laws of
electrical equilibrium ; but the effect of the external coating is
to dissimulate or render latent the electricity as it flows from
the conductor, so that the repulsion of the part of it which
remains free is less than the expansive force of the electricity
of the conductor, and a stream of the fluid continues to flow
accordingly from the conductor to the internal coating; and
this process continues until the increasing force of the free
electricity on the internal coating of the jars becomes so great,
that the force of the fluid on the condenser can no longer over-
come it, and thus the flow of electricity to the jars from the
conductor will cease.
It follows, therefore, that during the process of charging the
jars, the depth or tension of the electricity on the conductor is
just so much greater than that of the free electricity on the in-
terior of the jars, as is sufficient to sustain the flow of electricity
from the one to the other ; and as this is necessarily so extremely
minute an excess as to be insensible to any measure which
could be applied to it, it may be assumed that the depth of
electricity on the conductor is always equal to that of the free
electricity on the interior of the jars. If e therefore express
the actual depth of the electric fluid at any time on the interior
coating, (1 m 2 ) x e will express the depth of the free electricity;
and since, throughout the process, m does not change its value,
it follows that the actual depth of electricity, and therefore the
actual magnitude of the charge, is proportionate to the depth of
free electricity on the interior of the jar, which is sensibly the
same as the depth of free electricity on the conductor. It follows,
therefore, that the magnitude of the charge, whether of a single
jar or several, will always be proportionate to the depth of
electricity on the conductor of the machine from which the charge
is derived. If, therefore, during the process of charging a jar
or battery, an electrometer be attached to the conductor, this
instrument will at first give indications of a very feeble elec-
234 ELECTRICITY.
tricity, the chief part of the fluid evolved being dissimulated on
the inside of the jars; but as the charge increases, the indications
of an increased depth of fluid on the conductor become apparent ;
and at length, when no more fluid can pass from the conductor
to the jars, the electrometer becomes stationary, and the fluid
evolved by the machine escapes from the points or into the
circumjacent air.
The quadrant electrometer, described in (1757), is the indi-
cator commonly used for this purpose, and is inserted in a hole
on the conductor. When the pith ball attains its maximum
elevation, the charge of the jars may be considered as complete.
The charge which a jar is capable of receiving, besides being
limited by the strength of the glass to resist the mutual attrac-
tion of the opposite fluids, and the imperfect insulating force of
that part of the jar which is not coated, is also limited by the
imperfect insulating force of the air itself. If other causes,
therefore, allowed an unlimited flow of electricity to the jar,
its discharge would at length take place by the elasticity of
the free electricity within it surmounting the confining pres-
sure of the air, and accordingly the fluid of the interior would
pass over the mouth of the jar, and unite with the opposite
fluid of the exterior surface.
CHAP. VIII.
LAWS OF ELECTRICAL FORCES.
1769. Electric forces investigated by Coulomb. It is not
enough to ascertain the priciples which govern the decomposition
of the natural electricity of bodies, and the reciprocal attraction
and repulsion of the constituent fluids. It is also necessary to
determine the actual amount of force exerted by each fluid in
repelling fluid of the like or attracting fluid of the opposite
kind, and how the intensity of this attraction is varied by vary-
ing the distance between the bodies which are invested by the
attracting or repelling fluids.
By a series of experimental researches, which rendered his
name for ever memorable, COULOMB solved this difficult and
LAWS OF ELECTRICAL FORCES. 235
delicate problem, measuring with admirable adroitness and pre-
cision these minute forces by means of his electroscope or
balance of torsion, already described (1756).
1770. Proof-plane. The electricity of which the force was
to be estimated was taken up from the surface of the electrified
c body upon a small circular disk c, fig. 504., coated
with metallic foil, and attached to the extremity of a
delicate rod or handle A B of gum-lac. This disk,
called a PROOF-PLANE, was presented to the ball sus-
pended in the electrometer of torsion (1756), and the
intensity of its attraction or repulsion was measured
by the number of degrees through which the suspend-
A
ing fibre or wire was twisted by it.
Fig. 504. The extreme degree of sensibility of this apparatus
may be conceived, when it is stated that a force equal to the 340th
part of a grain was sufficient to turn it through 360 degrees ;
and since the reaction of torsion is proportional to the angle
of torsion, the force necessary to make the needle move through
one degree would be only the 122,400th part of a grain. Thus
this balance was capable of dividing a force equal to a single
grain weight into 122,400 parts, and rendering the effect of
each part distinctly observable and measurable.
1771. Law of electrical force similar to that of gravitation.
By these researches it was established, that the attraction
and repulsion of the electric fluids, like the force of gravitation,
and other physical influences which radiate from a centre, vary
according to the common law of the inverse square of the dis-
tance ; that is to say, the attraction or repulsion exerted by a
body charged with electricity, or, to speak more correctly, by
the electricity with which such a body is charged, increases in
the same proportion as the square of the distance from the body
on which it acts is diminished, and diminishes as the square of
that distance is increased.
In general, if f express the force exerted by any quantity of
electric fluid, positive or negative, at the unit of distance,
^ will express the force which the same quantity of the same
fluid will exert at the distance D.
In like manner, if the quantity of fluid taken as the unL
f
exercise at the distance Dthe force expressed by-^j, the quantity
236
ELECTRICITY
expressed by E, will exert at the same distance D the force
expressed by
fXE
V*s-'
D 2
These formulae have been tested by numerous experiments
made under every possible variety of conditions, and have
been found to represent the phenomena with the greatest
precision.
1772. Distribution of the electric fluid on conductors. The
distribution of electricity upon conductors can be deduced as
a mathematical consequence of the laws of attraction and re-
pulsion which have been explained above, combined with the
property in virtue of which conductors give free play to these
forces. The conclusions thus deduced may further be verified
by the proof plane and electrometer of torsion, by means of
which the fluid diffused upon a conductor may be gauged, so
that its depth or intensity at every point may be exactly ascer-
tained ; and such depths and intensities have accordingly been
found to accord perfectly with the results of theory.
1773. It is confined to their surfaces. Numerous facts
suggest the conclusion that the electricity with which a con-
ductor is charged is either superficial, or very nearly so.
If an electrified conductor be pierced with holes a little
greater than the proof plane (Jig. 504.) to different depths,
that plane, inserted so as to touch the bottom of these holes,
will take up no electricity.
If a spheroidal metallic body
A, Jig. 505., suspended by a silken
thread, be electrified, and two
thin hollow caps BB and B'B'
made to fit it, coated on their
inside surface with metallic foil,
and having insulating handles
cc' of gum-lac, be applied to it,
on withdrawing them the sphe-
roid will be deprived of its elec-
tricity, the fluid being taken off by the caps.
Although it follows, from these and other experimental tests,
as well as from theory, that the diffusion of electricity on con-
Fig. 505.
LAWS OF ELECTRICAL FORCES. 237
ductors is nearly superficial, it is not absolutely so. If one
end of a metallic rod, coated with sealing-wax, be presented to
any source of electricity, the fluid will be received as freely
from the other end, as if its surface were not coated with a
non-conductor. It follows from this that the electricity must
pass along the rod sufficiently within the surface of the metal
which is in contact with the wax to be out of contact with the
wax, which, by its insulating virtue, would arrest the progress
of the fluid.
1774. How the distribution varies. It remains, however, to
ascertain how the intensity of the fluid, or its depth on different
parts of a conductor, varies.
There are some bodies whose form so strongly suggests the
inevitable uniformity of distribution as to render demonstration
needless. In the case of a sphere, the symmetry of form alone
indicates the necessity of an uniform distribution. If, then,
the fluid be regarded as having an uniform depth on every
part of a conducting sphere, exactly as a liquid might be uni-
formly diffused over the surface of the globe, the total quantity
of fluid will be expressed by multiplying its depth by the super-
ficial area of the globe.
1775. Distribution on an ellipsoid. If the electrified con-
ductor be not a globe, but an elliptical spheroid, such as A A',
fig. 506., the fluid will be found to be accumulated in greater
quantity at the small ends A and A' than at
the sides BB', where there is less curvature.
} This unequal distribution of the fluid is re-
presented by the dotted line in the figure.
Fig. 506. It follows from theory, and it is confirmed
by observation, that the depth of the fluid at A and A' is greater
than at BB' in the ratio of the longer axis A A' of the ellipse to
the shorter axis BB'.
If, therefore, the ellipsoid be very elongated, as in fig. 506.,
the depth of the fluid at the ends A and A'
will be proportionally greater.
1776. Effects of edges and points
Fig. 507. ^ tne conductor be a flat disk, the
depth of the fluid will increase from its
centre towards its edges. The depth will, however, not vary
sensibly near the centre, but will augment rapidly in ap-
238 ELECTRICITY.
preaching the edge, as represented \i\fig. 508., where A and B
are the edges, and c the centre of the disk, the depth of the
fluid being indicated by the dotted line.
Fig. 508.
It is found in general that the depth of the fluid increases in
a rapid proportion in approaching the edges, corners, and ex-
tremities, whatever be the shape of the conductor. Thus,
when a circular disk or rectangular plate has any considerable
magnitude, the depth of the electricity is sensibly uniform at
all parts not contiguous to the borders ; and whatever be the
form, whether round or square, if only it be terminated by
sharp angular edges, the depth will increase rapidly in approach-
ing them.
If a conductor be terminated, not by sharp angular edges,
but by rounded sides or ends, then the distribution will become
more uniform. Thus, if a cylindrical conductor of considerable
diameter have hemispherical ends, the distribution of the elec-
tricity upon it will be nearly uniform ; but if its ends be flat,
with sharp angular edges, then an accumulation of the fluid
will be produced contiguous to them. If the sides of a flat
plate of sufficient thickness be rounded, the accumulation of
fluid at the edges will be diminished.
The depth of the fluid is still more augmented at corners
where the increase of depth due to two or more edges meet
and are combined ; and this effect is pushed to its extreme
limit if any part of a conductor have the form of a POINT.
The pressure of the surrounding air being the chief, if not
the only force, which retains the electric fluid on a conductor,
it is evident that if at the edges, corners, or angular points,
the depth be so much increased that the elasticity of the fluid
exceeds the restraining pressure of the atmosphere, the elec-
tricity must escape, and in that case will issue from the edge,
corner, or point, exactly as a liquid under strong pressure
would issue from &jet d'eau.
1 1 77. Experimental illustration of the effect of a point. Let
T,fig> 509., be a metallic point attached to a conductor c, and let
LAWS OF ELECTRICAL FORCES.
239
1
Fig. 509.
perpendicular n"p.
of the atmosphere
the perpendicular n express the
thickness or density of the electric
i fluid at that place ; this thickness
will increase in approaching the
point P, so as to be represented
by perpendiculars drawn from
the respective points of the curve
n, n', n" to AF, so that its density
at P will be expressed by the
Experience shows that in ordinary states
very moderate charge of electricity given
to the conductor c will produce such a density of the electric
fluid at the point P as to overcome the pressure of the atmo-
sphere, and to cause the spontaneous discharge of the electricity.
The following experiments will serve to illustrate this escape
of electricity from points.
Let a metallic point, such as AP,Jig 509., be attached to a
conductor, and let a metallic ball of two or three inches in dia-
meter, having a hole in it corresponding to the point P, be stuck
upon the point. If the conductor be now electrified, the elec-
tricity will be diffused over it, and over the ball which has
been stuck upon the point P. The electric state of the con-
ductor may be shown by a quadrant electrometer being attached
to it. Let the ball now be drawn off the point P by a silk
thread attached to it for the purpose, and let it be held sus-
pended by that thread. The electricity of the conductor c will
now escape by the point P, as will be indicated by the electro-
meter, but the ball suspended by the silk thread will be elec-
trified as before.
1778. dotation produced by the reaction of points. Let two
wires AB and cv,Jig. 510., placed at right angles, be supported
by a cap E upon a fine point at the top of an
insulating stand, and let them communicate by
a chain F with a conductor kept constantly elec-
trified by a machine. Let each of the four
arms of the wires be terminated by a point in a
horizontal direction at right angles to the wire,
each point being turned in the same direction,
as represented in the figure. When the elec-
tricity comes from the conductor to the wires,
Fig. 5io. it will escape from the wires at these four points
240
ELECTRICITY.
Fig. 511.
respectively ; and the force with which it leaves them will be
attended with a proportionate recoil, which will cause the wire
to spin rapidly on the centre E.
1779. Another experimental illustration of this principle.
An apparatus supplying another illustration of this principle is
represented \nfig. 511. : a square
wooden stand T has four rods of
glass inserted in its corners, the
rods at one end being less in
height than those at the other.
The tops of these rods having
metal wires A B and c D stretched
between them, across these wires
another wire E F is placed, having
attached to it at right angles another wire G H, having two
points turned in opposite directions at its extremities, so that
when G H is horizontal these two points shall be vertical, one
being presented upwards, and the other downwards. A chain
from A communicates with a conductor kept constantly elec-
trified by a machine.
The electricity coming from the conductor by the chain,
passes along the system of wires, and escapes at the points G
and H. The consequent recoil causes the wire G H to revolve
round E F as an axis, and thereby causes E F to roll up the in-
clined plane.
1780. Electrical orrery. An apparatus called the electrical
orrery is represented in fig- 512. A
metallic ball A rests upon an insu-
lating stand by means of cap within
it, placed upon a fine metallic point
forming the top of the stand.
From the ball A an arm D A pro-
ceeds, the extremity of which is
turned up at E, and formed into a
fine point.
A small ball B rests by means of
a cap on this point, and attached to
it are two arms extended in opposite directions, one terminated
with a small ball c, and the other by a point P presented in the
horizontal direction at right angles to the arm. Another point
p', attached at right angles to the arm D A, is likewise presented
Fig. 512.
MECHANICAL EFFECTS OF ELECTRICITY. 241
in the horizontal direction. By this arrangement the ball A
together with the arm D A is capable of revolving round the
insulating stand, by which motion the ball B will be carried in
a circle round the ball A. The ball B is also capable at the same
time of revolving tin the point which supports it, by which
motion the ball C will revolve round the ball B in a circle. If
electricity be supplied by the chain to the apparatus, the balls A
and B and the metallic rods will be electrified, and the electricity
will escape at the points r and P'. The recoil produced by this
escape will cause the rod D A to revolve round the insulating
pillar, and at the same time the rod P c together with the ball
B to revolve on the extremity of the arm D A. Thus, while the
ball B revolves in a circular orbit round the ball A, the ball C
revolves in a smaller circle round the ball B, the motion re-
sembling that of the moon and earth with respect to the sun.
CHAP. IX.
MECHANICAL EFFECTS OF ELECTRICITY.
1781. Attractions and repulsions of electrified bodies. If a
body charged with electricity be placed near another body, it
will impress upon such body certain motions, which will vary
according as the body thus affected is a conductor or a non-con-
ductor ; according as it is in its natural state or charged with
electricity; and in fine, if charged with
electricity, according as the electricity
is similar or opposite to that with which
the body acting upon it is charged.
Let &,f,g. 513., be the body charged
with electricity, which we shall sup-
pose to be a metallic ball supported on
an insulating column. Let B be the
body upon which it acts, which we
shall suppose to be a small ball sus-
pended by a fine silken thread. We
shall consider successively the cases
above mentioned.
1782. Action of an electrified body
Fig. 513.
242 ELECTRICITY.
on a non-conductor not electrified. 1. Let B be a non-conductor
in its natural state.
In this case no motion will be impressed on B. The elec-
tricity with which A is charged will act by attraction and re-
pulsion on the two opposite fluids which compose the natural
electricity of B, attracting each molecule of one by exactly the
same force as it repels the molecule of the other. No decom-
position of the fluid will take place, because the insulating
property of B will prevent any motion of the fluids upon it,
and will therefore prevent their separation. Each compound
molecule therefore being at once attracted and repelled by equal
forces, no motion will take place.
1783. Action of an electrified body on a non-conductor
charged with like electricity. 2. Let B be charged with elec-
tricity similar to that with which A is charged.
In this case B will be repelled from A. For, according to
what has been explained above, the forces exerted on the natural
electricity of B will be in equilibrium, but the electricity of A
will repel the similar electricity with which B is charged; and
since this fluid cannot move upon the surface of B because of
its insulating virtue, and cannot quit the surface because of the
restraining pressure of the surrounding air, it must adhere
to the surface, and, being repelled by the electricity of A, must
carry with it the ball B in the direction of such repulsion. The
ball B therefore will incline from A, and will rest in such a
position that its weight will balance the repulsive force.
1784. Its action on a non-conductor charged with opposite
electricity. 3. Let B be charged with electricity opposite to
that with which A is charged.
In this case B will be attracted towards A, the distribution of
the fluid upon it not being changed, for the same reasons as in
the last case.
1785. Its action on a conductor not electrified. 4. Let B
be a conductor in its natural state.
In this case the action of the fluid on A attracting one con-
stituent of the natural electricity of B, and repelling the other,
will tend to decompose and separate them ; and since the con-
ducting virtue of B leaves free play to the movement of the
fluids upon it, this attraction and repulsion will take effect, the
attracted fluid moving to the side of B nearest to A, and the
repelled fluid to the opposite side.
MECHANICAL EFFECTS OF ELECTRICITY. 243
To render the explanation more clear, let us suppose that A is
charged with positive electricity.
In that case, the negative fluid of B will accumulate on the
side next A, and the positive fluid on the opposite side. The
negative fluid will therefore be nearer to A than the positive
fluid ; and since the force of the attraction and repulsion in-
creases as theequare of the distance is diminished (1771), and
since the quantity of the negative fluid on the side next A is
equal to the quantity of positive fluid on the opposite side, the
attraction exerted on the former will be greater than the repul-
sion exerted on the latter; and since the fluids are prevented
from leaving B by the restraining pressure of the air, the fluids
carrying with them the ball B will be moved towards A and
will rest in equilibrium, when the inclination of the string is
such that the weight of B balances and neutralizes the attractioa.
If A were charged with negative electricity, the same effects
would be produced, the only difference being that, in that case,
the positive fluid on B would accumulate on the side next A, and
the negative fluid on the opposite side.
Thus it appears that a conducting body in its natural state
is alway attracted by an electrified body, with whichever species
of electricity it be charged.
1786. Its action upon a conductor charged with like electri-
city. 5. Let B be a conductor charged with electricity similar
to that with which A is charged.
In this case the effect produced on B will depend on the re-
lative strength of the charges of electricity of A and B.
The electricity of A will repel the free electricity of B, and
cause it to accumulate on the side of B most remote from A.
But it will also decompose the natural electricity of B, attracting
the fluid of the contrary kind to the side near A, and repelling
the fluid of the same kind to the opposite side. It will follow
from this, that the quantity of the fluid of the same name accu-
mulated at the opposite side of B will be greater than the quan-
tity of fluid of the contrary name collected at the side near A.
While, therefore, the latter is more attracted than the former, by
reason of its greater proximity, it is less attracted by reason of
its lesser quantity. If these opposite effects neutralize each
other, if it lose as much force by its inferior quantity as it
gains by its greater proximity, the attractions and repulsions
of A on B will neutralize each other, and the ball B will not
M 2
244 ELECTRICITY.
move. But if the quantity of electricity with which B is charged
be so small that more attraction is gained by proximity than is
lost by quantity, then the ball B will move towards A. If, how-
ever, the quantity of electricity with which B is charged be so
great that the effect prevail over that of distance, the ball B
will be repelled.
It follows, therefore, from this, that in order* to ensure the
repulsion of the ball B in this case, the charge of electricity
must be so strong as to prevail over that attraction which
would operate on the ball B if it were in its natural state. A very
small electrical charge is, however, generally sufficient for this.
1787. Its action upon a conductor charged icith opposite
electricity. 6. Let B be charged with electricity of a contrary
name to that with which A is charged.
In this case B will always be attracted towards A, for the at-
traction exerted on the fluid with which it is charged will be
added to that which would be exerted on it if it were in its
natural state.
The free electricity on B will be attracted to the side next A,
and the natural fluid will be decomposed, the fluid of the same
name accumulating on the side most remote from A, and the fluid
of the contrary name collecting on the side nearest to A, and
there uniting with the free fluid with which B is charged.
There is therefore a greater quantity of fluid of the contrary
name on that side, than of the same name on the opposite side.
The attraction of the former prevails over the repulsion of the
latter therefore at once by greater quantity and greater proxi-
mity, and is consequently effective.
1788. Attractions and repulsions of pith balls explained.
What has been explained above will render more clearly under-
stood the attractions and repulsions manifested by pith balls
before and after their contact with electrified bodies (1697).
Before contact, the balls, being in their natural state, and being
composed of a conducting material, are always attracted, what-
ever be the electricity with which the body to which they are
presented is charged (1785); but after contact, being charged
with the like electricity, they are repelled (1786).
When touched by the hand, or any conductor which com-
municates with the ground, they are discharged and restored to
their natural state, when they will be again attracted.
If they be suspended by wire or any other conducting thread,
MECHANICAL EFFECTS OF ELECTRICITY. 245
and the stand be a conductor communicating with the ground,
they will lose their electricity the moment they receive it,
The electric fluid in passing through bodies, especially if they
be imperfect conductors, or if the space they present to the fluid
bear a small proportion to its quantity, produces various and
remarkable mechanical effects, displacing the conductors some-
times with great violence.
1789. Strong electric charges rupture imperfect conductors.
Card pierced by discharge of jar. The current of elec-
tricity discharged from a Ley den jar will penetrate several
leaves of paper or card.
A method of exhibiting this effect is represented in fig. 514.
The chain A communicates with the outside coating
of the jar. The card c is placed in such a position
that two metallic points touch it on opposite sides,
terminating near each other. The pillar G, being
glass, intercepts the electricity. The ball of the dis-
charger being put in communication with the inside
coating of the jar, is brought into contact with the
ball B, so that the two points which are on opposite
sides of the card, being in connection with the two
coatings of the jar, are charged with contrary fluids,
which exert on each other such an attraction that they
rush to each other, penetrating the card, which is
Fig. 514. f oun( j } n t ki s case pi erce (j by a hole larger than that
produced by a common pin.
It is remarkable that the burr produced on the surface of the
card is in this case convex on both sides, as if the matter pro-
ducing the hole, instead of passing through the card from one
side to the other, had either issued from the middle of its thick-
ness, emerging at each surface, or as if there were two distinct
prevailing substances passing in contrary directions, each
elevating the edges of the orifice in issuing from it.
The accordance of this effect with the hypothesis of two
fluids is apparent.
1790. Curious fact observed by M. Tr emery. A fact has
been noticed by M. Tremery for which no explanation has
yet been given. That observer found that when the two
points on opposite sides of the card are placed at a certain
distance, one above the other, the hole will not be midway
between them. When the experiment is made in the
M 3
246
ELECTRICITY.
atmosphere, the hole will always be nearer to the negative
fluid. When the apparatus is placed under the receiver of an
air-pump, the hole approaches the positive fluid as the rare-
faction proceeds.
If several cards be placed between the knobs of the universal
discharger (1744), they may be pierced by a strong charge of
ajar or battery, having more than one square foot of coated
surface.
1791. Wood and glass broken by discharge. A rod of wood
half an inch thick may be split by a strong charge transmitted
in the direction of its fibres, and other imperfect conductors
pierced in the same manner.
If a leaf of writing-paper be placed on the stage of the dis-
charger, the electricity passed through it will tear it.
The charge of a jar will penetrate glass. An apparatus for
exhibiting this effect is shown in Jig. 515. It may also be
exhibited by transmitting the charge through the side of a
phial, fig. 516.
A strong charge passed through water scatters the liquid in
all directions around the points of discharge, fig. 517.
1792. Electrical bells. The alternate attraction and re-
pulsion of electrified conductors is prettily illustrated by the
electrical bells.
Fig. 517.
AB and CD, Jig. 518., are two metal rods supported on a glass
pillar. From the ends of these rods four bells A' B' c' D' are
MECHANICAL EFFECTS OF ELECTRICITY. 247
suspended by metallic chains. A central bell G is supported on
the wooden stand which sustains the glass pillar EF, and this
central bell communicates by a chain GK with the ground.
From the transverse rods are also suspended, by silken threads,
four small brass balls H. The transverse rods being put in
communication with the conductor of an electrical machine,
the four bells A' B' c' D' become charged with electricity.
They attract and then repel the balls H, which when repelled
strike the bell G, to which they give up the electricity they
received by contact with the bells A'B'C'D', and this electricity
passes to the ground by the chain G. The bells will thus
continue to be tolled as long as any electricity is supplied by
the conductor to the bells A' B' c' D'.
1793. Repulsion of electrified threads. Let a skein of linen
thread be tied in a knot at each end, and let one end of it be
attached to some part of the conductor of the machine. When
the machine is worked the threads will become electrified and
will repel each other, so that the skein will swell out into a
form resembling the meridians drawn upon a globe.
1 794. Curious effect of repulsion of pith ball. Let a metallic
point be inserted into one of the holes of the prime conductor,
so that, in accordance with what has explained, a jet of electri-
city may escape from it when the conductor is electrified. Let
this jet, while the machine is worked, be received on the
interior of a glass tumbler, by which the surface of the glass
will become charged with electricity.
If a number of pith balls be laid upon a metallic plate com-
municating with the ground, and the tumbler be placed with
its mouth upon the plate, including the balls within it, the balls
will begin immediately leaping violently from the metal and
striking the glass, and this action will continue till all the
electricity with which the glass was charged has been carried
away.
This is explained on the same principle as the former ex-
periments. The balls are attracted by the electricity of the
glass, and when electrified by contact, are repelled. They give
up their electricity to the metallic plate from which it passes
to the ground ; and this process continues until no electricity
remains on the glass of sufficient strength to attract the balls.
1795. Electrical dance. Let a disk of pasteboard or wood,
coated with metallic foil, be suspended by wires or threads of
248 ELECTRICITY.
linen from the prime conductor of an electrical machine, and
let a similar disk be placed upon a stand capable of being
adjusted to any required height. Let this latter disk be placed
immediately under the former, and let it have a metallic com-
munication with the ground. Upon it place small coloured
representations in paper, of dancing figures, which are prepared
for the purpose. When the machine is worked, the electricity
with which the upper disk will be charged will attract the
light figures placed on the lower disk, which will leap upwards ;
and after touching the upper disk and being electrified, will be
repelled to the lower disk, and this jumping action of the figures
will continue so long as the machine is worked. An electrical
dance is thus exhibited for the amusement of young persons.
1796. Curious experiments on electrified water. Let a small
metallic bucket B, Jig. 519., be suspended from the prime con-
ductor of a machine, and let it have a capillary tube c D of the
siphon form immersed in it ; or let
it have a capillary tube inserted in
the bottom ; the bore of the tube
being so small, that water cannot
escape from it by its own pressure.
When the machine is put in opera-
tion, the particles of water becom-
ing electrified, will repel each
other, and immediately an abun-
dant stream will issue from the
tube ; and as the particles of water
F>g- 5 '9- after leaving the tube still exercise
a reciprocal repulsion, the stream will diverge in the form of a
brush.
If a sponge saturated with water be suspended from the
prime conductor of the machine, the water, when the machine
is first worked, will drop slowly from it ; but when the con-
ductor becomes strongly electrified, it will descend abundantly,
and in the dark will exhibit the appearance of a shower of
luminous rain.
1797. Experiment with electrified sealing-wax. Let a piece
of sealing-wax be attached to the pointed end of a metallic rod ;
set fire to the wax, and when it is in a state of fusion blow out
the flame, and present the wax within a few inches of the
prime conductor of the machine. Strongly electrified myriads
THERMAL EFFECTS OF ELECTRICITY. 249
of fine filaments will issue from the wax towards the conductor,
to which they will adhere, forming a sort of net-work resem-
bling wool. This effect is produced by the positive electricity
of the conductor decomposing the natural electricity of the wax ;
and the latter being a conductor when in a state of fusion, the
negative electricity is accumulated in the soft part of the wax
near the conductor, while the positive electricity escapes along
the metallic rod. The particles of wax thus negatively elec-
trified being attracted by the conductor, are drawn into the
filaments above mentioned.
1798. Electrical see-saw. The electrical see-saw a b, Jig.
520., is a small strip of wood covered over with silver leaf
or tinfoil, insulated on c like a balance. A slight prepon-
derance is given to it at a, so that it rests
on a wire having a knob m at its top ; p
is a similar metal ball insulated. Connect
p with the interior, and m with the ex-
Fig. 520. terior coating of the jar, charge it, and
the see-saw motion of a b will commence from causes similar
to those which excited the movements of the pith balls.
CHAP. X.
THERMAL EFFECTS OF ELECTRICITY.
1799. A current of electricity passing over a conductor raises
its temperature. If a current of electricity pass over a con-
ductor, as would happen when the conductor of an electrical
machine is connected by a metallic rod with the earth, no change
in the thermal condition of the conductor will be observed, so
long as its transverse section is so considerable as to leave
sufficient space for the free passage of the fluid. But, if its
thickness be diminished, or the quantity of fluid passing over it
be augmented, or, in general, if the ratio of the fluid to the
magnitude of the space afforded to it be increased, the con-
ductor will be found to undergo an elevation of temperature,
which will be greater the greater the quantity of the electricity
and the less the space supplied for its passage.
250 ELECTRICITY.
1800. Experimental verification. Wire heated, fused, and
burned. If a piece of wire of several inches in length be
placed upon the stage of the universal discharger (1744), a feeble
charge transmitted through it will sensibly raise its tempera-
ture. By increasing the strength of the charge, its temperature
may be elevated to higher and higher points of the thermo-
metric scale ; it may be rendered incandescent, fused, vaporized,
and, in fine, burned.
With the powerful machine of the Taylerian Museum at
Haarlem, Van Marum fused pieces of wire above 70 feet in
length.
Wire may be fused in water ; but the length which can be
melted in this way is always less than in air, because the liquid
robs the metal of its heat more rapidly than air.
A narrow ribbon of tinfoil, from 4 to 6 inches in length,
may be volatilized by the discharge of a common battery. The
metallic vapour is in this case oxidized in the air, and its fila-
ments float like those of a cobweb.
1801. Thermal effects are greater as the conducting power
is less. These thermal effects are manifested in different
degrees in different metals, according to their varying conduct-
ing powers. The worst conductors of electricity, such as pla-
tinum and iron, suffer much greater changes of temperature by
the same charge than the best conductors, such as gold and
copper. The charge of electricity, which only elevates the
temperature of one conductor, will sometimes render another
incandescent, and will volatilize a third.
1802. Ignition of metals. If a fine silver wire be extended
between the rods of the universal discharger (1744), a strong
charge will make it burn with a greenish flame. It will pass
off in a greyish smoke. Other metals may be similarly ignited,
each producing a flame of a peculiar colour. If the experi-
ments be made in a receiver, the products of the combustion
being collected, will prove to be the metallic oxides.
If a gilt thread of silk be extended between the rods of the
discharger, the electricity will volatilize or burn the gilding,
without affecting the silk. The effect is too rapid to allow the
time necessary for the heat to affect the silk.
A strip of gold or silver leaf placed between the leaves of
paper, being extended between the rods of the discharger, will
be burnt by a discharge from ajar having two square feet of
THERMAL EFFECTS OF ELECTRICITY. 251
coating. The metallic oxide will in this case appear on the
paper as a patch of purple colour in the case of gold, and of
grey colour in that of silver.
A spark from the prime conductor of the great Haarlem
machine burnt a strip of gold leaf twenty inches long by an inch
and a half broad.
1803. Effect on fulminating silver. The heat developed in
the passage of electricity through combustible or explosive
substances, which are imperfect conductors, causes their com-
bustion or explosion.
A small quantity of fulminating silver placed on the point of
a knife, explodes if brought within a few feet of the conductor
of an electrical machine in operation. In this case the explo-
sion is produced by induction.
1804. Electric pistol. The electrical pistol or cannon is
charged with a mixture of hydrogen and oxygen gases, in the
proportion necessary to form water. A conducting wire ter->
minated by a knob is inserted in the touch-hole, and the gases
are confined in the barrel by the bullet. An electric spark
imparted to the ball at the touch-hole, causes the explosion of
the gases. This explosion is produced by the sudden combi-
nation of the gases, and their conversion into water, which, in
consequence of the great quantity of heat developed, is instantly
converted into steam of great elasticity, which, by its expansion,
forces the bullet from the barrel in the same manner as do the
gases which result from the explosion of gunpowder.
1805. Ether and alcohol ignited. Ether or alcohol may be
fired by passing through it an electric discharge. Let cold
water be poured into a wine-glass, and let a thin stratum of
ether be carefully poured upon it. The ether being lighter will
float on the water. Let a wire or chain connected with the
prime conductor of the machine be immersed in the water, and,
while the machine is in action, present a metallic ball to the
surface of the ether. The electric charge will pass from the
water through the ether to the ball, and will ignite the ether.
Or, if a person standing on an insulating stool, and holding in
one hand a metal spoon filled with ether, present the surface of
the ether to a conductor, and at the same time apply the other
hand to the prime conductor of a machine in operation, the
electricity will pass from the prime conductor through the
body of the person to the spoon, and from the spoon through
252 ELECTRICITY.
the ether, to the conductor to which the ether is presented, and
in so passing will ignite the ether.
1806. Resinous powder burned The electric charge trans-
mitted through fine resinous powder, such as that of colophony,
will ignite it. This experiment may be performed either
by spreading the powder on the stage of the discharger (1744),
or by impregnating a hank of cotton with it ; or, in a still more
striking manner, by sprinkling it on the surface of water con-
tained in an earthenware saucer.
1807. Gunpowder exploded Gunpowder may, in like
manner, be ignited by electricity. This experiment is most
conveniently exhibited by placing the powder in a small wooden
cup, and conducting the electric charge along a moist thread,
six or seven inches long, attached to the arm of the discharger,
which is connected with the negative coating of a jar, and
the charge, in its passage from one rod of the discharger to the
other, will ignite the powder.
1808. Electric mortars. The electric mortar, fig. 521., is
an apparatus by which the gunpowder is ignited
by passing an electric charge through it. The
mixed gases may also be used in this instru-
Fig. 521. ment .
Common air or gas, not being explosive, is heated so suddenly
and intensely by transmitting through it an electric charge,
that it will expand so as to project the ball from
the mortar.
1809. Kinnersley's electrometer. Kinnersley's
electric thermometer,^. 522., is an instrument in-
tended to measure the degree of heat developed in
the passage of an electric charge by the expansion
of air. The discharge takes place between the two
balls bb' in the glass cylinder, and the air confined
in the cylinder being heated expands, presses upon
the liquid contained in the lower part of the cy-
linder, and causes the liquid in the tube tf to rise.
The variation of the column of liquid in the tube
ft' indicates the elevation of temperature.
LUMINOUS EFFECTS OF ELECTRICITY. 253
CHAP. XL
LUMINOUS EFFECTS OF ELECTRICITY.
1810. Electric fluid not luminous. The electric fluid is not
luminous. An insulated conductor, or a Ley den jar or battery,
however strongly charged, is never luminous so long as the
electric equilibrium is maintained and the fluid continues in
repose. But if this equilibrium be disturbed, and the fluid
move from one conductor to another, such motion is, under
certain conditions, attended with luminous phenomena.
1811. Conditions under which light is developed by an elec-
tric current. One of the conditions necessary to the develop-
ment of light by the motion of the electric fluid is, that the
electricity should have a certain intensity. If the conductor of
an ordinary electric machine while in operation be connected
with the ground by a thick metallic wire, the current of the
fluid which flows along the wire to the ground will not be
sensibly luminous ; but if the machine be one of great power,
such for example as the Taylerian machine of Haarlem, an
iron wire of 60 or 70 feet long communicating with the ground
and conducting the current will be surrounded by a brilliant
light. The intensity of the electricity necessary to produce
this effect depends altogether on the properties of the medium
in which the fluid moves. Sometimes electricity of feeble in-
tensity produces a strong luminous effect, while in other cases
electricity of the greatest intensity developes no sensible de-
gree of light.
It has been already explained that the electric fluid with
which an insulated conductor is charged is retained upon it
only by the pressure of the surrounding air. According as
this pressure is increased or diminished, the force necessary to
enable the electricity to escape will be increased or diminished,
and in the same proportion.
When a conductor A in communication with the ground ap-
proaches an insulated conductor B charged with electricity,
the natural electricity of B will be decomposed, the fluid of the
same name as that which charges A escaping to the earth, and
the fluid of the opposite name accumulating on the side of B
254 ELECTRICITY.
next to A. At the same time, according to what has been ex-
plained (1785), the fluid on A accumulates on the side nearest
to B. These two tides of electricity of opposite kinds exert a
reciprocal attraction, and nothing prevents them from rushing
together and coalescing, except the pressure of the intervening
air. They will coalesce, therefore, so soon as their mutual
attraction is so much increased as to exceed the pressure of
the air.
This increase of mutual attraction may be produced by
several causes. First, by increasing the charge of electricity
upon the conductor A, for the pressure of the fluid will be pro-
portional to its depth or density. Secondly, by diminishing the
distance between A and B, for the attraction increases in the
same ratio as the square of that distance is diminished ; and
thirdly, by increasing the conducting power of either or both
of the bodies A and B, for by that means the electric fluids, being
more free to move upon them, will accumulate in greater quan-
tity on the sides of A and B which are presented towards each
other. Fourthly, by the form of the bodies A and B, for ac-
cording to what has been already explained (1776), the fluids
will accumulate on the sides presented to each other in greater
or less quantity, according as the form of those sides approaches
to that of an edge, a corner, or a point.
When the force excited by the fluids surpasses the restrain-
ing force of the intervening air, they force their passage through
the air, and, rushing towards each other, combine. This move-
ment is attended with light and sound. A light appears to be
produced between the points of the two bodies A and B, which
has been called the electric spark, and this luminous phenomenon
is accompanied by a sharp sound like the crack of a whip.
1812. The electric spark. The luminous phenomenon called
the electric spark does not consist, as the name would imply, of
a luminous point which moves from the one body to the other.
Strictly speaking, the light manifests no progressive motion.
It consists of a thread of light,
which for an instant seems to con-
nect the two bodies, and in gene-
ral is not extended between them
in one straight unbroken direction
like a thread which might be
LUMINOUS EFFECTS OF ELECTRICITY. 255
stretched tight between them, but has a zig-zag form resembling
more or less the appearance of lightning,^. 523.
1813. Electric aigrette. If the part of either of the bodies
A or B which is presented to the other have the form of a
point, the electric fluid will escape, not in the form of a spark,
but as an aigrette or brush light, the diverging rays of which
sometimes have the length of two or three inches. A very
feeble charge is sufficient to cause the .escape of the fluid when
the body has this form (1776).
1814. The length of the spark. If the knuckle of the finger
or a metallic ball at the end of a rod held in the hand be pre-
sented to the prime conductor of a machine in operation, a spark
will be produced, the length of which will vary with the power
of the machine.
By the length of the spark must be understood the greatest
distance at which the spark can be transmitted.
A very powerful machine will so charge its prime conductor
that sparks may be taken from it at the distance of 30 inches.
1815. Discontinuous conductors produce luminous effects.
Since the passage of the electricity produces light wherever the
metallic continuity, or more generally wherever the continuity
of the conducting material is interrupted, these luminous effects
may be multiplied by so arranging the conductors that there
shall be interruptions of continuity arranged in any regular or
desired manner.
1816. Various experimental illustrations. If a number of
metallic beads be strung upon a thread of silk, each bead being
separated from the adjacent one by a knot on the silk so as to
break the contact, a current of electricity sent through them will
produce a series of sparks, a separate spark being produced be-
tween every two successive beads. By placing one end of such
a string of beads in contact with the conductor of the machine,
and the other end in metallic communication with the ground,
a chain of sparks can be maintained so long as the machine is
worked.
The string of beads may be disposed so as to form a variety
of fancy designs, which will appear in the dark in characters of
light.
Similar effects may be produced by attaching bits of metallic
foil to glass. Sparkling tubes and plates are contrived in this
256 ELECTRICITY.
manner, by which amusing expe-
riments are exhibited. A glass
plate is represented \njig. 524. by
which a word is made to appear
in letters of light in a dark room.
The letters are formed by attaching
lozenge-shaped bits of tinfoil to the glass, disposed in the proper
form. In the same manner designs may be formed on the inner
surface of glass tubes, or, in fine, of glass vessels of any form.
In these cases the luminous characters may be made to ap-
pear in lights of various colours, by using spangles of different
metals, since the colour of the spark varies with the metal.
1817. Effect of rarefied air, When the electric fluid passes
through air, the brilliancy and colour of the light evolved de-
pends on the density of the air. In rarefied air the light is
more diffused and less intense, and acquires a reddish or violet
colour. Its colour, however, is affected, as has been just stated,
by the nature of the conductors between which the current
flows. When it issues from gold the light is green, from silver
red, from tin or zinc white, from water deep yellow inclining
to orange.
It is evident that these phenomena supply the means of pro-
ducing electrical apparatus by which an infi-
nite variety of beautiful and striking luminous
effects may be produced.
When the electricity escapes from a metallic
point in the dark, it forms an aigrette, ./fy. 525.,
which will continue to be visible so long as the
machine is worked.
The luminous effect of electricity in rarefied
air is exhibited by an apparatus,^. 526., consisting of a glass
Fig. 526.
receiver bb', which can be screwed upon the plate of an air-
pump and partially exhausted. The electric current passes
LUMINOUS EFFECTS OF ELECTRICITY. 257
between two metallic balls attached to rods, winch slide in air-
tight collars in the covers of the receiver bb'.
It is observed that the aigrettes formed by the negative fluid
are never as long or as divergent as those formed by the posi-
tive fluid, an effect which is worthy of attention as indicating a
distinctive character of the two fluids.
1818. Experimental imitation of the auroral light. This
phenomenon may be exhibited in a still more remarkable
manner by using, instead of the receiver bb', a glass tube two or
three inches in diameter, and about thirty inches in length. In
this case a pointed wire being fixed to the interior of each of the
caps, one is screwed upon the plate of the air-pump, while the
external knob of the other is connected by a metallic chain
with the prime conductor of the electrical machine. When the
machine is worked in the dark, a succession of luminous phe-
nomena will be produced in the tube, which bear so close a
resemblance to the aurora borealis as to suggest the most pro-
bable origin of that meteor. When the exhaustion of the tube
is nearly perfect, the whole length of the tube will exhibit a
violet red light. If a small quantity of air be admitted, luminous
flashes will be seen to issue from the two points attached to the
caps. As more and more air is admitted, the flashes of light
which glide in a serpentine form down the interior of the tube
will become more thin and white, until at last the electricity
will cease to be diffused through the column of air, and will ap-
pear as a glimmering light at the two points.
1819. Phosphorescent effect of the spark. The electric
spark leaves upon certain imperfect conductors a trace which
continues to be luminous for several seconds, and sometimes
even so long as a minute after the discharge of the spark.
The colour of this species of phosphorescence varies with the
substances on which it is produced. Thus white chalk pro-
duces an orange light. With rock crystal the light first red
turns afterwards white. Sulphate of barytes, amber, and loaf
sugar render the light green, and calcined oyster-shell gives all
the prismatic colours.
1820. Leichtenberg's figures. The spark in many cases
produces effects which not only confirm the hypothesis of two
fluids, but indicate a specific difference between them. One of
these has been already noticed. The experiment known as
258 ELECTRICITY.
Leichtenberg's figures presents another example of this. Let
two Leyden jars be charged, one with positive, the other with
negative electricity; and let sparks be given by their knobs to
the smooth and well-dried surface of a cake of resin. Let the
surface of the resin be then slightly sprinkled with powder of
Semen lycopodii, or flowers of sulphur, and let the powder thus
sprinkled be blown off. A part will remain attached to the
spots where the electric sparks were imparted. At the spot
which .received the positive spark, the adhering powder will
have the form of a radiating star ; and at the point of the nega-
tive spark it will have that of a roundish clouded spot.
1821. Experiments indicating specific differences between the
two fluids. If lines and figures be traced in like manner on
the cake of resin, some with the positive and some with the
negative knob, and a powder formed of a mixture of sulphur
and minium be first sprinkled over the cake and then blown
off, the adhering powder will mark the traces of the two fluids
imparted by the knobs, the traces of the positive fluid being
yellow, and those of the negative red. In this case the sul-
phur is attracted by the positive electricity, and is therefore
itself negative ; and the minium by the negative electricity, and
is therefore itself positive. The mechanical effects of the two
fluids are also different, the sulphur powder being arranged in
divergent lines, and the minium in more rounded and even
traces.
Let two Leyden jars, one charged with positive and the
other with negative electricity, be placed upon a plate of glass
coated at its under surface with tinfoil at a distance of six or
eight inches asunder, and let the surface of the. glass between
them be sprinkled with semen lycopodii. Let the jars be then
moved towards each other, and let their inner coatings be
connected by a discharging rod applied to their knobs. A
spark will pass between their outer coatings through the
powder, which it will scatter on its passage. The path of the
positive fluid will be distinguishable from that of the negative
fluid, as before explained, by the peculiar arrangement of the
powder ; and this difference will disappear near the point where
the two fluids meet, where a large round speck is sometimes
seen bounded by neither of the arrangements which charac-
terize the respective fluids.
1822. Electric light above the barometric column. The
LUMINOUS EFFECTS OF ELECTRICITY.
259
Fig. 5-27.
electric light is developed in every form of elastic fluid and
vapour when its density is very inconsiderable.
A remarkable example of this is presented in
the common barometer. When the mercurial
column is agitated so as to oscillate in the
tube, the space in the tube above the column
becomes luminous, and is visibly so in the
dark. This phenomenon is caused by the
effect of the electricity developed by the fric-
tion of the mercury and the glass upon the
atmosphere of mercurial vapour which fills the
space above the column in the tube.
1823. Cavendish's electric barometer. The
electric barometer of Cavendish, Jig. 527., il-
lustrates this in a striking manner. Two ba-
rometers are connected at the top by a curved
tube, so that the spaces above the two columns
communicate with each other. When the in-
strument is agitated so as to make the columns oscillate,
electric light appears in the curved tube.
1824. Luminous effects produced by imperfect conductors.
The electric spark or charge transmitted by means of the
universal discharger and Leyden jar or battery through various
imperfect conductors, produces luminous effects which are
amusing and instructive.
Place a small melon, citron, apple, or any similar fruit on the
stand of the discharger ; arrange the wires so that their ends
are not far asunder, and at the moment when the jar is dis-
charged the fruit becomes transparent and luminous. One or
more eggs may be treated in the same manner if a small wooden
ledge be so contrived that their ends may just touch, and the
spark can be sent through them all. Send a charge through a
lump of pipe-clay, a stick of brimstone, or a glass of water, or
any coloured liquid, and the entire mass of the substance will
for a short time be rendered luminous. As the phosphorescent
appearance induced is by no means powerful, it will be neces-
sary that these experiments should be performed in a dark
room, and indeed the effect of the other luminous electrical
phenomena will be heightened by darkening the room.
1825. Attempt to explain electric light, the thermal hypo-
thesis. No explanation of the physical cause of the electric
260 ELECTRICITY.
spark, or of the luminous effects of electricity, has yet been
proposed which has commanded general assent. It appears
certain, for the reasons already stated, and from a great variety
of phenomena, that the electric fluids themselves are not lu-
minous. The light, therefore, which attends their motion must
be attributed to the media, or the bodies through which or
between which the fluids move. Since it is certain that the
passage of the fluids through a medium developes heat in
greater or less quantity in such medium, and since heat, when
it attains a certain point, necessarily developes light, the most
obvious explanation of the manifestation of light was to ascribe
it to a momentary and extreme elevation of temperature, by
which that part of the medium, or the body traversed by the
fluid, becomes incandescent.
According to this hypothesis, the electric spark and the flash
of lightning are nothing more than the particles of air, through
which the electricity passes, rendered luminous by intense heat.
There is nothing in this incompatible with physical analogies.
Flame we know to be gas rendered luminous by the ardent heat
developed in the chemical combinations of which combustion is
the effect.
1826. Hypothesis of decomposition and recomposition.
According to another hypothesis, first advanced by Hitter and
afterwards adopted by Berzelius, Oersted, and Sir H. Davy,
the electric fluids have strictly speaking no motion of transla-
tion whatever, and never in fact desert the elementary mole-
cules of matter of which, according to the spirit of this hypo-
thesis, they form an essential part. Each molecule or atom
composing a body is supposed to be primitively invested with
an atmosphere of electric fluid, positive or negative, as the case
may be, which never leaves it. Bodies are accordingly classed
as electro-positive or electro-negative, according to the fluid
attracted to their atoms. Those atoms which are positive
attract so much negative fluid, and those which are negative
so much positive fluid, as is sufficient to neutralize the forces
of their proper electricities, and then the atoms are unelectrized
and in their natural state.
When a body is charged with positive electricity, its atoms
act by induction upon the atoms of adjacent bodies, and these
upon the atoms next beyond them, and so on. The fluids in
the series of atoms through which the electricity is supposed to
PHYSIOLOGICAL EFFECTS OF ELECTRICITY. 261
pass, assumes a polar arrangement such as that represented in
fig. 528.
123456789
Fig. 528.
The first atom of the series being surcharged with + electricity
acts by induction on the second, and decomposes its natural
electricity, the negative fluid being attracted to the side near
the first atom, and the positive repelled to the side near the
third atom. The same effect is produced by atom 2 on atom 3,
by atom 3 on atom 4, and so on. The surplus positive fluid on
1 then combines with and neutralizes the negative fluid on 2 ;
and, in like manner, the positive fluid on 2 combines with and
neutralizes the negative fluid on 3, and so on until the last
atom of the series is left surcharged with positive electricity.
Such is the hypothesis of decomposition and recomposition
which is at present in most general favour with the scientific
world.
The explanation which it affords of the electric spark and
other luminous electric effects, may be said to consist in trans-
ferring the phenomenon to be explained from the bodies them-
selves to their component atoms, rather than in affording an
explanation of the effect in question inasmuch as the produc-
tion of light between atom and atom by the alternate decompo-
sition and recomposition of the electricities stands in as much
need of explanation as the phenomenon proposed.
1827. Cracking noise attending electric spark. The sound
produced by the electric discharge is obviously explained by
the sudden displacement of the particles of the air, or other
medium through which the electric fluid passes.
CHAP. XII.
PHYSIOLOGICAL EFFECTS OF ELECTRICITY.
1828. Electric shock explained. The material substances
which enter into the composition of the bodies of animals are
262 ELECTRICITY.
generally imperfect conductors. When such a body, therefore,
is placed in proximity with a conductor charged with electricity,
its natural electricity is decomposed, the fluid of a like name
being repelled to the side more remote from, and the fluid of the
contrary name being attracted to the side nearest to, the elec-
trified body. If that body be very suddenly removed from or
brought near to the animal body, the fluids of the latter will
suddenly suffer a disturbance of their equilibrium, and will
either rush towards each other to recombine, or be drawn from
each other, being decomposed; and owing to the imperfection of
the conducting power of the fluids and solids composing the
body, the electricity in passing through it will produce a mo-
mentary derangement, as it does in passing through air, water,
paper, or any other imperfect conductor. If this derangement
do not exceed the power of the parts to recover their position
and organization, a convulsive sensation is felt, the violence of
which is greater or less according to the force of electricity
and the consequent derangement of the organs ; but if it exceed
this limit, a permanent injury, or even death, may ensue.
1829. Secondary shock. It will be apparent from this, that
the nervous effect called the electric shock does not require
that any electricity be actually imparted to, abstracted from, or
passed through the body. The momentary derangement of the
natural electricity is sufficient to produce the effect with any
degree of violence.
The shock produced thus by induction, without transmitting
electricity through the body, is sometimes called the secondary
shock.
The physiological effects of electricity are extremely various,
according to the quantity and intensity of the charge, and ac-
cording to the part of the body affected by it, and according to
the manner in which it is imparted.
1830. Effect produced on the skin by proximity to an electri-
fied body. When the back of the hand is brought near to the
glass cylinder of the machine, at the part where it passes from
under the silk flap, and when therefore it is strongly charged
with electricity, a peculiar sensation is felt on the skin, re-
sembling that which would be produced by the contact of a cob-
web. The hairs of the skin being negatively electrified by in-
duction, are attracted and drawn against their roots with a slight
force.
PHYSIOLOGICAL EFFECTS OF ELECTRICITY. 263
1831. Effect of the sparks taken on the knuckle. The effect
of the shock produced by a spark taken from the prime con-
ductor by the knuckle is confined to the hand ; but with a very
powerful machine, it will extend to the elbow.
1832. Methods of limiting and regulating the shock by ajar,
The effects of the discharge of a Leyden jar extend through
the whole body. The shock may, however, be limited to any
desired part or member by placing two metallic plates con-
nected with the two coatings of the jar on opposite sides of the
part through which it is desired to transmit the shock.
1833. Effect of discharges of various force. The violence
of the shock depends on the magnitude of the charge, and may
be so intense as to produce permanent injury. The discharge
of a single jar is sufficient to kill birds, and other smaller
species of animals. The discharge of a moderate-sized battery
will kill rabbits, and a battery of a dozen square feet of coated
surface will kill a large animal, especially if the shock be trans-
mitted through the head.
1834. Phenomena observed in the autopsis after death by the
shock. When death ensues in such cases, no organic lesion or
other injury or derangement has been discovered by the autopsis ;
nevertheless, the violence of the convulsions which are mani-
fested when the charge is too feeble to destroy life, indicates a
nervous derangement as the cause of death.
1835. Effects of a long succession of moderate discharges.
A succession of electric discharges of moderate intensity, trans-
mitted through certain parts of the body, produce alternate
contraction and relaxation of the nervous and muscular organs,
by which the action of the vascular system is stimulated and
the sources of animal heat excited.
1836. Effects upon a succession of patients receiving the
same discharge. The electric discharge of a Leyden jar may
be transmitted through a succession of persons placed hand in
hand, the first communicating with the internal, and the last
with the external coating of the jar.
In this case, the persons placed at the middle of the series
sustain a shock less intense than those placed near either ex-
tremity, another phenomenon which favours the hypothesis of
two fluids.
1837. Remarkable experiments of Nollet, Dr. Watson, and
others. A shock has in this manner been sent through a
264 ELECTRICITY.
regiment of soldiers. At an early period in the progress of
electrical discovery, M. Nollet transmitted a discharge through
a series of 180 men ; and at the convent of Carthusians a chain
of men being formed extending to the length of 5400 feet, by
means of metallic wires extended between every two persons
composing it, the whole series of persons was affected by the
shock at the same instant.
Experiments on the transmission of the shock were made in
London by Dr. Watson, in the presence of the Council of the
Royal Society, when a circuit was formed by a wire carried from
one side of the Thames to the other over Westminster Bridge.
One extremity of this wire communicated with the interior of a
charged jar, the other was held by a person on the opposite bank
of the river. This person held in his other hand an iron rod,
which he dipped in the river. On the other side near the jar
stood another person, holding in one hand a wire communicating
with the exterior coating of the jar, and in the other hand an
iron rod. This rod he dipped into the river, when instantly
the shock was received by both persons, the electric fluid
having passed over the bridge, through the body of the person
on the other side, through the water across the river, through
the rod held by the other person, and through his body to the
exterior coating of the jar. Familiar as such a fact may now
appear, it is impossible to convey an adequate idea of the
amazement bordering on incredulity with which it was at that
time witnessed.
CHAP. XIII.
CHEMICAL AND MAGNETIC EFFECTS OF ELECTRICITY.
1838. Phenomena which supply the basis of the electro-chemical
theory. If an electric charge be transmitted through certain
compound bodies they will be resolved into their constituents,
one component always going in the direction of the positive,
and the other of the negative fluids. This class of phenomena
has supplied the basis of the electro-chemical hypothesis already
briefly noticed (1826). The constituent which goes to the posi-
CHEMICAL EFFECTS OF ELECTRICITY. 265
tive fluid is assumed to consist of atoms which are electrically
negative, and that which goes to the negative fluid, as con-
sisting of atoms electrically positive.
1839. Faraday's experimental illustration of this. This
class of phenomena is more prominently developed by voltaic
electricity, and will be more fully explained in the following
Book. For the present it will therefore be suflicient to indicate
an example of this species of decomposition by the electricity of
the ordinary machine. The following experiment is due to
Professor Faraday.
Lay two pieces of tinfoil T T',Jig. 528. a, on a glass plate, one
being connected with the prime conductor of the machine, and
Fig. 528 a.
the other with the ground. Let two pieces of platinum wire
PP', resting on the tinfoil, be placed with their points on a drop
of the solution of the sulphate of copper c, or on a piece of
bibulous paper wetted with sulphate of indigo in muriatic acid,
or iodide of potassium in starch, or litmus paper wetted with a
solution of common salt or of sulphate of soda, or upon tur-
meric paper containing sulphate of soda.
In all these cases the solutions are decomposed : in the first,
the copper goes to the positive wire ; in the second the indigo is
bleached by the chlorine discharged at the same wire ; in the
third the iodine is liberated at the same wire ; in the fourth
the litmus paper is reddened by the acid evolved at the positive
wire, and when muriatic is used, it is bleached by the chlorine
evolved at the same wire ; and, in fine, in the fifth case, the
turmeric paper is reddened by the alkali evolved at the negative
wire.
1840. Effect of an electric discharge on a magnetic needle.
When a stream of electricity passes over a steel needle or
bar of iron, it produces a certain modification in its magnetic
state. If the needle be in its natural state it is rendered mag-
netic. If it be already magnetic its magnetism is modified,
II. N
266 ELECTRICITY.
being augmented or diminished in intensity, according to
certain conditions depending on the direction of the current
and the position of the magnetic axis of the needle ; or it may
have its magnetism destroyed, or even its polarity reversed.
This class of phenomena, like the chemical effects just men-
tioned, are, however, much more fully developed by voltaic
electricity ; and we shall therefore reserve them to be explained
in the following Book. Meanwhile, however, the following
experiments will show how common electricity may develop
them.
1841. Experimental illustration of this. Place a narrow
strip of copper, about two inches in length, on the stage of the
universal discharger, and over it place a leaf of any insulating
material, upon which place a sewing needle transversely to the
strip of copper. Transmit several strong charges of electricity
through the copper. The needle will then be found to be mag-
netized, the end lying on the right of the current of electricity
being its north pole.
If the same experiment be repeated, reversing the position of
the needle, it will be demagnetized. But by repeating the
electric discharges a greater number of times, it will be mag-
netized with the poles reversed.
67
BOOK THE FOURTH.
VOLTAIC ELECTRICITY.
CHAP. I.
SIMPLE VOLTAIC COMBINATION.
1842. Discovery of galvanism. In tracing the progress of
physical science, the greatest discoveries are frequently found
to originate, not in the sagacity of observers, but in circum-
stances altogether fortuitous. One of the most remarkable
examples of this is presented by Voltaic Electricity. Speaking
of the voltaic pile, Arago, in his Eloge de Volta, says, that
" this immortal discovery arose in the most immediate and
direct manner, from an indisposition with which a Bolognese
lady was affected in 1790, for which her medical adviser
prescribed frog-broth."
Galvani, the husband of the lady, was Professor of Anatomy
in the University of Bologna. It happened that several frogs,
prepared for cooking, lay upon the table of his laboratory, near
to which his assistant was occupied with an electrical machine.
On taking sparks from time to time from the conductor, the
limbs of the frogs were affected with convulsive movements
resembling vital action.
This was the effect of the inductive action of the electricity
of the conductor upon the highly electroscopic organs of the
frogs ; but Galvani was not sufficiently conversant with this
branch of physics to comprehend it, and consequently regarded
it as a new phenomenon. He proceeded to submit the limbs of
frogs to a course of experiments, with the view to ascertain the
cause of what appeared to him so strange. For this purpose,
he dissected several frogs, separating the legs, thighs, and
lower part of the spinal column from the remainder, so as to lay
bare the lumbar nerves. He then passed copper hooks through
that part of the dorsal column which remained above the
junction of the thighs, without any scientific object, but merely
268 VOLTAIC ELECTRICITY.
for the convenience of suspending them until required for
experiment. It chanced, also, that he suspended these copper
hooks upon the iron bar of the balcony of his window, when,
to his inexpressible astonishment, he found that whenever the
wind or any other accidental cause brought the muscles of the
leg into contact with the iron bar, the limbs were affected by
convulsive movements similar to those produced by the sparks
taken from the conductor of the electric machine.
This fact, reproduced and generalized, supplied the founda-
tion of the theory of animal electricity propounded by Galvani,
and for a considerable time universally accepted. In this
theory it was assumed, that in the animal economy there exists
a specific source of electricity; that at the junction of the
nerves and muscles this electricity is decomposed, the positive
fluid passing to the nerve, and the negative to the muscle ; and
that, consequently, the nerve and muscle are in a state of
relative electrical tension, analogous to that of the internal and
external coatings of a charged Leyden jar. When, under these
circumstances, rods of metal z c, fig. 529., are applied, one to
the nerve, and the other to the muscle, the opposite electricities
rush towards each other along the conducting rods ; a dis-
charge of the nerve and muscle takes
place, like that of the Leyden jar ;
and this momentary derangement of
the electrical condition of the organ
produces the convulsive movement.
1843. Yalta's correction of Gal-
vanfs theory. Volta, then Professor
of Natural Philosophy at Como, and
afterwards at Pavia, repeating the
Fig. 529. experiments of Galvani, overturned
his theory by various ingenious experimental tests, one of which
consisted in showing that the effects of the electric shock
were equally produced when both metallic rods were applied
to the muscle, neither touching the nerve. He contended that
Galvani, in taking the nerve and muscle to represent the coat-
ings of the Leyden jar, and the metallic rods the discharging
conductor, had precisely inverted the truth, for that the rods
represented the jar, and the nerve and muscle the conductor.
If the rods, as Galvani supposed, played the part of the
metallic conductor, communicating between the opposite elec-
SIMPLE VOLTAIC COMBINATION. 269
tricities imputed to the nerve and muscle, a single rod of one
uniform metal would serve this purpose, not only as well, but
better than two rods of different metals ; whereas, the presence
of two different metals in contact, was essential to the develop-
ment of the phenomenon.
In fine, Volta maintained, and ultimately proved, that the
electricity decomposed, was not that of the nerve and muscle,
but that of the metallic rods ; that the seat of the decomposition
was not the junction of the nerve and muscle, but the junction
of the two metals ; that the positive and negative fluids passed,
not upon the nerve and muscle, but upon the iron and copper
forming the rods flowing in opposite directions from their point
of junction; and that, in fine, the nerve and muscle, or the latter
alone, served merely as the conductor by which the opposite
electricities developed on the metals were recomposed, exactly
as they would if placed between the internal and external coat-
ings of a charged Leyden jar.
1844. Theory of animal electricity exploded. After a con-
flict of some years' duration, the animal electricity of Galvani
fell before the irresistible force of the reasoning and experiments
of Volta, whose theory obtained general acceptation. This form
of electric agency has since been denominated indifferently,
GALVANISM Or VOLTAIC ELECTRICITY.
1845. Contact hypothesis of Volta. According to the hy-
pothesis of Volta, now known as the CONTACT THEORY, any two
different metals, or, more generally, any two different bodies
which are conductors of electricity, being placed in contact, a
spontaneous decomposition of their natural electricity will be
effected at their surface of contact, the positive fluid moving
from such surface and diffusing itself over the one, and the ne-
gative moving in the contrary direction and diffusing itself over
the other, the surface of contact constituting a neutral line
separating the two fluids.
1846. Electro-motive force. This power of electric decom-
position was called by Volta, ELECTRO-MOTIVE FORCE.
Different bodies placed in contact manifest different electro-
motive forces, the energy of the electro-motive force being
measured by the quantity of electricity decomposed.
Its direction and intensity. The electro- motive force acts
on the two fluids in opposite directions, but it will be convenient
to designate its direction by that of the positive fluid.
3
270
VOLTAIC ELECTRICITY.
To indicate, therefore, the electro-motive force developed when
any two conductors are placed in contact, it is necessary to
assign the energy and direction of such force, which is done by
showing the intensity of the electricity developed, and the
conductor towards which the positive fluid is directed.
1847. Classification of bodies according to their electro-
motive property . Although the results of experimental research
are not in strict accordance on these points, the electric tensions
produced by the mere contact of heterogeneous conductors
being in general so feeble as to elude the usual electroscopic
tests, it has nevertheless been found, that bodies may be arranged
so that any one placed in contact with another holding a lower
place in the series, will receive the positive fluid, the lower
receiving the negative fluid, and so that the electro-motive force
of any two shall be greater the more distant they are from each
other in the series. How far the results of experimental re-
searches are in accordance on these points, will be seen by com-
paring the following series of electromoters given by Volta,
Pfaff, Henrici, and Peclet :
Volta.
Pfaff.
Henrici.
Peclet.
Zinc.
Zinc.
Zinc.
Zinc.
Lead.
Lead.'
Lead.
Lead.
Tin.
Cadmium.
Tin.
Tin.
Iron.
Tin.
Antimony.
Bismuth.
Bismuth.
Antimony.
Bismuth.
Graphite.
Charcoal.
Cobalt.
Arsenic.
Brass.
Copper.
Copper.
Silver.
Crystallized Amber.
Copper.
Silver.
Mercury.
Gold.
Platinum.
Platinum.'
Gold.
Gold.
Platinum.
Mercury.
Silver.
Charcoal.
To which Pfaff adds the following mineral substances in the
order here given : Argentum vitreum (vitreous silver ore), sul-
phurous pyrites, cuprum mineralisatum pyritaceum (yellow
copper ore), galena, crystallized tin, niccolum sulphuratum arse-
nicum pyritaceum (arsenical mundick), molydena, protoxyde of
uranium, oxyde of titanium, graphite, wolfram (tungstate of iron
and manganese), gypsum stillatium, crystallized amber, peroxyde
of lead (?).
It is to be understood, that, according to the results of the
experimental researches of the observers above named, the
SIMPLE VOLTAIC COMBINATION. 271
electro-motive force produced by the contact of any two of the
bodies in the preceding series will be directed from that which
holds the lower to that which holds the higher place, and that
the energy of such electro-motive force will be greater the more
remote the one body is from the other in the series.
1848. Relation of electro-motive force to susceptibility of oxy-
dation. The mere inspection of these several series will
suggest the general conclusion, that the electro-motive force is
directed from the less to the more oxydable body, and that the
more the one exceeds the other in its susceptibility of oxyda-
tion, the more energetic will be the electro-motive force. Thus,
a combination of zinc with platinum produces more electro-
motive energy than a combination of zinc with any of the more
oxydable metals.
If several electromotors of the series be placed in contact in
any order, the total electro-motive force developed is found to
be the same as if the first were immediately in contact with the
last. The intermediate elements are therefore in this case
inefficient.
1849. Analogy of electro-motive action to induction. It
appears, therefore, that when two pieces of different metals
taken from the series of electromoters, such as zinc and copper
for example, are brought into contact, an electric state is pro-
duced in their combined mass similar to that which would be
produced by placing an insulated conductor charged with
positive electricity near the copper side of the combination.
The inductive action of such a conductor would decompose
the natural electricity of the combined mass, attracting the
negative fluid to the side near the conductor, that is, to the
copper element, and repelling the positive fluid to the opposite
side, that is, to the zinc element. But this is precisely the
effect of the electro-motive force of the two metals as already
described.
Let z and c, Jig. 530., be cylinders of zinc and copper placed
end to end. The former will, by the contact, be charged with
positive, and the latter with negative
electricity. Let the two cylinders,
being insulated, be separated, and the
one will be positively, and the other
negatively electrified ; but in this case
Fig. 530. t ] ie intensity of the electricity deve-
272 VOLTAIC ELECTRICITY.
loped upon them will be so feeble, that it cannot be rendered
manifest by any of the ordinary electroscopic tests. Let it,
however, be imparted to the collecting plate of a powerful con-
densing electroscope, and, after the two cylinders z and c are
discharged, let them be again placed in contact. They will be
again charged by their electro-motive action, and their charges
may, as before, be imparted to the collecting plates of the
electroscopes ; and this process may be repeated until the
electricities of each kind accumulated in the plates of the
electroscopes becomes sensible.
1850. Electro-motive action of gases and liquids. Several
German philosophers have recently instituted elaborate experi-
mental researches to determine the electro-motive action of
liquids, and even of gases, on solids and on each other. The
labours of Pfaff have been especially directed to this inquiry,
and have enabled him to arrive at the following general con-
clusions respecting the electro-motive force developed by the
contact of solid with liquid conductors.
The electro-motive force produced by the contact of alkaline
liquids with the metals, is generally directed from the metal to
the liquid, and its energy is greater the higher is the place held
by the metal in the series of electromoters (1847). Thus, tin,
antimony, and zinc, in contact with caustic potash, caustic
soda, or ammonia, have a more energetic action than platinum,
bismuth, or silver.
The electro-motive force of nitric acid in contact with a
metal, is invariably directed from the acid to the metal. In
this acid, iron and platinum are the most powerful, and zinc the
most feeble electromoters.
Sulphuric and hydrochloric acid, in contact with those
metals which stand at the head of the series (1847), develop a
force directed from the acid to the metal, and in contact with
those at the lowest part of the series, produce a force directed
from the metal to the acid. Thus, these acids in contact with
the less oxydable metals, as gold, platinum, copper, give an
electro-motive force directed from the acid to the metal ; but in
contact with the more oxydable, as antimony, tin, or zinc, give
a force directed from the metal to the acid.
When the metals are placed in contact with weak acid, or
saline solutions generally, the electro-motive force is directed
from the metal to the liquid, the energy of the force being in
SIMPLE VOLTAIC COMBINATION. 2/3
general greater the higher is the place of the metal in the series
of electrometers (1847). In the case of the metals holding the
lowest places in the series, the electro-motive force is in some
instances directed with feeble intensity from the liquid to the
metal.
1851. Differences of opinion as to the origin of electro-motive
action. Since the date of the discoveries of Volta to the
present day, opinion has been divided in the scientific world as
to the actual origin of that electrical excitation which is here
expressed by the term electro-motive force, and which, as has
been explained, Volta ascribed to the mere mechanical contact
of heterogeneous conductors. Some have contended and
among them many of the most eminent recent discoverers in
this branch of physics that the real origin of the electro-
motive force is the chemical action which takes place between
the solid and liquid conductors ; and that, in the cases where
there is an apparent development of electricity by the contact
of heterogeneous solid conductors, its real source has been the
unperceived chemical action of moisture on the more oxydable
electromoter. Others, without disputing the efficacy of chemical
action, maintain that it is a secondary agent, merely exciting
the electro-motive energy of the solid conductors. Thus,
Martens holds that liquids are not properly electrometers at all,
but rather modify ^the electro-motive force of the metals in
contact with them ; so that they may be considered as some-
times augmenting and sometimes diminishing the effect of the
two metals. It is admitted by the partisans of the theory of
contact, that the liquids which most powerfully influence the
electro -motive force of the solids are those which act chemically
on them with greatest energy. But it is contended, that liquids
which produce no chemical change on the metal with which
they are in contact, do nevertheless affect its electro-motive
action.
It fortunately happens, that this polemic can produce no
obstacle to the progress of discovery, nor can it affect the
certitude of the general conclusions which have been based
upon observed facts ; while, on the other hand, the spirit of
the opposition arising from the conflicting theories, has led to
experimental results of the highest importance.
Whatever, therefore, be the origin of the electricity developed
under the circumstances which have been described, we shall
N 5
274
VOLTAIC ELECTRICITY.
continue to designate it by the term electro-motive force, by
which it was first denominated by its illustrious discoverer ;
and we shall invariably designate as the direction of this force
that which the positive fluid takes in passing from one element
to another in the voltaic combination.
1852. Polar arrangement of the fluids in all electro-motive
combinations. In every voltaic combination, therefore, the
effect of the electro-motive force is a polar arrangement of the
decomposed fluids ; the positive fluid being driven towards that
extremity of the system to which the electro-motive force is
directed, and the negative fluid retiring towards the other
extremity.
1853. Positive and negative poles. These extremities are
therefore denominated the POLES of the system ; that towards
which the electro-motive force is directed, and where the positive
fluid is collected, being the POSITIVE, and the other the NEGATIVE
pole.
1854. Electro-motive effect of a liquid interposed between two
solid conductors. When a liquid conductor is placed in contact
with and between two solid conductors, an electrical condition
is induced, the nature of which will be determined by the
quantities and direction of the electro-motive forces developed
at the two surfaces of contact. The several varieties of con-
dition presented by such a voltaic arrangement are represented
\njig. 531. to fig. 536.
Fig. 535. Fig. 536.
Let z and c represent the solid, and L the liquid conductors ;
and let the arrows directed from the two surfaces of contact
represent in each case the direction of the electro-motive forces.
SIMPLE VOLTAIC COMBINATION. 275
If the electro-motive forces be both directed to the same pole,
as in figs. 531, 532., that pole receiving all the positive fluid
transmitted by the conductors will be the positive pole, and the
other, receiving all the negative fluid transmitted, will be the
negative pole.
The quantity of electricity with which each pole will be
charged, will be the sum of the quantities developed by the
electro-motive forces at the two surfaces, diminished by the sum
of the quantities intercepted by reason of the imperfect con-
ducting power of the liquid and solids, and by reason of the
quantity intercepted in passing from the liquid to the solid
conductors at their common surface.
If the electro-motive forces be directed to opposite poles, that
pole to which the more energetic is directed will be the positive
pole. The varieties of conditions presented by this case are
represented in figs. 533, 534, 535, and 536. Each pole in
these cases receives positive fluid from one surface, and negative
from the other. That to which the more energetic electro-
motive force is directed receives more positive than negative
fluid, and is therefore charged with positive fluid equal to their
difference, and is, consequently, the positive pole. The other
receives more negative than positive fluid, and is, consequently,
the negative pole.
In the case represented in fig. 533., the electro-motive force
between z and L is the more energetic. A greater quantity of
positive fluid is received by c from the surface ZL than of
negative fluid from the surface CL, and the surplus of the former
above the latter constitutes the free electricity of the positive
pole c. In like manner, the quantity of negative fluid received
by the pole z from the surface ZL predominates over the quantity
of positive fluid received from the surface CL, and the surplus
of the former over the latter constitutes the free electricity of
the negative pole z.
The like reasoning, mutatis mutandis, will be applicable to
figs. 534., 536., in which the electro-motive force between z and
L is the more energetic, and to fig. 535., in which the electro-
motive force between c and L is the more energetic.
In all these cases, the quantity of electricity with which the
poles are charged is the difference between the actual quantities
developed by the two electro-motive forces, diminished by the
N 6
276 VOLTAIC ELECTRICITY.
difference between the quantities intercepted by the imperfection
of the conduction of the liquid and solid, and in passing through
the surface which separates the liquid and solid conductors.
1855. Electro-motive action of two liquids between two solids.
The quantity of electricity developed may be augmented by
placing different liquid conductors in contact with the two solid
conductors. In this case, however, it is necessary to provide
some expedient by which the two liquids, without being allowed
to intermingle, may nevertheless be in contact, so that the
electricities transmitted from the electro-motive surfaces may
pass freely from the one liquid to the other. This may be
accomplished by separating the liquids by a diaphragm or
partition composed of some porous material, which is capable
of imbibing the liquids without being
z I. P I. C sufficiently open in its texture to allow
I the liquids to pass in any considerable
i^Ji quantity through it. A partition of un-
Fig. 537. glazed porcelain is found to answer this
purpose perfectly. Such an arrange-
ment is represented in fig. 537., where z and c are the solid
electrometers, L and i/ the two liquids, and P the porous par-
tition separating them.
1856. Practical examples of such combinations. As a
practical example of the application of these principles, let the
liquid \^,fig. 531., be concentrated sulphuric acid placed between
a plate of zinc z, and a plate of copper c. In this case the
electro-motive force is directed from z to L, and from L to C ;
and, consequently, the tension of the negative electricity on z,
and the positive electricity on c, will be the sum of the tensions
transmitted from the two surfaces, and z will be the negative,
and c the positive pole (1854).
If the liquid be a dilute solution of acid or salt, or a strong
alkaline liquid, the electro-motive forces are both directed from
the metal to the liquid, but that of the zinc is more energetic
than that of the copper ; consequently z, fig. 533., will in this
case be the negative, and c the positive pole, the energy of the
combination being proportional to the difference of the two
electro-motive forces.
If the liquid be concentrated nitric acid, the electro-motive
forces will be both directed from the liquid to the metals. In
this case the zinc z, _/?. 535., being the more feeble electrometer,
SIMPLE VOLTAIC COMBINATION. 277
the copper element c will be the positive, and the zinc z the
negative pole.
If two different liquids be interposed between plates of the
same metal, the conditions which affect the development of
electricity may be determined by similar reasoning.
If z and c, fig. 537., be two plates of the same metal, and L
and i/ be two liquids, between which and the metal there are
unequal electro-motive forces, the effect of such an arrange-
ment will be a polar development, the positive pole being that
to which the electro-motive forces are directed if they have a
common direction, and that of the more energetic if they act in
opposite directions. The intensity of the charge at the poles
will be in the one case the sum, and in the other the difference
of the quantities of fluid transmitted.
As a practical example of the application of this principle,
let the metals z and c be both platinum, and let L be an
alkaline solution, and i/ concentrated nitric acid. In this case
the electro-motive forces will be directed from z to L, and from
L' to c, and the effect of the arrangement will be similar to that
represented vbfig. 531.
1857. Most powerful combinations determined. The most
powerful voltaic arrangements are produced by taking two
metals from the extremes of the electro-motive series (1847), and
interposing between them two liquids, the electro-motive force
of one being directed from the metal to the liquid, and of the
other from the liquid to the metal, and so selecting the liquids,
subject to this latter condition, as to have the greatest possible
electro-motive action on the respective metals.
Observing these principles, voltaic combinations of extra-
ordinary power have been produced by interposing dilute
sulphuric ^,fig. 537., and concentrated nitric acid i/, between
zinc z, and carbon or platinum c. In such a combination,
strong electro-motive forces are developed, directed from the
zinc to the acids, and from the acids to the carbon or platinum.
The zinc is therefore the negative, and the carbon or platinum
the positive pole of the system.
1858. Form of electro-motive combination. We have se-
lected the form of parallel plates or columns, which has been
supposed in the arrangements here described, merely because of
the clearness and simplicity which it gives to the exposition of
the principles upon which all voltaic combinations act. This
278 VOLTAIC ELECTRICITY.
form, although it was that of the earliest voltaic systems, and
is still in some cases adhered to, is neither essential to the
principle of such arrangements, nor convenient where the
development of great force is required. In order to obtain as
great an extent of electro-motive surface in as small a volume
as is practicable, the form of hollow cylinders of varying
diameters, placed concentrically in cylindrical vessels a little
larger, and containing the exciting liquid, is now generally
preferred.
1859. Yalta's first combination. The simple arrangements
first adopted by Volta consisted of two equal discs of metal, one
of zinc, and the other of copper or silver, with a disc of cloth
or bibulous card, soaked in an acid or saline solution, between
them. These were usually laid, with their surfaces horizontal,
one upon the other.
1860. Wollastorfs combination The late Dr. Wollaston
proposed an arrangement, in which the copper plate was bent
into two parallel plates, a space between them being left for the
insertion of the zinc plate, the contact of the plates being pre-
vented by the interposition of bits of cork or other non-con-
ductor. The system thus combined was immersed in dilute
acid contained in a porcelain vessel.
1861. Hare's spiral arrangement. This consists of two
metallic plates, one of zinc and the other of copper, of equal
length, rolled together into the form of a spiral, a space
of a quarter of an inch being left between them. They are
maintained parallel without touching, by means of a wooden
cross at top and bottom, in which notches are provided at
proper distances, into which the plates are inserted, the two
crosses having a common axis. This combination is let into
a glass or porcelain cylindrical vessel of corresponding mag-
nitude, containing the exciting liquid.
This ai'rangement has the great advantage of providing a
very considerable electro-motive surface with a very small
volume.
The exciting liquid recommended for these batteries when
great power is desired, is a solution in water of 2 per cent of
sulphuric, and 2 per cent of nitric acid. A less intense but
more durable action may be obtained by a solution of common
salt, or of 3 to 5 per cent of sulphuric acid only.
1862. Amalgamation of the zinc. Whatever be the form
SIMPLE VOLTAIC COMBINATION.
279
of the arrangement, its force and uniformity of action will be
promoted by amalgamating the zinc element, which may be best
accomplished in the following manner.
Immerse the rough plate or cylinder of zinc in a solution of
sulphuric acid containing from 12 to 16 per cent, of acid, until
the thin film of oxyde which usually collects on the surface of the
metal be dissolved. Then wash it well in water, and immerse
it in a dilute solution of the nitrate of mercury. After a short
time a perfectly uniform amalgam will be formed on the surface
of the zinc. Let the zinc be then washed in water and rubbed
dry with saw-dust.
1863. Cylindrical combination with one fluid. Voltaic
systems of the cylindrical form usually consist of two hollow
cylinders of different metals, one of which, however, is always
zinc. The exciting liquid being placed in a cylindrical vessel
a little longer than the greater of the two hollow metallic
cylinders, these are immersed in it concentrically with it and
with each other. A part of each projecting from the top of the
vessel becomes the pole of the system.
Such a combination is represented in vertical section in fig.
538., where vv is a vessel of glazed porcelain, containing the
acid or saline solution, zz is a hollow cylinder of zinc, and cc
a similar hollow cylinder of copper, each being open at both
ends, and separated from each other by a space of a quarter to
half an inch. Strips of metal CP and ZN represent the poles,
that connected with the zinc being the negative, and that con-
nected with the copper being the positive pole.
In some cases the porcelain vessel vv is dispensed with, and
the acid solution is placed in a cylindrical copper vessel, in
Fig. 538.
Fig. 539.
280
VOLTAIC ELECTRICITY.
Fig. 540.
which the hollow cylinder of zinc is immersed, resting upon
some non-conducting support. Such an arrangement is re-
presented \nfig. 539. in vertical section, cc being the copper
vessel, zz the zinc cylinder, and p and N the poles.
1864. Cylindrical combinations with two fluids. Cylin-
drical arrangements with two exciting liquids are made in the
following manner. The hollow
cylinder of zinc z z, open at both
ends as already described, is
placed in a vessel of glazed
porcelain *vv,fig. 540. Within
this is placed a cylindrical ves-
sel vv, of unglazed porcelain,
a little less in diameter than
the zinc z z, so that a space of
about a quarter of an inch
may separate their surfaces.
In this vessel vv, is inserted
a cylinder cc of platinum, open
at the ends, and a little less than vv, so that their surfaces may
be about a quarter of an inch asunder. Dilute sulphuric acid
is then poured into the vessel vv, and concentrated nitric
acid into vv. According to what has been already explained
(1857), P proceeding from the platinum will then be the positive,
and N proceeding from the zinc the negative pole.
1865. Grove's battery. This arrangement is known as
GROVE'S BATTERY. Various modifications have been suggested
with the view to increase the electro-motive surface of the
platinum and economize expense. Gruel suggests the use of
thin platinum, attached by platinum wires to a central axis,
from which from 4 to 6 leaves or flaps diverge. Poggendorf
proposes a single leaf of platinum, greater in breadth than the
diameter of the vessel vv in the ratio of about 3 to 2, and bent
into the form of an S, so as to pass freely into it. Pfaff
proposes to coat the inner surface of the vessel vv with leaf
platinum. Peschel affirms, after having tried this expedient,
that it is less effective than the former.
In these systems it is recommended to use a solution of sul-
phuric acid containing from 10 to 25 per cent, of acid, and
nitric acid of the specific gravity of 1-33.
1866. Bunsen's battery. The voltaic system known as
SIMPLE VOLTAIC COMBINATION.
281
BUNSEN'S, is similar to the preceding, substituting charcoal for
platinum. The charcoal cylinder used for this purpose, is made
from the residuum taken from the retorts of gas-works. A
strong porous mass is produced by repeatedly baking the pul-
verized coke, to which the required form is easily imparted.
Messrs. Deleuil and Son, of Paris, have fabricated batteries on
this principle with great success. I have one at present in use
consisting of fifty pairs of zinc and carbon cylinders, the zinc
being 2^ inches diameter, and 8 inches high, which performs
very satisfactorily.
The electro-motive forces of Grove's and Bunsen's batteries
are considered to be, ceteris paribus, equal.
1867. Daniel's constant battery. The voltaic arrangement
known as Daniel's constant battery consists of a copper cylin-
drical vessel C C, fig. 541., widening near the top ad. In this
is placed a cylindrical vessel of unglazed
d porcelain p. In this latter is placed the
hollow cylinder of zinc z, already described.
The space between the copper and porce-
lain vessels is filled with a saturated solu-
tion of the sulphate of copper, which is
maintained in a state of saturation by crys-
tals of the salt placed in the wide cup abed,
in the bottom of which is a grating com-
posed of wire carried in a zigzag direction between two con-
centric rings, as represented in plan at G. The vessel p, con-
taining the zinc, is filled with a solution of sulphuric acid, con-
4. taining from 10 to 25 per cent, of acid
when greater electro-motive power is re-
quired, and from 1 to 4 per cent, when
more moderate action is sufficient.
1868. PouilleCs modification of Da-
niel's battery. The following modi-
fication of Daniel's system was adopted
by M. Pouillet in his experimental re-
searches. A hollow cylinder a, fig. 542.,
of thin copper, is ballasted with sand b,
having a flat bottom c, and a conical
top d. Above this cone the sides of
the copper cylinders are continued, and
terminate in a flange e. Between this
Fig. 541.
c,
h
j
i a i
- -., : -(, -;.,-_;.. -
./ . . ..._,. -
\:... - ">. \ ,
Fig. 542.
282 VOLTAIC ELECTRICITY.
flange and the base of the cone, and near the base, is a ring of
holes. This copper vessel is placed in a bladder which fits
it loosely like a glove, and is tied round the neck under the
flange e. The saturated solution of the sulphate of copper is
poured into the cup above the cone, and, flowing through the
ring of holes, fills the space between the bladder and the copper
vessel. It is maintained in its state of saturation by crystals
of the salt deposited in the cup.
This copper vessel is then immersed in a vessel of glazed
porcelain i, containing a solution of the sulphate of zinc or the
chloride of sodium (common salt). A hollow cylinder of zinc
//, split down the side so as to be capable of being enlarged or
contracted at pleasure, is immersed in this solution surrounding
the bladder. The poles are indicated by the conductors p and
n, the positive proceeding from the copper, and the negative
from the zinc.
M. Pouillet states that the action of this apparatus is sus-
tained without sensible variation for entire days, provided the
cup above the cone d is kept supplied with the salt, so as to
maintain the solution in the saturated state.
1 869. Advantages and disadvantages of these several systems.
The chief advantage of Daniel's system is that from which it
takes its name, its constancy. Its power, however, in its most
efficient state, is greatly inferior to that of the carbon or pla-
tinum systems of Bunsen and Grove. A serious practical in-
convenience, however, attends all batteries in which concen-
trated nitric acid is used, owing to the diffusion of nitrous
vapour, and the injury to which the parties working them are
exposed by respiring it. In my own experiments with Bunsen's
batteries the assistants have been often severely affected.
In the use of the platinum battery of Grove, the nuisance
produced by the evolution of nitrous vapour is sometimes miti-
gated by enclosing the cells in a box, from the lid of which a
tube proceeds which conducts these vapours out of the room.
In combinations of this kind, Dr. O'Shaugnessy substituted
gold for platinum, and a mixture of two parts by weight of
sulphuric acid to one of saltpetre for nitric acid.
1870. Smee's battery. The voltaic combination called SJIEE'S
BATTEUY, consists of a porcelain vessel A, Jig. 543., containing
an acid solution, which may be about 15 per cent, of sulphuric
acid in water. A plate of iron or silver s, whose surfaces are
SIMPLE VOLTAIC COMBINATION.
283
Fig. 543.
in Jig. 544.
platinized by a certain chemical process, is sus-
pended from a bar of wood a, between two
plates of zinc z, suspended from the same bar
without contact with the plate s. The electro-
motive action is explained on the same prin-
ciple as the combinations already described.
Mr. Smee claims, as an advantage for this sys-
tem, its great simplicity and power, the quantity
of electricity evolved being, ceteris paribus, very
great, and the manipulation easy.
1871. Wheatstone's system Professor Wheat-
stone has proposed the combination represented
A cylindrical vessel v v, of unglazed and half-baked
Fig. 544.
red earthenware, is placed in another w larger one of glazed
porcelain or glass. The vessel vv is filled
with a pasty amalgam of zinc, and the space
between the two vessels is filled with a satu-
rated solution of sulphate of copper. In the
latter solution is immersed a thin cylinder
of copper cc. A rod or wire of copper N is
plunged in the amalgam. The electro-motive
forces of this system are directed from the
amalgam to the copper solution ; so that P
proceeding from the copper cylinder is the
positive, and N proceeding from the amalgam,
is the negative pole.
The action of this system is said to be constant, like that of
Daniel, so long at least as the vessel vv allows equally free
passage to the two fluids, and the state of saturation of the
copper solution is maintained.
1872. Bagratiori s system. A voltaic
arrangement suggested by the Prince
Bagration, and said to be well adapted
to galvano-plastic purposes, consists of
parallel hollow cylinders,^. 545., of zinc
and copper, immersed in sand contained
in a porcelain vessel. The sand is kept
wet by a solution of hydrochlorate of
ammonia.
1873. BecquereVs system. M. Bee-
Fig. 545. querel has applied the principle of two
284
VOLTAIC ELECTRICITY.
fluids and a single metal, explained in (1856) in the following
manner :
A porcelain vessel v, fig. 546., contains concentrated nitric
acid. A glass cylinder T, to which is attached a bottom of un-
glazed porcelain, is immersed in it. This cy-
linder contains a solution of common salt. Two
plates of platinum are immersed, one in the
nitric acid, and the other in the solution of
salt. The electro-motive forces take effect, the
conduction being maintained through the porous
bottom of the glass vessel T, the positive pole
being that which proceeds from the nitric acid,
and the negative that which proceeds from the
salt.
1874. Schonbein's modification of Bunsen's
battery. M. Schonbein proposes the following
modification of Bunsen's system. In a vessel
Fig. 546.
of cast-iron rendered passive, he places a mixture of three
parts of concentrated nitric with one of sulphuric acid. In
this he immerses the cylindrical vessel of unglazed porcelain
which contains the zinc, immersed in a weak solution of sul-
phuric acid. In this arrangement the cast-iron vessel plays
the part of Bunsen's cylinder of charcoal. The positive pole
is therefore that which proceeds from the cast-iron vessel,
and the negative that which is connected with the zinc.
1875. Grove's gas electro-motive apparatus. We shall con-
clude this synopsis of the simple voltaic combinations with the
gas electro-motive apparatus of Mr. Grove, one
of the most curious and interesting that has been
contrived. Two glass tubes h and o,fig. 547., are
inverted in a vessel containing water slightly
acidulated with sulphuric acid. Hydrogen gas h
is admitted into one of these, and oxygen o into
the other in the usual way. A narrow strip of
platinum passes at the top of each tube through
an aperture which is hermetically closed around
it, the strip descending near to the bottoms
of the tubes. An electro-motive force is de-
veloped between the platinum and the gases,
which is directed from the platinum to the
oxygen, and from the hydrogen to the platinum. The end of
Fig. 547.
VOLTAIC BATTERIES. 285
the platinum which issues from the hydrogen is therefore the
positive, and that which issues from the oxygen the negative,
pole of the system.
CHAP. II.
VOLTAIC BATTERIES.
1876. Volta's invention of the pile. Whatever may be the
efficacy of simple combinations of electrometers compared
one with another, the electricity developed even by the most
energetic among them is still incomparably more feeble than
that which proceeds from other agencies, and indeed so feeble
that without some expedient by which its power can be aug-
mented in a very high ratio, it would possess very little im-
portance as a physical agent. Volta was not slow to perceive
this ; but having also a clear foresight of the importance of the
consequences that must result from it if its energy could be
increased, he devoted all the powers of his invention to discover
an expedient by which this object could be attained, and happily
not without success.
He conceived the idea of uniting together in a connected and
continuous series, a number of simple electro-motive combi-
nations, in such a manner that the positive electricity developed
by each should flow towards one end of the series, and the
negative towards the other end. In this way he proposed to
multiply the power of the extreme elements of the series by
charging them with all the electricity developed by the inter-
mediate elements.
In the first attempt to realize this conception, circular discs
of silver and copper of equal magnitude (silver and copper coin
served the purpose), were laid one over the other, having inter-
posed between them equal discs of cloth or pasteboard soaked
in an acid or saline solution. A pile was thus formed which
was denominated a VOLTAIC PILE ; and although this arrange-
ment was speedily superseded by others found more convenient,
the original name was retained.
Such arrangements are still called VOLTAIC FILES, and
286
VOLTAIC ELECTRICITY.
sometimes VOLTAIC BATTERIES, being related to a simple
voltaic combination in the same manner as a Leyden battery is
to a Leyden jar.
1877. Explanation of the principle of the pile. To explain
the principle of the voltaic battery, let us suppose several
simple voltaic combinations, z^c 1 , z 2 L 2 c 2 , z 3 L 3 c 3 , z 4 L 4 c 4 ,
fig. 548., to be placed, so that the negative poles z shall all
Fig. 548.
look to the left, and the positive C to the right. Let the
metallic plates c be extended, and bent into an arc, so as to be
placed in contact with the plates z. Let the entire series be
supposed to stand upon any insulating support, and let the
negative pole z 1 of the first combination of the series be put in
connection with the ground by a conductor.
If we express by E the quantity of positive electricity de-
veloped by z 1 !, 1 ^, the negative fluid escaping by the con-
ductor, this fluid E will pass to c 1 , and from thence along the
entire series to the extremity c 4 . The combination z 1 !, 1 ^ acts
in this case as the generator of electricity in the same manner
as the cushion and cylinder of an electrical machine, and the
remainder of the series z 2 L 2 c 2 , &c., plays the part of the con-
ductor, receiving the charge of fluid from z^c 1 .
The second combination z 2 L 2 c 2 being similar exactly to the
first, evolves an equal quantity of electricity E, the negative
fluid passing through z 1 L 1 c 1 , and the conductor to the ground.
The positive fluid passes from z 2 L 2 c 2 to the succeeding com-
binations to the end of the series.
In the same manner, each successive combination acts as a
generator of electricity, the negative fluid escaping to the
ground by the preceding combinations and the conductor, and
the positive fluid being diffused over the succeeding part of the
series.
It appears, therefore, that the conductor p connected with the
last combination of the series must receive from each of the
four combinations an equal charge E of positive fluid ; so that
the depth or quantity of electricity upon it will be four times
VOLTAIC BATTERIES. 287
that which it would receive from the single combination
z 4 L 4 c 4 acting alone and unconnected with the remainder of the
series.
In general, therefore, the intensity of the electricity received
by a conductor attached to the last element of the series, will
be as many times greater than that which it would receive
from a single combination as there are combinations in the
series. If the number of combinations composing the series be
n, and E be the intensity of the electricity developed by a
single combination, then n x E will be the intensity of the
electricity produced at the extremity of the series.
It has been here supposed, that the extremity z 1 of the series
is connected by the conductor N with the ground. If it be not
so connected, and if the entire series be insulated, the distribu-
tion of the fluids developed will be different. In that case, the
conductor P will receive the positive fluid propagated from
each of the electro-motive surfaces to the right, and the con-
ductor N will receive the negative fluid propagated from each
of these surfaces to the left, and each will receive as many
times more electricity than it would receive from a single
combination as there are simple combinations in the series.
If, therefore, E' express the quantity of fluid which each con-
ductor p and N would receive from a single combination Z I L I C I ,
then n x E' will be the quantity it would receive from a series
consisting of n simple combinations.
Since two different metals generally enter with a liquid into
each combination, it has been usual to call these voltaic com-
binations PAIRS ; so that a battery is said to consist of so many
PAIRS.
On the Continent these combinations are called ELEMENTS ;
and the voltaic pile is said to consist of so many ELEMENTS,
each element consisting of two metals and the interposing
liquid.
1878. Effect of the imperfect liquid conductors. In what
precedes we have considered that all the electricity developed
by each pair is propagated without resistance or diminution to
the poles P and N of the pile. This, however, could only occur
if the materials composing the pile through which the electri-
city must be transmitted were perfect conductors. Now, although
the metallic parts may be regarded as practically perfect con-
288 VOLTAIC ELECTRICITY.
ductors, the liquid through which the electricity must be trans-
mitted in passing from one metallic element to another is not
only an imperfect conductor, but one whose conducting power
is subject to constant variation. A correction would therefore
be necessary in applying the preceding reasoning, the electri-
city received by the poles P and N being less than n x E', by
that portion which is intercepted or lost in transmission through
the liquid conductors. The amount of the resistance to con-
duction proceeding from the conductors, liquid and metallic,
by which the electricity evolved at the generating surfaces is
transmitted to the poles of the pile, has not been ascertained
with any clearness or certainty.
Professor Ohm, who has investigated the question of the
resistance of the conductors composing a battery to the propa-
gation of the electricity through them, maintains that the in-
tensity of the electricity transmitted to the poles of the pile is
" directly as the sum of the electro-motive forces, and inversely
as the sum of all the impediments to conduction." We do not
find, however, that this law has been so developed and verified
by observation and experiment as to entitle it to a place in
elementary instruction.
1879. Method of developing electricity in great quantity.
If the object be to obtain a great quantity of electricity, the
elements of the pile should be combined by connecting the poles
of the same name with common conductors. Thus, if all the
positive poles be connected by metallic wires with one conductor,
and all the negative poles with another, these conductors will
be charged with as much electricity as would be produced by a
single combination, of which the generating surfaces would be
equal to the sum of the generating surfaces of all the elements
of the series ; but the intensity of the electricity thus de-
veloped would not be greater than that of the electricity de-
veloped by a single pair.
1880. Distinction between quantity and intensity important.
It is of great importance to distinguish between the quantity
and the intensity of the electricity evolved by the pile. The
quantity depends on the magnitude of the sum of all the sur-
faces of the electromoters. The intensity depends on the
number of pairs composing the series. The quantity is mea-
sured merely by the actual quantity of each fluid received at
VOLTAIC BATTERIES.
289
the poles. The intensity is proportional, cteteris paribus, to the
number of pairs transmitting electricity to the same pole, the
fluids being superposed at the poles, and the intensity being
produced by such superposition.
Voltaic piles have been composed and con-
structed in a great variety of forms by com-
bining together the various simple electro-
motive combinations which have been de^
scribed in the last chapter.
1881. Yalta's first pile. The first pile
constructed by Volta was formed as follows :
A disc of zinc was laid upon a plate of glass.
Upon it was laid an equal disc of cloth or
pasteboard soaked in acidulated water. Upon
this was laid an equal disc of copper. Upon
the copper were laid in the same order three
discs of zinc, wet cloth, and copper, and
the same superposition of the same combi-
nations of zinc, cloth, and copper was con-
tinued until the pile was completed. The
highest disc (of copper) was then the positive,
and the lowest disc (of zinc) the negative pole,
according to the principles already explained.
It was usual to keep the discs in their places
by confining them between rods of glass.
Such a pile, with conducting wires con-
nected with its poles, is represented in Jig. 549.
des tasses. The next arrangement
Fig. 549.
1882. The couronne
proposed by Volta formed a step towards the form which the
pile definitively assumed, and is known under the name of
Fig. 550.
the COURONNE DES TASSES (ring of cups) : this is represented
in Jig. 550., and consists of a series of cups or glasses con-
II. O
290
VOLTAIC ELECTRICITY.
taining the acid solution. Rods of zinc and copper zc, soldered
together end to end, are bent into the form of arcs, the ends
being immersed in two adjacent cups, so that the metals may
succeed each other in one uniform order. A plate of zinc, to
which a conducting wire N is attached, is immersed in the first ;
and a similar plate of copper, with a wire P, in the last cup.
The latter wire will be the positive, and the former the negative,
pole.
1883. CruikslianKs arrangement, The next form of vol-
taic pile proposed was that of Cruikshank, represented \\\fig.
551. This consisted of a trough of glazed earthenware divided
into parallel cells corresponding in number and magnitude to
the pairs of zinc and copper plates which were attached to a
bar of wood, and so connected that, when immersed in the cells,
each copper plate should be in connexion with the zinc plate of
the next cell. The plates were easily raised from the trough
when the battery was not in use. The trough contained the
acid solution.
1884. Wollaston's arrangement. In order to obtain within
the same volume a greater extent of electro-motive surface, Dr.
Wollaston doubled the copper plate round the zinc plate, without
however allowing them to touch. In this case the copper plates
have twice the magnitude of the zinc plates. The system, like
the former, is attached to a bar of wood, and being similarly
connected, are either let down into a trough of earthenware
Fig. 551.
Fig. 555
divided into cells, as represented vnfig. 552., or into separate
glass or porcelain vessels, as represented in Jig. 553. The latter
VOLTAIC BATTERIES.
291
method has the advantage of affording greater facility for dis-
charging and renewing the acid solution.
1885. Heliacal pile of Faculty of Sciences at Paris. The
heliacal pile is a voltaic arrangement adapted to produce
electricity of low tension in great quantity.
This pile, as constructed for the Faculty of
Sciences at Paris under the direction of M.
Pouillet, consists of a cylinder of wood b,Jig.
554., of about four inches diameter and fifteen
inches long, on which is rolled spirally two thin
leaves of zinc and copper separated by small
bits of cloth, and pieces of twine extended
parallel to each other, having a thickness a
little less than the cloth. A pair is formed in
this manner, having a surface of sixty square
feet. A single combination of this kind evolves
electricity in large quantity, and a battery
composed of twenty pairs is an agent of pro-
digious power.
The method of immersing the combination
in the acid solution is represented \r\fig. 555.
1886. Piles are formed by connecting to-
gether a number of any of the simple electro-
motive combinations described in the last
chapter, the conditions under which they are
connected being always the same, the positive
pole of each combination being put in me-
Fig. 555.
tallic connexion with the negative pole of the succeeding one.
o 2
292 VOLTAIC ELECTRICITY.
When the combinations are cylindrical, it is convenient to set
them in a framing, which
will prevent the acci-
dental fracture or strain of
the connexions. A bat-
ill ' \A W I/.I ULiil^ tery often pairsof Grove's
r "" ^ [/ or Bunsen sis represented
F; 556 with its proper connex-
ions in fig. 556.
1887. Conductors connecting the elements. Whatever be
the form or construction of the pile, its efficient performance
requires that perfect metallic contact should be made and
maintained between the elements composing it by means of
short and good conductors. Copper wire, or, still better, strips
cut from sheet copper from half an inch to an inch in breadth,
ai-e found the most convenient material for these conductors,
as well as for the conductors which carry the electricity from
the poles of the. pile to the objects to which it is to be con-
veyed. In some cases, these conducting wires or strips are
soldered to metallic plates, which are immersed in the exciting
liquid of the extreme elements of the pile, and which, there-
fore, become its poles. In some cases, small mercurial cups are
soldered to the poles of the pile, in which the points of the con-
ducting wires, being first scraped, cleaned, and amalgamated,
are immersed. Many inconveniences, however, attend the use
of quicksilver, and these cups have lately been very generally
superseded by simple clamps constructed in a variety of
forms, by means of which the conducting wires or strips may
be fixed in metallic contact with the poles of the pile, with each
other, or with any object to which the electricity is required to
be conveyed. Where great precaution is considered necessary
to secure perfect contact, the extremities of the conductors at
the points of connexion are sometimes gilt by the electrotyping
process, which may always be done at a trifling cost. I have
not, however, in any case found this ne-
cessary, having always obtained perfect con-
tact by keeping the surfaces clean, and using
screw clamps of the form in fig. 557. This
is represented in its proper magnitude.
1888. file may be placed at any distance
Fig. 557. from place of experiment It is generally
VOLTAIC BATTERIES. 293
found to be inconvenient in practice to keep the pile in the
room where the experiments are made, the acid vapours being
injurious in various ways, especially where nitric acid is used.
It is therefore more expedient to place it in any situation
where these vapours have easy means of escaping into the
open air, and where metallic objects are not exposed to them.
The situation of the pile may be at any desired distance from
the place where the experiments are made, communication
with it being maintained by strips of sheet copper as above
described, which may be carried along walls or passages, contact
between them being made by doubling them together at the
ends which are joined, and nailing the joints to the wall. They
should of course be kept out of contact with any metallic object
which might divert the electric current from its course. I have
myself a large pile placed in an attic connected by these means
with a lower room in the house, by strips of copper which
measure about fifty yards.
1889. Memorable piles: Davy's pile at the Royal Institu-
tion. Among the apparatus of this class which have obtained
celebrity in the history of physical science, may be mentioned
the pile of 2000 pairs of plates, each having a surface of 32
square inches, at the lloyal Institution, with which Davy
effected the decomposition of the alkalies, and the pile of the
Royal Society of nearly the same magnitude and power.
1890. Napoleon's pile at Polytechnic School. In 1808, the
Emperor Napoleon presented to the Polytechnic School at
Pai'is a pile of 600 pairs of plates, having each a square foot
of surface. It was with this apparatus that several of the
most important researches of Gay Lussac and Thenard were
conducted.
1891. Children's great plate battery. Children's great
plate battery consisted of 16 pairs of plates constructed by
Wollaston's method, each plate measuring 6 feet in length and
2| feet in width, so that the copper surface of each amounted
to 32 square feet ; and when the whole was connected, there
was an effective surface of 512 square feet.
1892. Hare's deflagrator. The pile of Dr. Hare of Phila-
delphia, called a deflagrator, was constructed on the heliacal
principle, and consisted of 80 pairs, each zinc surface measui'-
ing 54 squai'e inches, and each copper 80 square inches.
1893. Stratingfis deflagrator. Stratingh's deflagrator con
294 VOLTAIC ELECTRICITY.
sisted of 100 pairs on Wollaston's method. Each zinc surface
measured 200 square inches. It was used either as a battery of
100 pairs or as a single combination (1879), presenting a total
electromotive surface of 277 square feet of zinc and 544 of
copper.
1894. Pepys' pile at London Institution. Mr. Pepys con-
structed an apparatus for the London Institution, each element
of which consisted of a sheet of copper and one of zinc,
measuring each fifty feet in length and two feet in width.
These were wound round a rod of wood with horsehair be-
tween them. Each bucket contained fifty-five gallons of the
exciting liquid.
1895. Powerful batteries on Daniel and Grove's principles.
These and all similar apparatus, powerful as they have been,
and memorable as the discoveries in physics are to which
several of them have been instrumental, have fallen into
disuse, except in certain cases, where powerful physiological
effects are to be produced; since the invention of the piles of
two liquids, which, with a number of elements not exceeding
forty, and a surface not exceeding 100 square inches each,
evolve a power equal to the most colossal of the apparatus
above described.
The most efficient voltaic apparatus are formed by combining
Daniel's, Grove's, or Bunsen's single batteries, connecting their
opposite poles with strips of copper as already described.
Grove's battery, constructed by Jacobi of St. Petersburg,
consists of 64 platinum plates, each having a surface of 36
square inches; so that their total surface amounts to 16 square
feet. This is considered to be the most powerful voltaic ap-
paratus ever constructed. According to Jacobi's estimate, its
effect is equal to a Daniel's battery of 266 square feet, or to a
Hare's deflagrator of 5500 square feet.
1896. Dry piles. The term DRY PILE was originally in-
tended to express a voltaic pile composed exclusively of solid
elements. The advantages of such an apparatus were so ap-
parent, that attempts at its invention were made at an early
stage in the progress of electrical science. In such a pile,
neither evaporation nor chemical action taking place, the ele-
ments could suffer no change ; and the quantity and intensity
of the electricity evolved would be absolutely uniform and in-
variable, and its action would be perpetual.
VOLTAIC BATTERIES. 295
1897. Deluc's pile. The first instrument of this class con-
structed was tlie dry pile of Deluc, subsequently improved
by Zamboni. This apparatus is prepared by soaking thick
writing-paper in milk, honey, or some analogous animal fluid,
and attaching to its surface by gum a thin leaf of zinc or tin.
The other side of the paper is coated with peroxide of man-
ganese. Leaves of this are superposed, the sides similarly
coated being all presented in the same direction, and circular
discs are cut of an inch diameter by a circular cutter. Several
thousands being laid over one another, are pressed into a close
and compact column by a screw, and the sides of the column
are then thickly coated with gum-lac.
The origin of the electromotive force of the pile is various.
Besides the contact of heterogeneous substances, chemical
action intervenes in several ways. The organic matter acts
upon the zinc as well as upon the manganese, reducing the
latter to a lower state of oxidation.
1898. Zamboni's pile. Piles, having two elements only,
have been constructed by Zamboni. These consist of one
metal and one intermediate conductor, either dry or moist. If
the former, the discs are of silver paper laid with their metal
faces all looking the snrno way ; if the latter, a number of pieces
of tinfoil, with one end pointed and the other broad, are laid
in two watch-glasses which contain water, in such a manner,
that the pointed part lies in one glass and the broad part in the
other. After some time, they develope at their poles a feeble
electricity, which they retain for several days, the metal pole
being positive in the dry pile, and the pointed end of the zinc
in the moist one.
1899. Piles of a single metal. Piles of a single metal have
been constructed by causing one surface to be exposed to a
chemical action different from the other. This may be effected
by rendering one surface smooth and the other rough. A pile
of this kind has been made with sixty or eighty plates of zinc
of four square inches surface. These are fixed in a wooden
trough parallel to each other, their polished faces looking the
same way, and an open space of the tenth to the twentieth of
an inch being left between them, these spaces being merely
occupied by atmospheric air. If one extremity of this apparatus
be put in communication with the ground, the other pole will
sensibjy affect an electroscope.
296 VOLTAIC ELECTRICITY.
In this case, the electromotive action takes place between
the air and the metal.
1900. Hitter's secondary piles. The secondary piles, some-
times called HITTER'S PILES, consist of alternate layers of homo-
geneous metal plates, between which some moist conducting
substance is interposed. When they stand alone, no electro-
motive force is developed ; but, if they be allowed to continue
for a certain time in connexion with the poles of a battery,
and then disconnected, positive electricity will be found to
be accumulated at that end which was connected with the
positive pole, and negative electricity at the other end ; and
this polar condition will continue for a certain time, which will
be greater the less the electrical tension imparted. This phe-
nomenon has not been satisfactorily explained, but would seem
to arise from the low conducting power of the strata of liquid
interposed between the plates.
CHAP. III.
VOLTAIC CURRENTS.
1901. The voltaic current. The voltaic pile differs from the
electrical machine inasmuch as it has the power of constantly
reproducing whatever electricity may be drawn from it by con-
ductors placed in connexion with its poles, without any manipu-
lation, or the intervention of any agency external to the pole
itself. So prompt is the action of this generating power, that
the positive and negative fluids pass from the respective poles
through such conductors in a continuous and unvarying stream,
as a liquid would move through pipes issuing from a reser-
voir. The pile may indeed be regarded as a reservoir of the
electric fluids, with a provision by which it constantly replenishes
itself.
If two metallic wires be connected at one end with the poles
r and ff,fig. 558., of the pile, and at the other with any con-
ductor o, through which it is required to transmit the electri-
city evolved in the pile, the positive fluid will pass from p
VOLTAIC CURRENTS. 297
along the wire to o, and the negative fluid in like manner from
N to o. The positive fluid will therefore form a stream or
Fig. 558.
current from p through o to N, and the negative fluid a con-
trary current from N through o to P.
It might be expected that the combination of the two
opposite fluids in equal quantity would reduce the wire to its
natural state ; and this would, in fact, be the case, if the fluids
were in repose upon the wire, which may be proved by de-
taching at the same moment the ends of the wires from the poles
p and N. The wires and the conductor o will, in that case,
show no indication of electrical excitement. If the wire be
detached only from the negative pole N, it will be found, as well
as the conductor o, to be charged with positive electricity ; and
if it be detached from the positive pole P, they will be charged
with negative electricity, the electricity in each case being in
repose. But when both ends of the wire are in connexion
with the poles P and N, the fluids, being in motion in contrary
directions along the wire and intermediate conductors, impart
to these qualities which show that they are not in the natural
or unelectrified state, but which have nothing in common with
the qualities which belong to bodies charged with the electric
fluid in repose. Thus, the wire or conductor will neither
attract nor repel pith balls, nor produce any electroscopic
effects. They will, however, produce a great variety of other
phenomena, which we shall presently notice.
The state of the electricities in thus passing between the
poles of the piles through a metallic wire or other conductor
exterior to the pile, is called a VOLTAIC CURRENT.
1902. Direction of the current. Although, according to
what has been stated, this current consists of two streams
flowing in contrary directions, it receives its denomination ex-
clusively from the positive fluid ; and, accordingly, the DIUEC-
298 VOLTAIC ELECTRICITY.
TIOX OF THE CURRENT is always from the positive pole through
the wire or other conductor to the negative pole.
It is necessary, however, to observe, that in passing through
the pile itself from element to element, it moves from the
negative pole to the positive pole. The direction and course of
the current is indicated \\\fig. 558. by the arrows.
1903. Poles of the pile, how distinguished. In designat-
ing the poles of the pile, much confusion and obscurity, and
consequent difficulty to students, has arisen from identifying
the poles of the pile with the extreme plates of metal com-
posing it. In the piles first constructed by Volta, the last plate
at the positive end was zinc, and the last at the negative end
copper ; and such an arrangement was often retained at more
recent periods. Hence the positive pole was called the zinc
pole, and the negative pole the copper pole. The extreme
plates being afterwards dispensed with, the final plate at the
positive end became copper, and that at the negative end zinc ;
and, consequently, the positive pole was then the copper pole,
and the negative pole the zinc pole.
This confusion, however, may be avoided, by observing that
the poles, positive or negative, are not dependent on the plates
or cylinders of metal with which the pole may terminate, but
on the direction of the electromotive forces of its elements. In
general, in a pile composed of zinc and copper elements, the
zinc plates of each pair all look towards the positive pole, and
the copper plates towards the negative pole. It must, how-
ever, be observed, that, in the ordinary arrangement, one ele-
ment of a pair is placed at each extremity of the pile, and con-
stitutes its pole, the pair being only completed when the poles
are united by the conducting wire. Thus, the pole to which
the copper elements look, terminates in a zinc plate in contact
with the exciting liquid, but r.ot with the adjacent copper
plate ; and the pole to which the zinc plates look terminates
in like manner with a copper plate in contact with the ex-
citing liquid, but not with the adjacent zinc plate. The single
extreme plates of zinc and copper thus forming the poles of the
pile being connected by the conducting wire, form, in fact, a
pair through which the current passes exactly as it passes
through any other pair in the series.
1904. Voltaic circuit. When the poles are thus connected
by the conducting wire, the VOLTAIC CIRCUIT is said to be
VOLTAIC CURRENTS.
299
complete, and the current continually flows, as well through the
pile as through the conducting wire. In this state the pile
constantly evolves electricity at its electromotive surfaces, to
feed and sustain the current ; but if the voltaic circuit be not
completed by establishing a continuous conductor between pole
and pole, then the electricity will not be in motion, no current
will flow ; but the wire or other conductor which is in con-
nexion with the positive pole will be charged with positive, and
that in connexion with the negative pole will be charged with
negative electricity, of a certain feeble tension, and in a state
of repose. Since, in such case, the electricity with which the
pile is charged has no other escape than by the contact of the
surrounding atmosphere, the electromotive force is in very
feeble operation, having only to make good that quantity which
is dissipated by the air. The moment, however, the voltaic
circuit is completed, the pile enters into active operation, and
generates the fluid necessary to sustain the current.
These are points which it is most necessary that the student
should thoroughly study and comprehend ; otherwise, he will
find himself involved in great obscurity and perplexity as he
attempts to proceed.
1905. Case in which the earth completes the circuit. If the
conducting wires connected with the poles p and N, instead of
being connected with the conductor Q,fig. 558., be connected with
the ground, the earth itself \vi\[ take the place and play the part
of the conductor o in relation to the current. The positive fluid
will in that case flow by the wire P E, fig. 559., and the
Fig. 559.
negative fluid by the wire N E to the earth E ; and the two fluids
will be transmitted through the earth E E in contrary directions,
300 VOLTAIC ELECTRICITY.
exactly in the same manner as through the conductor O. In
this case, therefore, the voltaic circuit is completed by the
earth itself.
1906. Methods of connecting the poles with the earth. In
all cases, in completing the circuit, it is necessary to ensure
perfect contact wherever two different conductors are united.
"We have already explained the application of mercurial cups
and metallic clamps for this purpose, where the conductors to
be connected are wires or strips of metal. When the earth is
used to complete the circuit, these are inapplicable. To ensure
the unobstructed flow of the current in this case, the wire is
soldered to a large plate of metal, having a surface of several
square feet, which is buried in the moist ground, or, still better,
immersed in a well or other reservoir of water.
In cities, where there are extensive systems of metallic pipes
buried for the conveyance of water or gas, the wires proceeding
from the poles P and N may be connected with these.
There is no practical limit to the distance over which a
voltaic current may in this manner be carried, the circuit being
still completed by the earth. Thus, if while the pile PN, Jig.
559., is at London, the wire P E is carried to Paris or Vienna
(being insulated throughout its entire course), and is put in
communication with the ground at the latter place, the current
will return to London through the earth E E as surely and as
promptly as if the points E E were only a foot asunder.
1907. Various denomination of currents. Voltaic currents
which pass along wires are variously designated, according to
the form given to the conducting wire. Thus they are RECTI-
LINEAR CURRENTS when the wire is straight ; INDEFINITE
CURRENTS when it is unlimited in length ; CLOSED CURRENTS
when the wire is bent so as to surround or inclose a space ;
CIRCULAR or SPIRAL CURRENTS when the wire has these
forms.
1908. The electric Jluid forming Ihe current not necessarily
in motion. Although the nomenclature which has been adopted
to express these phenomena implies that the electric fluid has a
motion of translation along the conductor similar to the motion
of liquid in a pipe, it must not be understood that the existence
of such motion of the electric fluid is necessarily assumed, or
that its non-existence, if proved, could disturb the reasoning
VOLTAIC CURRENTS. % 301
or shake the conclusions which form the basis of this branch
of physics. Whether an actual motion of translation of the
electric fluid along the conductor exist or not, it is certain that
the effect which would attend such a motion is propagated along
the conductor ; and this is all that is essential to the reasoning.
It has been already stated, that the most probable hypothesis
which has been advanced for the explanation of the phenomena
rejects the motion of translation, and supposes the effect to be
produced by a series of decompositions and recompositions of
the natural electricity of the conductor (1826).
1909. Method of coating the conducting wires. When the
wires by which the current is conducted are liable to touch
other conductors, by which the electricity may be diverted from
its course, they require to be coated with some non-conducting
substance, under and protected by which the current passes.
Wires wrapped with silk or linen thread may be used in sucli
cases, and they will be rendered still more efficient if they are
coated with a varnish of gum-lac.
When the wires are immersed in water, they may be pro-
tected by enclosing them in caoutchouc or gutta percha.
If they are carried through the air, it is not necessary to
surround them with any coating, the tension of the voltaic
electricity being so feeble, that the pressure of the air and its
non-conducting quality are sufficient for its insulation.
1910. Supports of conducting wire. When the wire is
carried through the air to such distances as would render its
weight too great for its strength, it requires to be supported at
convenient intervals upon insulating props. Rollers of por-
celain or glass, attached to posts of wood, are used for this
purpose in the case of telegraphic wires.
1911. Ampere's reotrope to reverse the current. In experi-
mental inquiries respecting the effects of currents, it is frequently
necessary to reverse the direction of a current, and sometimes
to do so suddenly, and many times in rapid succession. An
apparatus for accomplishing this, contrived by Ampere, and
which has since undergone various modifications, has been
denominated a commutator, but may be more appropriately
named a REOTROPE, the Greek words pt'oc (reos) signifying a
current, and rpoTrof (tropos}, a turn.
Let two grooves rr, fig. 560., about half an inch in width
302
VOLTAIC ELECTRICITY.
and depth, be cut in a board, and
between them let four small cavi-
ties v, t, v', t' be formed. Let these
cavities be connected diagonally in
pairs by strips of copper II' and
mm', having at the place where
they cross each other a piece of
cloth or other non-conducting sub-
stance between them, so as to pre-
vent the electricity from passing
from one to the other. Let the
grooves r and r', and the four cavi-
ties, be masticated on their surfaces with resin, so as to render
them non-conductors.
These grooves and cavities being filled with mercury, let the
apparatus represented in jig. 561. be placed upon the board.
A horizontal axis a a' moves in two holes oo' made in the up-
right pieces pp'. It carries four rectangular pieces of metal
e, c', d, d', so adapted, that
when they are pressed
downwards one leg of
each will dip into the mer-
cury in the groove, and
the other into the adjacent
cavity. The arms uniting
the rectangular metallic
pieces are of varnished wood, and are therefore non-conductors.
When this apparatus is in the position represented in the
figure, it will connect the groove r with the cavity v, and the
groove r' with the cavity t. When the ends dd! are depressed,
and therefore cc' elevated, it will connect the groove r with the
cavity t', and the groove r' with the cavity t.
The conductor which proceeds from the positive pole of the
pile is immersed in the mercury in r, and that which comes
from the negative pole is immersed in the mercury in r'. Two
strips of copper bb' connect the mercury in the cavities t' and
v', with the wire ww f which carries the current.
The apparatus being arranged as represented in^z^. 560., the
current will pass from the pile to the mercury in r; thence to
v by the conductor c ; thence to v' by the diagonal strip of
metal IV ; thence to w by the metal b f , and will pass along the
VOLTAIC CURRENTS. 303
wire as indicated by the arrows to b ; thence it will pass to the
mercury in t; thence by the diagonal strip m' in to t! ; thence
by the conductor c' to the mercury in the groove r' ; and thence,
in fine, to the negative pole of the pile.
If the ends dd' be depressed, and the ends cc' elevated, the
course of the current may be traced in like manner, as
follows : from r to t r ; thence by b to w; thence along the
conducting wire in a direction contrary to that of the arrows
to b' ; thence to v; thence to r; and thence to the negative
pole of the pile.
1912. PohFs reotrope. Various forms have more recently
been given to reotropes, one of the most convenient of which
is that of Pob.1, in which the use of mercury is dispensed with.
Four small copper columns A, B, c, D, Jig. 562., about inch
diameter, are set in a square board, and connected diagonally,
A with D, and B with c, by two bands of
copper, which intersect without contact.
ft tkjrf^57Ll/ 7] These pillars correspond to the four cavi-
ff ~
tiefl v, t/, t, t' in Ampere's reotrope. An
horizontal axis crosses the apparatus
similar to Ampere's; the ends of which
are copper, and the centre wood or ivory.
On each of the copper ends a bow a c, b d of copper rests, so
formed, that when depressed on the one side or the other, it
falls into contact with the copper pillars A, B, c, D. Two
metallic bands connect the pillars A and B with clamps or
binding screws p and m, to which the ends of the wire carrying
the current are attached. The ends of the horizontal axis are
attached to conductors which proceed from the poles of the pile.
The course of the current may be traced exactly as in the
reotrope of Ampere.
The arrangement and mode of operation of the metallic bows,
by depressing one end or the other of \\hich the direction
of the current is charged, is represented in
Wn^ Jig. 563., where ac is the bow, A and c the
\r\ A T\.^\r\ c two copper pillars with which it falls into con-
Fi- 563 tact n ^ ie ne S '^ e r ^ 1G ot ^ er ' ant * P tne
binding screw connected with the wire which
carries the current.
1913. Electrodes. The designation of POLES being usually
limited to the extreme elements of the pile, and the ne-
304
VOLTAIC ELECTRICITY.
cessity often arising of indicating a sort of secondary pole,
more or less remote from the pile by which the current enters
and leaves certain conductors, Dr. Faraday has proposed the
use of the term ELECTRODES to express these. Thus in the
reotrope of Ampere, the electrodes would be the mercury in
the grooves r' r', fig. 560. In the reotrope of Pohl, the electrodes
would be the ends of the horizontal axis p and M.
This term electrode has reference, however, more especially to
the chemical properties of the current, as will appear hereafter.
1914. Floating supports for conducting wire. It happens
frequently in experimental researches respecting the effects of
forces affecting voltaic currents or developed by them, that the
wire upon which the current passes requires to be supported
or suspended in such a manner as to be capable of changing its
position or direction in accordance with the action of such
forces. This object is sometimes attained by attaching the
wire, together with a small vessel containing zinc and copper
plates immersed in dilute acid, to a cork float, and placing the
whole apparatus on water or other liquid, on which it will be
capable of floating and assuming any position or direction which
the forces acting upon it may have a tendency to give to it.
1915. Ampere's apparatus for supporting movable currents.
A more convenient and generally useful apparatus for this
purpose, however, is that contrived by Ampere ; which consists
of two vertical copper rods v v', Jig. 564., fixed in a wooden
stage TT', the upper parts being bent at right angles and termi-
nated in two mercurial cups yy', one below the other in the
same vertical line. The horizontal parts are rolled with silk
or coated with gum-lac, to prevent the electricity passing from
Fig. 565.
INFLUENCE OF CURRENTS AND MAGNETS. 305
one to the other. Two small cavities rr' filled with mercury,
being connected with the poles of a battery, become the elec-
trodes of the apparatus. These may be connected at pleasure
with two mercurial cups ss', which are in metallic communi-
cation with the rods v v'. The reotrope may be applied to this
apparatus, so as to reverse the connexions when required.
The wire which conducts the current is so formed at its ex-
tremities as to rest on. two points in the cupsyy', and to balance
itself so as to be capable of revolving freely round the vertical
line passing through yy' as an axis.
A wire thus arranged is represented in fig. 565., having its
ends resting in the cups yy', the current passing from the cup
y' through the wire, and returning to the cupy. If the reotrope
be reversed, it will pass from y through the wire and return
toy.
CHAP. IV.
RECIPROCAL INFLUENCE OF RECTILINEAR CURRENTS AND
MAGNETS.
1916. Mutual action of magnets and currents. When a
voltaic current is placed near a magnetic needle, certain mo-
tions are imparted to the needle or to the conductor of the
current, or to both, which indicate the action of forces exerted
by the current on the poles of the needle, and reciprocally by
the poles of the needle on the current. Other experimental
tests show that the magnets and currents affect each other in
various ways ; that the presence of a current increases or
diminishes the magnetic intensity, imparts or effaces magnetic
polarity, produces temporary magnetism where the coercive
force is feeble or evanescent, or permanent polarity where it
is strong ; that magnets reciprocally affect the intensity and
direction of currents, and produce or arrest them.
1917. Electro-magnetism. The body of these and like
phenomena, and the exposition of the laws which govern them,
constitute that branch of electrical science which has been
denominated ELECTRO-MAGNETISM.
306
VOLTAIC ELECTRICITY.
To render clearly intelligible the effects of the mutual action
of a voltaic current and a magnet, it will be necessary to con-
sider separately the forces exerted between the current and
each of the magnetic poles ; for the motions which ensue, and
the forces actually manifested, are the resultants of the sepa-
rate actions of the two poles.
1918. Direction of the mutual forces exerted by a rectilinear
current and the pole of a magnet Tosimplify the explana-
tion, we shall, in the first instance, consider only the case of
rectilinear currents.
Let c c', Jig. 566., represent the
C wire along which a voltaic current
passes, directed from c to c 7 , as in-
dicated by the arrows. Let N N 7 be
a straight line which is parallel to
the current c C 7 , and which passes
through the magnetic pole. We
shall call this the line of direction
of the magnetic pole. Let a plane
\X b e imagined to pass through these
lines c c' and N N', and let a line A B
be drawn in this plane at right
angles to c c' and N N'.
The force exerted by the current
upon the magnetic pole, and re-
ciprocally the force exerted by the
magnetic pole upon the current,
; will have a direction at right angles
Fi". 566. * the plane passing through the
direction c c' of the current, and
the line of direction N N' of the magnetic pole.
Thus the line of direction N N' will be impelled by a force in
the direction of the line L R, and the current C c' by a force in
the direction of the line I/ R' ; these lines L R and L' R' being
understood to be drawn at right angles to the plane passing
through c c' and N N'.
But it is necessary to show in which direction on the lines
L R and L 7 R 7 these forces respectively act.
This direction will depend on and vary with the name of
the magnetic pole and the direction of the current on the line
cc'.
INFLUENCE OF CURRENTS AND MAGNETS. 307
It' we suppose the magnetic pole to be an austral or north
pole, and the current to descend on the line c c', as indicated
by the arrows, let an observer be imagined to stand with his
person in the direction c c' of the current, looking towards
N N', and the current consequently passing from his head to his
feet. In such case the direction of the force impressed by the
current on the line N N' will be directed to the right of such
observer, that is, from A towards R.
If the observer stand in the direction of the line of direction
N N' of the magnetic pole, looking towards the current c c', the
force impressed by the magnetic pole upon the current will, as
before, be directed to his right, that is, from B towards R'.
If the magnetic pole of which N N' is the line of direction be
a boreal or south pole, these directions will be reversed, each
line NN' and cc' being impelled to the left of the observer,
who looks from the other line. Thus, in such case, N N' will
be impelled by a force directed from A towards L, and c c' by
a force directed from B towards i/.
If the current ascend on the line c c', the directions of the
forces will be the reverse of those produced by a descending
current. Thus, when the current ascends, the line N N' will
be impelled to the left of the observer at c c' if the pole be
austral or north, and to his right if it be boreal or south ;
and in the same case the current c c' will be likewise impelled
to the left of the observer at N N' if the pole be austral or
north, and to his right if it be boreal or south.
To impress the memory with these various effects, it will be
sufficient to retain the directions of the forces produced be-
tween a descending current and a north magnetic pole. The
directions will be the same for an ascending current and a
south magnetic pole ; they will be reversed for a descending
current and south pole, or for an ascending current and north
pole.
Thus if the lines of direction of the current and the pole be
supposed to be both perpendicular to the surface of this paper,
and that the line of direction of the pole pass through the
paper at p, and that of the current at c, the directions of the
forces impressed on the lines of direction of the current and
the pole for a descending current and north magnetic pole, or
an ascending current and a south magnetic pole, are indicated
by the arrows \\\fig. 567., and their directions for a descending
308 VOLTAIC ELECTRICITY.
current and south magnetic pole, or an ascending current and
a north magnetic pole, are indicated in jig. 568.
p
Fig. 567. Fig. 568.
For example, if a current descend on a vertical wire, and the
austral or north pole of a magnet be placed so that its line of
direction shall be to the north of the current, the wire of the
current will be impelled by a force directed to the west, and the
line of direction of the magnetic pole by a force directed to the
east.
If the current ascend, or if the pole be a south pole, the wire
of the current will be impelled to the east, and the line of
direction of the pole to the west.
1919. Circular motion of magnetic pole round a fixed
current. If the line of direction of the current be fixed, and
that of the magnetic pole be movable, but so connected with
the line of the current as to remain always at the same distance
from it, the line of direction of the pole will be capable only of
moving round the surface of a cylinder whose axis is the direc-
tion of the current. In this case the force impressed by the
current on the line of direction of the pole, being always at
right angles to that line, and always on the same side of it as
viewed from the current, will impart to the line of direction of
the pole a motion of continued rotation round the current as an
axis. This rotation, as viewed on the side from which the
current flows, will be in the same direction as the motion of
the hand of a watch, where the pole is north, as represented in
fig. 569., and in the contrary direction as represented in fig.
570., where the pole is south.
1920. Circular motion of a current round a magnetic pole.
A similar motion of continued rotation will be imparted to
the wire conducting the current if the line of direction of the
magnetic pole be fixed, and the wire be similarly connected
with it. In this case the motion imparted by a north pole on
INFLUENCE OF CURRENTS AND MAGNETS. 309
a descending current is represented in Jig. 569., and that im-
pressed by a south pole \nfig. 570.
Fig. 569.
1921. Apparatus to illustrate experimentally these effects.
A great variety of apparatus and experimental expedients has
been contrived to illustrate and verify these laws.
1922. Apparatus to exhibit the direction of the force im-
pressed by a rectilinear current on a magnetic pole. To
demonstrate the direction of the force impressed by a rectilinear
current on a magnetic pole, let a light bar,
fig. 571., of ivory, or any other substance
not susceptible of magnetism, made flat at
the upper surface, be balanced like a compass
needle on a fine point, so as to be free to
move round it in an horizontal plane. Let a
magnetic needle, N s, be placed upon one
arm of it, so that one of the poles, the boreal s for example, be
exactly over the point of support ; and let a counterpoise, w, be
placed upon the other arm. Let the magnet be rendered
astatic, so as not to be affected by the earth's magnetism by
any of the methods already explained (1695).
Let the needle thus suspended be supposed to play round s,
Jig. 572., in the plane of the paper, and let a voltaic current
pass downwards along a wire perpendicular to the paper, c re-
presenting the intersection of such wire with the paper. The
needle, after some oscillations, will come to rest in the position
Fig. 571.
310
VOLTAIC ELECTRICITY.
-"c
Fig. 572.
s N, so that its direction shall be at right angles to the line ON,
drawn from the current to the pole
P' N, and so that the centre s shall be
to the left of N as viewed from c.
\ It follows, from what has been
-fetf already explained, that the force ex-
\ erted by the current c on the pole N
I has the direction indicated by the
/' arrow from s to N. This force is
/' therefore directed to the right of N
as viewed from c.
If the wire carrying the current
be moved round the circle c c' c" c'",
the pole N will follow it, assuming
always such positions, N', N", N'", that SN', SN", s N w shall be
at right angles to c' N', C"N", c'" N'". It follows, therefore, that
whatever position may be given to the current, it will exert a
force upon the austral or north pole N of the magnet, the di-
rection of which will be at right angles to the line drawn from
the current to the pole, and to the right of the pole as viewed
from the current.
If the position of the needle be reversed, the pole N being
placed at the centre of motion, the
same phenomena will be manifested,
but in this case the needle will
place itself to the right of the pole s as
viewed from the current c, as repre-
sented in fig. 573. It follows there-
fore, in this case, that whatever po-
sition be given to the current, it will
exert a force upon the boreal or south
pole of the magnet, the direction of
which will be at right angles to the
line drawn from the current to the
pole, and to the left of the pole as viewed from the current.
The current has here been supposed to descend along the
wire. If it ascend the effects will be reversed. It will exert
a force on the austral pole directed to the left, and on the
boreal pole one directed to the right.
1923. Apparatus to measure intensity of this force. Having
indicated the conditions which determine the directions of the
INFLUENCE OF CURRENTS AND MAGNETS. 311
forces reciprocally exerted between magnetic poles and a cur-
rent, it is necessary to explain those which affect their intensity.
Let sx,Jig. 574., be an astatic needle affected by the current
c, whose direction is per-
pendicular to the- paper,
as already explained. If
, N be displaced it will 1
,W oscillate on the one side
~| "N"" || and the other of its po-
sition of rest, and its os-
Fig. 574. cillations will be governed
by the laws already explained in the case of the pendulum (256).
The intensity of the force impressed on it in the direction of
the arrows by the current c, will be proportional to the squares
of the number of vibrations per minute.
1924. Intensity varies inversely as the distance. If the
distance of c from N be varied, it will be found that the square
of the number of vibrations per minute will increase in the
same proportion as the distance CN is diminished, and vice versa.
It follows, therefore, that the force impressed by the current on
the pole is increased in the same ratio as the distance of the
current from the pole diminishes, and vice versa.
In the case here contemplated, the length of the wire carrying
the current being considerable, each part of it exercises a
separate force on N, and the entire force exerted is consequently
the resultant of an infinite number of forces, just as the weight
of a body is the resultant of the forces separately impressed by
gravity on its component molecules. LAPLACE has shown
that the indefinitely small parts into which the current may be
supposed to be divided, exert forces which are to each other
in the inverse ratio of the squares of their distances from the
pole, and that by the composition of these a resultant is pro-
duced, which varies in the inverse proportion of the distances
as indicated by observation.
From what has been stated, it is evident that if the current
^^ fig. 575. be placed at the centre s of the circle
round which the north pole of a magnet is free
to move, it will impart to the pole a continuous
7 motion of rotation in that circle. If the current
be supposed to move downwards, the pole N will
Fig. 575. t> e constantly driven to the right as viewed from
312 VOLTAIC ELECTRICITY.
the centre s (1918), and consequently the magnet will move in
the direction of the hand of a watch, as indicated by the arrows.
If the north pole N be placed at the centre, as in fig. 576.,
the current still descending, the force exerted on
the south pole s will be constantly directed to the
left as viewed from the centre, and the magnet
will accordingly move contrary to the hand of a
watch, as indicated by the arrows.
If the current ascend, these motions will be
reversed, the north pole moving contrary, and the
south according to the hand of a watch, as
indicated in figs. 577., 578.
Descending current acting on north pole
(fid- 575.).
p. 5 _ Descending current acting on south pole
g * (fig. 576.).
Ascending current acting on south pole
Ascending current acting on north pole
(fig. 578.).
Fig. 578.
1925. Case in which the current is icitkin, but not at the
centre of the circle in which the pole revolves. If the current
be within the circle described by the free pole, but not at its
centre, the pole will still revolve ; but the force which impels it
will not be uniform, as it is when the current is at the centre.
Since the force exerted by the current on the pole is inversely
as its distance from the pole, that force will be necessarily
uniform when the current is at the centre, the distance of the
pole from it being always the same. But when the current is
within the circle at a point c,fig. 579., different from the centre,
the distance CN will vary, and the force ex-
erted on N will vary in the inverse proportion,
increasing as the distance is diminished, and
decreasing as the distance is increased. The
rotation will nevertheless equally take place,
and in the same direction as when the current
c is at the centre.
1926. Action of a current on a magnet, both poles being free.
Having thus explained the mutual action of the current, and
INFLUENCE OF CURRENTS AND MAGNETS. 313
each pole of the needle separately, we shall now consider the
case in which a magnetic needle suspended as usual on its
centre is exposed to the action of the current.
1927. Case in which the current is outside the circle described
by the poles. Let G,figs. 580, 581., as before, be a descending
current placed outside the circle in which the poles of the needle
NS play. The forces exerted by the current on the two poles
N and s have in this case opposite effects on the needle, and
consequently it will turn in the direction of that which has the
greater effect, and will be in equilibrium when the effects are
equal.
If the poles be placed at N' and s', fig. 580., the force exerted
on them will move N' towards N, and s' towards s, and the
needle will turn in the direction of the arrows by the combined
effects of both forces. When N' arrives at N", the force being
in the direction ON" will be ineffective ; but the force acting on
s", the opposite pole, will continue to turn the needle towards
the position NS, where it is at right angles to co. After
passing N", the force on N is effective in opposition to that on
s, but in a very small degree, so that the effect on s prepon-
derates until the needle arrives at the position SN. Here, the
poles N and s being equally distant from c, the forces are equal,
and being equally inclined to SN, have equal effects in opposite
directions. The needle is therefore held in equilibrium. If the
needle be moved beyond this position, the effect of the force
on N predominating over that of the force on s, the needle
will be brought back to the position SN, and will oscillate on
the one side and the other of this direction, showing that it
is the position of stable equilibrium (299).
If the pole s be placed at s',fig. 581., the effects on the two
314
VOLTAIC ELECTRICITY.
poles s' and N' will, as before, combine to turn the needle as
indicated by the arrows, moving the south pole towards s, and
the north towards N.
It follows, therefore, that a downward current c acting as in
figs. 580, 581. outside the circle described by the poles, will
throw the needle into a direction SN at right angles to the line
CO drawn from the current to the centre of the needle, the
north pole being on the right as viewed from the current.
An ascending current will produce the contrary effects, the
north pole being thrown to the left as viewed from the
current.
1928. Case in which the current passes through the circle.
If the current pass through the circle described by the poles,
the needle will rest indifferently in any direction that may be
given to it. In this case, fig. 582., let c be the current. The
forces it exerts on s and N being in the in-
verse ratio of the distances, that which affects
s will be to that which affects N as CN is to
cs. The moment of the force on s will
therefore be CNXCS, and the moment of the
force on N will be CSXCN. These moments
being equal, the forces must be in equilibrium
(426), and the needle will therefore remain
at rest whatever position be given to it.
1929. Case in which the current passes
within the circle. Let the current pass within the circle
described by the poles. In this case the effects of the current on
the two poles s and N is opposite in every position. If the pole be
at t*\jig. 583., the point of the circle nearest
to c, the force on N' being greater than the
force on s' in the ratio of cs' to CN'(1923),
the effect of the force on N' will predomi-
nate, and N' will be moved towards N, and
s' towards s. The effects on N will continue
to predominate until they arrive at the
position NS, where the effects become equal
and the needle is in equilibrium ; for here
the distances of s and N from C being equal, the forces are equal ;
and since they are equally inclined to the needle, they have
equal effects to turn it in contrary directions. After passing N,
the effect on s predominates ; the needle will be brought back
Fig. 583.
INFLUENCE OF CURRENTS AND MAGNETS. 315
to the position NS, and will oscillate on the one side and the
other, indicating stable equilibrium (299).
If the needle be placed with the pole s' at the point nearest
toe, fig. 584., the effect on s' predominating,
s' will be moved towards s, and N' towards
N, and the needle will attain the same
position as in the former case.
It follows, therefore, that when a de-
scending current passes within the circle
described by the poles, as represented in
figs. 583, 584., the needle will be thrown
into a direction SN at right angles to the line CO drawn from
the current to the centre of the needle, the north pole pointing
to the left as viewed from the current.
An ascending current will produce the contrary effect,
throwing the north pole to the right.
It will be observed that the direction of the poles, when the
current is within the circle, is opposite to its direction when
it is outside the circle.
1930. Apparatus to illustrate electro-magnetic rotation.
A variety of interesting and instructive apparatus has been
contrived to illustrate experimentally the reciprocal forces
manifested between currents and magnets. These may be
described generally as exhibiting a magnet revolving round a
current, or a current revolving round a magnet, or each
revolving round the other, impelled by the forces which the
current and the poles of the magnet exert upon each other. It
will be conducive to brevity in describing these effects to
designate a motion of rotation which is from left to right, or
according to that of the hand of a watch, as direct rotation,
and the contrary as retrograde rotation. It will therefore
follow from what has been explained, that if N and s express
the north and south poles of the magnet, and A and D express
an ascending and descending current, the rotation of each
round the other in every possible case will be as follows :
'
j Retrograde.
We shall classify the apparatus according to the particular
manner in which they exhibit the action of the forces.
p 2
316
VOLTAIC ELECTRICITY.
1931. To cause either pole of a magnet to revolve round a
fixed voltaic current. Let two bar magnets be bent into the
form shown 'mfig. 585., so that a small
part at the middle of their length shall
be horizontal. Under this part an
agate cap is fixed, by which the mag-
net is supported on a pivot. Above
the horizontal part a small cup contain-
ing mercury is fixed. The magnets
are thus free to revolve on the pivots.
A small circular canal of mercury sur-
rounds each magnet a little below the
rectangular bend, into which the amal-
gamated point of a bent wire dips.
These wires are connected with two
vertical rods, which, turning at right
angles above, terminate in a small cup
Two similar mercurial cups communicate
If the upper cup be put
Fig. 585.
containing mercury.
with the circular mercurial canals.
in communication with the positive pole of a battery, and the
lower cups with the negative pole, descending currents will be
established on the vertical rods ; and if the upper cup be put
in communication with the negative, and the lower with the
positive, the currents will ascend. The two magnets may be
placed either with the same or opposite poles uppermost. The
currents pass from the vertical rods to the mercury in the
circular canals, thence to the lower cups, and thence to the
negative poles.
When the descending current passes on the rods, the north
pole of the magnet revolves with direct, and the
south pole with retrograde motion. When the cur-
rent ascends, these motions are reversed.
1932. To cause a moveable current to revolve
round the fixed pole of a magnet. Let a glass
vessel,^. 586., be nearly filled with mercury. Let
a metallic wire suspended from a hook over its
centre be capable of revolving while its end rests
upon the surface of the mercury. A rod of metal
enters at the bottom of the vessel, and is in con-
tact with a magnetic bar fixed vertically in the
Fig. 586. centre of the vessel. When one of the poles of the.
INFLUENCE OF CURRENTS AND MAGNETS. 317
battery is put in communication with the moveable wire, and
the other with the fixed wire connected with the magnet, a
current will pass along the moveable wire, either to the mer-
cury or from it, according to the connexion made with the
poles of the battery ; and the moveable wire will revolve round
the magnet, touching the surface of the mercury with a motion
direct or retrograde, according as the current descends or
ascends, and according to the name of the magnetic pole fixed
in the centre (1930).
Let zz',fig. 587., represent a section of a circular trough
containing mercury, having an opening at the
centre, in which is inserted a metallic rod,
terminating at the top in a mercurial cup c.
A. wire at a b b' a' is bent so as to form three
sides of a rectangle, the width b b' correspond-
ing with the diameter of the circular trough
z z' . A point is attached to the middle of b b 1 ,
which rests in the cup c, so that the rectangle
is balanced on the rod t, and capable of re-
Fig. 587. volving on the pivot as a centre.
If the mercury in the circular trough be
connected by a wire with the negative, while the cup c is
connected with the positive pole of a battery, descending
currents will be established along the vertical wires b a and
b' a'; and if the connexions be reversed, these currents will
ascend.
If, when these currents are established, the pole of a magnet
be applied under the centre P, it will act upon the vertical
currents, and will cause the rectangular wire at abb' a' to
revolve round c, with a motion direct or retrograde, according
to the direction of the current and the name of the magnetic
pole (1920).
The points of contact of the revolving wires with the
mercury may be multiplied by attaching the ends a a' of the
wires to a metallic hoop, the edge of which will rest in contact
with the metal ; or the wires a b and a' b' may be altogether
replaced by a thin copper cylinder balanced on a point in the
cup at c.
Another apparatus for illustrating this is represented in
fig. 588. A bar magnet is fixed vertically in the centre of a
circular trough containing mercury. A light and hollow cy-
318
VOLTAIC ELECTRICITY.
linder of copper is suspended on a point rest-
ing in an agate cup placed on the top of the
magnet, and having a vertical wire proceed-
ing from it, which terminates in a small mer-
curial cup p at the top. Another wire con-
nects the mercury in the trough with a
mercurial cup N. When the cups p and N
are put in communication with the poles of
the battery, a current is established on the
sides of the copper cylinder c c, and rotation
takes place as already described.
A double apparatus of this kind, erected on
the two poles of a horse-shoe magnet, is re-
presented in fig. 589.
Fig. 58!).
Fig. 590.
1933. Amperes method. Ampere adopted the following
method of exhibiting the revolution of a current round a
magnet. A double cylinder of copper, c C,Jig. 590., about 2^-
in. diameter and 2iy in. high, is supported on the pole of a
bar magnet by a plate of metal passing across the upper orifice
of the inner cylinder. A light cylinder of zinc z z, supported
on a wire arch A, is introduced between the inner and outer
cylinders of copper, a steel point attached to the wire arch
resting upon the plate by which the copper cylinders are sup-
ported. On introducing dilute acid between the copper cy-
linders, electromotive action takes place, the current passing
from the zinc to the acid, thence to the copper, and thence
INFLUENCE OF CURRENTS AND MAGNETS. 319
through the pivot to the zinc. The zinc being in this case free
to revolve, while the copper is fixed, and the current descending
on the former, the rotation will be direct or retrograde accord-
ing as the magnetic pole is north or south.
If the copper were free to revolve as well as the zinc, it would
turn in the contrary direction, since the current ascends upon
it, while it descends on the zinc. Mr. J. Marsh modified Am-
pere's apparatus, so as to produce this effect by substituting a
pivot, resting in a cup at the top of the magnet, for the metallic
arch by which, in the former case, the copper vessel was
sustained.
A double arrangement of this kind is given in fig. 591.,
where the double cylinders are supported on pivots on the two
poles of a horse-shoe magnet. The rotation of the corresponding
cylinders on the two opposite magnetic poles will be in con-
trary directions.
Fig. 591.
Fig. 592.
1934. To make a magnet turn on its own axis by a current
parallel to it. The tendency of the conductor on which a
current passes to revolve round a magnet will not the less exist,
though the current be so fixed to the magnet as to be in-
capable of revolving without carrying the magnet with it. In
fig. 592., the magnet M is sunk by a platinum weight p ; its
upper end being fixed to the copper cylinder ww, a current
passing from p to N causes the cylinder to rotate, carrying with
it the magnet.
v 4
320
VOLTAIC ELECTRICITY.
Since a magnetic bar is itself a conductor, it is not necessary
to introduce any other ; and a current passing
along the bar will give rotation to it. An ap-
paratus for exhibiting this effect is represented
in fig. 593., where a magnetic bar is supported
in the vertical position between pivots which
play in agate cups. A circular mercurial
canal is placed at the centre of the magnet,
and another round the lower pivot. Mercurial
cups communicate with these two canals.
When these cups are put in communication
with the poles of a battery, the current will
pass between the two canals along the lower
pole of the magnet, in the one direction or the
other, according to the mode of connexion ;
and the magnet will turn on its own axis with
a direct or retrograde rotation, according to
the name of the pole on which the current runs, and to the
direction of the current.
Fig. 593.
CHAJP. V.
RECIPROCAL INFLUENCE OF CIRCULATING CURRENTS AND
MAGNETS.
IF a wire PABCDN, figs. 594., 595., be bent into the form of any
geometrical figure, the extremities being brought near each other
\\
C +**& B
Fig. 594. Fig. 595.
without actually touching, a current entering one extremity and
departing from the other, is called a CIRCULATING CURRENT.
CIRCULATING CURRENTS AND MAGNETS. 321
1935. Front and back of circulating current. If such a
current be viewed on opposite sides of the figure formed by the
wire, it will appear to circulate in different directions, on one
side direct, and on the other retrograde (1930). That side on
which it appears direct is called the FRONT, and the other the
BACK of the current.
1936. Axis of current. If the current have a regular figure
having a geometrical centre, a straight line drawn through this
centre perpendicular to its plane is called the AXIS of the
current.
1937. Reciprocal action of circulating current and magnetic
pole. To determine the reciprocal influence of a circulating
current and a magnetic pole placed anywhere upon its axis, let
D^ the axis be xcx', fig. 596.,
"* J^ 1 t ^ ie pl ane f tne current being
o7"-^vNT X at right angles to the paper,
| c s^\L ' A being the point where it as-
* cends, and D the point where
4 it descends through the paper.
F 'S- 596 - 1. Let N be a north mag-
netic pole placed in front of the current.
The part of the current at D will exert a force on N in the
direction NM' at right angles to DN, and the part at A will exert
an equal force in the direction N M at right angles to AN. These
two forces being compounded, will be equivalent to a single
force N o (152) directed from N along the axis towards the
current.
It may be shown that the same will be true for every two
points of the current which are diametrically opposed.
2. Let a south magnetic pole $,fig- 597., be similarly placed
in front of a circulating current. The part D will exert upon
D it a force in the direction sil
*> x perpendicular to SD and to
'^S/ \u K the left of s as viewed from D,
~^ \ , and the part A will exert an
M\ equal force in the direction
s M' to the right of S as viewed
Flg< 597 ' from A. These two equal
forces will have a resultant so directed from the current ; and
the same will be true of every two points of the current which
are diametrically opposed,
p 5
322 VOLTAIC ELECTRICITY.
If the magnetic pole be placed at the back of the current, the
contrary effects ensue.
The same inferences may be deduced with respect to any cir-
culating current which has a centre, that is, a point within it
which divides into two equal parts all lines drawn through it
terminating in the current.
It may therefore be inferred generally that when a magnetic
pole is placed upon the axis of a circulating current, attraction
or repulsion is produced between it and the current ; attraction
when a NORTH pole is before, or a SOUTH pole BEHIND, and re-
pulsion when a SOUTH pole is before, or a north pole BKHIND.
1938. Intensity of the force vanishes ivhen the distance of
the pole bears a very great ratio to the diameter of current.
Since the intensity of the attraction between the component
parts of the current and the pole decreases as the square of the
distance is increased, and since the lines NM and NM/, fig. 596.,
and SM and SM', Jig. 597., form with each other a greater angle
as the distance of the pole from the current is increased, it is
evident that when the diameter AD of the current bears an
inconsiderable ratio to the distance of the pole N or s from it,
the attraction or repulsion ceases to produce any sensible
effect.
1939. But the directive power of the pole continues. This,
however, is not the case with relation to the directive power of
the pole upon the current. The tendency of the forces im-
pressed by the pole upon the current is always to bring the
plane of the current at right angles to the line drawn from the
pole to its centre. There is, in short, a tendency of the line of
direction of the pole to take a position coinciding with or
parallel to the axis of the current, and this coincidence may be
produced either by the change of position of the pole or of the
plane of the current, or of both, according as either or both are
free to move.
1940. Spiral and heliacal currents. The force exerted by
a circulating current may be indefinitely augmented by causing
the current to circulate several times round its centre or axis.
If the wire which conducts the current be wrapped with silk or
coated with any non-conducting varnish, so as to prevent the
electricity from escaping from coil to coil when in contact, cir-
culating currents may be formed round a common centre or
axis in a ring, a spiral, a helix, or any other similar form, so
CIRCULATING CURRENTS AND MAGNETS. 323
that the forces exerted by all their coils on a single magnetic
pole may be combined by the principle of the composition of
force ; and hence an extensive class of electro-magnetic phe-
nomena may be educed, which supply at the same time im-
portant consequences and striking experimental illustrations of
the laws of attraction and repulsion which have been just
explained.
1941. Expedients to render circulating currents moveable.
Ampere's and Delarive's apparatus. Two expedients have
been practised to render a circulating current moveable.
1. By the apparatus of AMPERE already described (1915),
the wire conducting the current being bent
at the ends, as represented in Jig. 598., may
be supported in the cups yy 1 as represented
in fig. 564., so that its plane being vertical,
it shall be capable of revolving round the
line yy 1 as an axis. By this arrangement
the plane of the current can take any di-
rection at right angles to an horizontal
plane, but it is not capable of receiving any
Fig. 598. progressive motion.
2. The latter object is attained by the floating apparatus of
M. Delarive.
-. Let a coated wire be formed into a circular
g ( 1 N Tin S composed of several coils. Let one end of
V / it be attached to a copper cell, fig. 599., and
:'if* the other to a slip of zinc which descends
into this celt. The cell being filled with aci-
| dulated water, a current will be established
through the wire in the direction of the
\ arrows. The copper cell may be inclosed in
a glass vessel, or attached to a cork so as to
float upon water, and thus be free to assume
Fig. 599. an y position which the forces acting upon the
current may tend to give it.
1942. Rotatory motion imparted to circular current by a
magnetic pole. If a magnetic north pole be presented in front
of a circular current, fig. 598., suspended on Ampere's frame,
fig. 564., the ring will turn on its points of suspension until its
axis pass through the pole. If the pole be carried round in a
circle, the plane of the ring will revolve with a corresponding
324 VOLTAIC ELECTRICITY.
motion, always presenting the front of the current to the pole,
the axis of the current passing through the pole.
If a south magnetic pole be presented to the back of the
current, like effects will be produced.
If a north magnetic pole be presented to the back, or a south
to the front of the current, the ring will, on the least disturb-
ance, make half a revolution round its points of suspension, so
as to turn its point to the north and its back to the south mag-
netic pole.
1943. Progressive motion imparted to it. If c, fig. 600.,
represent a floating circular current,
a north magnetic pole placed anywhere
B on its axis will cause the ring con-
ducting it to move in that direction in
which its front is presented ; for if the
Fig. eoo. pole be before it at A it will attract the
current, and if behind it at B it will repel it (1937). In either
case the ring will move in the direction in which its front looks.
If a south magnetic pole be similarly placed, it will cause the
current to move in the contrary direction ; for if it be placed
before the current at A it will repel it, and if behind it at B it
will attract it. In either case the ring will move in the di-
rection to which the back of the current looks.
1944. Reciprocal action of the current on the pole. If the
magnetic pole be moveable and the current fixed, the motion
impressed on the pole by the action of the current will have a
direction opposite to that of the motion which would be im-
pressed on the current, being moveable, by the pole being fixed.
A north magnetic pole placed on the axis of a fixed circular
current will therefore be moved along the axis in that direction
in which the back of the current looks, and a south magnetic
pole in that direction in which the front looks.
1945. Action of a magnet on a circular floating current.
If a bar magnet s N,
fig. 601., be placed in
a fixed position with
its magnetic axis in
Fig- 601. the direction of a
floating circular current A, its north pole x being directed to
the front of the current, the current will be attracted by x and
repelled by s ; but the force exerted by x will predominate in
CIRCULATING CURRENTS AND MAGNETS. 325
consequence of its greater proximity to A, and the current will
accordingly move from A towards N. After it passes N, the
bar passing through the centre of the ring, it will be repelled
by N and also by s (1937) ; but so long as it is between N and
the centre c of the bar, as at B, the repulsion of N will pre-
dominate over that of s in consequence of the greater proximity
of N, and the current will move towards c. Passing beyond c to
B', the repulsion of s predominates over that of N, and it will be
driven back to C, and after some oscillations on the one side
and the other it will come to rest in stable equilibrium, with its
centre at the centre of the magnet, its plane at right angles to
it, the front looking towards s and the back towards N.
1946. Reciprocal action of the current on the magnet. If
the current be fixed and the magnetic bar moveable, the latter
will move in a direction opposite to that with which the cur-
rent would move, the bar being fixed. Thus, if the current
were fixed at A, the bar would move to it in the direction of N A,
and the pole N passing through the ring, the bar would come
to rest, after some oscillations, with its centre at the centre of
the ring.
1947. Case of instable equilibrium of the current. If the
ring were placed with its centre at c and its front directed to
N, it would be in instable equilibrium, for if moved through any
distance, however small, towards N or s, the attraction of the
pole towards which it is moved would prevail over that of the
other pole which is more distant, and the ring would conse-
quently be moved to the end of the bar and beyond that point,
when, being still attracted by the nearest pole, it would soon be
brought to rest. It would then make a half revolution on its
axis and return to the centre of the bar, where it would take
the position of stable equilibrium.
All these are consequences which easily follow from the ge-
neral principles of attraction and repulsion established in (1937).
J948. Case of a spiral current. If the wire which con-
ducts the current be bent into the form of a spiral,
fig. 602., each convolution will exert the force of
\\ a circular current, and the effect of the whole will
be the sum of the forces of all the convolutions.
Such a spiral will therefore be subject to the con-
Fig. 602. ditions of attraction and repulsion which affect a
circular current (1937).
326 VOLTAIC ELECTRICITY.
1949. Circular or spiral currents exercise the same action as
a magnet. In general it may be inferred that circulating
currents exercise on a magnetic pole exactly the same effects as
would be produced by another magnet, the FRONT of the current
playing the part of a SOUTH pole, and the BACK that of a NORTH
pole.
1950. Case of heliacal current. It has been shown that a
helix or screw is formed by a point which is at the same time
affected by a circular and progressive motion, the circular motion
being at right angles to the axis of the helix, and the progressive
motion being in the direction of that axis (496). In each con-
volution the thread of the helix makes one revolution, and at
the same time progresses in the direction of the axis through a
space equal to the distance between two successive convolutions.
1951. Method of neutralizing the effect of the progressive
motion of such a current. If a current therefore be transmitted
on a heliacal wire, it will combine the characters of a circular
and rectilinear current. The latter character, however, may be
neutralized or effaced by transmitting a current in a contrary
direction to the progression of the screw, on a straight wire
extended along the axis of the helix. This rectilinear current
being equal, parallel, and contrary in direction to the pro-
gressive component of the heliacal current, will have equal and
contrary magnetic properties, and the forces which they exert
together on any magnetic pole within their influence will
counteract each other.
1952. Right-handed and left-handed helices. Helices are
TWWVWWy???'??^ of two forms : those in which the wire
HHimilD turns like the thread of a cork9creW)
lg< ' that is, in the direction of the hands of a
HffiSHHEffiH watch, fig. 603. ; and those in which it
Fig. 6O4. turns in a contrary direction,^. 604.
1953. Front of current on each kind. If a current traverse
a right-handed helix, its front will be directed to the end at
which it enters, and in the left-handed helix to the end at
which it departs.
1954. Magnetic properties of heliacal currents their poles
determined. Hence it follows, that in a right-handed heliacal
current, the end at which the current enters, and which is the
positive pole, has the magnetic properties of a south pole ; and
in a left-handed helix this end has the properties of a north pole.
CIRCULATING CURRENTS AND MAGNETS. 327
1955. Experimental illustration of these properties. The
magnetic properties of spiral and heliacal currents may be illus-
trated experimentally by means of Ampere's arrangement,^.
564., or by a floating apparatus constructed on the same prin-
ciple as that represented in^. 599.
The manner of forming spiral currents adapted to Ampere's
apparatus is represented \njigs. 605. and 606. In Jig. 605. the
Fig. 605.
spirals are both in the same plane, passing through the axis of
suspension yy'. In Jig- 606. they are in planes parallel to this
axis, and at right angles to the line joining their centres, which
is therefore their common axis.
1956. The front of a circulating current has the properties
of a south, and the back those of a north, magnetic pole. Ac-
cording to what has been explained, the front of such a spiral
current will have the properties of a south magnetic pole, and
will therefore attract and be attracted by the north, and repel
and be repelled by the south pole of a magnet. If the spirals
in fig. 605., therefore, be so connected with the poles of a voltaic
system, as to present their fronts on the same side, they will be
both attracted by the north pole, and both repelled by the
south pole of a magnet presented to them, that which is nearer
to the magnet being more attracted or repelled than the other.
If the magnetic pole be equally distant from them, they will
be in equilibrium, and the equilibrium will be stable if they
are both repelled, and instable if they are both attracted by
the magnet.
To demonstrate this, let s, fig. 607., be the south pole of a
magnet placed in front of the two spirals, whose centres are at
A and B, equally distant from s. It is evident that a perpen-
dicular so drawn from s to AB will in this case pass through
the middle of AB. The pole s will therefore, according to what
.328
VOLTAIC ELECTRICITY.
has been already explained, repel the two spirals with equal
forces. If the spirals be re-
moved from this position to
the positions A'B', A', being
nearer to S than B', will be
repelled by a greater force,
and therefore A' will be driven
back towards A, and B' to-
wards B. In like manner, if
they were removed to the
positions A"B", the force re-
pelling B" would be greater
than that which repels A",
F 'S- 607 - and therefore B" will be driven
back to B, and A" to A.
It follows, therefore, that the position of equilibrium of AB is in
this case such that the system will return to it after the slightest
disturbance on the one side or the other, and is therefore stable.
If the pole s were the north pole, it would attract both
currents, and in that case A' would be more strongly attracted
than B', and B" than A", and consequently the spirals would de-
part further from the position A after the least disturbance. The
equilibrium would therefore be instable.
It will be found, therefore, that when a NORTH POLE is pre-
sented BEFORE, or a south pole BEHIND, such a pair of spiral
currents, the system,^. 605., will, on the least disturbance from
the position of instable equilibrium, turn on its axis yy' through
half a revolution, presenting the fronts of the currents to the
south pole, and will there come to rest after some oscillations.
In the position of stable
equilibrium, the front of the
currents must therefore be
presented to the south pole
of the magnet, or the back
to the north pole.
1957. Adaptation of an
heliacal current to Ampere's
and Delarive's apparatus.
The manner of adapting
an heliacal current to Am-
pere's arrangement,^. 564.,
Fig. 608.
is represented in fig. 608.
CIRCULATING CURRENTS AND MAGNETS. 329
and the manner of adapting it to the floating method is repre-
sented in Jiff. 609.
The positive wire is carried down from y,fig. 608., and then
coiled into an helix from the centre to
, . mu -i j
the extremity. Thence it is carried in a
straight direction through the centre of
the helix to the other extremity, from
whence it is again conducted in heliacal
coils back to the centre, where it is bent
upwards and terminates at the negative
pole#'. In one half of the helix the cur-
rent therefore enters at the centre and
The forces which a voltaic current impresses upon the poles of
a permanent magnet, being similar in all respects to those with
which the same poles would be affected by another magnet, it
may be expected that the natural magnetism of an unmagnetized
body would be decomposed, and polarity imparted to it by the
approach of a voltaic current, in the same manner as by the
approach of a magnet. Experiment accordingly confirms this
consequence of the analogy suggested by the phenomena. It is,
in fact, found that a voltaic current is capable of decomposing
the natural magnetism of magnetic bodies, and of magnetizing
them as effectually as the most powerful magnets.
Soft iron rendered magnetic by voltaic currents. If the wire
upon which a voltaic current flows be immersed in filings of
soft iron, they will collect around it, and attach themselves
to it in the same manner as if it were a magnet, and will
continue to adhere to it so long as the current is maintained
upon it ; but the moment the connexions with the battery are
broken, and the current suspended, they will drop off.
Sewing needles attracted by current. Light steel sewing
needles being presented to the wire conducting a current will
instantly become magnetic, as will be apparent by their as-
suming a position at right angles to the wire, as a magnetic
needle would do under like circumstances. When the current
is suspended or removed, the needles will in this case retain the
magnetism imparted to them.
1960. Magnetic induction of an heliacal current. To exhibit
these phenomena with greater effect and certainty, the needles
should be exposed to the influence not of one, but of several
currents, or of several parts of the same current flowing at
right angles to them. This is easily effected by placing them
within an heliacal current.
Let a metallic wire coated with silk or other non-conductor
332 VOLTAIC ELECTRICITY.
be rolled heliacally on a glass tube, fig. 612., and the current
being made to pass along the wire, let a needle or bar
of steel or hard iron be placed within the tube. It
will instantaneously acquire all the magnetism it is
capable of receiving under these circumstances.
On testing the needle it will be found that its boreal
or south pole is at that end to which the front of
the current is presented; and, consequently, for a right-
handed helix, it will be towards the positive, and for a
left-handed helix towards the negative pole. It ap-
pears, therefore, that the needle acquires a polarity
identical with that which the helix itself is proved to
.1961. Polarity produced by the induction of heli-
acal current. In the case of the right-handed helix,
Fig. 612. represented \nfig. 612., the current passes in the di-
rection indicated by the arrows, and consequently the austral
pole will be at a and the boreal pole at b. In the case
of the left-handed helix, fig. 613., the position of these
poles a and b is reversed in relation to the direction of
the current, but the boreal pole b is in both cases at
that end to which the front of the current looks.
1962. Consequent points produced. If the helix
ID be reversed once or oftener in passing along the tube,
iv; being alternately right-handed and left-handed, as re-
presented in fig. 614., a consequent point will be
produced upon the bar at each change of direction
of the helix.
1963. Inductive action of common electricity pro-
duces polarity. It is not only by the induction of
the voltaic current that magnetic polarity rnay be
. ' imparted. Discharges of common electricity trans-
lg ' 6 3 ' mitted along a wire, especially if it have the form
of an helix, will produce like effects. If the wire be straight,
the influence is feeble. Sparks taken from the prime con-
ductor produce sensible effects on very fine needles ; but if the
wire be placed in actual contact with the conductor at one end
and the cushion at the other, so that a constant current shall
pass along it Irom the conductor to the cushion, no effect is
produced. The effect produced by the spark is augmented as
the spark is more intense and taken at a greater distance from
the conductor.
ELECTRO-MAGNETIC INDUCTION. 333
If the wire be formed into an helix, magnetic polarity will
be produced by a continuous current, that is, by ac-
tually connecting the ends of the wire with the con-
ductor and the cushion ; but these effects are much
more feeble than those produced under like circum-
r stances by the spark.
All these effects are rendered much more intense
when the discharge of a Leyden jar, and still more
that of a Leyden battery, is transmitted along the wire.
When these phenomena were first noticed, it was
assumed that the polai-ity thus imparted by common
electricity must necessarily follow the law which pre-
vails in the case of a voltaic current, and that in the
case of helices the boreal or south pole wpuld be pre-
sented towards the front of the current. Savary,
Fig. 614. however, showed that the effects of common electricity
obey a different principle, and thus established a fundamental
distinction between the voltaic current and the electi-ic discharge.
1964. Conditions on which a needle is magnetized positively
and negatively. When an electric discharge is transmitted
along a straight wire, a needle placed at right angles to the
wire acquires sometimes the polarity of a magnetic needle,
which under the influence of a voltaic current would take a
like position ; that is to say, the austral or north pole will be
to the right of an observer who looks at the needle from the
current, his head being in the direction from which the current
flows. The needle is in this case said to be magnetized posi-
tively. When the opposite polarity is imparted to the needle,
it is said to be magnetized negatively.
1965. Results of Savory's experiments. Savary showed
that needles are magnetized by the discharge of common elec-
tricity, positively or negatively, according to various conditions,
depending on the intensity of the discharge, the length of the
conducting wire, supposing it to be straight, its diameter, the
thickness of the needles, and their coercive force. In a series
of experiments, in which the needles were placed at distances
from the current increasing by equal increments, the magnetiz-
ation was alternately positive and negative ; when the needle
was in contact with the wire, it was positive ; at a small dis-
tance negative, at a greater distance no magnetization was
produced ; a further increase of distance produced positive
334 VOLTAIC ELECTRICITY.
magnetism ; and after several alternations of this kind, the
magnetization ended in being positive, and continued positive
at all greater distances.
The number and frequency of these alternations are de-
pendent on the conditions above-mentioned, but no distinct law
showing their relation to those conditions has been discovered.
In general it may be stated, that the thinner the wire which
conducts the current, the lighter and finer the needles, and the
more feeble their coercive force is, the less numerous will be
those periodical changes of positive and negative magnetization.
It is sometimes found, that when these conditions are observed,
the magnetization is positive at all distances, and that the pe-
riodic changes only affect its intensity.
Similar effects are produced upon needles placed in tubes of
wood or glass, upon which an heliacal current is transmitted.
In these cases, the mere variation in the intensity of the dis-
charge produces considerable effect.
1966. Magnetism imparted to the needle affected by the non-
magnetic substance ivhich surrounds it. Savary also ascer-
tained a fact which, duly studied, may throw much light on the
theory of these phenomena. The quantity of magnetism im-
parted to a needle by an electric discharge, and the character
of its polarity, positive or negative, are affected by the non-
magnetic envelope by which the needle is surrounded. If a
needle be inserted in the axis of a very thick cylinder of
copper, an heliacal current surrounding the cylinder will not
impart magnetism to it. If the thickness of the copper en-
velope be gradually diminished, the magnetization will be ma-
nifested in a sensible degree, and it will become more and more
intense as the thickness of the copper is diminished. This
increase, however, does not continue until the copper envelope
disappears, for when the thickness is reduced to a certain limit,
a more intense magnetization is produced than when the un-
covered needle is placed within the helix.
Envelopes of tin, iron, and silver placed around the needle are
attended with analogous effects, that is to say ; when they con-
sist of very thin leaf metal they increase the quantity of mag-
netism which can be imparted to the needles by the current ;
but when the metallic envelope is much thicker, they prevent
the action of the electric discharge altogether. Cylinders formed
of metallic filings do not produce these effects, while cylinders
ELECTRO-MAGNETIC INDUCTION. 335
formed of alternate layers of metallic and non-metallic sub-
stances do produce them. It is inferred from this that solutions
of continuity at right angles to the axis of the needle, or to that
of the cylinder, have an influence on the phenomena.
1967. Formation of powerful electro-magnets. The in-
ductive effect of a spiral or heliacal current on soft iron is still
more energetic than on steel or other bodies having more or
less coercive force. The property enjoyed by soft iron, of
suddenly acquiring magnetism from any external magnetizing
agent, and as suddenly losing its magnetism upon the suspen-
sion of such agency, has supplied the means of producing the
temporary magnets which are known under the name of
ELECTRO-MAGNETS.
The most simple form of electro-magnet is represented in
fig. 615. It is composed of a bar of soft iron bent into the
form of a horse-shoe, and of a wire wrapped
with silk, which is coiled first on one arm,
proceeding from one extremity to the bend
of the horse-shoe, and then upon the other
from the bend to the other extremity, care
being taken that the convolutions of the
spiral shall follow the same direction in
passing from one leg to the other, since,
otherwise, consequent points would be
Fig. 615. produced. An armature is applied to the
ends of the horse-shoe, which will adhere to them so long as a
voltaic current flows upon the wire, but which will drop off the
moment that such current is discontinued.
1968. Conditions which determine the force of the magnet.
The force of the electro-magnet will depend on the dimensions
of the horse-shoe and the armature, the intensity of the current,
and the number of convolutions with which each leg of the
horse-shoe is wrapped.
1969. Electro-magnet of Faculty of Sciences at Paris. In
1830 an electro-magnet of extraordinary power was constructed
under the superintendence of M. Pouillet at Paris. This appa-
ratus, represented in fig. 616., consists of two horse-shoes, the
legs of which are presented to each other, the bends being
turned in contrary directions. The superior horse-shoe is fixed
in the frame of the apparatus, the inferior being attached to
a cross-piece which slides in vertical grooves formed in the
336
VOLTAIC ELECTRICITY.
sides of the frame. To this cross-
piece a dish or plateau is suspended
in which weights are placed, by the
effect of which the attraction which
unites the two horse-shoes is at length
overcome. Each of the horse-shoes
is wrapped with 10,000 feet of co-
vered wire, and they are so arranged
that the poles of contrary names shall
be in contact. With a current of
moderate intensity the apparatus is
capable of supporting a weight of
several tons.
1970. Form of electro-magnets in general. It is found
more convenient generally to construct electro-magnets of two
straight bars of soft iron, united at one end by a straight bar
transverse to them, and attached to them by screws, so that the
form of the magnet ceases to be that of a horse-shoe, the end at
which the legs are united being not curved but square. The
conductor of the heliacal current is usually a copper wire of
extreme tenuity.
1971. Electro-magnetic power applied as a mechanical
agent. The property of electro-magnets by which they are
capable of suddenly acquiring and losing the magnetic force
has supplied the means of obtaining a mechanical agent which
may be applied as a mover of machinery. An electro- magnet
and its armature, such as that represented in fig. 615., or
two electro-magnets such as those represented in fig. 616., are
placed so that when the electric current is suspended they will
rest at a certain distance asunder, and when the current passes
on the wire they will be drawn into contact by their mutual
attraction. When the current is again suspended they will
separate. In this manner, by alternately suspending and trans-
mitting the current on the wire which is coiled round the
electro-magnet, the magnet and its armature, or the two mag-
nets, receive an alternate motion to and from each other
similar to that of the piston of a steam-engine, or the foot of a
person who works the treddle of a lathe. This alternate
motion is made to produce one of continued rotation by the
same mechanical expedients as are used in the application of
any other moving power.
ELECTRO-MAGNETIC INDUCTION. 337
The force with which the electro-magnet and its armature
attract each other, determines the power of the electro-motive
machine, just as the pressure of steam on the piston determines
the power of a steam-engine. This force, when the magnets
are given, varies with the nature and magnitude of the galvanic
pile which is employed.
1972. Electro-motive power applied in the workshop of M.
Froment. The most remarkable and beautiful application of
electro-motive power as a mechanical agent which has been
hitherto witnessed is presented in the workshops of M. Gustave
Froment, of Paris, so celebrated for the construction of in-
struments of precision. It is here applied in various forms to
give motion to the machines contrived by M. Froment for
dividing the limbs of astronomical and surveying instruments
and microscopic scales. The pile used for the lighter descrip-
tion of work is that of Daniel, consisting of about 24 pairs.
Simple arrangements are made by means of commutators, reo-
meters, and reotropes, for modifying the current indefinitely in
quantity, intensity, and direction. By merely turning an index
or lever in one direction or another, any desired number of
pairs may be brought into operation, so that a battery of
greater or less intensity may be instantly made to act, subject
to the major limit of the number of pairs provided. By
another adjustment, the copper elements of two or more pairs,
and at the same time their zinc elements, may be thrown into
connexion, and thus the whole pile, or any portion of it, may
be made to act as a single pair, of enlarged surface. By
another adjustment, the direction of the current can be re-
versed at pleasure. Other adjustments, equally simple and
effective, are provided, by which the current can be turned on
any particular machine, or directed into any room that may be
required.
The pile used for heavier work, is a modification of Bunsen's
charcoal battery, in which dilute sulphuric acid is used in the
porous porcelain cell containing the charcoal, as well as in the
cell containing the zinc. By this expedient the noxious fumes
of the nitric acid are removed, and although the strength of the
battery is diminished, sufficient power remains for the purposes
to which it is applied.
The forms of the electro-motive machines constructed by
ii. Q
338 VOLTAIC ELECTRICITY.
M. Froment are very various. In some the magnet is fixed and
the armature moveable ; in some both are moveable.
In some there is a single magnet and a single armature.
The power is in this case intermittent, like that of a single
acting steam-engine, or of the foot in working the treddle of a
lathe, and the continuance of the action is maintained in the
same manner by the inertia of a fly-wheel.
In other cases two electro-magnets and two armatures are
combined, and the current is so regulated, that it is established
on each during the intervals of its suspension on the other.
This machine is analogous in its operation to the double-acting
steam-engine, the operation of the power being continuous, the
one magnet attracting its armature during the intervals of sus-
pension of the other. The force of these machines may be
augmented indefinitely, by combining the action of two or more
pairs of magnets.
Another variety of the application of this moving principle,
presents an analogy to the rotatory steam-engine. Electro-
magnets are fixed at equal distances round a wheel, to the
circumference of which the armatures are attached at corre-
sponding intervals. In this case the intervals of action and
intermission of the currents are so regulated, that the magnets
attract the armatures obliquely, as the latter approach them,
the current, and consequently the attraction, being suspended
the moment contact takes place. The effect of this is, that all
the magnets exercise forces which tend to turn the wheel on
which the armatures are fixed constantly in the same direction,
and the force with which it is turned is equal to the sum of the
forces of all the electro-magnets which act simultaneously.
This rotatory electro-motive machine is infinitely varied, not
only in its magnitude and proportions, but in its form. Thus
in some the axle is horizontal, and the wheel revolves in a
vertical plane ; in others the axle is vertical, and the wheel
revolves in an horizontal plane. In some the electro-magnets
are fixed, and the armatures moveable with the wheel ; in
others both are moveable. In some the axle of the wheel
which carries the armatures is itself moveable, being fixed
upon a crank or excentric. In this case the wheel revolves
within another, whose diameter exceeds its own by twice the
length of the crank, and within this circle it has an hypo-
cycloidal motion.
ELECTRO-MAGNETIC INDUCTION.
339
Each of these varieties of the application of this power, as
yet novel in the practical operations of the engineer and manu-
facturer, possesses peculiar advantages or convenience, which
render it more eligible for special purposes.
1973. Electro-motive machines constructed by him. To
render this general description of M. Froment's electro-motive
machines more clearly understood, we shall add a detailed ex-
planation of two of the most efficient and useful of them.
In the machine represented in fig. 617., a and b are the two
legs of the electro-magnet ; c d is the transverse piece uniting
Fig. 617.
them, which replaces the bend of the horse-shoe ; ef is the
armature confined by two pins on the summit of the leg a
Q 2
340 VOLTAIC ELECTRICITY.
(which prevent any lateral deviation), the end / being jointed
to the lever g h, which is connected with a short arm projecting
from an axis k by the rod i. When the current passes round
the electro-magnet, the lever /is drawn down by the attraction
of the leg b, and draws with it the lever gh, by which i and the
short lever projecting from the axis k are also driven down.
Attached to the same axis k is a longer arm m, which acts by a
connecting rod n upon a crank o and a fly-wheel v. When the
machine is in motion, the lever gh and the armature /attached
to it recover their position by the momentum of the fly-wheel,
after having been attracted downwards. When the current is
again established, the armature /and the lever gh are again
attracted downwards, and the same effects ensue. Thus, during
each half-revolution of the crank o, it is driven by the force of
the electro-magnet acting on/ and during the other half-revo-
lution it is carried round by the momentum of the fly-wheel.
The current is suspended at the moment the crank o arrives at
the lowest point of its play, and is re-established when it returns
to the highest point. The crank is therefore impelled by the
force of the magnet in the descending half of its revolution, and
by the momentum of the fly-wheel in the ascending half.
The contrivance called a distributor, by which the current is
alternately established and suspended at the proper moments,
is represented in Jig. 618., where y represents the transverse
section of the axis of the fly-wheel ; r, a spring
which is kept in constant contact with it ; x, an ex-
centric fixed on the same axis y, and revolving with
it and r another spring similar to r, which is acted
upon by the excentric, and is thus allowed to press
_ = against the axis y during half the revolution, and
Fi 618 remove d from contact with it during the other half-
revolution. When the spring r' presses on the axis
y the current is established ; and when it is removed from it
the current is suspended.
It is evident that the action of this machine upon the lever
attached to the axis k is exactly similar to that of the foot on
the treddle of a lathe or a spinning-wheel ; and as in these
cases, the impelling force being intermittent, the action is un-
equal, the velocity being greater during the descending motion
of the crank o than during its ascending motion. Although the
inertia of the fly-wheel diminishes this inequality by absorbing
ELECTRO-MAGNETIC INDUCTION.
341
a part of the moving power in the descending motion, and re-
storing it to the crank in the ascending motion, it cannot alto-
gether efface it.
Another electro-motive machine of M. Froment is represented
in elevation in fig. 619., and in plan in^. 620. This machine
Fig. 619.
has the advantage of producing a perfectly regular motion of
rotation, which it retains for several hours without sensible
change.
A drum, which revolves on a vertical axis xy, carries on its
circumference eight bars of soft iron a placed at equal distances
asunder. These bars are attracted laterally, and always in the
same direction, by the intermitting action of six electro-magnets
b, mounted in a strong hexagonal frame of cast iron, within
which the drum revolves. The intervals of action and sus-
pension of the current upon these magnets are so regulated that
it is established upon each of them at the moment one of the
bars of soft iron a is approaching it, and it is suspended at the
moment the bar begins to depart from it. Thus the attraction
accelerates the motion of the drum upon the approach of the
piece a towards the magnet b, and ceases to act when the piece
a arrives in face of b. The action of each of the six impelling
forces upon each of the eight bars of soft iron attached to the
Q 3
342 VOLTAIC ELECTRICITY.
drum is thus intermitting. During each revolution of the
drum, each of the eight bars a receives six impulses, and there-
Fig. 620.
fore the drum itself receives forty-eight impulses. If we suppose
the drum to make one revolution in four seconds, it will there-
fore receive a succession of impulses at intervals of the twelfth
part of a second, which is practically equivalent to a con-
tinuous force.
. The intervals of intermission of the current are regulated by
a simple and ingenious apparatus. A metallic disc c is fixed
upon the axis of rotation. Its surface consists of sixteen equal
divisions, the alternate divisions being coated with non-con-
ducting matter. A metallic roller h, which carries the current,
presses constantly on the surface of this disc, to which it im-
parts the current. Three other metallic rollers efg press against
the edge of the disc, and, as the disc revolves, come alternately
into contact with the conducting and non-conducting divisions of
it. When they touch the conducting divisions, the current is
transmitted ; when they touch the non-conducting divisions,
the current is interrupted.
Each of these three rollers efg is connected by a conducting
wire with the conducting wires of two electro-magnets diame-
ELECTRO-MAGNETIC INDUCTION. 343
trically opposed, as is indicated in fig. 620., so that the current
is thus alternately established and suspended on the several
electro-magnets, as the conducting and non-conducting divisions
of the disc pass the rollers e, f, and g.
M. Froment has adapted a regulator to this machine, which
plays the part of the governor of the steam-engine, moderating
the force when the action of the pile becomes too strong, and
augmenting it when it becomes too feeble.
A divided circle mn,fiy. 619., has been annexed to the ma-
chine at the suggestion of M. Pouillet, by which various im-
portant physical experiments may be performed.
1974. Applied as a sonometer. This machine has been
applied with much success as a sonometer, to ascertain and
register directly the number of vibrations made by sonorous
bodies in a given time.
1975. Momentary current by induction. If a wire A, on
which a voltaic current is transmitted, be brought into proximity
with and parallel to another wire B, the ends of which are in
metallic contact either with each other, or with some continuous
system of conductors, so as to form a closed circuit, the electric
equilibrium of the wire B will be disturbed by the action of the
current A, and a current will be produced upon B in a direction
opposite to that which prevails on A. This current will, how-
ever, be only momentary. After an instant the wire B will
return to its natural state.
If the wire A, still carrying the current, be then suddenly
removed from the wire B, the electric equilibrium of B will be
again disturbed, and as before, only for a moment ; but in this
case the current momentarily produced on B will have the same
direction as the current on A.
If the contact of the extremities of the wire B, or either of
them with each other, or with the intermediate system of con-
ductors which complete the circuit, be broken, the approach or
removal of the current A will not produce these effects on the
wire B.
If, instead of moving the wire A to and from B, the wires,
both in their natural state, be placed parallel and near to each
other, and a current be then suddenly transmitted on A, the
same effect will be produced on B as if A, already bearing the
current, had been suddenly brought into proximity with B.
And in the same way it will be found that if the current es-
344 VOLTAIC ELECTRICITY.
tablished on A be suddenly suspended, the same effect will
be produced as if A, still bearing the current, were suddenly
removed.
These phenomena may be easily exhibited experimentally, by
connecting the extremities of the wire A with a voltaic pile, and
the extremities of B with the wires of a reoscope. So long as
the current continues to pass without interruption on A, the
needle of the reoscope will remain at rest, showing that no cur-
rent passes on B. But if the contact of A with either pole of
the pile be suddenly broken, so as to stop the current, the
needle of the reoscope will be deflected for a moment in the
direction which indicates a current similar in direction to that
which passed on A, and which has just been suspended ; but this
deflection will only be momentary. The needle will imme-
diately recover its position of rest, indicating that the cause of
the disturbance has ceased.
If the extremity of A be then again placed suddenly in con-
tact with the pile, so as to re-establish the current on A, the
needle of the reoscope will again be deflected, but in the other
direction, showing that the current produced on B is in the con-
trary direction to that which passes on A, and, as before, the
disturbance will only be momentary, the needle returning im-
mediately to its position of rest.
These momentary currents are therefore ascribed to the
inductive action of the current A upon the natural electricity of
the wire B, decomposing it and causing for a moment the positive
fluid to move in one direction, and the negative in the other.
It is to the sudden presence and the sudden absence of the
current A, that the phenomena must be ascribed, and not to any
action depending on the commencement of the passage of the
current on A, or on its discontinuance, because the same effects
are produced by the approach and withdrawal of A while it
carries the current, as by the transmission and discontinuance
of the current upon it.
1976. Experimental illustration. The most convenient form
of apparatus for the experimental exhibition of these moment-
ary currents of induction, consists of two wires wrapped with
silk, which are coiled round a cylinder or roller of wood or metal,
as represented in fig. 621. The ends are separated in leaving
the roller, so that those of one wire may be carried to the
pile, and those of the other to the reoscope. The effect of
ELECTRO-MAGNETIC INDUCTION.
345
the inductive action is augmented in proportion to the length
of the wires brought into prox-
imity, other things being the
same. It is found that the
wire B, which receives the in-
ductive action, should be much
finer and longer than that, A,
which bears the primary cur-
Flg> 621- rent. Thus, for example, while
J50 feet of wire No. 18. were used for A, 2000 feet of No. 26.
were used for B.
The effect of the induction is greatly augmented by intro-
ducing a cylinder of soft iron, or, still better, a bundle of soft
iron wires, into the core of the roller. The current on A
renders this mass of soft iron magnetic, and it reacts by in-
duction on the wires conducting the currents.
1977. Momentary currents produced by magnetic induction.
Since, as has been shown, a magnetic bar and an heliacal
current are interchangeable, it may naturally be inferred that
if an heliacal current produces by induction momentary currents
upon an heliacal wire placed in proximity with it, a magnet
must produce a like effect. Experiment has accordingly con-
firmed this inference.
1978. Experimental illustrations. Let the extremities of a
covered wire coiled on a roller, fig, 622., be
connected with a reoscope, and let the
pole of a magnet AB be suddenly inserted
in the core of the coil. A momentary
deflection of the needles will be produced,
similar to that which would attend the
sudden approach of the end of an heliacal
current having the properties of the mag-
netic pole which is presented to the coiL
Thus the boreal pole will produce the same
deflection as the front, and the austral
pole as the back of an heliacal current.
In like manner, the sudden removal of a magnetic pole from
proximity with the heliacal wire will produce a momentary
current on the wire, similar to that which would be produced
by the sudden removal of an heliacal current having like
magnetic properties.
Q 5
Fig. 622.
346 VOLTAIC ELECTRICITY.
The sudden presence and absence of a magnetic pole within
the coil of wire on which it is desired to produce the induced
current may be caused more conveniently and efficiently by
means of the effects of magnetic induction on soft iron. The
manner of applying this principle to the production of the
induced current is as follows :
Let a b,fig. 623., be a powerful horse-shoe magnet, over which
is placed a similar shoe of soft iron, round
which the conducting wire is coiled in the
usual manner, the direction of the coils
being reversed in passing from one leg of
the horse-shoe to the other, so that the
current in passing on each leg may have
its front presented in opposite directions.
The extremities of the wire are connected
with those of a reoscope at a sufficient
distance from the magnet to prevent its
indications from being disturbed by the
influence of the magnet.
If the poles ab of the magnet be suddenly brought near the
ends of the legs of the horse-shoe men, the needle of the reo-
scope will indicate the existence of a momentary current on the
coil of wire, the direction of which will be opposite to that which
would characterize the magnetic polarity imparted by induction
to the horse-shoe men. If the magnet ab be then suddenly
removed, so as to deprive the horse-shoe men of its magnetism,
the reoscope will again indicate the existence of a momentary
current, the direction of which will now, however, be that
which characterizes the polarity imparted to the horse-shoe
men.
It appears, therefore, as might be expected, that the sudden
decomposition and recomposition of the magnetic fluids in the
soft iron contained within the coil has the same effect as the
sudden approach and removal of a magnet.
1979. Inductive effects produced by a permanent magnet
revolving under an electro-magnet. If the magnet ab were
mounted so as to revolve upon a vertical axis passing through
the centre of its bend, and therefore midway between its legs,
its poles might be made to come alternately under the ends of
the horse-shoe men, the horse-shoe men being stationary.
During each revolution of the magnet ab, the polarity imparted
by magnetic induction to the horse- shoe would be reversed.
ELECTRO-MAGNETIC INDUCTION. 347
"When the austral pole a passes under m, and therefore the
boreal pole under n, m would acquire boreal and n austral
polarity. After making half a revolution b would come under
m, and a under n, and m would acquire by induction austral
and n boreal polarity. The momentary currents produced in
the coils .of wire would suffer corresponding changes of direction
consequent as well on the commencement as on the cessation of
each polarity, austral and boreal.
To trace these vicissitudes of the inductive current produced
upon the wire, it must be considered that the commencement of
austral polarity in the leg m, and that of boreal polarity in the
leg n, give the same direction to the momentary inductive
current, inasmuch as the wire is coiled on the legs in contrary
directions. In the same manner it follows that the commence-
ment of boreal polarity in m, and of austral polarity in n, pro-
duce the same inductive current.
The same may be said of the direction of the inductive
currents consequent on the cessation of austral and boreal
polarity in each of the legs. The cessation of austral polarity
in m, and of boreal polarity in n, or the cessation of boreal
polarity in m, and of austral polarity in n, produce the same
inductive current. It will also follow, from the effects of the
current and the reversion of the coils in passing from one leg
to the other, that the inductive current produced by the cessa-
tion of either polarity on one leg of men will have the same
direction as that produced by the commencement of the same
polarity in the other.
If the magnet ab were made to revolve under men, it would
therefore follow that during each revolution four momentary
currents would be produced in the wire, two in one direction
during one semi-revolution, and two in the contrary direction
during the other semi-revolution. In the intervals between
these momentary currents the wire would be in its natural
state.
It has been stated that if the extremities of the wire were not
in metallic contact with each other, or with a continuous system
of conductors, these inductive currents would not be produced.
This condition supplies the means of producing in the wire an
intermitting inductive current constantly in the same direction.
To accomplish this, it will be only necessary to contrive means
to break the contact of either extremity of the coil with the
348 VOLTAIC ELECTRICITY.
intermediate conductor during the same half of each successive
revolution of the magnet. By this expedient the contact may
be maintained during the half revolution in which the com-
mencement of austral polarity in the leg m, and of boreal in the
leg n, and the cessation of boreal polarity in the leg ?, and of
austral in the leg n, respectively take place. All these changes
produce momentary currents having a common direction. The
contact being broken during the other semi-revolution, in which
the commencement of boreal polarity in m, and of austral in M,
and the cessation of austral polarity in m, and of boreal in n,
respectively take place, the contrary currents which would
otherwise attend these changes will not be produced.
1980. Use of a contact breaker. If it be desired to reverse
the direction of the intermitting current, it will be only neces-
sary to contrive a contact breaker which will admit of such an
adjustment that the contact may be maintained at pleasure,
during either semi -revolution of the magnet a b, while it is
broken during the other.
1981. Magneto-electric machines. Such are the principles
on which is founded the construction of magneto-electric
machines, one form of which is represented in jig. 624. The
purpose of this apparatus is to produce by magnetic induction
an intermitting current constantly in the same direction, and
to contrive means by which the intervals of intermission shall
succeed each other so rapidly that the current shall have prac-
tically all the effects of a current absolutely continuous.
A powerful compound horse-shoe magnet A is firmly attached
by bolts and screws upon an horizontal bed, beyond the edge of
which its poles a and b extend. Under these is fixed an electro-
magnet XY, with its legs vertical, and mounted so as to revolve
upon a vertical axis. The covered wire is coiled in great
quantity on the legs XY, the direction of the coils being re-
versed in passing from one leg to the other ; so that if a voltaic
current were transmitted upon it, the ends x and Y would ac-
quire opposite polarities.
The axis upon which this electro-magnet revolves has upon
it a small grooved wheel f, which is connected by an endless
cord or band n, with a large wheel R driven by a handle m.
The relative diameters of the wheels K and f is such that an
extremely rapid rotation can be imparted to XY by the hand
applied at m.
ELECTRO-MAGNETIC INDUCTION.
349
The two extremities of the wire proceeding from the legs x
and Y are pressed by springs against the surfaces of two rollers,
c and d, fixed upon the axis of the electro-magnet. These
Fig. 624.
rollers themselves are in metallic connexion with a pair of
handles P and N, to which the current evolved in the wire of
the electro-magnet XT will thus be conducted.
If the electro-magnet XY be now put in rotation by the
handle m, the handles P and N being connected by any con-
tinuous conductor, a system of intermitting and alternately
contrary currents will be produced in the wire and in the con-
ductor by which the handles P and N are connected. But if the
rollers c and d are so contrived that the contact of the ends of
the wire with them shall be only maintained during a semi-
revolution in which the intermitting currents have a common
direction, then the current transmitted through the conductor
connecting the handles P and N will be intermitting, but not
350 VOLTAIC ELECTRICITY.
contrary ; and by increasing the velocity of rotation of the
electro-magnet XY, the intervals of intermission may be made
to succeed each other with indefinite celerity, and the current
will thus acquire all the character of a continuous current.
The contrivances by which the rollers c and d are made to
break the contact, and re-establish it with the necessary regu-
larity and certainty, are various. They may be formed as
excentrics, so as to approach to and recede from the ends of the
wire as they revolve, touching them and retiring from them at
the proper moments. Or, being circular, they may consist
alternately of conducting and non-conducting materials. Thus
one half of the surface of such roller may be metal, while the
other is wood, horn, or ivory. When the end of the wire
touches the latter the current is suspended, when it touches the
former it is maintained.
1982. Effects of this machine its medical use. All the
usual effects of voltaic currents may be produced with this
apparatus. If the handles P and N be held in the hands, the
arms and body become the conductor through which the cur-
rent passes from P to N. If XY be made to revolve, shocks
are felt, which become insupportable when the motion of XY
acquires a certain rapidity.
If it be desired to give local shocks to certain parts of the
body, the hands of the operator, protected by non-conducting
gloves, direct the knobs at the ends of the handles to the parts
of the body between which it is desired to produce the voltaic
shock.
1983. Inductive effects of the successive convolutions of the
same helix. The inductive effect produced by the commence-
ment or cessation of a current upon a wire, forming part of a
closed circuit placed near and parallel to it, would lead to the
inference that some effect may be produced by one coil of an
heliacal current upon another at the moment when such cur-
rent commences or ceases. At the moment when the current
commences, it might be expected that the inductive action of
one coil upon another, having a tendency to produce a moment-
ary current in a contrary direction, would mitigate the initial
intensity of the actual current, and that at the moment the cur-
rent is suspended the same inductive action, having a tendency
to produce a momentary current in the same direction, would,
on the contrary, have a tendency to augment the intensity of
the actual current.
INFLUENCE OF TERRESTRIAL MAGNETISM. 351
The phenomena developed when the contact of a closed
circuit is made or broken, are in remarkable accordance with
these anticipations.
If the wires which connect the poles of an ordinary pile, con-
sisting of a dozen pairs, be separated or brought together, a very-
feeble spark will be visible, and no sensible change in the in-
tensity of this spark will be produced when the length of the
wire compassing the circuit is augmented so much as to amount
to 150 or 200 yards. If this wire be folded or coiled in any
manner, so long as the parts composing the folds or coils are
distant from each other by a quarter of an inch or more, no
change of intensity will be observed. But if the wire be coiled
round a roller or bobbin, so that the successive convolutions
may be only separated from each other by the thickness of the
silk which covers them, a very remarkable effect will ensue.
The spark produced when the extremities of the wire are
brought together will still be faint ; but that which is manifest
when, after having been in contact, they are suddenly separated,
will have an incomparably greater length, and a tenfold or even
a hundredfold splendour. The shock produced, if the ends of
the wire be held in the hands when the contact is broken, has
also a great intensity.
1984. Effects of momentary inductive currents produced
upon revolving metallic discs : researches of Arago, Herschel,
Babbage, and Faraday. It was first ascertained by Arago
that if a circular disc of metal revolve round its centre in its
own plane under a magnetic needle, the needle will be deflected
from the magnetic meridian, and the extent of its deflection will
be augmented with the velocity of rotation of the disc. By in-
creasing gradually that velocity, the needle will at length be
turned to a direction at right angles to the magnetic meridian.
If the velocity of rotation be still more increased, the needle
will receive a motion of continuous rotation round its centre in
the same direction as that of the disc.
That this fact does not proceed from any mechanical action
of the disc upon the intervening stratum of air, is proved by
the fact that it is produced in exactly the same manner where a
screen of thin paper is interposed between the needle and the
disc.
Sir John Herschel and Mr. Babbage made a series of ex-
periments to determine the relative power of discs composed of
352
VOLTAIC ELECTRICITY.
different metals to produce this phenomenon. Taking the
action of copper, which is the most intense, as the unit, the fol-
lowing are the relative forces determined for discs of other
metals:
Copper
Zinc -
Tin -
- 1-00
- 0-93
- 0-46
Lead -
Antimony -
Bismuth
- 0-25
- 0-09
- 0-O2
Professor Barlow ascertained that iron and steel act more
energetically than the other metals. The force of silver is con-
siderable, that of gold very feeble. Mercury holds a place
between antimony and bismuth.
Herschel and Babbage found that if a slit were made in the
direction of a radius of the disc it lost a great part of its force ;
but that when the edges of such a slit were soldered together
with any other metal, even with bismuth, which itself has a
very feeble force, the disc recovered nearly all its force.
The motion of rotation of the needle is an effect which would
result from a force impressed upon it parallel to the plane of
the disc and at right angles to its radii. It was also ascertained,
however, that the disc exercises on the needle forces parallel to
its own plane in the direction of its radii, and also perpendicular
to its plane.
A magnetic needle, mounted in the manner of a dipping-
needle, so as to play on a horizontal axis in a vertical plane,
was placed over the revolving disc, so that the plane of its play
passed through the centre of the disc. The pole of the needle
which was presented downwards was attracted to or repelled from
the centre of the disc according to its distance from that point.
Placed immediately over the centre, no effect, either of at-
traction or repulsion was manifested. As it was moved from
the centre along a radius, attraction to the centre was mani-
fested. This attraction was diminished rapidly as the distance
from the centre was increased, and, at a certain point, it became
nothing, the pole of the needle resting in its natural position.
Beyond this distance repulsion was manifested, which was con-
tinued even beyond the limits of the disc. These phenomena
indicate the action of a force directed parallel to the plane of
the disc and in the direction of its radii.
A magnetic needle was suspended vertically by one of its
extremities, and, being attached to the arm of a very sensitive
balance, was accurately counterpoised. It was then placed sue-
INFLUENCE OF TERRESTRIAL MAGNETISM. 353
cessively over different parts of the disc, and was found to be
every where repulsed, whichever pole was presented downwards.
These phenomena indicate the action of a repulsive force di-
rected at right angles to the plane of the disc.
All these phenomena have been explained with great clearness
and felicity by Dr. Faraday, by the momentary inductive
currents produced upon the disc by the action of the poles of
the magnet, and the reaction of those currents on the moveable
poles themselves. By the principles which have been ex-
plained (1977), it will be apparent that upon the parts of the
disc which are approaching either pole of the magnet, moment-
ary currents will be produced in directions contrary to those
which would prevail upon an electro-magnetic helix substituted
for the magnet, and having a similar polarity ; while upon
the parts receding from the pole, momentary currents will be
produced, having the same direction.
These currents will attract or repel the poles of the magnet
according to the principles explained and illustrated in (1977);
and thus all the motions, and all the attractions and repulsions
described above, will be easily understood.
CHAP. VII.
INFLUENCE OF TERRESTRIAL MAGNETISM ON VOLTAIC CURRENTS.
1985. Direction of the earth's magnetic attraction. The laws
which regulate the reciprocal action of magnets and currents in
general being understood, the investigation of the effects pro-
duced by the earth's magnetism on voltaic currents becomes
easy, being nothing more than the application of these laws to
a particular case. It has been shown that the magnetism of
the earth is such, that in the northern hemisphere the austral
pole of a magnet freely suspended is attracted in the direction
of a line drawn in the plane of the magnetic meridian, and in-
clined below the horizon at an angle which increases gradually
in going from the magnetic equator, where it is nothing, to the
magnetic pole, where it is 90. In this part of Europe the
direction of the lower pole of the dipping-needle, and therefore
354 VOLTAIC ELECTRICITY.
of the magnetic attraction of the earth, is that of a line drawn
in the magnetic meridian at an angle of about 70 below the
horizon, and therefore at an angle of about 20, with a vertical
line presented downwards.
1986. In this part of the earth it corresponds to that of the
boreal pole of an artificial magnet. Now, since the magnetism
of the earth in this part of the globe attracts the austral pole of
the needle, it must be similar to that of the boreal or southern
pole of an artificial magnet (1656). To determine, therefore,
its effects upon currents, it will be sufficient to consider it as a
southern magnetic pole, placed below the horizon in the direc-
tion of the dipping-needle, at a distance so great that the
directions in which it acts on all parts of the same current are
practically parallel.
1987. Direction of the force impressed by it upon a current.
To ascertain the direction, therefore, of the force impressed
by terrestrial magnetism on a current, let a line be imagined to
be drawn from any point in the current parallel to the dipping-
needle, and let a plane be imagined to pass through this line
and the current. According to what has been explained of the
reciprocal action of magnets and currents, it will follow that
the direction of the force impressed on the current will be that
of a line drawn through the same point of the current perpen-
dicular to this plane.
Let cc', fig. 625., be the line of
direction of the current, and draw
OP parallel to the direction of the
dip. Let LOR be a line drawn
through o, at right angles to the
plane passing through OP and cc".
This line will be the direction of
the force impressed by the mag-
netism of the earth on the current
Fig. 625. cc'. If the current pass from c to
c', this force will be directed from o towards L, since the effect
produced is that of a southern magnetic pole placed in the line
OP. If the current pass from c' to c, the direction of the force
impressed on it will be from o towards R (1918).
It follows, therefore, that the force which acts upon the
current is always in a plane perpendicular to the dipping-
needle. This plane intersects the horizontal plane in a line
INFLUENCE OF TERRESTRIAL MAGNETISM. 355
directed to the magnetic east and west, and therefore perpen-
dicular to the magnetic meridian ; and it intersects the plane
of the magnetic meridian in a line directed north and south,
making, in this part of the earth, an angle with the horizon of
20 elevation towards the north, and depression towards the
south.
1988. Effect of terrestrial magnetism on a vertical current.
If the current be vertical, the plane passing through its direc-
tion and that of the dipping-needle will be the magnetic meri-
dian. The force impressed upon the current will therefore be at
right angles to the plane of the magnetic meridian, and directed
eastward when the current descends, and westward when it
ascends.
1989. Effect upon a horizontal current directed north and
south. If the current be horizontal, and in the plane of the
magnetic meridian, and therefore directed in the line of the
magnetic north and south, the force impressed on it will be
directed to the magnetic east and west, and will therefore be
also horizontal. It will be directed to the east, if the current
pass from north to south ; and to the west, if it pass from south
to north. Tiiis will be apparent, if it be considered that the
effect of the earth's magnetism is that of a south magnetic pole
placed below the current.
1990. Case of an horizontal current directed east and west.
If the current be horizontal and at right angles to the mag-
netic meridian, the force impressed on it will be directed north
and south in the plane of the magnetic meridian, and inclined
to the horizontal plane at an angle of 20 in this part of the
earth. This may be resolved into two forces, one vertical and
the other horizontal (154). The former will have a tendency to
remove the current from the horizontal plane, and the latter will
act in the horizontal plane in the direction of the magnetic
north and south. It will be directed from the south to the
north, if the current pass from west to east, and from the north
to the south, if the current pass from east to west. This will
also be apparent, by considering the effect produced upon a
horizontal current by a south magnetic pole placed below it.
1991. Case of a horizontal current in any intermediate
direction. If a horizontal current have any direction inter-
mediate between the magnetic meridian and a plane at right
angles to it, the force impressed on it, being still at right angles
356 VOLTAIC ELECTRICITY.
to the dipping-needle, and being inclined to the horizontal plane
at an angle less than 20, may be resolved into other forces
(154), one of which will be at right angles to the current, and
will be directed to the left of the current, as viewed from below
by an observer whose head is in the direction from which the
current passes (1918). '
1992. Effect of the earth's magnetism on a vertical current
which turns round a vertical axis. It follows, from what has
been here proved, that if a descending vertical rectilinear cur-
rent be so suspended as to be capable of turning freely round a
vertical axis, the earth's magnetism will impress upon it a force
directed from west to east in a plane at right angles to the
magnetic meridian ; and it will therefore move to such a posi-
tion, that the plane passing through the current and the axis
round which it moves shall be at right angles to the magnetic
meridian, the current being to the east of the axis.
If the current ascend, it will for like reasons take the position
in the same plane to the west of the axis, being then urged by
a force directed from east to west.
1993. Effect on a current which is capable of moving in a
horizontal plane. If a vertical current be supported in such a
manner that, retaining its vertical direction, it shall be capable
of moving freely in a horizontal plane in any direction what-
ever, as is the case when it floats on the surface of a liquid, the
earth's magnetism will impart to it a continuous rectilinear
motion in a direction at right angles to the plnne of the mag-
netic meridian, and directed eastward if the current descend,
and westward if it ascend.
If a horizontal rectilinear current be supported, so as to be
capable of revolving in the horizontal plane round one of its
extremities as a centre, the earth's magnetism will impart to it
a motion of continued rotation, since it impresses on it a force
always at right angles to the current, and directed to the same
side of it. If in this case the current flow towards the centre
round which it revolves, the rotation imparted to it will be
direct; if from the centre, retrograde, as viewed from above
(1920).
1994. Experimental illustrations of these effects. Pouillefs
apparatus A great variety of experimental expedients have
been contrived to verify these consequences of the principle of
the influence of terrestrial magnetism on currents.
INFLUENCE OF TERRESTRIAL MAGNETISM. 357
To exhibit the effects of the earth's magnetism on vertical
currents, M. Pouillet contrived an apparatus consisting of two
circular canals, represented in their vertical section in^. 626.,
one placed above the other, the lower canal
A o It' having a greater diameter than the upper.
In the opening in the centre of these canals
a metallic rod t is fixed in a vertical position,
supporting a mercurial cup c. A rod hh',
composed of a non-conducting substance, is
supported in the cup e by a point at its centre.
The vertical wires v v' are attached to the
ends of the rod hh', and terminate in points,
which are turned downwards, so as to dip
Fig. 626. into the liquid contained in the upper canal,
while their lower extremities dip into the
liquid contained in the lower canal. A bent wire connects
the mercury contained in the cup c with the liquid in the upper
canal.
The liquid on the upper and lower canals is acidulated water
or mercury. If the liquid in the lower canal be put in com-
munication with the positive, and the rod t with the negative
pole, the current will pass from that canal up the two vertical
Avires v v', thence to the liquid in the upper canal, thence by
the connecting wire to the mercury in the cup c, and thence by
the rod t to the negative pole.
By this arrangement the two vertical currents vv', which
both ascend, are moveable round the rod t as an axis.
When this apparatus is left to the influence of the earth's
magnetism, the currents vv' will be affected by equal and
parallel forces directed westward at right angles to the mag-
netic meridian (1988). The equal and parallel forces being at
equal distances from the axis t, will be in equilibrium in all
positions (421), and the wires will therefore be astatic (1695).
If the point of the wire v' at h' be raised from the upper
canal, the current on v' will be suspended. In that case, the
wire t; being impelled by the terrestrial magnetism westward at
right angles to the magnetic meridian, the system will take a
position at right angles to that meridian, the wire on which the
current passes being to the west of the axis t. If the point at
h' be turned down so as to dip into the liquid, and the point at
h be turned up so as to suspend the current on h and establish
358 VOLTAIC ELECTRICITY.
that on h', the system will make half a revolution and will
place the wire h' on which the current runs to the west of t.
If by the reotrope the connexions with the poles of the bat-
tery be reversed, the currents on vv' will descend instead of as-
cending. In that case the system will be astatic as before, so
long as both currents are established on the wires v v'. But
if the connexion of either with the superior canal be removed,
the wire on which the remaining current passes being impelled
eastwards, the system will take a position in the plane of the
magnetic meridian, the wire on which the current runs being
east of the axis t.
When the currents on the wires v v' are both passing, the
system will be astatic only so long as the currents are equally
intense, and both in the same plane with the axis t. If while
the latter condition is fulfilled one of the wires be even in a
small degree thicker than the other, it will carry a stronger
current, and in that case it will turn to the magnetic east or
west, according as the currents descend or ascend, just as
though the current on the other wire were suppressed ; for in
this case the effective force is that due to the difference of the
intensities of the currents acting on that which is the stronger.
If the two wires be not in the same plane with the axis, the
forces which act upon them being equal, and parallel to the
plane of the magnetic meridian, the position of equilibrium will
be that in which the plane passing through them will be
parallel to the latter plane.
The position of equilibrium will be subject to an infinite
variety of changes, according as the plane of the wires vv',
their relative thickness, and their distances from the axis of
rotation are varied, and in this way a great number of inter-
esting experiments on the effects of the earth's magnetism may
be exhibited.
1995. Its application to show the effect of terrestrial mag-
netism on a horizontal current. To show experimentally
the effect of the earth's magnetism on a horizontal current,
M. Pouillet contrived an arrangement on a similar principle,
consisting of a circular canal, the ver-
tical section of which is represented in
fig. 627. A horizontal wire ab is sup-
ported by a point at its centre which
Fig. 627. regtg j n a mercur i a i CU p fixed upon a
INFLUENCE OF TERRESTRIAL MAGNETISM. 359
metallic rod, like t, fig. 626. Two points, a and b, project from
the wire and dip into the liquid in the canal, the small weights
c and d being so adjusted as to keep the wire a b exactly balanced.
If the central rod be connected with the positive, and the
liquid in the canal with the negative pole, the current will
ascend on the central rod, and will pass along the horizontal
wire in both directions from its centre to the points a and b, by
which it will pass to the liquid in the canal, and thence to the
negative pole. If by the reotrope the connexions be reversed
and the names of the poles changed, the current will pass from
a and b to the centre, and thence by the central rod to the
negative pole.
In the former case, the wire a b will revolve with retrograde,
and in the latter with direct rotation, in accordance with what
has been already explained (1918).
1996. Its effect on vertical currents shown by Ampere's
apparatus. If a rectangular current, such as that represented
injtf^. 595., be suspended in Ampere's frame, fig. 564., it will,
when left to the influence of terrestrial magnetism, take a
position at right angles to the magnetic meridian, the side on
which the current descends being to the east. For in this case
the horizontal currents which pass on the upper and lower sides
of the rectangle, being contrary in direction, will have a ten-
dency to revolve, one with direct, and the other with retrograde
motion round yy'\ These forces, therefore, neutralize each
other. The vertical descending current will be attracted to the
east, and the ascending current to the west (1992).
1997. Its effect on a circular current shown by Ampere's
apparatus. If a circular current, such as that represented in
fig. 594., be suspended in Ampere's frame, fig. 564., and sub-
mitted to the influence of terrestrial magnetism, each part of it
may be regarded as being compounded of a vertical and hori-
zontal component. The horizontal components in the upper
semicircle, flowing in a direction contrary to those in the lower
semicircle, their effects will neutralize each other. The vertical
components will descend on one side and ascend on the other.
That side on which they descend will be attracted to the east,
and that at which they ascend to the west ; and, consequently,
the current will place itself in a plane at right angles to the
magnetic meridian, its front being presented to the south.
1998. Its effect on a circular or spiral current shown by
360 VOLTAIC ELECTRICITY.
Delarive's floating apparatus. If a circular or spiral current
be placed on a floating apparatus, it will assume a like position
at right angles to the magnetic meridian, with its point to the
south ; and the same will be true of any circulating current.
1999. Astatic currents formed by Ampere's apparatus. To
construct a system of currents adapted to Ampere's frame,
which shall be astatic, it is only necessary so to arrange them
that there shall be equal and similar horizontal currents
running in contrary directions, and equal and similar vertical
currents in the same direction, and that the latter shall be at
equal distances from the axis on which the system turns; for in
that case the horizontal elements, having equal tendencies to
make the system revolve in contrary directions, will equilibrate,
and the vertical elements being affected by equal and parallel
forces at equal distances from the axis of rotation, will also
equilibrate.
By considering these principles, it will be evident that the
system of currents represented mfig. 628.,
adapted to Ampere's frame, fig. 564., is
astatic.
2000. Effect of earth's magnetism on
spiral currents shown by Ampere's appa-
ratus. If the arrangement of spiral
currents represented in fig. 605. be so dis-
posed that the current after passing through
one only of the two spirals shall return to
the negative pole, the earth's magnetism
Fig. 628. W JH affect it so as to bring it into such a
po ition that its plane will be at right angles to the magnetic
meridian. If the descending currents be on the side of the
spiral more remote from the axis of motion, the system will
arrange itself so that the spiral on which the current flows shall
be to the east of the axis. If the descending currents be on
the side nearer to the axis, the spiral on which the current
flows will throw itself to the west of the axis. In each case,
the front of the current is presented to the magnetic south, and
the descending currents are on the east side of the spiral.
If the current pass through both spirals in fig. 605., and
their fronts be on the same side, the earth's magnetism will
throw them into the plane at right angles to the magnetic me-
ridian, their fronts being presented te the south.
INFLUENCE OF TERRESTRIAL MAGNETISM. 361
If their fronts be on different sides, the system will be astatic,
and will rest in any position independent of the earth's mag-
netism, which in this case will produce equal and contrary
effects on the two spirals.
If the system of spiral currents represented in fig. 606. be
suspended in Ampere's frame, subject to the earth's magnetism,
the fronts of the currents being on the same side of the two
spirals, it will take such a position that the centres of the two
spirals will be in the magnetic meridian, their planes at right
angles to it, and the fronts of the currents presented to the
south. If in this case the fronts of the currents be on opposite
sides, the system will be astatic.
2001. Effect on an horizontal current shown by Pouillet's
apparatus. The rotation of the horizontal current produced
with the apparatus^. 627., may be accelerated, retarded, ar-
rested, or inverted by presenting the pole of an artificial magnet
above or below it, at a greater or less distance. A south
magnetic pole placed below it, or a north magnetic pole above,
producing forces identical in direction with those produced by
terrestrial magnetism, will accelerate the rotation in a greater
or less degree, according to the power of the artificial magnet,
and the greater or less proximity of its pole to the centre of
rotation of the current.
A north magnetic pole presented below, or a south pole above
the centre of rotation, producing forces contrary in their direction
to those resulting from the earth's magnetism,
will retard, arrest, or reverse the rotation ac-
cording as the forces exerted by the magnet are
less than, equal to, or greater than those im-
pressed by terrestrial magnetism.
If the system of currents represented in
fig. 629. be suspended on Pouillet's apparatus,
represented in^. 626., it will receive a motion
of continued rotation from the influence of the
Fig. 629. earth's magnetism. In this case the vertical
currents being in the same direction will be in equilibrium
(1994) ; and the horizontal currents passing either from the
centre of the upper horizontal wire to the extremities, or vice
versa, according to the mode of connexion, will receive a mo-
tion of rotation direct or retrograde (1995). This motion of
rotation may be affected in the manner above described by the
II. K
362 VOLTAIC ELECTRICITY.
pole of a mngnet applied in the centre of the lower circular
canal, fig. 626.
2002. Effect of terrestrial magnetism on an heliacal current
shown by Amperes apparatus. An heliacal current such as
that represented in Jig. 608., being mounted on Ampere's
frame, or arranged upon a floating apparatus, fig. 609., will be
acted on by the eai'th's magnetism. The several convolutions
will, like a single circulating current, take a position at right
angles to the magnetic meridian, their fronts being presented
to the south. The axis of the helix will consequently be di-
rected to the magnetic north and south ; and it will, in fine,
exhibit all the directive properties of a magnetic needle, the
end to which the front of the currents is directed being its
south pole.
If such a current were mounted on a horizontal axis at right
angles to the plane of the magnetic meridian, it would, under
the influence of the earth's magnetism, take the direction of the
dipping-needle, the front of the currents corresponding in
direction to the south pole of the needle.
2003. The dip of a current illustrated by Amperes rectangle.
The phenomenon of the dip may also be experimentally
illustrated by Ampere's electro-magnetic rectangle, fig. 630.,
which consists of a horizontal axis xv, which is a tube of wood
or other non-conductor, at right angles to which is fixed a
Fig. 630.
RECIPROCAL INFLUENCE OF CURRENTS. 363
lozenge-shaped bar az, composed also of a non-conductor.
Upon this cross is fixed the rectangle ABDC, composed of wire.
The rectangle rests by steel pivots at M and N on metallic
plates, which communicate by wires with the mercurial cups at
s and K. These latter being placed in connexion with the
poles of a voltaic battery, the current will pass from the positive
cup s up the pillar and round the rectangle, as indicated by the
arrows. At x it passes along a wire through the tube xv to v,
and thence by the steel point, the plate M, and the pillar, to the
negative cup R.
The axis MN being placed at right angles to the magnetic
meridian, and the connexions established, the rectangle will be
immediately affected by the earth's magnetism, and after some
oscillations, will settle into a position at right angles to the
direction of the dipping-needle.
In this case the forces impressed by the earth's magnetism on
the parts of the current forming the sides AC and BD, will pass
through the axis MN, and will therefore be resisted. The forces
impressed on AB and CD will be equal, and will act at the middle
points a and z, at right angles to AB and CD, and in a plane at
right angles to the direction of the dip. These forces will
therefore be in directions exactly opposed to each other when
the line az takes the direction of the dip, and will therefore be
in equilibrium.
CHAP. VIII.
RECIPROCAL INFLUENCE OF VOLTAIC CURRENTS.
2004. Results of Ampere's researches. The mutual attrac-
tion and repulsion manifested between conductors charged with
the electric fluids in repose, would naturally suggest the in-
quiry whether any analogous reciprocal actions would be mani-
fested by the same fluids in motion. The experimental analysis
of this question led Ampere to the discovery of a body of phe-
nomena which he had the felicity of reducing to general laws.
The mathematical theory raised upon these laws has supplied the
means by which phenomena, hitherto scattered and unconnected,
R 2
364
VOLTAIC ELECTRICITY.
and ascribed to a diversity of agents, are traced to a common
source.
Although the limits within which a treatise so elementary as
this manual is necessarily confined excludes any detailed ex-
position of these beautiful physico-mathematical researches,
they cannot be altogether passed over in silence. We shall
therefore give as brief an exposition of them as is compatible
with their great importance, and that clearness without which
all exposition would be useless.
2005. Reciprocal action of rectilinear currents. If two
rectilinear currents be parallel, they will attract or repel each
other according as they flow in the same or opposite directions.
This is verified experimentally by the apparatus represented
\nftg. 631., which is on the principle of Ampere's frame. The
mercurial cup marked + receives
the current from the positive pole.
The current passes as indicated by
the arrows upwards on the pillar t,
and thence to the cup x, from
which it flows round the rectangle,
returning to the cup y, and thence
to the pillar v, by which it de-
scends to the cup, which is con-
nected with the negative pole.
If the rectangle thus arranged be
placed with its plane at any angle
Fl S- 631> -with the plane of the pillars t and
v, upon which the ascending and descending currents pass, it
will turn upon its axis until its plane coincides with the plane
of the pillars t and v, the side of the rectangle de on which the
current ascends being next the pillars t, on which it ascends.
If by means of the reotrope (1911) the connexion be reversed, so
that the current shall descend on *and de, and shall ascend on v
and be, it will still maintain its position. But if the connexions
at x and y be reversed, the connexions of the cups + and
remaining unchanged, the current will descend on ed while it
ascends on t, and will ascend on be while it descends on v. In
this case t will repel de and attract be, and v will repel be and
attract de, and accordingly the rectangle will make a half revo-
lution, and be will place itself near t, and de near v.
2006. Action of a spiral or heliacal current on a rectilinear
RECIPROCAL INFLUENCE OF CURRENTS. 365
current A sinuous, spiral, or heliacal current, provided its
convolutions are not considerable in magnitude, impresses on
another current in its neighbourhood the same force as a straight
current would produce, whose direction would coincide with
the axis of the sinuous or spiral current. This is proved ex-
perimentally by the fact that a spiral current which has a re-
turning straight current passing along its axis, will exercise no
force either of attraction or repulsion on a straight current
parallel to it. Now since on suspending the spiral current the
straight current will attract or repel a parallel straight current,
it follows that the spiral current exactly neutralizes the effect of
the straight current flowing in the opposite direction, and con-
sequently it will be equivalent to a straight current flowing in
the same direction.
2007. Mutual action of diverging or converging rectilinear
currents. Rectilinear currents which diverge from or con-
verge to a common point mutually attract. Those, one of which
diverges, and the other converges, mutually repel ; that is to
c say, if two rectilinear
currents cc' and cc',
fig. 632., which inter-
sect at o, both flow to-
wards or from o, they
will mutually attract ;
Fig- 632 - but if one flow towards,
and the other from o, they will mutually repel. The currents,
being supposed to flow in the direction of the arrows, oc and o
will mutually attract, as will also oc' andoc'; while oc' and oc
will repel, as will also oc and oc'.
If the wires conducting the currents were moveable on o as a
pivot, they would accordingly close, the
angle coc diminishing until they would
coincide.
2008. Experimental illustration of
this. This may be experimentally illus-
trated by the apparatus represented in
fig. 633. in plane, and in fig. 634. in sec-
tion, consisting of a circular canal filled
with mercury or acidulated water sepa-
rated into two parts by partitions at a
Fig. 633. an( j _ Two wires cd and ef, suspended
366 VOLTAIC ELECTKICITY.
on a central pivot, move freely one over and independent of
the other, like the hands of a watch, the points being at right
angles, so as to dip into the canal. The
mercurial cup x being supposed to be con-
nected with the positive, and y with the
Fig. 634. negative pole, the current passing to the
liquid will flow along the wires as indicated by the arrows from
the liquid in one section to that of the other, and will pass to the
negative cup y. When the wires cd and efihus carrying the
current are left to their mutual influence, the angle they form
will close, and the directions of the wires will coincide, so that
the currents shall flow in the same direction upon them.
In these and all similar experiments, the phenomena will
necessarily be modified by the effects produced by the earth's
magnetism. In some cases the apparatus can be rendered
astatic (1695); and in others, the effect due to the terrestrial
magnetism being known, can be allowed for, so that the phe-
nomena under examination may be eliminated.
2009. Mutual action of rectilinear currents which are not in
the same plane. If two rectilinear currents be not in the same
plane, their directions cannot intersect although they are not pa-
rallel. In this case a line may always be drawn, which is at the
same time perpendicular to both. To assist the imagination in
conceiving such a geometrical combination, let a vertical rod be
supposed to be erected, and from two different points of this
rod let lines be drawn horizontally, but in different directions,
one, for example, pointing to the north, and the other to the
east. If voltaic currents pass along two such lines, they will
mutually attract, when they flow both to or both from the ver-
tical rod ; they will mutually repel, when one flows to the ver-
tical rod and the other from it.
In either case the mutual action of such currents will have
a tendency to turn them into the same plane and to parallelism.
If they mutually attract, their lines of direction
turning round the vertical line will take
position parallel to each other, and at the same
side of that line. If they mutually repel, they
will turn on the vertical line in contrary di-
rections, and will take a position parallel to
Fig. 635. each other, but at opposite sides of it.
In fig. 635., AB and CD represent two currents which are not in
RECIPROCAL INFLUENCE OF CURRENTS. 367
the same plane. Let PO be the line which intersects them both
at right angles, and let planes be supposed to pass through
their directions respectively, which are parallel to each other,
and at right angles to PO. If, in this case, CD be fixed and AB
moveable, the latter will be turned into the direction a b pa-
rallel to CD; or if CD were free and AB fixed, CD would take
the position cd ; if both were free they would take some po-
sition parallel to each other ; and if free to change their planes,
they would mutually approach and coalesce. It follows from
this, that if the direction of either of the two currents be re-
versed, the directions of the forces they exert on each other
will be also reversed ; but if the directions of both currents be
reversed, the forces they exert on each other will be unaltered.
2010. Mutual action of different parts of the same current.'
Different parts of the same current exercise on each other a re-
pulsive force. This will follow immediately as a consequence
of the general principle which has been just established. Since
a repulsive action takes place between oc and oc',jig. 632.,
and such action is independent of the magnitude of the angle
coc', it will still take place, however great that angle may be,
and will therefore obtain when the angle occ' becomes equal to
180 ; that is, when oc' forms the continuation of CO, or coalesces
with oc'. Hence, between oc and oc' there exists a mutually
repulsive action.
2011. Ampere's experimental verification of this. Inde-
pendently of this demonstration, M. Ampere has reduced the
repulsive action of different pai-ts of the same rectilinear current
to the following experimental proof:
Let A B c D, Jig. 636., be a glass or porcelain dish, separated
into two divisions by a partition AC, also of glass ; and let it be
filled with mercury on both sides
of AC. Let a wire, wrapped
with silk, be formed into two
parallel pieces united, by a se-
micircle whose plane is at right
Fig- 636. angles to that of the straight
parallel parts, and let these two parallel straight parts be placed
floating on the surface of the mercury at each side of the
partition AC, over which the semicircle passes. The mer-
cury in the divisions of the dish is in metallic communication
with the mercurial cups E and F placed in the direction of
368 VOLTAIC ELECTRICITY.
the straight arms of the floating conductor. When the cups
E and F are put in connexion with the poles of a voltaic bat-
tery, a current will pass from the positive cup to the end of
the floating conductor, from that along the arm of the con-
ductor, then across the partition by the semicircle, then along
the other floating arm, and from thence through the mercury
to the negative cup. There is thus on each side of the par-
tition a rectilinear current, one part of which passes upon
the mercury, and the other part upon the straight arm of the
floating conductor. When the current is thus established, the
floating conductor will be repelled to the remote side of the
dish. This repulsion is effected by that part of the straight
current which passes upon the mercury acting on that part
which passes along the wire.
2012. Action of an indefinite rectilinear current on a finite
rectilinear current at right angles to
it. A finite rectilinear current a b,
fig. 637., which is perpendicular to an
indefinite rectilinear current cd lying
all at the same side of it, will be acted
on by a force tending to move it parallel
to itself, either in the direction of the
Y\f. 637. indefinite current, or in the contrary
direction, according to the relative di-
rections of the two currents.
If the finite current do not meet the indefinite current, let its
line of direction be produced till it meets it at a. Take any
two points c and d on the indefinite current at equal distances
from a, and draw the lines cb and db to any point on the finite
current.
First case. Let the finite current be directed towards the in-
definite current. Hence the point b will be attracted by d and
repelled by c (2007) ; and since db=cb, the attraction will be
equal to the repulsion. Let the equal lines be and bf represent
this attraction and repulsion. By completing the rectangle,
the diagonal bg will represent the resultant of these forces ;
and this line bg is parallel to cd, and the resultant is contrary
in direction to the indefinite current.
The same may be proved of the action of all points on the
indefinite current on the point b, and the sum of all these re-
sultants will be the total action of the indefinite current on b.
RECIPROCAL INFLUENCE OF CURRENTS. 369
The same may be proved respecting the action of the de-
finite current on all the points of the indefinite current.
Hence the current a b will be urged by a system of forces
acting at all its points parallel to cd, and in a contrary direction.
Second case. Let the finite current be directed from the in-
definite current. The point b will then be attracted by c and
repelled by d, and the resultant bg' will be contrary to its
former direction.
Hence the current a b will be urged by a system of forces
parallel to c d, and in the same direction as the indefinite
current.
Since the action of the two currents is reciprocal, the in
definite current will be urged by a force in its line of direction,
either according or contrary to its direction, as the finite current
runs from or towards it.
2013. Case in which the indefinite current is circular. If
the indefinite current cd be supposed to be bent into a circular
form so as to surround a cylinder, on the side of which is placed
the vertical current a b, it is evident that the same reciprocal
action will take place ; but in that case the motion imparted will
be one of rotation round the axis of the cylinder as a centre.
2014. Experimental verification of these principles. These
principles are experimentally verified by the apparatus,^. 638.,
where azsb repi'esents a rib-
bon of copper coated with
silk and carried round the
copper circular canal v. A
conductor connects the mer-
curial cup c with the central
metallic pillar which supports
a mercurial cup p. In this
cup the metallic point m is
Fio . 63g placed. The mercurial cup
d is in metallic communica-
tion with the acidulated water in the circular canal v. A hoop
of metal h is supported by the point m by means of the rect-
angular wire, and is so adjusted that its lower edge dips into
the liquid in the canal v.
Let the mercury in a be connected with the positive pole of
the battery, and the mercury in d with the negative pole. The
current entering at a will pass round the circular canal upon
R 5
370 VOLTAIC ELECTRICITY.
the coated ribbon of copper, and, arriving at b, it will pass to c
by a metallic ribbon or wire connecting these cups. From c it
will pass to the central pillar, and thence to the cup p. It will
then pass from m as a centre in both directions on the wire,
and will descend to the hoop k, from which it will pass into the
liquid in the canal v, and thence to the cup d, with which the
liquid is in metallic communication, and, in fine, from d it will
pass to the negative pole of the battery.
By this arrangement, therefore, a circular current flows
round the exterior surface of the vase v, while two descending
currents constantly flow upon the wire at right angles to this
circular current. The circular current being fixed, and the
vertical currents being moveable, the latter will receive a
motion of continued rotation by the action of the former ; and
in the case here supposed, this rotation will be in a direction
contrary to the direction of the circular current. If the con-
nexions be reversed by the reotrope, the direction of the cir-
cular current will be reversed, but at the same time that of the
vertical currents on the wire will be also reversed ; and, con-
sequently, no change will take place in the direction of the
rotation. These changes of direction of the two currents neu-
tralize each other. But if, while d is still connected with the
negative pole, b be connected with the positive pole, the con-
nexion between b and c being removed, and a connexion be-
tween a and c being established, then the direction of the
circular current being from * to z will be reversed ; while that
of the vertical cm-rents remains still the same, the direction of
the rotation will be reversed.
2015. To determine in general the action of an indefinite
rectilinear current on a finite rectilinear current. First. Let
, n ^ it be supposed that the finite current A B, Jig. 639.,
has a length so limited that all its points may be
considered as equally distant from the indefinite
current, and therefore equally acted on by it.
In this case the current AB may be replaced by
two currents, AD perpendicular and AC parallel
to the indefinite current, and the action of the
Fig. 639. indefinite current on AB will be equivalent to its
combined actions on AD and AC.
If A be supposed to be the positive end of the finite current,
it will also be the positive end of the component currents AD
RECIPROCAL INFLUENCE OF CURRENTS. 371
and AC. Supposing the indefinite current parallel to AC to
run in the same direction as AC, then AD will be urged in the
direction AC (2012), and AC in the direction AC', by forces pro-
portional to AD and AC. Hence, if AD'=AD, and AC'=AC, AD'
and AC' will express in magnitude and direction the two forces
which act on the component currents. The resultant of these
two forces AD' and AC' will be the diagonal AB', which is evi-
dently perpendicular to AB and equal to it.
Secondly. Let the finite current have any proposed length,
and from its positive end A, Jig. 640., let a line AO be drawn
perpendicular to the indefi-
nite current x'x, this cur-
rent being supposed to run
from x' to x.
If the distance OA be
greater than AB, that cur-
rent AB, whatever be its
position, will lie on the same
^ side of x'x, and the action
~o X of x'x on every small ele-
Fig. 64O. ment of AB will be perpen-
dicular to AB, as has been just demonstrated. The current AB
will therefore be acted on by a system of parallel forces perpen-
dicular to its direction. The resultant of these forces will be a
single force equal to their sum, and parallel to their common
direction. Hence the indefinite current x'x will act on the
finite current AB by a single force R in the direction CD.
If the current AB be supposed to assume successively different
positions, B a , B 2 , B 3 , &c., around its positive end A, the line CD
will represent in each position the direction of the action of
the current x'x upon it.
It is evident that when the indefinite current runs from x'
to x, the action on the finite current is such as would cause it
to turn round its positive end A with a direct, or round its ne-
gative end B with a retrograde rotation.
If the indefinite current run from x to x', the direction of its
action on AB, and the consequent motions of A B, would be reversed.
The point c of the current AB at which the resultant R acts
will vary with the position of the current AB, approaching more
towards x'x as AB approaches the position AB 3 ; but in every
position this resultant must be between A and B. The force
R 6
372 VOLTAIC ELECTRICITY.
producing the rotation therefore having a varying moment, the
rotation will not be uniform.
If the distance o A be very great compared with AB, the resultant
R will be sensibly constant, and will act at the middle point of AB.
In this case, if the middle point of AB be fixed, no rotation
can take place.
If the distance OA be less than AB, the current AB will in
certain positions intersect x'x, Jig. 641., and a part will be at
Fig. 641.
one side and a part at the other. In this case the action on AB,
in all positions in which it lies altogether above x'x, is the same
as in the former case.
When it crosses x'x, as in the positions AB 2 , AB 3 , AB 4 , the
action is different. In that case the forces which act on Am,
and those which act on WB, are in contrary directions, and their
resultant is in the one direction or in the other, according as
the sum of the forces acting on one part is greater or less than
the sum of the forces acting on the other part. If Am be in
every position of AB greater than mB, then the resultant will
be in every position in the same direction as if the current AB
did not cross x'x; and if the point A were fixed, a motion of
continued rotation would take place, in the same manner as in
the former case, except that the impelling force would be di-
minished as the line AB would approach the position AB S .
^ But if AO be less than
half AB, the circum-
stances will be different.
In that case there will
^ be two positions AB O and
B AE 4 , Jig. 642., at equal
distances from AB. 5 , at
^ which the line AB will be
Fig. 642. bisected by x'x.
In all positions of AB not included between AB 2 and AB 4 ,
RECIPROCAL INFLUENCE OF CURRENTS. 373
the action of the indefinite current upon it takes place in the
same direction as in the former cases.
But in the positions AB' and AB", where WE' and mv" are
greater than m A, the forces acting on m B' and m B" exceed
those acting in the contrary direction on m A, and consequently
the resultant of the forces on A B in all positions between A B 2
and AB 4 is contrary to its direction in every other position of
the line A B.
In the positions AB O and AB 4 the resultant of the forces in
one direction on Am is equal and contrary to the resultant of
the forces on B m. There will in these positions be no tendency
of the current AB to move except round its middle point.
If the indefinite current x' x pass through A, fig. 643., the
resultants of its action on A B will
be in contrary directions above and
-jj- below x'x, and will in each case
/\ ~ tend to turn the current A B round
' / \. i the point A so as to make it coin-
cide in direction with the indefinite
Fig. 643. current x'x.
2016. Experimental illustration of these principles. These
effects may be illustrated experimentally by means of the
apparatus, fig. 638., already described. The circular current
surrounding the canal v being removed, and the currents on the
wire m being continued, let an indefinite rectilinear current be
conducted under the apparatus at different distances from the
vertical line passing through the pivot, and the effects above
described will be exhibited.
2017. Effect of a straight indefinite current on a system of
diverging or converging currents. If any number of finite
rectilinear currents diverge from or converge to a common
centre, the system will be affected by an indefinite current near
it, in the same manner as a single radiating current would be
affected.
Thus if a number of straight and equal wires have a common
extremity, and are traversed by currents flowing between that
extremity, and the circumference of the circle in which their
other extremities lie, an indefinite current x'x placed in the
plane of the circle, as represented in^. 644., will cause the ra-
diating system of currents to revolve in the one direction or the
other, as indicated by the arrows in the figures.
374
VOLTAIC ELECTRICITY.
2018. Experimental illustration of this action. These
actions may be shown experimentally, by putting a vertical
wire, fig. 645., in communication with the centre of a shallow
circular metallic vessel of mercury v, and another wire N, com-
municating with the outside of the vessel,
into communication with the poles of a
battery: diverging currents will be trans-
mitted through the mercury in the one di-
rection or the other, according to the con-
nexion ; and if a straight conducting wire
CD, conveying a powerful electric current,
is brought near the vessel, a rotation will
Fig. 645. ^6 imparted to the mercury, the direction
of which will be in conformity with the principles just ex-
plained. Davy used a powerful magnet instead of the straight
wire.
2019. Consequences deducible from this action. The follow-
ing consequences respecting the action of finite and indefinite
rectilinear currents will readily follow from the principles which
have been established.
When a finite vertical conductor AB, moveable round an axis
oo', is subjected to the action of an indefinite horizontal current
MN, the plane ABo'owill place itself in the position O'OB'A',
when the vertical current descends, and the horizontal current
runs from N to t&,Jig. 646.
RECIPROCAL INFLUENCE OF CURRENTS. 375
Fig. 646.
If the direction of the
vertical or horizontal cur-
rent be reversed, the po-
sition of equilibrium of
the former will be OO'AB ;
but if the direction of
both be reversed, the po-
sition of equilibrium will
remain unaltered.
When two vertical conductors AB and A'B' are moveable
round a vertical axis oo', and connected together, they will
remain in equilibrium, whatever be their position, if they are
both traversed by currents of the same intensity in the same
direction, provided that the indefinite rectilinear current which
acts upon them be at such a distance and in such a position
that its distances from the points B and B' may be considered
always equal. When the wires AB and A'B' are traversed by
currents in opposite directions, one ascending and the other de-
scending, the system will then turn on its axis oo' until the
vertical plane through AB and A'B' becomes parallel to MN, the
descending current being on that side from which the inde-
finite current flows.
2020. Action of an indefinite straight current on a circulating
current, The circulating current, &.,fig. 647 v is affected by the
Fig. 647.
indefinite current PN in the same manner as would be affected
the rectangular current B. The current PN affects the de-
scending side a by a force contrary to, and the ascending side b
by an equal force according with, its own direction (2012).
In the same manner it affects the sides c and d with forces in
contrary directions, one towards, and the other from, PN. But
the side c, being nearer to PN than d, is more strongly affected ;
376
VOLTAIC ELECTRICITY.
and consequently the attraction, in the case represented in
fig. 646., will prevail over the repulsion. If the direction of
either the rectilinear or circulating current be reversed, the
repulsion will prevail over the attraction.
Thus it appears, that au indefinite current flowing from
right to left, under a circulating current having direct rotation,
or one moving from left to right under a circulating current
having retrograde rotation, will produce attraction; and two
currents moving in the contrary directions will produce
repulsion.
If the current A be fixed upon an horizontal axis a b on which
it is capable of revolving, that side c at which the current moves
in the same direction as PN will be attracted downwards, and
the plane of the current will take a position passing through PN,
the side c being nearest to that line.
If the current A be fixed upon the line cd as an axis, it will
turn into the same position, the side b on which the current
ascends being on the side towards which the current PN is
directed.
2021. Case in which the indefinite straight current is perpen-
dicular to the plane of the circulating current. If the rectilinear
current &'&,fig. 648., be perpendicular to the
circular current QNN, and within it, and be
moveable round the central line oo', a motion
of rotation will be impressed upon it con-
trary to that of the circular current. This
may be experimentally verified by an appa-
ratus constructed on the principles repre-
sented infig. 649., consisting of a wire frame
supported and balanced on a central point
in a mercurial cup. The current passing
between this point and the liquid in a circular canal will ascend
or descend on the vertical wires according to the arrangement
of the connexions. The circular current may
be produced by surrounding the circular
canal with a metallic wire, or ribbon coated
with a non-conductor, upon which the current
may be transmitted in the usual way. The
wire frame will revolve upon the central point
with direct or retrograde rotation, according to
Fig. 649. fhg directions of the currents. If the current
RECIPEOCAL INFLUENCE OF CURRENTS. 377
ascend on the wires, they will revolve in the same direction as
the circular current ; if it descend, in the contrary direction.
The circular current may also be produced by a spiral current
placed under the circular canal, and the wire frame may be re-
placed by a light hollow cylinder, supported on a central point.
The spiral in this case may be moveable and the cylinder fixed,
or vice versa, and the reciprocal actions will be manifested.
2022. Case in which the straight current is oblique to the
plane of the circulating current. Like effects will be produced
when the rectilinear current, in-
stead of being perpendicular to
the plane of the circular current,
is oblique to it.
Let the rectilinear current a c,
fig. 650., be parallel to the plane
of the circular current NQ. If
the current flow from a to c, the
part a b which is within the circle
will be affected by force opposite
to the direction of the nearest
part of the current NQ, and the
part be outside the circle will be affected by a force in the same
direction. If the current flow from c to a, contrary effects will
ensue.
If in this case the straight current be limited to ab, and be
capable of revolving round a in a plane parallel to that of the
circle, it will receive a motion of rotation in the same or in a
contrary direction to that of the circulating current, accordingly
as it flows from b to a, or from a to b. If the straight current
be limited to b c, it will, under the circumstances, receive rota-
tion in the contrary direction. If, in fine, it extend on both
sides of the circle, it will rotate in the one direction or the
other, according as the internal or external part predominates.
2023. Reciprocal effects of curvilinear currents. The
mutual influence of rectilinear and curvilinear currents being
understood, the reciprocal effects of curvilinear currents may be
easily traced. Each small part of such current may be regarded
as a short rectilinear current, and the separate effects of such
elementary parts being ascertained, the effects of the entire
extent of the curvilinear currents will be the resultants of these
partial forces.
378 VOLTAIC ELECTRICITY.
2024. Mutual action of curvilinear currents in general. An
endless variety of problems arise from the various forms that
curvilinear currents may assume, the various positions they may
have in relation to each other, and the various conditions which
may restrain their motions. The solution of all such problems,
however, presents no other difficulties than those which attend
the due application of the geometrical and mechanical prin-
ciples already explained in each particular case.
To take as an example one of the most simple of the infinite
variety of forms under which such problems are presented, let
the centres of two circular currents be fixed ; the planes of the
currents being free to assume any direction whatever, they will
turn upon their centres until they come into the same plane,
the parts of the currents which intersect the line joining their
centres flowing in the same direction. It is evident that upon
the least disturbance from this position, they will be brought
back to it by the mutual attraction of the parts of the circles
on the sides which are near each other. This is therefore their
position of stable equilibrium, and it is evident that the fronts
of the currents in this position are on opposite sides of their
common plane.
CHAP. IX.
VOLTAIC THEORY OF MAGNETISM.
2025. Circulating currents have the magnetic properties.
From what has been proved, it is apparent that an heliacal cur-
rent has all the properties of a magnet. Such currents exert
the same mutual attraction and repulsion, have the same po-
larity, submitted to the influence of terrestrial magnetism have
the same directive properties, and exhibit all the phenomena of
variation and dip as are manifested by artificial and natural
magnets. And it is evident that these properties depend on
the circulating and not on the heliacal character of the current,
inasmuch as the effect of the progression of the helix being
neutralized by carrying the current back in a straight direction
along its axis, the phenomena instead of being disturbed are
still more regular and certain.
VOLTAIC THEORY OF MAGNETISM. 379
These properties of circulating currents have been assumed by
Ampere as the basis of his celebrated theory of magnetism, in
which all magnetic phenomena are ascribed to the presence of
currents circulating round the constituent molecules of natural
and artificial magnets, and around the earth itself.
Let a bar magnet be supposed to be cut by a plane at
right angles to its length. Every molecule in its section is
supposed to be invested by a circulating current, all these cur-
rents revolving in the same direction, and consequently their
fronts being presented to the same extremity of the bar. The
forces exerted by all the currents thus prevailing around the
molecules of the same section may be considered as represented
by a single current circulating round the bar, and the same
being true of all the transverse sections of the bar, it may be
regarded as being surrounded by a series of circulating currents
all looking in the same direction, and circulating round the bar.
That end of the bar towards which the fronts of the currents
are presented will have the properties of a south or boreal pole,
and the other end those of a north or austral pole.
2026. Magnetism of the earth may proceed from currents.
In this theory the globe of the earth is considered to be traversed
by electric currents parallel to the magnetic equator. The
forces exerted by the currents circulating in each section of the
earth, like those in the section of an artificial magnet, are con-
sidered as represented by a single current equivalent in its
effect, and which is called the mean current of the earth, at
each place upon its surface. The magnetic phenomena indicate
that the direction of this mean current at each place is in a
plane at right angles to the dipping-needle, and that it is
directed in this plane from east to west, and at right angles to
the magnetic meridian.
2027. Artificial magnets explained on this hypothesis. In
bodies such as iron or steel, which are susceptible of mag-
netism, but which are not magnetized, the currents which
circulate round the constituent molecules are considered to cir-
culate in all possible planes and all possible directions, and
their forces thus neutralize each other. Such bodies, therefore,
exert no forces of attraction or repulsion on each other. But,
when such bodies are magnetized, the fronts of some or all of
these currents are turned in the same direction, and their
forces, instead of being opposed, are combined. The more
380 VOLTAIC ELECTRICITY.
perfect the magnetization is, the greater proportion of the cur-
rents will thus be presented in the same direction, and the mag-
netization will be perfect when all the molecular currents are
turned towards the same direction.
2028. Effect of the presence or absence of coercive force.
If the body thus magnetized be destitute of all coercive force,
like soft iron, the currents which are thus temporarily turned
by the magnetizing agent in the same direction will fall into
their original confusion and disorder when the influence of that
agent is suspended or removed, and the body will consequently
lose the magnetic properties which had been temporarily im-
parted to it. If, on the contrary, the body magnetized have
more or less coercive force, the accordance conferred upon the
direction of the molecular currents is maintained with more or
less persistence after the magnetizing agency has ceased ; and
the magnetic properties accordingly remain unimpaired until
the accordance of the currents is deranged by some other cause.
2029. This hypothesis cannot be admitted as established
until the existence of the molecular currents shall be proved.
To establish this theory according to the rigorous principles
of inductive science, it would be necessary that the actual ex-
istence of the molecular voltaic currents, which form the basis
of the theory, should be proved by some other evidence than
the class of effects which they are assumed to explain. Until
such proof shall be obtained, they cannot be admitted to have
the character of a vera causa, and the theory must be regarded
as a mere hypothesis, more or less probable, and more or less
ingenious, which may be accepted provisionally as affording an
explanation of the phenomena, and thus reducing magnetism to
the dominion of electricity.
CHAP. X.
REOSCOPES AND GEOMETERS.
2030. Instruments to ascertain the presence and to measure
the intensity of currents. It has been shown that when a vol-
taic current passes over a magnetic needle freely suspended, it
REOSCOPES AND REOMETERS. 381
will deflect the needle from its position of rest, the quantity of
this deflection depending on the force, and its direction on the
direction of the current.
If the needle be astatic, and consequently have no directive
force, it will rest indifferently in any direction in which it may
be placed. In this case the deflecting force of the current will
have no other resistance to overcome than that of the friction
of the needle on its pivot ; and if the deflecting force of the
current be greater than this resistance, the needle will be de-
flected, and will take a position at right angles to the current,
its north pole being to the left of the current (1918).
If the needle be not astatic it will have a certain directive
force, and, when not deflected by the current, will place itself
in the magnetic meridian. If, in this case, the wire con-
ducting the current be placed over and parallel to the needle,
the poles will be subject at once to two forces ; the directive
force tending to keep them in the magnetic meridian, and the
deflecting force of the current tending to place them at right
angles to that meridian. They will, consequently, take an in-
termediate direction, which will depend on the relation between
the directive and deflecting forces. If the latter exceed the
former, the needle will incline more to the magnetic east and
west ; if the former exceed the latter, it will incline more to the
magnetic north and south. If these forces be equal, it will take
a direction at an angle of 45 with the magnetic meridian.
The north pole of the needle will, in all cases, be deflected to
the left of the current (1918).
If while the directive force of the needle remains unchanged
the intensity of the current vary, the needle will be deflected at
a greater or less angle from the magnetic meridian, according
as the intensity of the current is increased or diminished.
2031. Expedient for augmenting the effect of a feeble cur-
rent, It may happen that the intensity of the current is so-
feeble as to be incapable of producing any sensible deflection
even on the most sensitive needle. The presence of such a
current may, nevertheless, be detected, and its intensity mea-
sured, by carrying the wire conducting it first over and then
under the needle, so that each part of the current shall exer
cise upon the needle a force tending to deflect it in the same
direction. By this expedient the deflecting force exercised by
the current on the needle is doubled.
382 VOLTAIC ELECTRICITY.
Such an arrangement is represented vafig. 651. The wire
passes from n to z over, and from
y to x under the needle; and it is
evident from what has been ex-
plained (1918), that the part zn
and the part y x exercise deflecting
forces in the same direction on the
poles of the needle, both tending
Fig. 651. t o deflect the north or austral pole
a to the left of a person who stands at z and looks towards n.
It may be shown in like manner that the vertical parts of the
current g x and y z have the same tendency to deflect the north
pole a to the left of a person viewing it from z.
2032. Method of constructing a reoscope, galvanometer, or
multiplier. The same expedient may be carried further. The
wire upon which the current passes may be carried any number
of times round the needle, and each successive coil will equally
augment its deflecting force. The deflecting force of the
simple current will thus be multiplied by twice the number of
coils. If the needle be surrounded with an hundred coils of
conducting wire, the force which deflects it from its position of
rest will be two hundred times greater than the deflecting
force of the simple current.
The wire conducting the current must in such case be
wrapped with silk or other non-conducting coating, to prevent
the escape of the electricity from coil to coil.
Such an apparatus has been called a multiplier, in con-
sequence of thus multiplying the force of the current. It has
been also denominated a galvanometer, inasmuch as it supplies
the means of measuring the force of the galvanic current.
We give it by preference the name reoscope or reometer, as
indicating the presence and measuring the intensity of the
current.
To construct a reometer, let two flat bars of wood or metal be
united at the ends, so as to leave an open space between them of
sufficient width to allow the suspension and play of a magnetic
needle. Let a fine metallic wire of silver or copper, wrapped
with silk, and having a length of eighty or a hundred feet, be
coiled longitudinally round these bars, leaving at its extremities
three or four feet uncoiled, so as to be conveniently placed in
connexion with the poles of the voltaic apparatus from which
KEOSCOPES AND REOMETERS.
383
the current proceeds. Over the bars on which the conducting
wire is coiled, is placed a dial upon which an index plays,
which is connected with the magnetic needle suspended between
the bars, and which has a common motion with it, the direction
of the index always coinciding with that of the needle. The
circle of the dial is divided into 360, the index being directed
to or 180, where the needle is parallel to the coils of the
conducting wire.
Such an instrument, mounted in the usual manner and co-
vered by a bell-glass to protect it
from the disturbances of the air, is
represented in^. 652.
The needle is usually suspended by
a single filament of raw silk. If the
length of wire necessary for a single
coil be six inches, fifty feet of wire
will suffice for a hundred coils. To
detect the presence of very feeble cur-
rents, however, a much greater number
p ~~ of coils are frequently necessary, and
Fig. 652. in some instruments of this kind there
are several thousand coils of wire.
2033. Nobili's reometer. Without multiplying inconveni-
ently the coils of the conducting wire, Nobili contrived a reo-
scope which possesses a sensibility sufficient for the most
delicate experimental researches. This arrangement consists
of two magnetic needles fixed upon a common centre parallel to
each other, but with their poles reversed as represented in
fig. 653. If the directive forces of
these needles were exactly equal,
such a combination would be as-
tatic ; and although it would indi-
cate the presence of an extremely
feeble current, it would supply no
means of measuring the relative
forces of two such currents. Such
reoscopic, but not reometric. To
Fig. 653.
an apparatus would be
impart to it the latter property, and at the same time to confer
on it a high degree of sensibility, the needles are rendered a
little, and but a little, unequal in their directive force. The
directive force of the combination being the difference of the
384 VOLTAIC ELECTRICITY.
directive forces of the two needles, is therefore extremely
small, and the system is proportionately sensitive to the in-
fluence of the current.
2034. Differential reometer. In certain researches a dif-
ferential reometer is found useful. In this apparatus two wires
of exactly the same material and diameter are coiled round the
instrument, and two currents are made to pass in opposite di-
rections upon them so as to exercise opposite deflecting forces
on the needle. The deviation of the needle in this case mea-
sures the difference of the intensities of the two currents.
2035. Great sensitiveness of these instruments illustrated.
The extreme sensitiveness and extensive utility of these reo-
scopic apparatus will be rendered apparent hereafter. Mean-
while it may be observed that if the extremities p and n of the
conducting wires be dipped in acidulated water, a slight che-
mical action will take place, which will produce a current by
which the needle will be visibly affected.
In all cases it is easy to determine the direction of the
current by the direction in which the north pole of the needle
is deflected.
CHAP. XL
THERMO-ELECTRICITY.
2036. Disturbance of the thermal equilibrium of conductors
produces a disturbance of the electric equilibrium. If a piece
of metal B, fig. 654., or other
conductor, be interposed be-
tween two pieces c, of a dif-
ferent metal, the points of
contact being reduced to dif-
Fig. 654. ferent temperatures, the na-
tural electricity at these points will be decomposed, the positive
fluid passing in one direction, and the negative fluid in the
other. If the extremities of the pieces c be connected by a
wire, a constant current will be established along such wire.
The intensity of this current will be invariable so long as the
temperatures of the points of contact of B with c remain the
THERMO-ELECTRICITY. 385
same ; and it will in general be greater, the greater the dif-
ference of these temperatures. If the temperatures of the
points of contact be rendered equal, the current will cease.
These facts may be verified by connecting the extremities of
c with the wires of any reoscopic apparatus. The moment a
difference of temperature is produced at the points of contact,
the needle of the reoscope will be deflected ; the deflection will
increase or diminish with every increase or diminution of the
difference of the temperatures ; and if the temperatures be equal-
ized, the needle of the reoscope will return to its position of
rest, no deflection being produced.
2037. Thermo-electric current. A current thus produced
is called a thermo-electric current. Those which are pro-
duced by the ordinary voltaic arrangements are called for dis-
tinction hydro-electric currents, a liquid conductor always
entering the combination.
2038. Experimental illustration. A convenient and simple
apparatus for the experimental illustration of a thermo-electric
current is represented in fig. 655., consisting of a narrow strip
of copper bent so as to form
three sides of a rectangle, the
fourth part of which is a cy-
linder of bismuth, about half
an inch in diameter, which
is soldered at both ends to
the copper so as to ensure
perfect contact. A magnetic
needle is placed within the
Fig. 655. rectangle, which is directed
in the plane of the magnetic meridian, so that the needle, when
undisturbed by the current, shall rest in the direction of the
rectangle, its north pole pointing to the zinc cylinder.
If a lamp be placed under the end of the bismuth cylinder, so
as to raise its temperature above that of the upper end, the
needle will be immediately deflected, and the deflection will in-
crease as the difference of the temperatures of the lower and
upper end of the zinc cylinder is increased.
2039. Conditions which determine the direction of the cur-
rent. When the temperature of the lower end of the bismuth
cylinder is more elevated than that of the upper end, the north
pole of the needle is deflected towards the east, from which it
386 VOLTAIC ELECTRICITY.
appears that the current in this case flows from the upper to the
lower end of the cylinder, and passes round the rectangle in
the direction represented by the arrows.
If the heat be applied to the upper end of the bismuth, or,
what is the same, if cold be applied to the lower end, the north
pole of the needle will be deflected to the west, showing that
the direction of the current will be reversed, the positive fluid
always flowing towards the warmer end of the bismuth.
2040. A constant difference of temperature produces a con-
stant current. If means be taken to maintain the extremities
of the bismuth at a constant difference of temperature, the needle
will maintain a constant deflection. Thus, if one end of the
bismuth be immersed in boiling water and the other in melting
ice, so that their temperatures shall be constantly maintained at
212 and 32, the deflection of the needle will be invariable.
If the temperature of the one be gradually lowered, and the
other gradually raised, the deflection of the needle will be
gradually diminished ; and when the temperatures are equalized,
the needle will resume its position in the magnetic meridian.
2041. Different metals have different thermo-electric energies.
This property, in virtue of which a derangement of the electric
equilibrium attends a derangement of the thermal equilibrium,
is common to all the metals, and, indeed, to conductors generally;
but, like other physical properties, they are endowed with it in
very different degrees. Among the metals, bismuth and anti-
mony have the greatest thermo-electric energy, whether they are
placed in contact with each other, or with any other metal. If
a bar of either of these metals be placed with its extremities in
contact with the wires of a reometer, a deflection of the needle
will be produced by the mere warmth of the finger applied to
one end of the bar. If the finger be applied to both ends, the
deflection will be redressed, and the needle will return to the
magnetic meridian.
It has been ascertained that if different parts of the same
mass of bismuth or antimony be raised to different temperatures,
the electric equilibrium will be disturbed, and currents will be
established in different directions through it, depending on the
relative temperatures. These currents are, however, much less
intense than in the case where the derangement of temperature
is produced at the points of contact or junction of different con-
ductors.
2042. Pouillefs thermo-electric apparatus. M. Pouillet
THERMO-ELECTRICITY. 387
has with great felicity availed himself of these properties of
thermo-electricity to determine some important and interesting
properties of currents. The apparatus constructed and applied
by him in these researches is represented in jig. 606.
Two rods A and B of bismuth, each about sixteen inches in
length and an inch in thickness, are bent at the ends at right
angles, and being supported on vertical stands are so arranged
that the ends CD and EF may be let down into cups. The
cups c and E are filled with melting ice, and D and F with
boiling water, so that the ends c and E are kept at the constant
temperature of 32, and the ends D and F at the constant tem-
perature of 212.
A differential reometer (2033) is placed at M. Two conducting
circuits are formed either of one or several wires, one com-
Fig. 656.
mencing from F, and after passing through the wire of the reo-
meter M, returning to E ; the other commencing from D, and
after passing through the wire of the reometer in a contrary
direction to the former, returning to c. The wires conducting
the current are soldered to the extremities c, D, E, F of the
bismuth rods, which are immersed in the cups.
If the two currents thus transmitted, the one between F and
E, and the other between D and G, have equal intensities, the
needle of the reometer M will be undisturbed ; but if there be.
any difference of intensity, its quantity and the wire on which
the excess prevails will be indicated by the quantity and di-
rection of the deflection of the needle,
s 2
388 VOLTAIC ELECTRICITY.
The successive wires along which the current passes are
brought into metallic contact by means of mercurial cups, a, b,
c, d, &c., into which their ends are immersed.
The circuits through which the current passes may be simple
or compound. If simple, they consist of wire of one uniform
material and thickness. If compound, they consist of two or
more wires differing in material, thickness, or length.
The wire composing a simple circuit is divided into two
lengths, one extending from D or F to the cup e or d, where the
current enters the convolutions of the reometer, and the other
extending from the cup b or/, where the current issues from
the reometer to c or E, where it returns to the thermo-electric
source. The wires composing a compound current may consist
of a succession of lengths, the current passing from one to
another by means of the metallic cups. Thus, as represented
in the figure, the wires FC, cd, and/E, forming, with one wire
of the reometer, one circuit, and the wires ve, ba, and a c,
forming with the other wire of the reometer the other circuit,
may differ from each other in material, in thickness, and in
length.
The currents pass as indicated by the arrows, from the extre-
mity of the bismuth which has the higher temperature through
the wires to the extremity which has the lower temperature.
2043. Relation between the intensity of the current and the
length and section of the conducting wire. If the two circuits
be simple and be composed of similar wires of equal lengths,
the intensity of the two currents will be found to be equal, the
needle of the reometer being undisturbed. But if the length of
the circuit be greater in the one than in the other, the inten-
sities will be unequal, that current which passes over the
longest wire having a less intensity in the exact proportion in
which it has a greater length.
If the section of the wire composing one circuit be greater
than that of the wire composing the other circuit, their lengths
being equal, the current carried by the wire of greater section
will be more intense than the other in exactly the proportion
in which the section is greater.
If the wire composing one of two simple circuits have a length
less than that composing the other, and a section also less in
the same proportion than the section of the other, the currents
passing over them will have the same intensity, for the excess
THERMO-ELECTRICITY. 389
of intensity due to the lesser length of the one is compensated
by the excess due to the greater section of the other.
In general, therefore, if i and ^ express the intensities of the
two currents transmitted from D and r, Jig. 656., over two
simple circuits of wire of the same metal, whose sections are
respectively s and s', and whose lengths are L and i/, we shall
have :
that is to say, the intensities are directly as the sections and
inversely as the lengths of the wire.
If two simple circuits be compared, consisting of wires of
different metals, this proportion will no longer be maintained,
because in that case wires of equal length and equal section
will no longer give the currents equal intensities, because they
will not have equal conducting powers. That circuit which,
being alike in other respects, is composed of the metal of
greatest conducting power, will give a current of proportionally
greater intensity. The relative intensities, therefore, of the
currents carried by wires of different metals of equal length
and thickness are the exponents of the relative conducting
powers of these metals.
In general, if c and c' express the conducting powers of the
metals composing two simple circuits, we shall have :
s s'
i : i' : : c x - : c' x ->
Li it
2044. Conducting powers of metals. M. Pouillet ascertained
on these principles the conducting powers of the following
metals relatively to that of distilled mercury taken at 100 :
Metals. Conducting Power.
Mercury - 100
Iron ... . 6OO to 700
Steel
Brass
Platinum
Copper
Gold
Silver
Palladium
500 to SOO
200 to 900
850
3800
39OO
52OO
5800
2045. Current passing through a compound circuit of uni-
form intensity. The current which passes through a compound
s 3
390 VOLTAIC ELECTRICITY.
circuit is found to have an uniform intensity throughout its
entire course. In passing through a length of wire, which is
a bad conductor, its intensity is neither greater nor less than
upon one which is a good conductor, and its intensity on pieces
of unequal section and unequal length is in like manner exactly
the same.
2046. Equivalent simple circuit. A simple circuit composed
of a wire of any proposed metal and of any proposed thickness
can always be assigned upon which the current would have
the same intensity as it has on any given compound circuit ;
for by increasing the length of such circuit the intensity of the
current may be indefinitely diminished, and by diminishing its
length the intensity may be indefinitely increased. A length
may therefore be always found which will give the current any
required intensity.
The length of such a standard wire which would give the
current of a simple circuit the same intensity as that of a com-
pound circuit, is called the reduced length of the compound
circuit.
2047. Ratio of intensities in tico compound circuits. It is
evident, therefore, that the intensities of the currents on two
compound circuits are in the inverse ratio of their reduced
lengths, for the wires composing such reduced lengths are sup-
posed to be of the same material and to have the same thickness.
2048. Intensity of the current on a given conductor varies
ivith the thermo-electric energy of the source. In all that has
been stated above, we have assumed that the source of thermo-
electric agency remains the same, and that the changes of in-
tensity of the current are altogether due to the greater or less
facility with which it is allowed to pass along the conducting
wires from one pole of the thermo-electric source to the other.
But it is evident, that with the same conducting circuit, whether
it be simple or compound, the intensity of the current will vary
either with the degree of disturbance of the thermal equilibrium
of the system or with the thermo-electric energy of the sub-
stance composing the system.
In the case already explained, the ends of the cylinders A and
B have been maintained at the fixed temperatures of 32 and
212. If they had been maintained at any other fixed tempera-
tures, like phenomena would have been manifested, with this
difference only, that with the same circuit the intensity of the
THERMO-ELECTKICITY. 391
current would be different, since it would be increased if the
difference of the temperature of the extremities were increased,
and would be diminished if that difference were diminished.
In like manner, if, instead of bismuth, antimony, zinc, or any
other metal were used, the same circuit and the same tempera-
tures of the ends c and D or E and F would exhibit a current
of different intensity, such difference being due to the different
degree of thermo-electric agency with which the different
metals are endowed.
The relative thermo-electric agency of different sources of
these currents, whether it be due to a greater or less disturb-
ance of the thermal equilibrium, or to the peculiar properties of
the substance whose temperature is deranged, or, in fine, to both
of these causes combined, is in all cases proportional to the
intensity of the current which it produces in a wire of given
material, length, and thickness, or in general to the intensity of
the current it transmits through a given circuit.
The relative thermo-electric energy of two systems may be
ascertained by placing them as at A and B, fig. 656., and con-
necting them by simple circuits of similar wire with the differ-
ential reometer. Let the lengths of the wires composing the
two circuits be so adjusted, that the currents passing upon them
shall have the same intensity. The thermo-electric energy of
the two systems will then be in the direct ratio of the lengths
of the circuits.
2049. Thermo-electric piles. The intensity of a thermo-
electric current may be augmented indefinitely by combining
together a number of similar thermo-electric elements, in a
manner similar to that adopted in the formation of a common
voltaic battery. It is only necessary, in making such arrange-
ment, to dispose the elements so that the several partial cur-
rents shall all flow in the same direction.
Such an arrangement is represented in fig. 657., where the
two metals (bismuth and copper, for example) composing each
U,
Fig. 657.
B 4
392 VOLTAIC ELECTRICITY.
thermo-electric pair are distinguished by the thin and thick
bars. If the points of junction marked 1, 3, 5, &c. be raised to
212, while the points 2, 4, 6, &c. are kept at 32, a current will
flow from each of the points 1, 3, 5, &c. towards the points 2, 4, 6,
&c. respectively, and these currents severally overlaying each
other, exactly as in the voltaic batteries, will form a current
having the sum of their intensities.
2050. Thermo-electric pile of Nobili and Melloni Various
expedients have been suggested for the practical construction
of such thermo-electric piles, one of the most efficient of which
is that of MM. Nobili and Melloni. This pile is composed of a
series of thin plates of bismuth and antimony, bent at their ex-
tremities, so that when soldered together they have the form
and arrangement indicated in Jig. 658. The
\ -rr ^ S p aces Between the successive plates are filled
' by pieces of pasteboard, by which the corn-
Fig. 658. bination acquires sufficient solidity, and the
plates are retained in their position without being pressed into
contact with each other. The pile thus formed is mounted in
a frame as represented in^/fy. 659., and its poles are connected
with two pieces of metal by which the
current may be transmitted to any conduc-
tors destined to receive it. It will be per-
ceived that all the points of junction of the
plates of bismuth and antimony which are
presented at the same side of the frame are
Fig. 659. alternate in their order, the 1st, 3rd, 5th, &c.
being on one side, and the 2nd, 4th, 6th, &c. on the other. If,
then, one side be exposed to any source of heat or cold from
which the other is removed, a corresponding difference of tem-
perature will be produced at the alternate joints of the metal, and
a current of proportionate intensity will flow between the poles
p and n upon any conductor by which they may be connected.
It is necessary, in the practical construction of this apparatus,
that the metallic plates composing it should be all of the same
length, so that when combined the ends of the system where
the metallic joints are collected should form an even and plain
surface, which it is usual to coat with lampblack, so as to
augment its absorbing power, and at the same time to render
it more even and uniform.
This was the form of thermo-electric pile used by M. Melloni
ELECTRO-CHEMISTRY. 393
in the series of exprimental researches adverted to in 1564,
and the manner in which it was applied is exhibited in all its
details in^. 451., where jaw are the poles of the system, and
p the reometer through which the current is transmitted.
CHAP. XII.
ELECTRO-CHEMISTRY.
2051. Decomposing power of a voltaic current. When a vol-
taic current of sufficient intensity is made to pass through cer-
tain bodies consisting of constituents chemically combined, it is
found that decomposition is produced attended by peculiar cir-
cumstances and conditions. The compound is resolved into
two constituents, which appear to be transported in contrary
directions, one with and the other against the course of the
current. The former is disengaged at the place where the
current leaves, and the other at the place where it enters the
compound.
All compounds are not resolvable into their constituents by
this agency, and those which are are not equally so ; some being
resolved by a very feeble current, while others yield only to one
of extreme intensity.
2052. Electrolytes and electrolysis. Bodies which are capa-
ble of being decomposed by an electric current have been called
ELECTROLYTES, and decomposition thus produced has been de-
nominated ELECTROLYSIS.
2053. Liquids alone susceptible of electrolysis. To render
electrolysis practicable, the molecules of the electrolyte must
have a perfect freedom of motion amongst each other. The
electrolyte must therefore be liquid. It may be reduced to this
state either by solution or fusion.
2054. Faraday's electro-chemical nomenclature. It has
been usual to apply the term poles either to the terminal ele-
ments of the pile, or to the extremities of the wire or other
conductor by which the current passes from one end and enters
the other. These are not always identical with the points at
which the current enters and leaves an electrolyte. The same
s 5
394 VOLTAIC ELECTRICITY.
current may pass successively through several electrolytes, and
each will have its point of entrance and exit ; but it is not
considered that the same current shall have more than two
poles. These and other considerations induced Dr. Faraday to
propose a nomenclature for the exposition of the phenomena
of electrolysis, which has to some extent obtained acceptation.
2055. Positive and negative electrodes. He proposed to
call the points at which the current enters and departs from
the electrolyte, ELECTRODES, from the Greek word 6(5 ABODE -> N
be connected by capillary siphons, which may be conveniently
formed of the fibres of asbestos or amianthus. Let any elec-
trolyte, a solution of a neutral salt for example, be placed in c,
and let the other cups be filled with distilled water. Let a
plate of platinum connected with the positive pole of a voltaic
battery be immersed in the cup A, and a similar plate connected
with the negative pole be immersed in E. The voltaic current
will then enter the series of cups at A, and passing successively
from cup to cup through the siphons, will issue from them at E,
as indicated by the arrows. Let the water in the cups A, B, D
and E be tinged by the juice of red cabbage, the property of
which is to be rendered red by the presence of an acid, and
green by that of an alkali.
The current thus established will, according to Sir H. Davy,
decompose the salt in the cup c. The acid will be transported
through the two siphons, and the water in B to the positive
electrode in A, where it will be liberated, and will enter into
solution with the tinged water. At the same time the alkali
will pass through the two siphons, and the cup D to the ne-
gative electrode, and will enter into solution with the water
in D.
The presence of the acid in A and of the alkali in E will be
rendered manifest by the red colour imparted to the contents of
the former, and the green to the latter.
2083*. While being transferred they are deprived of their
chemical property. Although to arrive at A and E respec-
tively, the acid must pass through B and the alkali through E,
their presence in these intermediate cups is not manifested by
any change of colour. It was therefore inferred by Sir
H. Davy, that so long as the constituents of the salt are under
the immediate influence of the current, they lose their usual
ELECTRO-CHEMISTRY. 411
properties, and only recover them when dismissed at the elec-
trodes by which they have been respectively attracted.
If the direction of the current be reversed, so that it shall
enter at E and issue from A, the constituents of the salt will
be transported back to the opposite ends of the series, the acid
which had been deposited in A, will be transferred successively
through the cups B, c, D, and the intermediate siphons to the
cup E, and the alkali in the contrary direction from E through
D, c, B, and the siphons to A. This will be manifested by the
changes of colour of the infusions. The liquid in A which had
been reddened by the acid, will first recover its original colour,
and then become green according as the ratio of the acid to
the alkali in it is diminished ; and in like manner the infusion
in E, which had been rendered green by the alkali, will gra-
dually recover its primitive colour, and then become red as the
proportion of the acid to the alkali in it is augmented.
During these processes no change of colour will be observed
in the intermediate cups B and D.
The intermediate cups B and D being filled with various che-
mical solutions for which the constituents of the salt had strong
affinities, and with which under any ordinary circumstances
they would immediately enter into combination, these consti-
tuents nevertheless invariably passed through the intermediate
vessels without producing any discoverable effect upon their
contents. Thus, sulphuric acid passed in this manner through
solutions of ammonia, lime, potash, and soda, without affecting
them. In like manner hydrochloric and nitric acids passed
through concentrated alkaline menstrua without any chemical
effect. In a word, acids and alkalis having the strongest mu-
tual affinities, were thus reciprocally made to pass each through
the other without manifesting any tendency to combination.
2084. Exception in the case of producing insoluble com-
pounds. Strontia and baryta passed in the same way through
muriatic and nitric acids, and reciprocally these acids passed
with equal facility through solutions of strontia and baryta.
But an exception was encountered when it was attempted to
transmit strontia or baryta through a solution of sulphuric acid,
or vice versa. In this case the alkali was arrested in transitu
by the acid, or the acid by the alkali, and the salt resulting
from their combination was precipitated in the intermediate
cup.
x 2
412 VOLTAIC ELECTRICITY.
The exception therefore generalised included those cases in
which bodies were attempted to be transmitted through men-
strua for which they have an affinity, and with which they
would form an insoluble compound.
2085. This transfer denied by Faraday. This transmission
of chemical substances through solutions with which they have
affinities by the voltaic current, those affinities being rendered
dormant by the influence of the current which appeared to be
established by the researches of Davy, published in 1807, and
since that period received by the whole scientific world as an
established principle, has lately been affirmed by Dr. Faraday
to be founded in error. According to Faraday no such transfer
of the constituents of a body decomposed by the current can or
does take place. He maintains that in all cases of electro-
lysation it is an absolutely indispensable condition that there
be a continuous and unbroken series of particles of the elec-
trolyte between the two electrodes at which its constituents are
disengaged. Thus, when water is decomposed, there must be
a continuous line of water between the positive electrode at
which the oxygen is developed and the negative electrode at
which the hydrogen is disengaged. In like manner, when the
sulphate of soda or any other salt is decomposed, there must be
a continuous line of particles of the salt between the positive
electrode at which the acid appears and the negative electrode
at which the alkali is deposited.
Dr. Faraday affirms, that in Davy's celebrated experiments,
in which the acid and alkaline constituents of the salt appear
to be drawn through intermediate cups containing pure water or
solutions of substances foreign to the salt, the decomposition
and apparent transfer of the constituents of the salt could not
have commenced until, by capillary attraction, a portion of the
salt had passed over through the siphons, so that a continuous
line of saline particles was established between the electrodes.
Dr. Faraday admits such a transfer of the constituents as may
be explained by the series of decompositions and recompo-
sitions involved in the hypothesis of Grotthus.
2086. Apparent transfer explained by him on Grotthus 1
hypothesis. It is also admitted by Dr. Faraday, that when
pure water intervenes between the metallic conductors pro-
ceeding from the pile and the electrolyte, decomposition may
ensue, but he considers that in this case the true electrodes
ELECTRO-CHEMISTRY. 413
are not the extremities of the metallic conductors, but the
points where the pure water ends and the electrolyte begins,
and that accordingly in such cases the constituents of the elec-
trolyte will be disengaged, not at the surfaces of the metallic
conductors, but at the common surfaces of the water and the
electrolyte. As an example of this he produces the following
experiment. Let a solution of the sulphate of magnesia be
covered with pure water, care being taken to avoid all ad-
mixture of the water with the saline solution. Let a plate of
platinum proceeding from the negative pole of a battery be
immersed in the water at some distance from the surface of the
solution on which the water rests, and at the same time let the
solution be put in metallic communication with the positive
pole of the battery. The decomposition of the sulphate will
speedily commence, but the magnesia, instead of being deposited
on the platinum plate immersed in the water, will appear at
the common surface of the water and the solution. The water,
therefore, and not the platinum, is in this case the negative
electrode.
2087. Faraday thinks that conduction and decomposition
are closely related. Dr. Faraday maintains that the con-
nection between conduction and decomposition, so far as relates
to liquids which are not metallic, is so constant that decom-
position may be regarded as the chief means by which the
electric current is transmitted through liquid compounds. Ne-
vertheless, he admits, that when the intensity of a current is
too feeble to effect decomposition, a quantity of electricity is
transmitted sufficient to affect the reoscope.
In accordance with those principles, Faraday affirms that
water which conducts the electric current in its liquid state,
ceases to do so when it is congealed, and then it also resists de-
composition, and in fine ceases to be an electrolyte. He holds
that the same is true of all electrolytes.
2088. Maintains that non-metallic liquids only conduct when
capable of decomposition by the current. The connection
between decomposition and conduction is further manifested,
according to Dr. Faraday, by the fact that liquids which do not
admit of electro-chemical decomposition, do not give passage
to the voltaic current. In short, that electrolytes are the only
liquid non-metallic conductors.
2089. Faraday's doctrine, not universally accepted Pouil-
414 VOLTAIC ELECTRICITY,
let's observations. These views of Dr. Faraday have not yet
obtained general acceptation ; nor have the discoveries of Davy
of the transfer and decantation of the constituents of electro-
lytes through solutions foreign to them, been yet admitted to be
overthrown. Peschel and other German authorities in full
possession of Faraday's views and the results of his experi-
mental researches, still continue to reproduce Davy's expe-
riments, and to refer to their results and consequences as
established facts. Pouillet, writing in 1847, and also in pos-
session of Faraday's researches, which he largely quotes, main-
tains nevertheless the transport of the constituents under
conditions more extraordinary still, and more incompatible with
Faraday's doctrine than any imagined by Davy. In electro-
chemical decomposition he says, " There is at once separation
and transport. Numberless attempts have been made to seize
the molecule of water which is decomposed, or to arrest en
route the atoms of the constituent gases before their arrival at
the electrodes, but without success. For example, if two cups
of water, one containing the positive and the other the ne-
gative wire of a battery, be connected by any conductor, singular
phenomena will be observed. If the intermediate conductor
be metallic, decomposition will take place independently in
both cups" (as already described), " but if the intermediate
conductor be the human body, as when a person dips a finger
of one hand into the water in one cup, and a finger of the
other hand into the other, the decomposition will sometimes
proceed as in the case of a metallic connection ; but more gene-
rally oxygen will be disengaged at the wire which enters the
positive cup, and hydrogen at the wire which enters the ne-
gative cup, no gases appearing at the fingers immersed in the
one and the other. It would thus appear that one or other of
the constituent gases must pass through the body of the ope-
rator in order to arrive at the pole at which it is disengaged.
And even when the two cups are connected by a piece of ice,
the decomposition proceeds in the same manner, one or other
gas appearing to pass through the ice, since they are disen-
gaged at the poles in the separate cups in the same manner."*
2090. Davy's experiments repeated and confirmed by Bec-
querel. The experiments of Davy, in which the transfer of
* Pouillet, Elements de Physique. Ed. 1847, vol. i. p. 598.
ELECTRO-CHEMISTRY. 415
the constituents of an electrolyte through water and through
solutions for which these constituents have affinities, was
demonstrated, have been repeated by Becquerel, who has ob-
tained the same results. The capillary siphons used by Bec-
querel were glass tubes filled with moistened clay. He also
found that the case in which the constituent transferred would
form an insoluble compound with the matter forming the inter-
mediate solution, forms an exception to this principle of transfer ;
but he observed that this only happens when the intensity of
the current is insufficient to decompose the compound thus
formed in the intermediate solution.*
2091. The electrodes proved to exercise different electrolytic
powers by Pouillet. The question whether the decomposing
agency resides altogether at one or at the other electrode, or is
shared between them, has been recently investigated by M.
Pouillet.
Let three tubes of glass having the form of the letter U,
, fig. 662., be prepared, each of the
vertical arms being about five
inches long, and half an inch in
diameter. Let the curved part
of the tubes connecting the legs
have a diameter of about the
twentieth of an inch when the so-
lutions used are good conductors,
but the same diameter as the tubes
themselves when the conducting
power is more imperfect. In this
latter case, however, the results are less exact and satisfactory.
Let platinum wire E and E' proceeding from the poles of a
voltaic battery be plunged in the first and last tubes, and let
the intermediate tubes be connected by similar wires 11' and
i" i'". Let acidulated water be poured into the tube E i, and
the solutions on which the relative effects of the two electrodes
are to be examined, into the other tubes 1 1" and i'" E'. After
the electrolysis has been continued for a certain time, the quan-
tity of the solution decomposed in each leg may be ascertained
by submitting the contents of each leg to analysis. The quan-
tity remaining undecomposed being thus ascertained and sub-
* Becquerel, Traite de Physique, vol. ii. p. 330. Ed. 1844.
T 4
416 VOLTAIC ELECTRICITY.
traded from the original quantity, the remainder will be the
quantity decomposed, since the fluids are prevented from inter-
mixing to any sensible extent by the smallness of the con-
necting tube, and by being nearly at the same level during the
process. It may be assumed that the decomposing agencies of
the two electrodes will be proportional to the quantities of the
solutions decomposed in the legs in which they are respectively
immersed.
2092. Case in which the negative electrode alone acts. The
current being first transmitted through a voltameter to indi-
cate the actual quantity of electricity transmitted, the tubes EI,
i' i" and i'" E' were filled, the first with a solution of the chlo-
ride of gold, the next with the chloride of copper, and the third
with the chloride of zinc. After the lapse of a certain interval
the contents of the tubes were severally examined, and it was
found that the solutions in legs in which the positive electrodes
were immersed had suffered no decomposition. The quantities
of the chlorides contained in them respectively were undimi-
nished, while the chloride in each of the legs containing the
negative electrodes was diminished by exactly the quantity
corresponding to the metal deposited in the negative wire, and
the chlorine transferred to the positive leg.
It was therefore inferred that in these cases the entire decom-
posing agency must be ascribed to the negative electrode.
The same results were obtained for the other metallic
chlorides.
2093. Cases in which the electrodes act unequally. The
alkaline chlorides showed somewhat different properties. In
the case of the chloride of magnesium the agency of the ne-
gative was found to be greater than that of the positive elec-
trode, but it was not exclusively efficacious. In the cases of
the chlorides of potassium, sodium, barium, &c., the agency
was also shared by the true electrodes, but the agency of the
positive electrode was found to be greater than the negative in
the ratio of about three to one.
2094. Liquid electrodes Series of electrolytes in immediate
contact In general, the electrodes by which the current
enters and departs from an electrolyte, are solid and most fre-
quently metallic conductors. In an experiment already cited
(2086.), Faraday has shown that water may become an elec-
trode, and Pouillet in some recent experiments has succeeded
ELECTRO-CHEMISTRY.
417
in generalising this result, and has shown not only that the
current may be transmitted to and received from an electrolyte
by liquid conductors, but that a series of different electrolytes
may become mutual electrodes, the current passing immediately
from one to the other without any intermediate conductor, solid
or liquid, and that each of them shall be electrolysed. Thus
suppose that the series of electrolytes are expressed by
a a' bb' cc' d
According to the observations of Walsh, who first submitted
this animal to exact inquiry, the following are its effects :
450 VOLTAIC ELECTRICITY.
If the finger or the palm of the hand be applied to any part
of the body of the animal out of the water, a shock will be felt
similar to that produced by a voltaic pile.
If, instead of applying the hand directly, a good conductor,
such as a rod of metal several feet in length, be interposed, the
shock will still be felt.
If non-conductors be interposed, the shock is not felt.
If the continuity of the interposed conductor be anywhere
broken, the shock is not felt.
The shock may be transmitted along a chain of several per-
sons with joined hands, but in this case the force of the shock
is rapidly diminished as the number of persons is increased.
In this case the first person of the chain should touch the
torpedo on the belly, and the last on the back.
When the animal is in the water, the shocks are less intense
than in the air.
It is evident that the development of electricity is produced
by a voluntary action of the animal. It often happens that in
touching it no shock is felt. But when the observer irritates
the animal, shocks of increasing intensity are produced in very
rapid succession. "Walsh counted as many as fifty electrical
discharges produced in this way in a minute.
2157. Observations of Becquerel and Breschet. In a series
of observations and experiments made on the torpedos of
Chioggia near Venice by MM. Becquerel and Breschet, it
was ascertained that when the back and belly were connected
by the wires of a sensitive reoscope, a current was indicated as
passing from the back to the belly. They also found that the
animal could at will transmit the current between any two
points of its body.
2158. Observations of Matteucci. In a series of experiments
made on the torpedos of the Adriatic, M. Matteucci confirmed
the results obtained by MM. Becquerel and Breschet, and also
succeeded in obtaining the spark from the current passing
between the back and belly.
2159. The electric organ. In the several species of fish
endowed with this quality, the structure of the organ in which
the electric fluids are developed is alike, differing only in its
form, magnitude, and position. In the torpedo, which has been
submitted to the most rigorous examination, it consists of two
parts symmetrically arranged at each side of the head and
EFFECTS OF THE VOLTAIC CURRENT. 451
resting against the gills. They fill all the thickness which
separates the two coats of the skin. On dissection it is proved
to consist of an extremely open cellular tissue, having the form
of a cylinder, or, more exactly, that of a five or six-sided prism.
It has been compared to the cellular structure of the honeycomb,
only that the partitions, instead of being thin membranes, are
fibres separated and extended in different directions.
Four or five hundred of these prisms are commonly counted
in each organ. Hunter in one case found 1182. They are
nearly at right angles to the surface of the skin, to which they
are strongly attached at the ends. When the structure of each
of these prisms is examined, they are found to consist of a
multitude of thin plates whose planes are perpendicular to the
axis of the prism, separated from each other by strata of mucous
matter, and forming a combination resembling the original
galvanic pile.
Four bundles of nerves of considerable volume are distributed
in the organ, and, according to Matteucci, the seat of the elec-
trical power is at their origin.
INDEX.
ABSORPTION of heat, 121.
Agonic lines, 172. ; American, 172. ; Asiatic,
172.
Air, rarefied, a conductor of electricity, 198.
Alcohol, congelation of, 81.
Alloys, liquefaction of, 73.
Ampere, method to reverse galvanic current,
301.; apparatus for supporting moveable
currents, 304. ; method of exhibiting revo-
lution of galvanic current round a magnet,
318. ; electro-magnetic rectangle, 362.
Animal heat, 138.
Animals, wool and fur of, their uses, 111.
Anion, 394.
Annealing, use of, 85.
Anode, 394.
Arago's electro-magnetic researches, 351.
Armatures, 185.
Armstrong's hydro-electrical machine, 210.
Astatic needle, 187.
Athermanous media, 123.
Atmosphere, low temperature of, superior
strata of, 54.; non-conductor of electricity,
198.
Attraction, magnetic, 152. ; electric, 189.
Aurora borealis, Influence of on magnetic
needle, 179.
Austral fluid, 155.
Axis, magnetic, 153. ; of circulating galvanic
current, 321.
Azimuth compass, 165.
Babbage's electro-magnetic researches, 351.
Bagration's galvanic system, 283.
Bain's electro-chemical telegraph, 436.
Balance-wheels, compensation of, 30.
Bar, magnetic, 161.
Battery, electric, 231 . ; voltaic, 280. 440.
Becquerel's galvanic system, 283. ; electro-
chemical researches, 405.
Bells, electric, 246.
Birds, plumage of, its uses, 110.
Blood, temperature of in human species,
Boiling point, 10. 99.
Boreal fluid, 155.
Buildings, metallic, 27.; warming, 38. 46.
106.; ventilation of, 38.
Bulb,thermometric, 7. ; liable to permanent
charge of capacity, 15.
Buusen's galvanic battery, 280.
Calcium, 418.
Caloric, 5.
Calorimeter, 48.
Calorimetry, 47. ; method of solving calori-
metric problems, 48.
Cavendish's electric barometer, 259.
Centigrade scale, 11.
Chemical effects of electricity, 264.
Chemistry, electro-, 393.
Children's great galvanic plate pile, 293.
Circuit, voltaic, 298. See Electricity.
Clothing, its properties, 111.
Cold, greatest natural, 81.
Combustibles, 3. 134.; illuminating power
of, 135.; constituents of, 135.; quantity of
heat developed by, 136.
Combustion, 3. 132. ; agency of oxygen in,
133.; combustibles, 134.; explained, 134.
temperature necessary to produce, 134.;
illuminating power of combustibles, 135.;
constituents of combustibles, 135.
Compass, azimuth, 165.; mariner's, 166.
Compensators, 27.
Compression of vapour, 96.
Coercive force, 156.
Coin, why stamped, not cast, 76.
Condensation, 2. 86.; of vapour, 105.
Condenser, electric, 214. ; principle of, 216. ;
forms of Culhbertson's, 218.
Conduction, heat, 107.; electricity, 196.;
magnetic, 3. 107.; electric, 196.
Conductibility, 3.
Conductors, heat, 4.; good and bad, 107.;
electric, 196. 237. ; connecting galvanic
elements, 292.
Congelation, 2. 67. ; latent heat rendered
sensible by, 65.; points of, 74.; of alcohol,
Contraction, 1.29 ; of solids, 26.; of mercury
in cooling, 76.
Coulomb's electroscope, 222.
Couronne des lasses, 289.
Crosse's electro-chemical researches, 406.
Cruikshank's galvanic arrangement, 290.
Currents, atmospheric, cause of, 41.; voltaic,
296. See Electricity ; rectilinear, 300. ;
indefinite, 300. ; closed, 300. ; circular or
spiral, 300. ; circulating, 320. ; spiral and
heliacal, 322. ; thermo-electric , 385.
Cuthbertson's electric condenser, 2. 218.
Dance, electric, 247.
Daniel's constant galvanic battery, 281. 294.
Davy, galvanic pile, 293. ; experiments in
electro-chemistry, 409. ; method of pre-
serving copper sheathing, 421. See Elec-
tricity.
Declination, 168. ; in different longitudes,
172. ; observed at Paris, 175.
Dena<*rator, galvanic, 293. 439.
Delarive's floating electro-magnetic appa-
ratus, 323.
Deluc's galvanic pile, 295.
Density, effect of relation of different strata
of same liquid, 44. ; relation of specific
heat to, 53.
Dew, principles of, 131.
453
Diathermanous media, 4. 123.
Dilatation. 1. 29.; of mercury, rate of, 13. ;
of solids, 22. ; of gases, 31 ; of gases differs
with change of pressure and temperature,
34.; of liquids, 42. ; rates of, of liquids, 43.
Dip, lines of equal, 171.; local, 173.; ob-
served at Paris, 175.
Dipping-needle. 167.
Discharging-rod, 213.
Distributor, electro-magnetic, 340.
Dutch tears, 85.
Earth, temperature of globe of, 114 ; analogy
of, to magnet, 164. 170. ; direction of mag-
netic attraction of, 353. ; magnetism, 379.
Ebullition, 99.
Electrical attractions and repulsions, 189.
Electrical machines, 207. See Machines.
Electric lamps, 445.
Electric light, 443.
Electricity, 189. ; attraction and repulsion,
189. ; origin of name, 189. ; fluid, 190. ;
. positive and negative, 191. ; single electric
fluid, 191.; two fluids, 192.; vitreous and
resinous and positive and negative fluids,
192. ; developed by various bodies, 193. ;
positive and negative substances, 194.;
method of producing by glass and silk,
195.; conduction, 196.; conductors and
non-conductors, 196.; insulators, 197.;
insulating stools, 197. ; induction, 202.;
electrical machines, 207. ; condenser and
electrophorous, 214.; dissimulated or la-
tent, 217. ; free, 217. ; electroscopes, 220. ;
Leyden jar, 224. ; charging a series of jars
by cascade, 231.; electric battery, 231.;
laws of electrical forces, 234. ; proof plane,
235. ; electrical orrery, 240. ; mechanical
effects of. 241. ; attractions and repulsions
of electrified bodies, 241. ; electrical bells,
246. ; electric dance, 247. ; electrical see-
saw, 249. ; thermal effects of, 249. ; igni-
tion of metals, 250. ; electric pistol, 251. ;
gunpowder exploded, 252. ; Kinnersley's
electrometer, 252. ; luminous effects of,
253.; electric spark, 254.; imitation of
auroral light, 257. ; Leichtenberg's figures,
257. ; experiments, indicating difference
between the two fluids, 258. ; Cavendish's
electric barometer, 259. ; thermal hypo-
thesis, 259. ; physiological effects of, 261. ;
chemical and magnetic effects of, 264.;
Voltaic, 67. ; discovery of galvanism, 67. ;
contact hypothesis of Volta, 269 ; electro-
motive force, 269. ; classification of bodies
as to electro-motive property, 270. ; elec-
tro-motive action of gases and liquids,
272. ; polar arrangement of fluids in elec-
tro-motive combinations, 274. ; positive
and negative poles, 274. ; Volta 's first
combination, 278.; Wollaston's combina-
tion, 278. ; Hare's spiral arrangement,
278. ; cylindrical combination with one
fluid, 279. ; with two fluids, 280. ; Grove's
battery, 280. ; Bunsen's battery, 280. ;
Daniel's constant battery, 281. ; Pouillet's
modification, 281. ; Smee's battery, 282. ;
Wheatstone's system, 283. ; Bagration's
system, 283. ; Becquerel's system, 283. ;
Schonbein's modification of Bunsen's bat-
tery, 284. ; Grove's gas electro-motive
apparatus, 284. ; Volta's invention of the
pile, 2S5.; Couronnedestasses,289.; Cruik-
shank's arrangement, 290. ; Wollaston's
arrangement, 290. ; heliacal pile of Faculty
of Sciences at Paris, 291 . ; conductors con-
necting the elements, 292. ; memorable
piles, 293. ; Davy's pile, 293. ; Napoleon's
pile, 293. ; Children's great plate batterr
293. ; Hare's deflagrator, 293. ; Stratingh's
deflagrator, 293. ; Pepys's pile, 294. ; bat-
teries on Daniel and Grove's principles
294.; dry piles, 294.; Deluc's pile 295. ^
/amboni's pile, 295. ; piles of a single
metal, 295. ; Hitter's secondary piles, 296. ;
Voltaic currents, 296. ; direction of curl
rent, 297.; poles of pile, 298.- voltaic
circuit, 298. ; method of coating conduct-
ing wires, 301 .; supports of wires, 301-304.;
Ampdre's method to reverse current, 301.
Pohl's reotrope, 303.; electrodes, 303.;
Ampere's apparatus for supporting move-
able currents, 304. ; reciprocal influence
of rectilinear currents and magnets, 305.
electro-magnetism, 305. ; effect of shock
on bodies recently deprived of life, 447
on a leech, 447. ; excitation of nerves of
taste, 448. ; of nerves of sight, 448. ; of
nerves of hearing, 448. ; supposed sources
of electricity in animal organization, 448.;
electrical fishes, 449. ; properties of the
torpedo, 449. ; the electric organ, 450.
Electricfty, thermo-, 384.; thermo-electric
current, 385.; Pouillet's thermo-electric
apparatus, 386.; conducting powers of me-
tals, 389.; thermo-electric piles, 391.-
electro-chemistry, 393.; electrolytes and
electrolysis, 393. ; Faraday's electro-che-
mical nomenclature, 393. ; positive and
negative element, 394. ; electrolysis of
water, 395.; Mitscherlich's apparatus, 397.-
compoundssusceptibleofelectrolysis,4()2.;
electro-negative bodies, 403. ; electro-posi-
tive bodies, 403. ; researches of Becquerel
and Crosse, 405. ; Faraday's voltameter
407. ; Faraday's law, 407. ; Sir H. Davy's
experiments, 409. ; Faraday's doctrine,
412. ; Pouillet's observations, 414. ; Davy's
experiments confirmed by Becquerel,414.
liquid electrodes, 416. ; electrolysis of the
alkalis and earths, 418. ; the series of new
metals, 418. ; Schoubein's experiments on
the passivity of iron, 419. ; tree of Saturn,
420. ; Davy's method of preserving copper
sheathing, 421.; calorific, luminous, and
physiological effects of voltaic current
438. ; Hare and Children's deflagrators,
439. ; Wollaston's thimble battery, 440. ;
Jacobi's fexperiments on conduction by
water, 441.; combustion of the metals,
442. ; electric light, 443. ; electric lamps,
445.
Electrodes, 303. 394.
Electro-chemistry, 393.
Electrolysis, 393. ; compounds, susceptible
of, 402.
Electrolytic classification of the simple
bodies, 403.
Electrolytes, 393.
Electro-magnets, 335.
Electro-magnetism : apparatus to exhibit
direction of force impressed by a rectili-
near current on a magnetic pole, 309.
apparatus to measure intensity of such
force, 310. ; apparatus to illustrate electro-
magnetic rotation, 315.; Ampere's method,
318. ; reciprocal influence of circulating
currents and magnets. 320. ; circulating
current, 320. ; axis of current, 321 . ; spiral
and heliacal currents, 322.; Ampere's and
454
Delarive's apparatus, 323. ; instable equi-
librium of current, 325. ; right-handed
and left-handed helices,326. ; electro-mag-
netic induction, 331. ; Savary's experi-
ments, 333.; electro-magnets, 335.; electro-
magnetic power employed as a mechanical
agent, 336. ; electro-motive power applied
by M. Froment, 337. ; electro-motive ma-
chines constructed by him, 339. ; distribu-
tor, 340. ; regulator, 343. ; use of a contact
breaker, 34S. ; magneto-electric machines,
348. ; effects of momentary inductive cur-
rents produced upon revolving metallic
disks, 351 .; researches of Arago, Herschel,
Babbage, and Faraday, 351. ; influence of
terrestrial magnetism on voltaic currents,
353. ; direction of earth's magnetic attrac-
tion, 353.; Pouillet's apparatus to exhibit
effects of (earth's magnetism, 3*7. ; Am-
pere's rectangle, 362. ; reciprocal influence
of voltaic currents, 363. ; voltaic theory of
magnetism, 378. ; reoscopes and reome-
ters, 3-*0. ; differential reometer, 384.
Electro-magnetic induction, 331.
Electro-metallurgy, 425. ; production of
metallic moulds, 428. ; production of ob-
jects in solid metal, 428. ; reproduction of
stereotypes and engraved plates, 429. ;
metallizing textile fabrics, 429. ; glypho-
graphy, 430. ; reproduction of Daguerre-
otype, 430.
Electro-motive force, 269.
Electro-negative bodies, 403.
Electro-positive bodies, 403.
Electrophorous, 214 219.
Electroscopes, 220. ; pith-ball, 221 . ; needle,
221.; Coulomb's, 222.; quadrant, 222. ;
gold leaf, 222. ; condensing, 223.
Electro-telegraphy, 431.; conducting wires,
432. ; earth best conductor, 432.; telegra-
phic signs, 433.; Morse's system, 436.;
electro-chemical telegraphy, 436. ; Bain's
telegraph, 436.
Element, positive, 394. ; negative, 394.
Equator, magnetic, 149. 170.
Evaporation, 86. ; mechancal force de-
veloped in, 92. ; heat absorbed In, 103.
Faraday's electro-magnetic researches, 351 . ;
electro-chemical nomenclature, 393. ; vol-
tameter, 407. ; electro-chemical law, 407.
See Electricity.
Fire-eaters, feats of, explained, 147.
Fireplaces, 39.
Fishes, electric, 449.
Flame, 133.
Fluid, magnetic, 155. ; electric, 190.; posi-
tive, 192. ; negative, 192. ; vitreous, 192. ;
resinous, 192. ; experiments indicating
specific difference between two electric,
258.
Fluxes, 81. j principle of, 81.; application,
Forces (electrical), laws of, 234.
Freezing mixtures, 77. 80. ; apparatus, 79.
French metrical system, 44.
Froment's electro-magnetic machines, 337.
Froast (hoar), principles of, 131.
Fusion 2. ; latent heat of, 70. ; points of, 70 ;
substances which soften before, 77. ; in-
fusible bodies, 82.
Galvanism, 269.
Galvanometer, 382.
Gases, dilatation of, 31 . ; liquefaction of, 55.
97. ; permanent, 97.'; solidification of, 97. ;
under extreme pressures, 99.; non-con-
ductors of heat, 109.
Graduation of thermometers, 9. ; of pyro-
meters, 18.
Gravity, specific, of liquid, 43.
Grove's galvanic battery, 280. 294.; gas
electro-motive apparatus, 284.
Hare's deflagrator, 293.
Hare's spiral galvanic arrangement, 278.
Harrison's pendulum, 28.
Heat, 1. 5. ; sensible, 1.; insensible, 1.,
latent, 1. 65. 101.; heating liquid, 45.;
does not descend in liquid, 45. ; propagation
of, through liquid by currents, 45. ; quan-
titative analysis of, 47. ; specific, 47. 58. ;
uniform and variable, 48. ; development
and absorption of by chemical combina-
tion, 56. 132.; specific of simple gases
equal under same pressure, 57. ; relation
between specific and atomic weight, 57. ;
rendered latent in liquefaction, 65. ; latent
rendered sensible by congelation, 65.; la-
tent, of fusion, 70. ; absorbed in evapora-
tion at different temperatures, 103. ; ra-
diation of, 115.; reflection of, 119. 12^.;
absorption of, 121.; transmission of, 123.;
decomposition of by absorption, 125.; re-
fraction of, 128.; polarization of, 128.;
Quantity of developed by combustibles,
36. ; animal, 138.; experiments to ascer-
tain rate of development of animal, 142. ;
total quantity of animal explained by
chemical laws, 143.; sensation of, 144.;
touch, fallacious measure of, 145.
Heliacal galvanic pile of Paris, 291.
Helices. See Electricity.
Herschel's electro-magnetic researches, 351 .
Hydraulic press, Britannia bridge, casting
of, 113.
Ice, method of preserving, 112.; production
of artificial, 79. 132.
Incandescence, 2.
Induction, magnetic, 156.; electric, 202.;
effects of, 179.; electro-magnetic, 331.
Influence of voltaic currents and magnets,
306. 363.
Influence of terrestrial magnetism on voltaic
currents, 353.
Infusible bodies, 82.
Inlaying, metallic, 27.
Insulating stools, 197. 212.
Insulators, 197.
Isogonic lines, 173.
Jacobi's experiments on electric conduction
by water, 441 .
Kathion, 394.
Kathode, 394.
Keepers, 185.
Kinnersley's electrometer, 252.
Lamp, Argand, 40. ; electric, 445.
Laplace's calorimeter, 48.
Lavoisier's calorimeter, 48.
Leyden jar, 22-1. ; improved form of, 230.
Light, solar, thermal analysis of, 115.; phy-
sical analysis of, 116.; electric, 443.
Lines of equal dip, 171.; agonic, 172.; iso-
gonic, 173. ; isodynamic, 176.
Liquefaction, 2. 67. ; of gases, 55. 97. ; ther-
mal phenomena attending, 63. ; facility of
455
proportional
to latent heat, 72. ; of alloys
Liquids, dilatation of, 42.; effects of tempe-
ratures of different climates on, 106. ; non-
conductors of heat, 109.; materials fitted
for vessels to keep warm, 129.
Loadstone, 149.
Luminous effects of electricity, 253.
Machines, electrical, 207.; parts of, 207.;
common cylindrical, 207. ; Nairne's cylin-
der, 209. ; common plate, 209. ; Arm-
strong's hydro-electrical, 210. ; appen-
dages to, 212.
Magnetic effects of electricity, 264.
Magnetism, 149. ; equator, 149. 170. ; poles,
149.; pendulum, 151.; attraction and re-
pulsion, 152. ; axis, 153.; boreal and aus-
tral fluids, 155. ; coercive force, 156. ; sub-
stances, 156. ; induction, 156. ; decompo-
sition of fluid, 159. ; effects of heat on,
162.; terrestrial, 164.; meridian, 168.
171.; declination or variation, 168.; va-
riation of dip, 170. ; lines of equal dip,
171.; agonic lines, 172.; isogonic lines,
173. ; local dip, 173. ; position of mag-
netic poles, 173. ; intensity of terrestrial,
175.; isodynamic lines, 176.; diurnal va-
riation of needle, 177. ; influence of au-
rora borealis, 179.; voltaic theory of, 378.
See Electricity.
Magnetization, 179.; effects of induction,
179.; method of single touch, 181.; of
double touch, 182.; magnetic saturation,
183.; limit of magnetic force, 183. ; effects
of terrestrial magnetism on bars, 184.;
armatures or keepers, 185. ; compound
magnets, 186.; influence of heat on mag-
netic bars, 186. ; astatic needle, 187.
Magnets, electric machines, 348. See Elec.
tricity.
Magnets, natural, 149.; artificial, 149. 180. ;
compound, 161. 186.; with consequent
points, 163. ; best material, form, and
method of producing artificial, 180. ; elec-
tro-magnet, 335. See Electricity.
Marble, fusible, 83.
Mariner's compass, 166.
Matting on exotics, its use, 112.
Mechanical effects of electricity, 241.
Melloni's thermoscopic apparatus, 123.;
thermo-electric pile, 392.
Mercury, preparation of, for thermometer,
7. ; introduction in tube, 8. ; rate of dila-
tation of, 13. ; qualities which render it a
convenient thermoscopic fluid, 15.
Meridian, magnetic, 168. 171.; true, 168.;
terrestrial, 168.
Metals, weldable, 77. ; ignition of, hy elec-
tricity, 250. ; electric conducting power of,
389. ; new, 418.
Mitscherlich's electro-chemical apparatus,
Moisture, deposit of on windows, 129.
Moulds for casting metal, 26. ; electro-me-
tallurgic, 428.
Morse's electric telegraph, 436.
Multiplier, electro-magnetic, 382.
Nairne's cylinder electrical machine, 209.
Napoleon, galvanic pile, 293.
Needle, magnetic, 161.; dipping, 167.; di-
urnal variation of, 179. ; influence of
aurora borealis on, 179. ; astatic, 187.
Nobili's reometer, 383.; Thermo-electric
pile, 392.
Non-conductors, electric, 196.
Oils, congelation of, 76.
Orrery, electric, 240.
Oxygen, its agency in combustion, 133.
Pendulum, compensating, 27. ; Harrison's
gridiron, 28.; magnetic, 151.
Pepys' galvanic pile, 294.
Physiological effects of electricity, 261.
Phosphorus, congelation of, 76.
Piles, galvanic. See Electricity Thermo-
electric, 391.
Pistol, electric, 251.
Platinum rendered incandescent, 135.
Pohl's reotrope, 303.
Point, standard, in thermometer, 9. ; freez-
ing, 10. ; boiling, 10. 99. ; of fusion, 70. ;
of congelation, 74.; consequent, 163.;
electric conductors with, 237. ; consequent,
electro-magnetic, 332.
Polarization, heat, 128.
Poles, magnetic, 149. 168. ; position of mag-
netic, 173. ; electric, positive and negative,
274. ; of galvanic pile, 298.
Potassium, 418.
Pouillet's modification of Daniel's battery,
281 .; apparatus to exhibit effects of earth's
magnetism, 357. ; thermo-electric appa-
ratus, 386.
Pressure, 31."
Proof-plane, 235.
Pyrometer, 3. 18. ; graduation of, 18.
Radiation, 4. 115. ; rate of heat, 120.; inten-
sity of, 120. ; influence of surface on, 120. ;
radiating powers, 121.
Reaumur's scale, 12.
Red-hot, 2.
Regnault's tables of specific heat, 58.
Regulator, electro-magnetic, 343.
Reflection, heat, 4. 119. 128.; reflecting
powers, 121.
Refraction, heat, 5. 128. ; light, 116.
Refractory bodies, 73.
Reometer, 380. ; differential, 384.
Reoscope, 380.
Reotrope, Pohl's, 303.
Repulsion, magnetic, 152. ; electric, 189.
241.
Ritter, secondary galvanic piles, 296.
Rod (discharging), 213.
Roofs, metallic, 27.
Scale: thermometric, 9.; centigrade.il.;
Reaumur, 12.
Schb'nbein's modification of Bunsen's bat-
tery, 284. ; experiments on the passivity
of iron, 419.
Sensation of heat, 144.
See-saw (electric), 249.
Simple voltaic combination, 267-
Smee's galvanic battery, 282.
Snow, perpetual, line of, 55. ; effect of on
soil, 111.
Sodium, 418.
Solar light, 115.
Solids, dilatation of, 22. ; contraction of, 26.
Solidification, 2. 63. ; of gases, 97.
Sonometer, application of electro-magnetic
machine as, 343.
Specific heat, 47. ; relation of to density, 53.
456
Standard points (thermometer), 9.
Steam, high pressure, expansion of, 54.
Steel, tempering, 86.
Stools, insulating, 197. 212.
Stoves, 39. ; unpolished, advantage of, 129.
Stratingh's galvanic deflagrator, 293.
Structures, metallic, 27.
Substances, magnetic, 156.
Sulphur, fusion of, 74.
Syringe, fire, 53.
Temperature, 5. 31.; methods of computing
according to different scales, 12. ; of great-
est density, 44. ; method of equalization of,
51. ; in liquids and gases, 109. ; of globe of
earth, 114. ; necessary to produce combus-
tion, 134. ; of blood in human species,
138. ; of blood in animals, 140. See Heat.
Terrestrial magnetism, 164. ; influence of on
voltaic currents, 353.
Thermal unit, 47.
Thermo-electricity, 384. See Electricity.
Thermometer, 3. ; mercurial, 6. ; tube, 7. ;
bulb, 7.; self-registering, 16.; spirit of
wine, 16. ; air, 16. ; differential, 17.
Thermometry, 5, 47.
Thermoscopic bodies, 3. 6. ; apparatus, 123.
Torpedo, electrical properties of, 449.
Tree of Saturn, 420.
Tube (thermometric), 7.
Unit (thermometric), 11. ; thermal, 47.
Vaporization, 2. 86.
Vapor, 86. ; apparatus for observing proper,
ties of, 86. ; elastic, transparent, and invi-
sible, 88. ; how pressure of indicated and
measured, 88. ; relation between its pres-
sure, temperature, and density, 91.; me-
chanical force of, 92. ; dilatable by heat,
95. ; properties of super-heated, 95. ;
cannot be reduced to liquid by mere com-
pression, 96. ; compression of, 96. ; latent
heat of, 101.
Variation, 16S. ; diurnal of needle, 177.
Ventilation of buildings, 38.
Vernier, 21.
Voltaic batteries, 285. See Electricity.
Voltaic currents, 296. 363.
Voltaic electricity, 67. See Electricity.
Volta's first galvanic combination, 278. ; in-
vention of the pile, 285.
Voltaic piles, 285.
Voltaic theory of magnetism, 378.
Voltameter (Faraday's), 407.
Warming buildings, 38. 46. 106.
Water, solidification of, 63. ; liquid below
32, 68. ; pressure, temperature, and den-
sity of the vapour of water, 91.; vapour
produced from at all temperatures, 91. ;
mechanical force of vapor of, 93.; tempe-
rature, volume, and density of vapor of,
corresponding to atmospheric pressures,
95.; latent heat of vapor of, 103,104. ; a
conductor of electricity, 199. ; composition
of, 394. ; electrolysis of, 395.
Weight, atonic, 57.
Wheatstone's galvanic system, 283.
Wine coolers, 112.
Wires (electric), 301. 304.
Wollaston's galvanic combination, 278290.;
thimble battery, 440.
Zamboni's galvanic pile, 295.
Zero of thermometric scale, 10.
THE END OF THE SECOND COURSE.
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