California
egional
3-cility
IMENT-MAKING
OR AMATEURS
T~> rj i f \
.
ELECTRICAL
INSTRUMENT MAKING
FOR AMATEURS.
& f rsrffral Ban&foHjk
BY
S. R. BOTTONE,
LATE OF THE COLLEGIO DEL CARMINE, TURIN J THE ISnTCTO
BELLINO, NOVAiA. CERTIFICATED SCIENCE TEACHER,
SOUTH KENSINGTON.
NEW YORK:
EXCELSIOR PUBLISHING HOUSE,
29 AKD 31 BEEKMAN STEBE-S?.
oc
PREFACE.
THE amateur, especially if he be young, inexperienced,
and unblest with " filthy lucre," must ever command
our sympathy and respect He, withput. any hope of
reward, except that gratification which follows the
acquirement of knowledge, works on, under the greatest
difficulties, to the attainment of his end an end which,
in nine cases out of ten, tends directly to the benefit of
his fellow-creatures.
Nearly all the really useful inventions and discoveries,
which have rendered the igth century so remarkable
as a season of progress, must be attributed to amateurs.
For this reason, if for no other, we should render every
assistance in our power to the 'bond-fide amateur, and
for this reason, along with another, have I attempted,
in the following pages, to guide the tyro in his attempts
at the construction of the more useful pieces of electrical
apparatus. The other reason is that " a fellow-feeling
makes us wondrous kind," and as I myself felt acutely
the need of assistance at the beginning of my amateur-
scientific career, so I am pleased to suggest when
PREFACE.
and how much trouble may be saved, and expense
spared by the adoption of certain simple modes of
procedure.
In the following pages no attempt has been made
to describe the production of such highly finished
" brass and glass " instruments as those which adorn
the windows of our opticians' shops. Such a high
degree of finish requires a technical knowledge of
French polishing, lacquering, burnishing, etc., as is not
usually possessed by the amateur. The tools used, also,
are supposed to be of the simplest description, such
as may be found in every home, however humble.
Not one of the instruments described necessitates the
employment of a lathe or other expensive tool in its
manufacture ; though, of course, much truer and finished
circular work can be done on the lathe than in any
other manner. But the instruments produced as
described in this book, may be relied upon to act
efficiently ; and this is, after all, the end for which every
instrument is 'constructed. It must be borne in mind
that this work does not profess to teach the science of
electricity : and no attempt is made to enter upon the
domain of scientific speculation,
INDEX TO SECTIONS.
SECTION PACK
SECTION PAGE
i Tools ....
I
72 Dynamos, Rules for
2 Materials ...
2
Winding . . . 108
3 Soldering . .
3
72 Ring Armature, Paci-
5 with Flame .
7
notti's . . . 109
6 Pivots ....
9
72 Wires, Resistances of . 112
7 Glass threads . .
ii
73 Resistance of Wire
8 Straws ....
ii
Table . . .112
9 Pith Balls .
ii
73 Wires, Carrying power
10 Electroscope .
12
of . . . .112
ii Gold Leaf .
14
74 Dynamos, Winding . 115
12 Coulomb's Torsion
75 Ammeter . . .116
Balance
20
75 Ampere, What it is .116
15 Electrophorus
23
79 Ammeter, To Grade . 119
17 Bertsch's Machine.
28
80 Voltmeter . . .121
1 8 Mounting a Glass Plate.
29
81 E.M.F. . . .121
20
82 Ohm's Law . . .122
without drilling .
34
86 Galvanometers .^ .131
25 Carre's Dielectric
91 Tangents and Arcs, "
Machine .
40
Table of . . .136
29 Holtz's Machine .
47
93 Thermopile . . . 139
34 Wimhurst
57
95 Batteries . ; . 146
42 Plate
65
95 Porous Cells . . 147
43 Condensers .
70
95 Binding Screws, and
44 Leyden Jar .
71
Substitutes . . 148
45 Franklin Plate
75
95 Amalgamation . .148
45 Fulminating Panes
75
95 Zinc .... r48
47 Fizeau's Condenser
76
97 Graphite Plates and
48 Microfarad .
79
Rods to Mount. . 150
49 Medical Coils
81
98 Batteries, Single Fluid. 151
49 Shocking Coil . .
81
98 Battery, Agitating
56 Induction Coils . .
88
Fluid in . . .151
58 Magneto Machine.
90
99 Batteries, Double Fluid 153
59 Shocking
90
99 Table of
65 Uni-direction
E.M.F. . . .155
Machine .
99
lob Telephone . . .156
72 Dynamos
108
102 Electro Motors . .164
72 Gramme.
1 08
103 The Phonograph . . 170
72 Manchester
1 08
104 The Microphone. . 180
ELECTRICAL INSTRUMENT MAKING
FOR AMATEURS.
I. TOOLS. The true amateur, as a rule, has not a
large assortment of tools. Little by little he gets
together, or constructs those which are necessary for
his purpose ; but he seldom aspires to the complete
paraphernalia of a workshop. Still there are certain tooto
that are indispensable, of which the following is a lJs>t In
order of utility :
i Large pocket-knife.
I Fine penknife.
I Archimedean drill and bits.
Pair of cutting pliers.
Pair of large scissors for metal.
Pair of small scissors.
Several files, large and smalL '
I Hammer.
I Mallet.
Bradawl, gimlet, pincers.
Small bench vice.
Small tenon saw.
Soldering iron.
Spirit lamp.
I Wheel glass cutter or diamond
Pair of compasses
aft. rule, s
B
^ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
I should like to have put a lathe at the heacl of this
list, for that is really the king of tools ; but I would not
deter the student from making electrical apparatus
because he has not a lathe, as most may be made well
without, though better with one.
Besides tools, the materials mentioned below will be
found useful. They need not be procured all at once,
but as occasion demands. If the amateur adopts the
plan of keeping up a little stock of his materials and
tools, as they are worn out or consumed, and more
especially if he remembers that, " Order is Nature's first
law," and that there chould be " a place for everything,
and everything in its place," he will turn out better work,
keep his temper, and v/ork better than if he allows him-
self to degenerate into a slipshod style of doing things.
Let him never say " that'll do " to anything capable of
improvement.
\ 2. MATERIALS. The following will be found useful
in carrying out the instructions given in "the ensuing
pages :
Glass rods from J to \ in. in diameter.
Ebonite rods from Jto \ in. in diameter.
Glass tubes from J to i in! in diameter.
Guttapercha.
Glass bottles, preferably green glass.
Sheets of glass ; every piece is useful.
Bottoms of broken wine-glasses as stands, &c.
Tinfoil
Sheet zinc and sheets of tinned iron.*
* Clean beef tins, sardine tins, &c., may be worked up very well and
econouvically,
MATERIALS. $
Sheet copper.
Sheet brass, and brass rod, iin. diameter*
Solder.
Chloride of zinc..
Rosin.
Needles.
Watch springs.
MVhite hard varnish.
Red lead.
Benzoline.
Burnt umber.
Copper wire of various sizes.*
Prout's elastic glue.
Methylated spirits of wine.
Having these materials at hand, the amateur will find
several operations are required so frequently as to render
a certain amount of technical skill absolutely necessary
if the work is to look neat and act satisfactorily. Among
the first of the -amateur's requirements must be placed
the power of soldering.
3. SOLDERING. For small work, an iron, shaped as
shown at Fig. i, will be found extremely useful. The
FIG. i.
amateur can easily construct this for himself by boring
a tff hole in a copper wedge 2in. long by Jin. thick, and
in. wide on the base. An iron rod, 15in. long, is
straightened out for the handle, and the end of this rod
. Thia had better tie procured ^s required.
4 ELECTRICAL INSTRUMENT MAKING FOR'AMATEURS*
is heated in the fire and hammered up until it can be thrust
through the hole in the copper wedge; what projects is
then hammered down so as to form a kind of boss or rivet
head, to prevent the copper bit from moving. The other
end of the rod may then be filed or hammered'to a point
and driven into a wooden handle. Five inches of broom-
handle will answer the purpose, if nothing better can be
got at
In order to solder successfully, four things are
essential. Firstly, the portions to be soldered must be
made scrupulously clean, either by scraping with a p\ece
of sharp knife blade (kept specially for this purpose), or
by sandpapering, or by filing, whichever is most
convenient. Secondly the portions that are to be
soldered must be raised nearly to the temperature of the
melted solder. For this reason the amateur would
surely fail were he to attempt to make a good joint, say,
Between two sad irons, with an ordinary bit, unless he
previously heated the irons to nearly the melting point
of solder. In ordinary small work, where wires are
soldered to wires, sheets to sheets, &c., the heat of the
soldering iron itself is generally sufficient, not only to
melt the solder, but also to raise the temperature of the
surrounding portions to be joined to the requisite point
Thirdly, the nose of the soldering iron must be perfectly
clean, and covered with melted solder, or, as it is tech-
nically called " tinned."
To insure this result the soldering iron must be placed
in a clear red fire until nearly red-hot (if allowed to get
too hot it will not take up the solder at all), then quickly
SOLDERING. 5
and lightly filed on one face, and at once plunged into a
rod of soft solder, on which a little powdered rosin has
been placed The heat of the soldering iron will cause
the solder to melt, and run into globules. If one of
these be allowed to run into the lid of a milk tin, or
other convenient tinned iron receptacle in which a little
powdered rosin is also placed, and the filed face of the
soldering iron rubbed briskly over the melted solder, the
surface of the copper bit will be found to have taken a
coating of solder as brilliant as a looking-glass. Should
this not be the case, the iron must again be heated and
filed and rubbed over the solder until a perfect coating
has been obtained.
When once the nose of the copper bit is well "tinned,"
it is sufficient after each heating (provided it has not
been overheated so as to burn off the solder) to rub the
iron briskly on a piece of old carpet, kept especially for
this purpose, just before touching the solder. Fourthly,
the surfaces to be united must be kept perfectly clean,
and unoxidised during the application of the heat
necessary for soldering, by the application of some
substance which prevents the access of air, &c., to the
heated surfaces.
These substances' are various, and some are more
adapted to one metal than to another. Thus rosin is
excellent for tinned iron and for copper. Hydrochloric
acid (spirits of salt) is perhaps the best for zinc.
Chloride of zinc (killed spirits of salt), again, is excellent
for iron, for copper, and for brass, where the surfaces can
afterwards be well washed.
6 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
A lump of sal-ammoniac (chloride of ammonia) is also
very useful for removing the oxidation from the copper
tint by rubbing it against the lump for a few seconds after
heating.
4. As an example of the mode of proceeding, let us
suppose we wish to solder a wire to the copper plate of a
Daniell battery. We begin by cleaning the copper plate
at the spot to which we wish to attach the wire, by
rubbing it with a piece of glass or sandpaper until the
surface is as brilliant as a mirror. In like manner we
polish and clean the end of the copper wire. Laying the
copper plate flat on the board which we keep expressly
for soldering on, we place the copper wire on the desired
spot.
We now put the soldering iron into the fire* (it having
been previously tinned as described), and watch it until
it shows by the melting of the solder on the surface
that it is hot enough. We then remove it from the fire,
give a rub on the sal-ammoniac or piece of carpet, then
take up a globule of solder by touching one with the"
cleaned nose of the bit, and, lastly, having quickly
touched the surfaces both of the copper plate and wire
with a feather dipped into the chloride of zinc solution,
rub the surface of the plate and wire simultaneously with
the soldering iron. As soon as the surfaces become
sufficiently heated, the solder will be seen to flow over
them. The wire must then be pressed into its desired
place, the melted solder rubbed well over the point of
Care must be taken that the fire is emitting no sulphurous smoke,
otherwise the mm will surely not take up the solder.
SOLDERING WITH A FLAME. 7
junction, the iron removed, while the wire is held motion-
less in its position, until a sudden dulling of the surface
of the solder shows that it has set and is solid enough to
hold the parts together. After soldering with chloride
of zinc (" killed spirits," " soldering fluid "), always wash
in plenty of water to prevent rusting.
5. SOLDERING WITH A FLAME. In many instances
a better joint and neater-looking work can be made over
the flame of a spirit or other lamp than with the
soldering iron. Thte is more especially the case in
small Work, such as joining wires, soldering pivots, &c
As an example, let us suppose we desire to make a
poised magnetic needle out of two similar pieces of
needle, joining them together by means of a short
tinned-iron junction, in which the pivot is inserted. (It
is evident that a pivot could not well be attached to an
ordinary^ sewing needle were the needle in one
piece.)
The needles (of which two are required) are broken
off of the desired length by means of a pair of nippers.
The heads may be the portions rejected, if a very light
needle is. required ; the points, if a heavier needle be net
objectionable. Care should be taken that the pieces be
of the same weight, to insure a well-balanced needle.
'A small piece of sheet tinned iron (" tin-plate ") about
1 in. square should now be procured and flattened out.
With a screw-drill, or small punch, a clean central hole
is made a trifle smaller than the largest external
diameter of the pivot (See 6 for pivots.) The piece
js now cut into the shape of a small lozenge, as shov/i*
8 ELECTRICAL INSTRUMENT MAKING FOR-AMATEURS,
at Fig. 2, A, and again flattened out by a light blow
with a flat-faced hammer. The needles are now to be
lightly sandpapered at the ends which r are to be
soldered to the lozenge, then these extremities immersed
Fio. a.
in the chloride of zinc soldering fluid. The soldering;
iron is then to be heated, and a little globule of solder
caused to adhere to the prepared extremity of each
needle by its aid. The needles should now present the
SOLDERING . WITH A FLAME. $
appearance shown in Fig. 2, B. The spirit lamp is now
to be brought into requisition. The flame should not
be too high for such work, so that the wick need not be
raised much. Taking up the lozenge-shaped piece in
the left hand, with a pair of tweezers (or longish piece of
wire bent to a tweezer shape), and allowing one half to
project, we apply with a feather a little soldering fluid to
the projecting point The needle , with its adhering
solder is now taken up in the right hand (a small piece
of paper being wrapped round it, to prevent the fingers
being burnt), and both the needle and the lozenge are
held for a few seconds over the flame of the spirit lamp, 1
in the position they are desired to remain (See Fig/ 2, c).
The solder will soon be seen to flow. As soon as this
is the case the pieces must be pressed firmly together, 3
and both hands removed from over the flame of the
lamp, care being taken to keep the parts constantly
pressed together 4in til a sudden dulling shows that the
solder is set. In 'like? manner the other needle is
soldered to the other point of the lozenge. The needle
may then be magnetised, and after magnetisation
balanced on' its pivot, as will be described further on.
Fig. 2,"D, shows the needle ready for pivoting.
6.- PIVOTS. These are* in ! constant requisition;
consequently, it is well to be able to make them of all
sizes. The amateur will need only, two kinds viz,
in brass and in glass. Brass pivots are very easily made
from stout, hard, brass wire. The most useful size is
about Ath in. in diameter. A short length of this is
placed in the jaws of a vice, with Jts_circular section
10 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
uppermost A bit, capable of cutting a i*th hole,
is placed in the screw-drill, and after being oiled, the
drill is worked over the brass rod, the bit resting quite
centrally, until the shoulders of the bit just touch the out-
side of the wire. This produces a good deep and evenly
conical hole. The wire can now be removed from the vice,
and the outside of the pivot also made conical by filing,
while still on the brass rod ; when the desired shape has
>een secured, it cam be cut off with a fret-saw or file.
Glass pivots are extremely useful in all electrical
experiments. The best way to make these is to soften
a glass tube over a spirit lamp, and gradually but
steadily pull it asunder. By so doing, two conical
pieces are the result If the extreme hair-like points of
these are held over the lamp, they fuse up into a round
globule, thus closing up the hole. The pivots thus
produced can easily be cut off (when the glass is cold.) by
making a mark round the tube with a sharp triangular
file, at the point where it is desired to break it off. On
applying a little pressure between the finger and thumb
of both hands, the pivot easily breaks away from the
remainder of the tube. The pivots may be made very
long and thin, by allowing the glass to soften well, before
beginning to pull asunder, and such long thin pivots are
very useful for astatic galvanometers, or other cases in
which two objects have to be poised on one pivot Short
pivots can be made by pulling asunder as soon as the
glass softens in the flame. Fig. 2, C, shows how the glass
tube should be held in the hands to soften, over the flame.
Fig. 2, E, gives the appearance of the tube after pulling
CLASS THREADS. SI
asunder. Fig. 2, F, indicates how the hair-like extremity
should be melted into a bead over the lamp, so as to close
the capillary aperture, and Fig. 2, G, shows finished pivots.
7. GLASS THREAD^ These are extremely useful
as insulators in small work, such as electroscopes,
carriers, torsion balances, &c. They are easily made
from rather thin glass rod, by heating it carefully over a
clear flame (spirit-lamp, or Bunsen burner) until soft, and
then pulling asunder rapidly if a very fine thread be
desired ; more slowly, if a coarser filament be required.
Similar threads may be made from shellac ; and these,
though somewhat more brittle, are even better insulators.
8. STRAWS. The straws of various grasses, more
especially the fine, straight, hair-like terminals of such
grasses as Agrostis spicavznti t Alopecnrus pratensis,
Phalaris arundinacca, and Aira cristata, if gathered soon
after the flowers are fully matured, cut to the length of
about 6 inches, dried, and then boiled in melted paraffin
wax, make excellent insulating supports, far stronger than
the glass or shellac ones mentioned above. The same
straws, not paraffined, are, from their lightness, well
adapted as pointers for such galvanometers as tangents
and others, which require a separate indicator.
9. PlTH-BALLS. These are much used in experi-
ments with frictional electricity. The best piths for
ordinary purposes are those of the elder, Sambucus nigra.
and of the Jerusalem artichoke, Helianthus tuberosus t
Before being shaped into balls or other figures, the pith
should be thoroughly, dried. With a penknife, reduced
to a razor-like sharpness, the pith can be cut to any
12 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
desired figure, and is easily rounded to an approximate
sphere. When a number of equal size have been made,
the final rounding may be given by lightly rolling them
with a smooth, flat board, on a level table. Care must
be taken not to press too heavily, otherwise the balls
will be flattened. Having now got together the few
things necessary for starting work, we may try our con-
structive abilities on the simpler forms of electroscopes.
Electroscopes are instruments employed for the
detection of the presence (and sometimes of the nature)
of electricity ; not for its measurement
$ 10. PITH-BALL ELECTROSCOPES. These are of two
FJG. 3.
kinds viz., those in which the balls are suspended by a
filament, and those in which the suspension consists of
a horizontal rod, pivoted at the centre. Fig. 3, A, shows,
the former ; B, the latter form.
PITH-BALL ELECTROSCOPES. 13
To make the former, a glass rod about Sin. long by
Jin. thick, should have one extremity heated over a
spirit lamp, and when quite soft, touched with another
piece, so as to enable the operator to draw out a filament,
which must be bent into the shape of a recurved hook.
The rod must then be heated at a point about 2in. from
this end, and, when sufficiently softened, bent neatly
(not too hurriedly) at right angles. The bent rod must
be allowed to cool gradually without touching anything,
otherwise it will be liable to fly to pieces. When quite
cold it may be fastened to the foot of a broken wine-
glass by means of a turn or two of stout brown paper,
previously brushed over with good Russian glue, as
shown at C. When this is quite dry it should receive (at
this joint only) a coating of red varnish, made by mixing
a few grains of red lead with a teaspoonful of white
hard varnish. (The white hard varnish may be pro-
cured at any oilman's.) A single filament of cocoon silk
is now to be procured, and a fine needle threaded with
it The needle is passed through the centre of a pith-
ball, the end of silk fibre moistened with a drop of glue,
and the needle pulled until the ball reaches the glued
portion of the silk. This will serve to fasten it to the
b ilL The other end of the fibre may be tied or glued to
the little hook of the bent glass arm. Two balls may be
thus suspended, if it is required to show the repulsive
effects of similarly charged bodies. To construct the hori-
zontally-pivoted form, it is only necessary to split a
straight paraffined straw ( 8) at its centre, with a sharp
penknife, insert a small glass pivot in the split, and
14 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
fasten it thereto by means of a single drop of hot glue.
When quite cold, a pith-ball must be attached to each
end of the straw, a small hole being made in each ball
with a pin, and the end of the straw (previously touched
with glue) inserted in the hole* Care must be taken
at this point that the balls balance one another. They
may be made to do this by sliding alo'ng the straw, until
when placed on a needle-point the arms of the electro-
scope remain perfectly level. Half of an ordinary sewing-
cotton reel maybe used as the foot of this electroscope.
After sawing in half, the upper portion should be filed,
rounded, and smoothed, a short length (say4in.) of cane,
glued, and thrust in the central hole ; a needle (point up-
wards) forced into the upper extremity of the cane ;
and, lastly, the whole wooden portion neatly varnished
with the red varnish as described.
ii. GOLD LEAF ELECTROSCOPE. This is a most
useful instrument for the detection of minute charges of
electricity. If well made, it also serves admirably to
show the phenomena of induction. The requisites are
a tall wide glass jar, a sheet of gold leaf, a couple of
strips of tinfoil, a short piece of brass rod, about
i-inch in diameter, a couple of beef tin bottoms, or
similar pieces of tinned iron, and an empty cigar
box.
For the glass jar, we may either use the straight glass
chimneys (about 3 inches in diameter by 7 inches in
height), that are used for large gas or paraffin burners,
and which may be obtained at any respectable iron-
mongers for a few pence (see Fig. 4, A), or the bell-
GOLD LEAF ELECTROSCOPE. jj
shaped chimneys (see Fig. 4, B). Whichever shape be
selected, the diameter should not be less than 3 inches
FIG. 4.
nor more than 4 inches. Preference should be given to
the bell form, when procurable^ as the " cap " is more
tf ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
easily fitted to the narrow neck. Having procured a
glass jar, the next step is to fit a cover to both extremi-
ties. If Fig. R be chosen, the wider extremity which Is
'destined to form the bottom, should be fitted with* a
tinned-iron cover. If the bottom of the glass be
3 inches diameter, it will be fitted exactly by the bottom
of a Swiss milk tin. The bottom must not fit too tightly,
if fitted in winter, or else the slightest increase in tem-
perature will expand the glass and cause it to crack. If
a tin " bottom " is not at hand to fit, it may readily be
made, by striking out on a piece of thin tinned-iron, a
circle of the same size as the outside of the glass jar.
Keeping about -inch outside this line, the circle is cut
out, and then the edge turned up square by hammering
lightly on the edge of an ordinary iron. The tin disc
should now show ^he appearance sketched at Fig. 4, C.
This bottom must not be fastened in until after the gold
leaves have been adjusted. The next operation consists
in making the cover to fit the other end of the jar or
cylinder. If A has been chosen, two circular discs
must be cut out of the cigar box wood ; one that will
just drop into the jar, the other precisely the diameter
of the outside of the^ar. The two circles had better be
measured, struck out with the compasses, and sawn with
a fret or band-saw. The two discs,, after being well
smoothed, should be glued together, as shown in Fig.
4, D. If B is our choice, half a sewing-machine cotton
reel will Be found to. fit with a trifling adjustment.'
All that will be necessary in this case -will be to cut of?
the narrow portion, leaving one of the coned heads, the
COLD LEAF ELECTROSCOPE. 17
rim of whicli can' be trimmed with a rasp, until it just
fits the small mouth of the chimney. Whichever form
be adopted, this wooden cover must have in it two per-
forations, one central, about J inch in diameter, wherein
will pass the brass rod from which depend the gold
leaves ; the other near the circumference, which will
serve as an exit for moist air. Fig. 4, E shows the glass
chimney fitted with its lower " tray " and upper " cap."
At exactly opposite diameters of the glass jar, two
strips of tinfoil, about | inch wide by 4 inches long, are -to
be pasted itisidt the jar, reaching from a little above
the middle downwards to the metal. tray. The object of
these is twofold ; they increase the sensibility of the in-
strument by heightening the apparent repulsive effect of
a charge ; and they serve to discharge the gold leaves
without tearing, if by inadvertence an overcharge be
given. This being done, the brass rod may be fitted to
its table. Maruy operators use a brass ball at the top of
the gold leaf electroscope ; for general purposes, a
metallic table will be found more sensitive and more
convenient. Having cut a tinned-iron or thin brass disc
3^ inches in diameter, let the edges be turned in by ham-
mering. To the centre of this metal disc, and perpendi-
cular to it, should be soldered about 6 inches of brass
rod, having a small hole drilled in it at half its length
say iinch in diameter. Care must be taken that
this rod is quite straight and smooth. The under por-
tion of the disc, as well as the rod to within \ inch of
its extremity, should now be varnished with red varnish
(see io),and set aside to dry. While this is drying,
iS ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
the upper wooden cap of the cylinder may be immersed
in melted paraffin wax, and allowed to remain therein
until thoroughly permeated with paraffin. The -brass
rod may now be passed into the central orifice of the
"cap," and tightly fitted thereto by means of a roll or
two of paraffined brown paper. (N.B. When paraffin
is mentioned in this work, paraffin wax is understood,
not paraffin oil.} The rod must be pushed down in the
orifice until the J-inch hole is just level with the top of
the cap. A short brad, or similar piece of wire, pushed
forcibly into this hole, will effectually prevent the rod
sinking through, if. any weight be afterwards placed on
the table above. A small, flat .piece t of brass, about
\ inch long by inch square section, must now be
soldered to the lower end of the rod, transversely to it,
so as to form a letter JL- It is to this transverse piece
that the gold leaves are to be attached. The gold leaves
should be about ineh wide by about 2 inches long.
To cut them neatly is not an 'easy job for the tyro ; still,
it may be managed in the following manner : >
Having procured a clean half-sheet of note paper, let
it be folded down the middle. This is to be placed open
close by the side of the book containing the gold leaf.
Holding his breath for a! few seconds, so as not to blow
away the leaf, the operator slides the edge of the note
paper under one of the leaves of gold, and assisting
operations with a pin point, or perfectly clean camel-
hair pencil, coaxes the gold leaf so that it lies flat and
square in the centre of the note paper. He then covers
it over with the bent half of the note paper, so that the
GOLD LEAF ELECTROSCOPE. ig
gold leaf lies between the two leaves of paper, forming the
half-sheet of note. Then, with a pair of perfectly clean
sctesors, he cuts the paper into strips, about \ inch wide
by 2 inches long. In so doing, he must take care not
to let the papers separate, otherwise his gold leaves will
get crumpled, or blow about. Of course, in cutting the
paper he cuts the gold leaf into the desired size. He
then touches the two sides of the "transverse piece of brass
(attached to the rod passing through the cap of the
electroscope) with the merest trace of gum, then care-
fully lifting the upper pieces of paper off the strips of
gold leaf, takes up, first on one side, and then on the
other (of the transverse piece) a strip of gold leaf, being-
careful that they hang straight down and parallel
Having succeeded in getting the two strips to hang
p auarely and free from one another, the operator next
inserts *hem very cautiously into the chimney. The
cap may now be cemented down on to the neck of the
chimney by means of a piece of silk ribbon moistened
with thin glue. When this is quite dry, and after the
bottom has been likewise cemented to the lower end of
the chimney or cylinder, a coating of red varnish is
applied, care being taken not to stop up the side air-
hole. This air-hole should be fitted with a small wooden
plug, furnished with a rounded knob, likewise varnished.
A small quantity of Prout's elastic glue, run round the
edge of the warmed tin, will be found the most effectual
way of fastening the bottom to the lower portion of the
glass chimney or cylinder. Care must be^ taken that the
two tinfoil strips are opposite the width of the gold leaves.
20 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
and that the said strips make metallic contact with the
tin bottom. If all has been carefully executed, the elec-
troscope will present the appearance shown at Fig. F,
and will be so delicate as to give a large divergence of
its leaves, if a rubbed rod of sealing-wax is held at a
distance of one foot from the " table," or upper plate.
12. COULOMB'S TORSION BALANCE. Apart from
the actual use of this instrument as an accurate measurer
of electric and magnetic force, it is extremely service-
able in calculating the laws of electrical attraction, and
repulsion, viz., that these are "inversely as the squares,
of the distance, and directly as the charges."
To make such an instrument, we select a' glass chimney
similar to that shown in our last section; Fig. 4, A.
This must be fitted with a .metal top, precisely like that
described for the bottom 'cover of the electroscope,
Fig/4, C.'^In the centre of this a small aper-
ture is made to admit of the introduction of about
I inch of thin brass tube, having about inch bore. This
is soldered neatly into the.c"over, so that the tube pro-
jects about \ inch on either ^ide of the cover. A block
of mahogany -or deal, about 4 inches square by inch
thick, is now planed up and made truly square. A cjr-
ular channel, about \ inch wide, and of the same depth,
is now cut in this base board for the glass to stand in.
If deal, this base should be stained black and varnished ;
if mahogany it should be polished.
\ 13. An efficient black stain for such pieces of white
wood may be made by working upon a slab a teaspoonful
Df lamp-black, with a tablespoonful of thin glue, until
COULOMB'S TORSION BALANCE.
quite smooth, with a muller. This stain may be applied
while warm to the wood, well rubbed in, and when quite
dry varnished with " white hard varnish," which will be
dry in about twelve hours.
14. A circular card, graduated to the 360* of a circle,
and of the same diameter as the interior of the glass
chimney, is now to be glued on to the central circle of
the base board ; the zero point coinciding with the
centre of one of the flat edges, not with the diagonal
corners. Previous to gluing down the card, a slot about
| in. deep, and \ in. wide, must be cut in the upper surface
of the base board, reaching from the position of the zero
point to the extreme edge of the board. (This serves for
the introduction of a soft iron rod, or of different
magnets.) The next step is to make a similar but
rather smaller graduated circle to slip over the pro-
jecting brass tube in the' upper cover. Tl^is circle
should.be cut out of a sheet of tinned-iron or zinc, and
the degrees of arc (of which there should also be 360)
marked clearly upon it by scratching deeply with a
sharp steel point. In the centre of this circle, a hole,
just sufficiently large to admit the passage of the piece
"of brass tube, should be punched. This circle is to be
slipped over the tube and lie flat on the cover, but must
not be fastened (Town. A short brass rod, about 2\ in. long,
and just thick enough to enter- freely into the brass tube,
is now procured, and fitted with a circular brass head
made by filing up a piece of in. sheet brass into \ in.
in diameter, and soldering the rod in the centre of the
circle. Or, the pinion of an old paraffin lamp burner.
22 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
with the teeth knocked off, may be used instead of this
rod, provided it fits the tube. Whichever is used, a
short piece of brass wire must be soldered at the upper
extremity of this brass rod, near the head.
This index serves two purposes : First, it prevents the
rod going too deeply into the tube ; secondly, it serves
to point out the amount of torsion, or twist, given to the
wire or fibre which supports the " stirrup " in which
is placed the magnetic needle or insulating rod, which
is used in magnetic or electric measurements. The
lower end of the rod must project just below the lower
end of the tube which passes through the cover, and to
this lower end of the rod must be soldered, a short length
of No. 40 German silver v/ire. To the other end of this
German silver wire must be attached, by soldering, a
" stirrup," in the form of a wide J. The length of the
wire, inclusive of the stirrup, must be such that it just
.swings clear of the lower graduated circle when a mag-
netic needle or other rod is placed across the stirrup, and
the cover is on the top of the glass cylinder. A well-
magnetisea needle, a little shorter than the diameter of
the cylinder, completes the instrument if it is to be used
as a magnetic measurer. The needle may be a piece of
a good steel knitting needle, carefully magnetised to
saturation. If required for electrical experiments, the
magnetic needle must be replaced by a light shellac rod,
carrying at one extremity a small disc of thin sheet brass
or copper. In this latter case also, the channel in the
base board must be fitted with a bent brass wire shaped
like the letter L, furnished with a small brass ball at-
COULOMB'S TORSION BALANCE.
each extremity. This rod must be carefully insulated by
being thickly coated v/ith good red sealing wax to the
depth of at least | in. all round, except at the two
extremities where the balls are^ situate.
Fig. 5 illustrates the different parts, and the complete
instrument. A is the base board, with the slot and
graduated circle attached ; B is the tin cover, with its
B
Fir.. 5.
trass tube, the whole being cemented to the top .of the
glass cylinder when once the wire has been soldered to
the milled head ; C is the upper graduated disc ; D the
rod carrying the index, and actuated by the brass head
to this rod is attached the wire and stirrup E ; F shows^
the instrument put together, and lettered homologously.
.15." VOLTA'S ELECTROPHORUS. This is essentially
the 11 amateur's electrical machine. It is at once the
simplest to construct, the most efficient in action, and
24 ELECTRICAL INSTRUMENT MAKING FOR AMATEUR'S.
its theory is most interesting as giving 'the key to the
continuous-action electrophori of the present day. I
allude to the Holtz, to the Bertsch, to the Carrd, and to
the Wimshurst machine.-
1 6. To make an electrophorus will be needed a
sheet of zinc, from which are cut two discs ; one being
,about 2ft. in diameter, the other about 4 in. less. The
zinc should not be more than A in., nor less than sV in. in
thickness. The outer edge of bxr , the discs must be
turned up by careful hammering* th a mallet, and a
round, soft, iron rod, a \ in. thick, mus be run round the
edge of each disc, and covered over with the upturned
edge of the disc. This operation requires a considerable
amount of care and patience to effect it neatly. It is
absolutely essential to the efficiency of the machine that
the edges should be perfectly round, without any sharp
angles; and this end can only
be attained by using a steel
tool, in shape something like
a solid punch, but having a
semi-circujar concavity at its
lower extremity (see Fig.'
6, A). By turning the edge
of the zinc over the rod by
the aid of this tool and a
hammer, a smooth border can
easily be produced. Should,
however, the amateur find
any difficulty in performing this operation, it would
be advisable for him to have recourse to the nearest
FIG. 6 A.
. VOLTAS ELECTROPHORUS. 25
tinman, who will execute the necessary sleight of hand
for a few pence. If well done the two discs should
present the appearance shown at Fig. 6, B and C. It
will be noticed that from the ce tre of the smaller disc
projects a short piece of tube. This is made of a short
length of in. brass or zinc tubing about I in. in length,
FIG. 6 B, c, D.
which is soldered centrally on the smaller disc. It tS
into this tube that the insulating handle is to be inserted,
so that the amateur may vary somewhat the diameter
of this tube to suit that of his handle ; but it should not
greatly'exceed \ in. in diameter, nor be less than a \ in.
Near one edge of the smaller disc should be soldered a
a6 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
short length of \ in. brass wire say, about 3 in. in length
bent into the form of a rounded L , and bearing at its
extremity a brass ball about I in. in diameter. (Such
balls may be obtained from most ironmongers at about
sixpence each.) If not easily procurable, it may be
replaced by a leaden bullet cast on to the end of the wire
(see Fig. 6, B). A handle of some insulating material
must now be provided. ' In shop-bought instruments
glass handles are generally seen; but these are not
nearly so efficient as ebonite, though very much more
beautiful. At most chemists ebonite stirring-rods, about
9 in. long .and somewhat over a \ in. thick, may .be
obtained for twopence or threepence each. One of
these will serve our purpose admirably ; and its round
end should be cemented into the central tube by the aid
of a little guttapercha or Prout's elastic glue, applied hot
to the end of the rod, and the extremity of the tube
slightly pinched round it to insure its not slipping out.
The larger disc must now be filled with some insulation
composition. Of these there are several ; the best is
perhaps ebonite; but it is expensive, being about 6s. 6d.
a pound ; however, it can be obtained in very thin
sheets, and this, of course, w lessens the expense. If
ebonite is used, a disc should be cut that will just fit
inato the larger zinc disc, and be fastened down to it
nicely 'and smoothly by means of Prout's elastic glue,
aided by equal pressure with a warm (not too hot) iron.
Another very good material is gun-paper. Gun- paper,.
or P<*Pyro*yline, is paper which has been immersed for a
few seconds in a mixture of nitric and sulphuric acids,
VOLTAS ELECTROPHORUS. VJ
and then washed in an abundance of water. A disc of
this papyroxyline, cut to reach quite to the bead of the
larger zinc disc, may be cemented to the said disc by
means. of any spirit varnish (say, white hard) and when
quite dry> the surface also varnished with the same, to
prevent the paper absorbing moisture, which it would
otherwise be apt to do. If papyroxyline is used, it
should be made from stout millboard of at least ^in.
thick, and \ will be better still. This material is prac-
tically inde'structible, and does not deteriorate, as does
the ebonite, by the oxidation of the surface, or as the
usual shellac composition, by splitting up from the
zinc.
The last and most usual composition for the larger
disc is a mixture of Venice turpentine, wax, and shellac.
To make it of even consistency, it is necessary to pro-
ceed as. follows : ^
Take best shellac .3 parts.
^fellow beeswax 2 parts.
Venice turpentine ........................ I part.
Place the shellac in an earthen pipkin, and stand it over
a gentle fire. Watch until melted. When melted add
the beeswax. When well mixed pour in the Venice
turpentine. Again stir, and when mixed, pour into the
large zinc disc, which must have previcusly been warmed
to nearly the same temperature and placed on a
perfectly level table. Should any difficulty be experienced
in getting the mixture to flow evenly over the disc, it
may bs accisted by ironing over with a tolerably hot
iron. Should this mixture be preferred to the ebonite,
28 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
or papyroxyline disc, then care must be taken to have a
sufficiency to fill the zinc disc up level with the wfre
beading, because if the coating of resinous mixture be
left too th'in, it will all split away on being beaten during
excitation. Fig. 6, D, shows the electropEorus in its
finished form, and in position ready for use.
To put in action, it is only necessary to Remove the
covering disc a\ by its handle b', and to excite the
resinoys cake (ebonite or gun-paper) by beating it with
about 'half a yard of warm, dry flannel, or better still, a
cat's skin. Then if the cover be placed on the centre
of the cake, the tipper disc touched with the finger, and
then rapidly raised to a height of 6 or 8 in. by means of
the insulating Candle b', care being taken not to let any
part- of the clothes or, body touch the disc, a fine 2 in.
spark may be drawn from the knob by approaching the
knuckle or another brass ball to it rapidly. Sparks may
be obtained almost indefinitely by again lowering the
dis'cs, touching, and again raising. I strongly recom-
mend every student to make an electrophorus, and not
to rest satisfied until he has Jully mastered the theory
of its action. When he understands this, he will have
got pretty deeply into the theory of induction, and will
be prepared to grasp the theory ;of the Wimshurst
machine a machine which will probably play a very
important part in future applications of electricity to
the arts.
17. BERT'SCH'S MACHINE, Though this is not the
best form of continuous-action electrophorus, yet as its
construction is extremely simple, and leads to a thorough
MOUNTING A GLASS PLATE. s$
comprehension of the principles on which the more
efficient forms are dependent, it will be advisable for
the student to undertake it. The Bertsch machine may
be made with the rotating plate either of glass or ebonite.
Both forms will be described, beginning with the glass
plate form, as the mode of mounting a .glass plate on a
spindle is generally regarded as a " poser " by amateurs.
1 8. MOUNTING A GLASS PLATE. There are two
modes open to the amateur viz., by drilling a hole
through the glass of sufficient size to take the spindle,
and screwing or cementing two cheeks against the glass
plate ; or by cementing two cheeks, one on either side of
the glass plate, exactly central, and opposite one another,
these cheeks having the. spindle (of which there are two
halves) inserted at their centres.
19. To drill a glass plate requires more patience
than skill, though both are needful. In the first place
the glass must be cut to an exact circle, and its centre
marked.^
To do this the amateur should procure a stout piece
of brown paper, and with the compasses strike out a
circle of the size of which he intends his glass plate to
be. (A very convenient size for most electrical machines
is 18 in. in diameter.)
Keeping this marked paper as a template, our student
cuts out a similar circle in rather stout millboard. He
then places his glass plate upon a perfectly flat table,
with the millboard disc over it. A small dab of hot
Prout's cement applied to the glass, will hold the mill-
board to the glass so that it shall not move during the
30 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
cutting of the circle. The operator now traces with a
diamond or glass-cutter a circle all round the millboard
guide, being careful not to press so heavily as to split
the glass, or so lightly as not to cut To a practised ear
the peculiar whistle tells when the glass is being cut,
and when only scratched. The circle being thus traced
on the glass with/ the diamond, the millboard guide is
removed, and the corners of the glass cut so as to
permit the curved pieces being pulled asunder. The
four diagonal cuts must, of course, come quite close to
the periphery of the circle cut on the glass. A cursory
examination of Fig. 7 will render this clear. A repre-
FIG. 7.
sents the circle first traced on the glass ; B the four
diagonal cuts, which enables the quadrantal segments C
to be detached. To insure the glass breaking where it
is cut, and nowhere else, a little care is necessary..
Firstly, the operator must see -that the glass is really
cut, and not simply scratched. To know this, when he
has traced his line or curve with the diamond or cutter,
he must turn the glass ove.r and examine the line on
the other side. If it looks like a mere white line, not
penetrating below the surface, it is but a scratch. If,
on the contrary, it shows as a glistening crack right
through, it is a true cut. By gently tapping with the
MOUNTING A GLASS PLATE. 3
cutter on the wrong side of the glass a scratch may
frequently be converted into a ait, but it is necessary to
raise the glass from the table "at one extremity while
tapping, and to tap on the spot only where it is required
to decide the fracture. When the line shows as a
glistening fracture in its entire length, then, by holding
the glass plate between the finger and thumb of the left
hand, with the "cut 'side uppermost, and the cut close to
the thumb, the operator will find no difficulty in detach-
ing the diagonals,^ and afterwards the quadrantal seg-
ments, by a steady downward pressure of the right
hand, applied to the other side *of the cut line. Should
any pieces refuse to Jareak* quite truly, they may be
" nibbled " t off Uy means of ., the wards of a key, or the
slots in the glass-cutter. When the circle has been cut
to'satisfaction, the edges should be smoothed by grind-
ing ; but as this is best done when the disc is mounted
on its spindle, the directions as to the mode of grinding
are deferred until the method of mounting has been
described. The glass disc must now be placed on drill-
ing-table, constructed specially for this purpose. It con-
sists, as shown in Fig. 8, in a flat board, A, about I in.
thick, 20 in. long,.nd about I2in. wide. From each
extremity of this board rises a standard B B' about 3 in.
wide by in. thick, and 10 in. or 12 in. high. There are
two cross pieces C C', joining these upright standards
together, one at the top, and one withiri an inch of the
bottom. These cross pieces have each a circular hole
perforated through the centre, of a trifle over the
diameter of the desired hole in the* glass (generally $ in.
'32 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
in diameter). Through these holes passes a copper tube,
D, \ in. in diameter and 14 in. long. Around the upper
extremity of this copper tube is cast a "aeavy flange of
lead, E, weighing, say, at least 2 Ib. Before using this tool,
the lower extremity of the tube must be " upset " id.
est t made irregular and broadened a little by hammer-
ing on its edges. This must be done to prevent* the
glass being split. To work this drill, the glass being
fastened in its place by means of three corks and screws
with its marked centre just under the centre of the
FIG. 8.
copper} tube, a small quantity of ordinary emery ig
poured into tne top of the tube, to which are added a feV
drops of oil of turpentin6 as a lubricator. A gut-band,
F, is now passed once around the tube, and fastened at
each end to a tolerably stiff bow, G. Moving the bow
backwards and forwards communicates a reciprocating
rotary motion to the drill, and this, aided by plenty of
patience _ and a little emery and turpentine, will cut a
clean round hole in the glass. This having been
effected, the next step . is to cement the spindle in its
MOUNTING A GLASS PLATE. 33
place. The spindle may be from in. to in. in diameter
and should be- furnished with wooden cheeks or washers
(consisting of cotton reels cut in half), which fit them
pretty tightly. One of the cheeks should be glued to
the spindle (nearly at- its centre) with Kay's coaguline,
or similar acetic glue.* " When quite dry and set firm'
the surface of the cheek- should be ' painted over with
"bicycle tire cement," and the glass plate immediately
slipped, over the spindle into its place. * The other cheelc
should now be treated m^the same manner; that is to
Fio.,9.
say, treatecTwith " bicycle cement ".on its face, and the
portion 'of the spindle near the glass plate -thickly
brushed over with acetic glue. The upper cheek must
now be~slipped^on to the spindle and pressed tightly
down upon the glass, the whole being left undisturbed
for some hours until quite set and firm. Fig. 9 will,
* This useful cement is made by soaking good glue in cold water until!
quite soft, pouring away the water and adding sufficient glacial acetic
acid to cover the glue. A slight heat will render the Whole liquid,
when .it should be poured into a bottle kept corked for use.
D
34 ELECTRICAL INSTR UMENT MAKING FOR AMA TEURS.\
illustrate the appearance of the disc' when the spindle,
and lower cheek have been affixed. A is the glass plate,
B the lower spindle, C the lower cheek, cemented, D the
upper cheek, ready to be pressed down. The length of
the spindle and the nature of its attachments will depend
on the use to which the plate will hereafter be put. In
the present instance, the glass plate being 18 in. diameter,
the spindle should be (a steel rod) at least i$in. long,
and should have a shoulder turned down at each end to
the length of about i in.
When the spindle and glass plate are thoroughly
cemented together, a small wooden pulley about i in.
diameter, similar to those used for roller blinds, must be
cemented close to one extremity of the spindle, just
before the shoulder. This serves to take the band or
gut, by means of which motion is dommunicated to the
glass" disc.
20. MOUNTING A GLASS PLATE WITHOUT DRILLING.
This method is much simpler than the former, and is
in motet cases preferable. It is certainly superior, in the
case of such machines as the Bertsch, the Carr<5, the
Holtz, the Wimshurst, and, indeed, all " induction "
machines ; and it is quite equal for frictional machines,
except when the plates are very large, say, over 2 ft. in
diameter. To mount a plate in this manner all ivhat is
necessary is to place the plate on the marked paper
which has been kept as a template (19), so as to be
able to find the exact centre. Over this must be pasted
(with bichromated paste) circlets of brown paper, one
on each side of the glass, of the same size as the
MOUNTING A GLASS PLATE WITHOUT DRILLING. 3$
wooden cheeks (made, as beforesaid, from a cotton reel
sawn in half). The bichromated paste is made as
follows : Flour, two teaspoonfuls ; water, four ounces ;
bichromate of potash, five grains. The flour must be
rubbed up to a smooth batter with the water, then placed
in a small saucepan over a source of heat, and kept stirred
until it "boils. The bichromate of potash, in powder,
is placed in a jam pot, and the boiling paste poured!
upon it, with constant stirring. This paste must be
kept in the dark. The brown paper used foi the
circlets should be of a good stiff fibrous texture, such as
is used for packing heavy goods, and should be well
soaked in the paste, previous to placing on the glass
disc. When the circlets have been fastened on, the
glass disc, with its paper circlets, must be exposed to
good sunlight for an hour or two. This sunning sets up
a chemical change in the bichromate, and renders the'
paste insoluble, so that it does not easily detach from
the glass plate. When quite dry, the wooden cheeks
(with the spindles) are to be glued to the paper circlets,
and the glue must contain a few grains of bichromate of
potash, so as to insure that it shall not be affected by
damp.
21. As there is no hole through the glass disc, the
amateur may find it rather difficult to get the spind!e
(which is now in two halves) to be exactly in a straight
line. The two halves are shown at Fig. 10, where a <t
are the two half reels, to which have been attacjhed with
acetic glue two steel rods, b b', about 7^ in. long. To
ensure these being perfectly opposite one another on
3b ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
the glass disc, it is only necessary to glue one on, and
let it dry perfectly; then, having placed the disc with
the spindle downwards on a block of wood, in which a
hole has been drilled of the same diameter as the
spindle (as shown at Fig. 10, <:), to glue the other on as
nearly central as one can judge by eye. On causing
the disc to rotate on the lower spindle as on a pivot, it
is easily seen whether the upper spindle is in a line
THE BER7SCH MACHINE. 37
with.it or not,' as it will in this .lattef case describe
a circle ' instead ' of remaining apparently motionless.
While the glue is still warm, any necessary correction in
position can be made. Both spindles being now glued
on, the disc should be allowed to dry if possible in the
sunshine ; and then the wooden cheeks neatly varnished
(especially round the junction between the glass and
the woo'd) with a varnish made by mixing good white
hard varnish with Chinese red.
22. The disc being now mounted in one "or oll^r
mode, we can proceed to fix it in the standards' on
which it is intended to rotate. To this end, a base
board, about 18 in. long by 12 in. wide, and about I in.
thick, is planed -up, and two uprights, about 16 in. in
height and I in. square, let into the sides and screwed
thereto, as shown at Fig. 10, d d. With a rat-tail file,
two semi-circular grooves are cut in the top of these
standards,- exactly opposite one another, in which thet
spindles of the disc must run easily. The glass disc
having been placed with, its spindle resting in these
grooves as bearings, ' two caps are fashioned out of
square pieces of wood to fit over the upright standards,
and are screwed in their places by means of two screws
The use of these, caps (of which one is figured at e) is to
prevent the spindle rising out of the groove during
rotation. The standards d and d' must be such a dis-
tance apart that the .ends of the spindle that have been
turned down (19, last paragraph but one) rest in the
grooves just up to the shoulder. : "
23. It will be remembered that it was recommended
38 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
to attach a small wooden pulley to one of the steel rods
( 19). It will now be necessary to place a driving-
wheel below this, which can be turned by means of a
projecting handle, and communicate its motion to the
small pulley by the aid of a gut. This driving-wheel
should be of wood, about 6 in. in diameter, \ in.
thick, and have a groove cut in its circumference
about -J in. deep.^ If our amateur has a lathe, this is
an easy matter ; if not, he can cut the wheel out of a
l-irch deal board with a handsaw, having first struck
a 6 in. circle on it with the compasses, as a guide in
sawing it. The bevel in the edge can be cut with a
good triangular file. This must be keyed to an iron
rod, about \ in. thick by 4 in. long, as shown at f t
Fig. 10, where it will be observed that the rod or
spindle projects more at the one extremity than at the
other. The longer extremity passes through a hole
in the standard d 1 , made \at such a height as to allow
this driving-wheel just to clear the base board. A
third standard, of the shape of a letter J_> is cut out ol
J-inch stuff planed up, reaching, when the head of the
is on the base board, to the same height as the other
two, g, Fig. IO. A hole is drilled through this third
standard to admit of the passage of the short end ol
the driving-wheel spindle ; and a channel (similar to
those in the other two standards) is filed in the top.
The driving-wheel having been put in its place, this
third standard is placed against it, leaving sufficient
room for it to rotate freely. The standard is then
fastened down to the base board by means of a screw
THE BERTSCH MACHINE. 39
in each branch of the J.- The glass plate is now put
in its place, the spindle resting in the grooves on 'the
tops of the standards. The caps e are then screwed on. A
gut band is attached so as to pass round the driving- whee?.
and the pulley fastened to the spindle of the qlass disc.
A small metal, or, better, wooden handle is screwed to
the projecting spindle of the driving-wheel (Fig. 11, H).
FIG. n.
24. A piece of sheet ebonite about 7 in. long by
4 in. wide is cut rounded at the top, and fastened at
the bottom fo. one side of a little wooden stand, which
can easily be effected by the aid of a couple of flat-
headed screws, the heads of w'.iich must be carefully
covered over with Prciut's clastic glue (Fig. n, v).
40 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
This littje piece of ebonite (technically known as the
"sector'*) and its stand must be attached to the base
board, parallel with and close to the glass disc, without
actually touching it As it is necessary to remove it
for the purpose of excitation, it must not be per-
manently fastened to the base board, but only held
there by means of a small thumb-screw. Opposite this
sector, but on the other side of the glass plate, is a
" comb," made by soldering half a dozen brass brads
to a stout brass rod about 4 in. long. A precisely
similar cornb faces the plate at its upper diameter
(see Fig. 1 1, c' and C). These combs are soldered by
their middles to stout brass rods R R'. The lower one
passes through the top of the ebonite rod E, which must
be solid, and forms a prolongation of the cap of the
standard s'. The lower rod R' terminates in a bras?
ball B : the upper one in a short piece of split brass
tube D, through which passes with some friction another
brass red r, to 'which is affixed at one end an ebonite
handle M, and at the other a second brass ball B'. The
split tube D allows this rod r to be approached to or
withdrawn from the ball B. It is usual, in order to
increase the effects, to hang a Leyden jar (or similar
condenser) between R and R', the inner coating being
in contact with the one, and the outer with the other,
of these rods. Contact with earth can also be made at
will by hanging a chain to the rod near the ball B, and
allowing it to touch the table, D'.
25. GARRETS DIELECTRJCAL MACHINE. The
principle of this machine is precisely similar to that of
CARK&S DIELECTRICAL MACHINE.- 41
the Bertsch, but it possesses the great advantage of
being less affected by the state of the atmosphere. 1
This is owing to the employment of a continuously
excited plate of glass t>r ebonite, instead of the "sector "
( 24) used in the Bertsch machine. With two plates,
respectively 15 in. and 19 in. in diameter, sparks from
6 in. to 7 in. in length may be obtained without a con-
denser'; and if a pair of Leyden jars be added, as in
most modern induction machines, this length may_- be
considerably exceeded.
26. With the knowledge acquired in mounting glass
plates on spindles ( 20) the amateur will find no difficulty
in constructing an effective Carr^ machine. The plates
may be either both ebonite, or both glass ; or one ebonite
and the other glass. Personally, I prefer both glass, pro-
vided the glass be of the required non-conducting quality.
Glass varies very much in its insulating power, and this
can only be ascertained by actual trial. Fortunately,'
this is not a difficult matter. It is only needful to dry
the sheet of glass cautiously before a moderate fire ; rub
it briskly with a silk handkerchief (previously dried) so
as to electrify the surface. If with an electrophorus
shield of about 12 in. in diameter ( 16), a spark of about
2 in. in length can be obtained, the glass may be con-
sidered sufficiently insulating for our purpose; if much
less, it had bettei be rejected. The suitable kind of glass
havjng been selected, it is cut, precisely as previously!
directed ( 19), into two discs, one 15 in., the other loin.i
in diameter. These two discs should be mounted in:
either of the modes previously Described ( 19, 20) ow
43 '\ ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
spindles, projecting 6 in. on each side of the discs,
These spindles should be made of J- in. circular iron rod,
inserted into the half-reels ; and afterwards the rods
should be carefully covered with a casing of thick brov/n
paper, which has previously been soaked' in melted
paraffin wax. This casing of paraffin paper must bs
neatly glued roun4 the rods, so as to form a smooth
surface, and when the whole is dry should receive a
coating of the red varnish mentioned at 10. The next
step is to cut out the base board from a piece of inch
stuff, which may be either deal or mahogany. In either
case, as the size must be II in. by 15 in., good, well*
seasoned wood must be used, otherwise warping will
take place, and 'spoil the whole instrument. In the
centre of the two long sides of this base board are
mortised two standards. Both these standards should
be cut from planed stuff, 2 in. wide by l\ in. thick one
(a) should be about 16 in. in length, the other (b) about
B in. Previous to being glued into the base board, the
necessary bearings, &c, to take the ends of the spindles,
should..be made in them.
27. As it is very convenient to be able to remove
and replace the glass plates at will, it is advisable to
make these bearings in the following manner : Having
placed the standards in their sockets temporarily, the
smaller glass disc is held with its spindle resting against
the two standards at such a height from the base board
that the" edge of the disc clears it by about I in. A
mark is made on both standards at the point where tile
spindles touch the standards. An assistant now holds
CARRES D1ELECTRICAL MACHINE.
43
fhe smaller disc in this position, while the operator takes
fhe larger glass disc, and holds its spindle against the
higher standard at such a height that the edge of this
clears the edge of the reel of the lower disc by about
J- jn. A glance at Fig. 12 will make this clear. The
position Jitjwhich the two discs must rest on the stan-
FIGS. 12 & 13.
dards to ride clear of the base board and of each other
having thus been found and marked, the standards are
removed, and a slot cut into them a little higher than
the first line, inclining a trifle downwards, and not ex-
ceeding in depth (when finished) the thickness of the
'iron spindle. Fig. 13 shows the position of/ the two
44 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
slots in the standards at a' and b' ; while c shows a third
slot in the centre of the top of the higher standard, and
reaching to the top line, in which the spindle of the
larger disc can ride. These slots must all be rounded
(where the spindles rest) with a rat-tail file. Square
pieces of wood, about J in. thick, are made to fit over
these slots, so as to keep the spindles from rising out of
their slots during rotation ; and these squares are held
in their places by screws, as shown at d d t Fig. 13. At
the top of the shorter standard (b} must now be cut,
with a centre bit, a circular hole, about three-quarters
of an inch in diameter, and reaching nearly, but not
quite, to the lateral slot in depth. Into this hole is
cemented, with marine glue, a solid ebonite rod f in.
diameter, and about 17 in. length. This rod serves to
insulate the upper and lower qombs ; and also as a
support for the spindle of the large disc. This rod is
shown at Fig. 14, e. At its lower half is a stout brass
ring e', which bears on one side the brass rod, co which
is attached the lower comb_/", and on the other a brass
rod carrying a ball g, about I in. in diameter, also in
brass. Three holes are bored in the sides of this ring,
two diametrically opposite to one another, which mu.st
be tapped and screwed to take the rods ; and a third
also tapped anJ screwed to take a small set-screw, to
hold the ring and its comb, &c., at the desired height.
About half-way up the ebonite rod, exactly opposite
the highest slot in the tallest standard, is a wooden
collar h t preferably of mahogany or other hard wood.
This is cemented to the ebonite rod, and has a small
orifice in its centre (facing the highest standard), in
CARRES D1ELECTRICAL MACHINE.
45
-which the end of the spindle of the longer disc can
enter.
A brass ball z, caps the ebonite rod, and this ball is
provided with a metal comb /, and varnished paper comb
f > at one extremity, and a ball through which passes
with friction a metal rod and knob k at the other.
This latter rod has an insulating handle /, by means of
FIG. 14.
which the knob k~ can be approached to, or removed
from, the ball g at will. These balls may either be in
brass, or in lead. In the latter case the amateur may
cast them on the brass rods, and drill a* hole through
the upper one for the sliding rod k\ to pass through. A
piece of wash-leather glued on one side of the hole with
.acetic glue will give sufficient friction to hold the sliding
46 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
rod in any position. If a condenser be used, it should
take the form of a double Leyden jar, and can easily be
made by coating the inside of two glass pomatum,
bottles, about 3 in. high, with tinfoil, to within i in. of
the top. The tinfoil should be cut to the height (about
2 in.X then-pasted on one side, and finally slipped into
the bottle and pressed into its place. The bottles are
covered in like manner, and to the same height with
tinfoil on their outsides, rare being taken to paste tinfoil
on the bottom as well as the side. When quite dry the
two bottles should be attached To one another by their
bottoms by means of a roll of brown paper glued round
both together ; but not reaching beyond the tinfoil.
When dry, the jars may be varnished all over outside
with red varnish. A cork is then fitted to each, and
through these corks are forced wires, one end of which
touches the tinfoil at the bottom of the jars, the other
end termina f mg in hooks, by means of which the double
jar can be hung on the upper rod /, and touch the lower
one g. Two pieces of wood, about 5 in. in height and of
the shape shown at n ri, are cut out of in. stuff, and
the inner sides of these are covered with a wash-leather
cushion, stuffed with horsehair and tinfoil. These
cushions, which serve to excite the lower disc, are fas-
tened to the base board by two screws, o d. Bosses,
made of reels sawn in halves, are geared to the spindles
p p' p" . The covering on the spindle ( 26) must be
removed at the points where they ride on the standards.
28. To give the requisite motion to the two discs, a
wooden pulley of about 6 in. in diameter, and in. thick,
HOLTZ STAZBINE. 47
having a handle r, projecting from it, is keyed or
screwed to the lower spindle ; and a small pulley, about
I in, in diameter, is in like manner keyed to the upper
spindle. A crossed gut-band, shown at a, completes
the driving apparatus. This band must be crossed, as
the plates must rotate in opposite directions.
29. HOLTZ MACHINE. Although this machine is
far outdone in point of efficiency by the Voss and the
Wimshurst form, yet, as it is historically interesting and
presents no particular difficulty in construction, a simple
form will be described in these pages.
The first thing to be prepared is a good sound base
board, about 16 in. long, by 1 1 in. wide, and i in. thick.
It is not material whether this be of pine or mahogany,
but it is essential that it should be thoroughly well sea-
soned. This base board, with the position of the mortise
holes to be cut in it, to admit the standard and glass
pillars, is shown at Fig. 1 5, where A is the hole in which
is inserted the main wooden standard that supports the
rotating disc. B and B' are two smaller apertures to|
receive the two shorter standards (also of wood) between!
which runs the driving pulley. C and c' are the holes
into which are cemented the two glass pillars that serve
to carry the combs and conductor rods. Two small
cleats are giued on at d and </, to hold the stationary
"disc" in position; but these had better be left until
after the " disc " is in position. I say " disc " because
in the older form of machine, the stationary plate really
vas a disc, but in this machine it will take the form of a
square plate ^6in. by i6"in.
4 8 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
30. The next operation consists in preparing the
wooden standards upon which turn the rotating plate
and the driving-pulley. The former should be cut or
turned in any hard wood (preferably mahogany), of a
circular shape, rather wider at the base, and terminating
in a square piece to fit the mortise-hole A, in the base
board. The height of this standard (exclusive of the
square portion that enters into the base board) should
be 7f in., and it should taper from 2 in. in diameter at
the bottom to I in. at the top, as shown at Fig. 15, E. A
piece of stout brass tubing, 3in. long, about -Jin. in
diameter (external), and \ in. bore, is next procured and
soldered at one extremity to a stirrup-shaped piece ot
brass made out of A in. stuff. This is first cut into a
long oval, i by I in., and then bent up in the middle to
admit the tube., 'The straight piede is shown at F, after
bending at G, and with the piece of tube soldered in its
place at H. Two holes are drilled and countersunk in
the projecting brass ears, and these serve to screw the
tube to the top of the standard. This tube forms the
" sleeve" in which the spindle that supports the rotating
disc turns. This main standard, with the "sleeve" screwed
in position, is figured at I. Our next step is to make
the two lesser uprights which support the driving-wheel.
These should be made in \ in. stuff, planed up nice and
smooth, about i-J in. wide by 4^ in. in height The top
of each standard should be rounded, and a slot, about
in. wide and f in. deep, cut in each. One of these
standards is shown at J, along with the small piece
which is placed in the slot, after the splindle has
HQL TZ MA CHIN2. 49
been put there. A , little pin, run through laterally
retains the whole in its place. Th.esc two uprights
are placed facing each other, at B and B', and sufficient
SO ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
space must be left between them for the driving-wheel
to turn freely. The driving-wheel should be made of
wood, preferably turned up on a lathe, and grooved
round its circumference to the depth of a in. At a
pinch, it may be made entirely by hand, without the use
of a lathe, by sav/ing a circular disc, 6 in. in diameter,
out of a piece of \ in. deal, and producing a groove in
the circumference by means of a red-hot iron rod carried
evenly all round periphery. The central hole should be
square, to admit of a square-shouldered wooden spindle
being glued therein, the projecting extremities of which
must be shaved down and rounded so as to run in the
slots of the lesser uprights J. The driving-wheel, with
its spindle and handle attached, is shown at K. We
may now proceed to m^unt a glass disc on a half-icel
and spindle, in the manner described at 20. This disc
should be 12 in. in diameter, and be fastened at its exact
centre to one spindle only, as illustrated at Fig. 15, L.
This spindle should be of steel, and should project
beyond the half-reel about 5 in. ; or, in other words,
the distance between the surface of the disc and the
extremity of the spindle should be about 6 in. This
cpindle must run freely (but without play) in the sleeve
at the top cf the main standard. About I in. of the
' spindle should project beyond the sleeve when the reel
end is quite against the sleeve at the other extremity.
, Over this projecting piece is placed a wooden pulley,
[about I in. in diameter, and this is keyed on to the
spindle so as to cause it to rotate rapidly when connected
. with a gut band to the driving-pulley K.
ffOLTZ MAC HIKE.
3 r. In the older form of mach!n< as already
mentioned, it was customary to employ a disc for the
16
FIG. 1 6.
stationary plate. In this disc were cut three apertures
one central^ to admit the spindle carrying the rotating
52 ELECTRICAL INSTRUMENT MAKING FOR AMA TEVR*.
disc, and two lateral ones, technically called "'windows,"
in vvnich were placed the pieces of paper that serve as
inductors. The cutting of these apertures was a serious
matter for amateurs. The plan herein recommended
will be found equally efficacious and very much simpler
in execution. A square sheet of glass, 16 in. by 16 in.;
is cut straight across the middle, so as to divide it into
two equal halves 16 in. by 8 in., as illustrated at
Fig. 16, A. When these two halves have been separated^
it will be found very easy to cut out two semi-ovals, as
shown, large enough to admit the passage of the spindle
and reel of the rotating disc. Anyone can cut these
semi-ovals with a corrjmon "wheel-cutter," or they may
be even nicked out with a key under water. Similar,
but longer and shallower semi-ovals, are cut out at about
2 in. each side of the central aperture. The length of
these cuts should be about 4 in., the depth about half an
inch, so that when the two cut sheets are placed in
juxtaposition there will be seen a central aperture about
:t in. by i in., flanked on either side by oval apertures,
4 in. by I in. The two half sheets are then cemented
together in position by means of two glass strips,
1 6 in. by 2 in., which are glued to the two sides of the
divided sheet with boiled and hot Canada balsam, the
said sheet being laid on a flat table, the strips loaded
with weights, and left until the balsam is set, which will
take three or four days. The general appearance of the
finished fixed sheet, with its " windows," central aperture,
and side strips (the position of which is indicated by
dotted lines), is sufficiently well shown at Fig. 16, A.
HOLTZ MACHINE.
S3
* 32. This 'plafe, when quite set and firm, may have
any excess 6r Canada balsam that may have exuded
under pressure scraped off and cleaned away with a rag
moistened with benzoline. It should then be placed on
the base bpjard, as shown at Fig. 16, B, and the cleats
FIG. 17."
/and d ^fastened thereto by glue and screws, the heads
of which must be covered with, guttapercha. The plate
must not be permanently fixed to the board until after
the glass pillars have been inserted ; but it is convenient
to place it between the cleats (as shown) to try whethci
54 ELECTRICAL INSTRUMENT MAKING FOR AMA TEURS.
the standard carrying the sleeve in which the spindle of
the movable plate turns is at the right height and
position. This also enables us to try whether the half-
reel, which is cemented to the movable glass disc and
spindle, allows the said movable disc to come near enough
to the fixed plate without actually touching it. The
distance between the two plates should be about % in.
If much more, the brass sleeve may be filed down until
the distance is sufficiently diminished ; if, on the
contrary, the disc runs too near the fixed plate, a brass
washer of sufficient thickness may be adjusted between
the end of the sleeve and the half-reel. It is, perhapsi
as well to point out that the side of the glass sheet
facing the rotating disc must be the one on which the
strips are not.
To insure good effects, the glass plates, as well as the
" armatures " or projecting paper combs, which act as
"inductors" through the windows, must be carefully
coated with good shellac varnish. Shellac dissolved in
methylated spirit is the best for this purpose.
33. The armatures must now be placed in position.
These consist in two portions of the periphery of a
circle, cut out of stout paper, as illustrated at Fig 16, C.
The scalloped portion must project through the windows,
so as to just graze the surface of the movable plate.
The armatures are fastened on the " back " of the fixed
plate (the side farthest from the rotating disc) one above
one window, and the other below the opposite window,
by means of good starch paste, and the whole carefully
varnished, as above recommended, when dry. This
BOLTZ MACHIN&, *$
having been satisfactorily performed, two glass standards,
about 8 in. in height, and in. thick, are fitted with
brass tube tops, to which have been previously soldered,
two cross arms, shaped like the letter T. The tops can
be securely fastened to the glass rods by means of plaster
of Paris. Along the top of each T are soldered a dozen
brass brads to serve as combs to collect the induced
electricity. To the leg of each T, beyond its junction
with the brass tube that fits on the glass rod, is soldered.
a hollow brass ball about ij in. in diameter, paving a
i in. hole right through it This hole serves for th*
passage of a sliding brass rod, furnished with a glass
handle on the outer extremity; and an inch brass ball ?-.t
the other. The outer extremity of each head of tha
T-piece must be furnished with a smooth wooden
prolongation at right angles with the T. This has a
deep " nick " in its inner surface, and serves to support
the fixed plate. The combs at the end T-pieces must be
at such a height that they come just opposite the
scalloped edges of the paper armature. A glass
standard, with its comb, ball, sliding rod, and projecting
wooden arm, is illustrated at Fig. 17, A.
After the rotating glass plate has been placed in
position, with" its spindle in the sleeve, and the small
driving pulley keyed or screwed at the other end, the
glass pillars may be cemented in position with Prout's
elastic glue, care being taken that the nicks in the
wooden prolongation of the T-pieces come into firm
contac.t with the fixed glass plate. A stout gut band
will be found best to drive with. The complete instru-
56 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
ment is shown at Fig. 17, B. It is usual to add Leyden
jars to these instruments, as in the Bertsch and Carr<$
machines, in order to increase the capacity of the two
conductors.
N.B. The Canada balsam for attaching the strips
to the plates is best thickened by placing in a shallow
FIG. 18.
saucer in a slow oven, and gently heating it until a
small portion withdrawn on a cold iron rod, sets ivJten
cold. It must be applied while warm, and the glasS
strips should also be warmed.
It is with considerable diffidence that the author
ventures to place before the reader the following in-
structions concerning the construction of the Wims
\TIIE WJMSHURST INFLUENCE MACHINE 57
hurst machine, since the machine has been so fully and
ably described by the inventor himself. However, to
render this series complete, a brief sketch of the mode
of making two simple forms will be given.
34. JHE WIMSHURST INFLUENCE MACHINE. The
portions that first demand cur attention are the plates.
These should be of glass good window-glass as flat
as can be got, and not too green in colour (as it is apt
to be poor in insulating power) is to be preferred. This
is to be cut into two discs, each 16 in. in diameter.
The thickness of these discs should not exceed ^th
of an inch. As in the Carr^ and Bertschmachines.it
will be actually better not to have holes drilled in the
plates, but to fasten the bosses to the plates as described
at 20. These bosses consist in two circular pieces of
mahogany or other well-seasoned wood, not less than
4 in. in length. (It is a great mistake 'to have these
too short, as then the glass discs come too near the
standards, and much electricity leaks away.) It is best
to turn these up in the lathe, of the form ajid dimen-
sions shown in Fig iB,a. While in the lathe, a perfectly
central hole nearly \ in. in diameter must be bored in
the small end of the boss (as shown in the cut), reaching
nearly, but not quite, to the thick end of the boss. This
hole must be bushed for its whole length with \ in. brass
tubing, and some stout steel wire which just-enters the
brass tube, selected and straightened out to serve as'
spindles on which the bosses are to turn. Mr. Wims-'
hurst, in his directions, says : " I must impress upon
those who make a machine that they cannot give too
58 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
much care in selecting the tubes, to fit properly, on
the steel wire ; it will save trouble throughout the
making, and the machine, when made, will work more
smoothly." The centre of the gla'ss plates having been
obtained as described in 20, the bosses are attached
to the plates precisely as described therein. When quite
set and dry, the plates should be varnished, in a warm,
dry room, on both sides, with good shellac varnish, and
the varnish dried by the fire.
35. As shellac varnish is in constant requisition for
insulating electrical apparatus, I give Mr. Wimshurst's
recipe for the preparation and preservation of this useful
compound. Take a large, wide-mouthed bottle say,
THE WIMSHURST INFLUENCE MACHINE. 59
a pickle bottle fit to it a soft wooden bung, bore a hole
through this bung, and in this hole tightly fix the
handle of a rather large brush (the brush end being in
the bottle) ; then about half-fill the bottle with good
shellac, cover the shellac with r methylated spirits, and
shake the bottle occasionally. In about 24 hours it
will be ready for use. By this means the brush is
always clean and serviceable.
36. While the varnish is drying on the glass discs
the operator may strike out on a piece of paper a circle
of the same diameter as the discs, and, by means of the
compasses, divide the circle into sixteen equal parts,
drawing radial lines at each division, from centre, to
circumference. This divided circle will, if placed on
a .flat table under the glass discs, enable the operator
to fasten down the tinfoil " sectors " or segments, at
equal distances from each other.
37. These sectors consist in wedge-shaped strips of
tinfoil, slightly rounded at the top and bottom, as
shdwn at b (Fig. 18), 3^ in. in length, by in. wide at
top, diminishing to J in. wide 'at bottom. These sectors
are easily attached to the glass plates by placing these
latter in turns upon the paper circle ( 36), and, having
rubbed a little thick shellac varnish over one surface
of the tinfoil sector, placing it (shellac side downwards)
on to the glass plate, just over one of the lines, care
being taken to leave about \ in. of clear glass between
the periphery of the disc and the circle of sectors.
The line showing this distance had better be struck
out with the compasses on the paper, as the perfect
fu ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
regularity of the circle of sectors adds much to the
neat appearance of the machine. One disc, mounted
on its boss, and fitted with sectors, is shown at Fig. iS^c.
When the sectors are firmly stuck down to the glass,
and the varnish quite dry, it will be well to run a
brush charged with varnish round the inner and outer
extremities of the tinfoil sectors. These rings of
varnish may extend in- inwards, but not more. They
serve to increase the adherence of the sectors to the
glass, ano! a ^ so to insulate slightly the extremities. On
the centre of each disc, exactly opposite the bosses,
must now be fastened with hot marine glue or Prout's
elastic glue, a. small ebonite washer, punched out of
sheet ebonite ^i in. thick. These washers are to
prevent the rotating glass discs from actually touching
during rotation.
38. The - stand next demands our attention. It
should be made of mahogany, walnut, or some other
well-seasoned wood. Six pieces will be required viz.,
two pieces 20 in. long by 3 in. wide, and I in. thick ; two
pieces 14 in. long by 3 in. wide, and two uprights 17 in.
in height, 3. in. square. These two latter must be cut
round the lower end to form a square tenon 2 in. long
by 2 in. square section, and two of their sides must be
made to slope away 2 in., this forming the top end,
which must be rounded, as shown at Fig. 18, d d '. A
hole of exactly the diameter of the steel spindle on
which the plates are to run must be drilled through each
standard, at about an inch from the top, care being
taken that these two holes come exactly opposite each
THE WIMSHVRST INFLUENCE MACHINE. 6l
other and at the same Jteight in the standards, otherwise
the plates will not run opposite each other. At about
3f in. from the tenon, and on the same side of each
standard, a long semi-cylindrical slot is cut, about \ in.
deep ; this serves for the spindle of the driving-wheels
to run in. All these pieces are shown in Fig. 1 8, where
d d' are the uprights,* e' the long cross pieces, and//"
the lower pieces of the stand ; these pieces must be
planed up so as to fit accurately ; mortice holes, cut in
the centre of the 20 in. strips to take the tenons of the
uprights. The whole is then joined together so as to
make a strong frame, with glue and screws ; the long
strips being screwed over the 14 in. pieces at each
extremity, so as to form a square 14 in. by 20 in. As
the two short pieces are placed below the- larger ones,
and as the uprights are morticed into these, it is neces-
sary to make the ends of the uprights project I in.
through the holes, so as to afford a support to the centfe
of the frame. With a spokeshave, or similar tool, the
sharp edges should be taken off the frame, so as to
prevent dispersion of electricity. The frame and
standard being put together, as shown at Fig. 19. A, it
will be well to cut out the holes in the centre of the
shorter pieces of the frames, which holes are to receive
the glass rods or jars (as the case may be) which support
the conductors and combs. These holes may be begun
with a centre-bit, and should be 2 in. in diameter if jars
are to be used, or I in. if glass rods only are to be
employed.
39. As the glass discs, when mounted, must rotale
62 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
in contrary directions, it is necessary to have two
driving-wheels on a spindle wherewith to drive them,
and to connect one driver with one boss ( 34) by means
of a straight band, while the other transmits its motion
by means of a crossed band. These driving-wheels
may be turned out of any suitable wood, 7^ in. in
diameter; they should have a centre-bit hole, \\ in.
in diameter, put through the centre ; a length of some
FIG. 20.
good hard wood should then be turned up to make
the spindle, on which the two wheels must be tightly
fitted and glued. Care must be taken that the edges
of the wheels, when glued to the wooden spindle, should
come exactly opposite the V groove in the bosses ( 34)
when the glass discs are in their places. These wheels
must have a groove turned in their edges, to take the
driving-band. The length of the wooden spindle must
THE WIMSHURST INFLUENCE MACHINE. 63
be such as to just not reach from standard to standard
say "j\ in. A central hole, about \ in. in diameter, js
now made through the entire length of this wooden
spindle, and through this wooden spindle is driven an
iron rod 15^ in. in length,- \ in. diameter, projecting
3 in. at one end, and 4 in. at the other. At the longest
end this rod is squared up to take a driving-handle.
If the wooden spindle docs not fit quite tight upon the
metal rod, it will be better to drill a hole through wood
and iron and drive in a metal pin. The, driving spindle
is placed in the two semi-cylindrical slots cut in the
sides of the standards ( 38), and is kept in its place v by
means of two small rounded cleats screwed thereto.
These driving-wheels, with their spindles, etc., are
represented at Fig. 19, -B.
40. The next step is to mount the comBs and con-
ductors on to the glass pillars, which are to insulate
them. These pillars, of which two are required, should
be I in. in diameter and 15 in. in length. Good white
glass that becomes readily electrical on friction should
be chosen. They must be fitted with brass collars I in.
long, at the upper extremities, and to these collars are
soldered brass balls 2 in. -in diameter. At the upper
portion of these balls is drilled a hole \ in. in diameter,
into which are fitted, but not fixed, brass rods \ itl. thick,
about 1 6 in. long. These rods are bent into quadrants,
and terminate in brass balls. It must be noted that
the two brass balls terminating these rods must be of
different sizes to obtain the best effects. In the sized
instrument under consideration, one should be f in. an4
64 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
the other I in. in diameter, and nicely polished. By
not having the rods as fixtures in the top of the glass
pillars, it is easy to exchange the rods if the direction of
the flow of the current in the machine is reversed, either
accidentally or' intentionally. The combs, which must
be screwed to the centre of each 2 in. ball that terminates
the glass pillars, consist in J in. brass rods 12 in. long,
terminating in brass balls about in. diameter and bent
into the shape of a horseshoe, so as to embrace both
the plates. In the interior of the horseshoe are soldered)
or otherwise affixed, four or five in. brass points. The
whole arrangement of the glass pillars, with its collecting
comb, large ball, movable end, and terminating ball, is
shown at Fig. 19, C.
41. The plates can now be mounted on the stan-
dards. To this end, each plate in turn is held with its
boss against the small hole at the top of the standard
on the steel rod, which was chosen as running freely in
the bushing of the boss ( 34), pushed through this hole
and into the hole in the boss as far as it will go. The
rod is then cut off, leaving an inch projecting on the
outside of the standard. With a file, a nick is cut in this
steel rod at about the centre of the portion that is to
remain in the standard, then a round-headed screw is
driven into the top of the standard to enter this nick and
keep the steel rod quite firm and immovable. To the
projecting ends of the steel rods are affixed the bent rods
carrying the brushes. These rods are technically known
as the "neutralising rods." They are made from J inJ
brass tod, about 17 in. in length. A hole J in. deep ant^
THE PLATE ELECTRIC MACHINE. t 5
about | in. wide,-is drilled at. the two extremities of each
of these 1 rods. A little tuft of about a dozen short
lengths of the fine wire used by the gilt lace manufac-
turers, is made into a brush by binding at the bottom
end with a ijttle of the same wire ; this same end is
pushed in the holes in brass rod, and wedged firmly into
place by means of a little wooden wedge. There are
two such rods ; and each rod must have a brush at eacn
end. A short length of brass tubing, fitting tightly on
to the projecting pieces of the steel spindles carrying the
plates, is now procured, and cut into two pieces \\ in.'
long. With a round file, a slot (sufficiently deep to
take the brass neutralising rods just finished) is produced
in these pieces, and each rod having been placed in this
slot, at its centre, is soldered thereto. At the same point,
o give a finish, and prevent dissipation of electricity, is
also soldered a small brass ball. Each rod is then bent
slightly in the shape of a bow, so that the " brushes "
'shall come into contact with the sectors, when the
neutralising rods are put in their places on the steel
spindle.
The appearance of 'the comclete machine,' with the
"brushes" in their right position for a right-handed
person to drive, is shown at Fig. 2O/
42. THE PLATE ELECTRIC MACHINE. The ordi-
nary frictional machine, although far behind the induc-
tion machines in point of efficiency, will always be a
favourite with amateurs, owing to the ease with which it
may be constructed.
A circular glass plate 12 in. in diameter, having been
1V> ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
cut but and mounted by either of the methods described
at iSet seq, is supported by its spindle, on two standards
(similar to those described and figured at 22), standing
about 8 in. high, 2 in. wide, and in. thick! v These are
let in, by mortising, to a base board 14 in. by 9 in. by
I in,, at a distance of about 2 in. from each other, with
their flatter sides facing as shown at 39, with this
difference that they must stand within an inch.on either
side of the centre of the base boarjd with the glass plate
between them. The wood to be used for all the parts of
this machine should be thoroughly well seasoned, and if
after being planed up it is well rubbed with a flannel
pledget dipped in melted paraffin wax, it will be much
( improved % The spindle on one side should not project
beyond the standard, while on the other it should extend
about i in. beyond the opposite standard, and on this end
should be filed up square to. take a handle by which it is
to be rotated. As in 19, so here, a shoulder must be
'turned at each end of the spindle to prevent the plate
riding backwards and forwards on the standards.^ Two
cushions are now made by cutting two thin smooth
pieces of wood 4 in. long, i in. wide, and a in. thick,
and covering these on one side^with tinfoil attached
with good paste'or thin glue.^ When dry, thej tin-foil is
.covered over to the depth of about f in. with^ horsehair,
'mixed with some short lengths of ^very fine iron wire,
such as is 'used by florists under the name of " binding
wire." A square of flannel of the same size as the little
board is now laid over the packing, and,* finally, the
whole is covered with good wash-leather, which is drawn
THE PLATE ELECTRIC MACHINE. 67
tightly round the edges and ghied thereto, tacks being
used to hold the leather in its place until the glue is dry.
N.B. The corners of the wood to which the leather is
attached should be raunded, to prevent dissipation of
electricity.
In order that the cushions may press firmly against
theglass plate, and yet at the same time give to any
inequality in surface, or want of elasticity, it is well not
to fix the rubbers or cushions in any permanent manner
to the standards. A mode which works admirably, is to
drill two holes at the centre of the edge of each cushion
to the depth ofabout f in. A stout piece of brass wire
(say \ in. in diameter) IO in. in length, is hammered at
its centre, to render it springy, and then bent into the
shape of a letter U, the legs standing at such a distance
apart as just to face the two holes just bored in the
cushions when these latter are held firmly against the
glass plate. The top of each leg of the U is then bent
sharply at right angles to the rest of the leg for a length
of f in., the bend in each leg facing and being parallel
to its neighbour. The U is then squeezed together at
its upper extremities until the two bent pieces just
touch.
These pieces *~are pushed into the holes in the
cushions, care being taken that the leathers of the
cushions face one another. The plate having been
placed in its position on the standards, the two cushions
(which are held face to face by the U spring) are slightly
separated, and caused to grip the glass plate between
them. They must be pushed so far towards the centre
68 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
of the glass plate as to clear it by about 2 in. The plate
is allowed to turn until the cushions come in a line with
the standards, and then, having decided in which direc-
tion the plate is intended eventually to be rotated, three
cleats of wood are glued to the inside of the standards
to prevent the cushions from being carried round during
rotation. These three cleats or stops are put together
like a letter E without the central stroke, so that the
mere act of rotating in the right direction causes the
cushions to hold in their right position on the standards,
while the U -spring pinches them against the plate.
When it is desired to remove the cushions for the pur-
pose of amalgamating, etc., it is only needful to give
the handle a half-turn backwards, when, of course, the
plate being gripped by the spring and cushions, brings
these along with it out from between the^ cleats. A
metallic chain should hang from the bottom of the U-
spring and touch the base board.
A solid glass rod 16 in. by f in. in diameter must now
be procured. This must be cemented into a wooden
foot about in. thick by 2 in. wide, and 3 in. or 4 in.
long, having a rather long slit through it, so that it can
be screwed down to the base board of the electric
machine by means of a thumbscrew, the slit being in-
tended to allow of a little adjustment in the way of
approach or recession from the plate. It is, perhaps,
needless to observe that this glass rod must be placed
on that side of the glass plate where the handle is not,
just in a line with the standards. This glass rod must
be surmounted with a brass or wooden ball, at least 4 in.
THE PLATE ELECTRIC MACHINE. 69
In diameter (a good skittle ball does admirably). If of
wood, it must be carefully and smoothly covered with
tinfoil. Besides the hole into which the glass rod enters
and to which it must be cemented, this ball has two,
other holes viz., one about J in. in diameter fadrrg the
glass plate ; another, also about \ in. in diameter, at the
top, opposite and perpendicular to the one in which the
glass rod enters. Into this latter is fixed a rod of metal
about Jin. in diameter, which extends straight upright for
a distance of 4 in. from the ball, and is then bent so as to
form a ring 12 in. in diameter. This ring, and the rod,
except at the extremity, where it enters into the ball,
should be carefully and neatly wrapped with good string,
in. thick, after the manner in which fencing foil
handles are wound. The coating of string should
bfe rubbed over with melted paraffin wax, and lastly,
carefully varnished with the red varnish previously
described.
The effect of this ring (known as Winter's ring) is
greatly to intensify and lengthen the spark.
All that now remains to be done is to make the
collectors. To th ; s end, two rings, 4 in. in diameter,
must be made out of \ in. galvanised iron wire,' allowing
a length of about 5 in. of wire to project beyond the
circles. A dozen sharp-pointed brads are now to be
Soldered at equal distances all round these rings so that
When the rings are lying flat on a table the points of the
brads, stand upright. The projecting wires, or stalks, of
these rings are now bent sharply at right angles to the
tings, at about \ in. from the rings, in such a manner
fo ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
that if placed with the stalks in the side hole in the
ball, the teeth in the one r;ng shall face the surface of
the glass plate nearer the handle, while the teeth of the
other ring shall face that surface of the plate which is
nearer the ball.
Having adjusted the rings so that the distance
between the teeth of the rings, and the glass plate shall
not be more than the in., nor less than \ in., the
operator will solder the two stalks together ; cut them
off to such a length as to enable them to enter the hole
in the ball, and yet retain the correct distance either
side of the plate. The stalks should then be wrapped
in tv/ine (except at the end that enters the ball), paraf-
fined and varnished as above. This end may now be
glued, wrapped in tinfoil, and forced tightly in the hole
in the ball. Such a machine, with cushions freshly
dressed with amalgam, will give, in dry weather, sparks
2 in. long.
43. CONDENSERS. These are instruments which
owe their peculiar power of retaining charges of electricity
to the fact that one conducting surface connected to
earth or other large conductor confers, by induction,
a greater capacity for .charge in a second insulated
conducting surface than this latter would have were it
not for the presence of the first surface.
Condensers may be conveniently divided into three
groups, according to their forms, and the nature of the
insulator used between the two conducting surfaces
1st, Bottle formed, or " Leyden jars"; 2nd, Coated
glass sheets, also called fulminating panes or Franklin's
THE LEYDENJAR. ft
plates ; 3rd, Coated sheets of paraffined paper or other
flexible insulator, known as Fizeau's condensers.
44. THE LEYDEN JAR. Every dabbler in elec-
tricity has, at one time or other, made a Leyden jar ;
not every one has made a good one. The first point
to be noted is the shape of the bottle or jar to be used.
It must have a neck sufficiently wide to allow the inner
coating to be easily placed in it and smoothly laid.
To this end the mouth should certainly not be less
than 2 in. in diameter for a jar 6 in. by 3 in., and so on
in proportion. The next point that demands attention
is that the bottle or jar should be of nearly uniform
thickness, and free from flaw or crack. To ascertain
this, it should be made to " ring." The last, but certainly
most essential point, electrically speaking, is that the
glass of which it is constructed should be really a good
insulator. The white, or rosy white glass (which con-
tains rnanganese) is very deficient in this regard. The
greener-coloured glasses are generally good insulators.
To test this point clean and dry the bottle. When
quite dry, but cold, rub it briskly on the outside with
a warm silk handkerchief. Reject all such jars as do
not easily and quickly become charged so as to give
a distinct spark. Choose all such as retain the charge
(once acquired) longer than the others. The jar or jars
chosen should now be fitted with a sound bung. This
requires cutting neatly above and below, so as to be
quite smooth, the upper surface being afterwards rather
thickly coated with good red sealing-wax. (N.B.
The coating of sealing-wax should not be applied
72 ELECTRICAL INSTRUMENT MAKING FOR A MA TEURS.
until the bung has been pierced centrally and a rod
fitted.) ;
It is essential 1 that the bung should be new, or at
least unused, for if it has served for acid or salts, it will
be found to attract moisture, and greatly detract from
the efficiency of the jar. The knob of the jar should be
one of those brass balls which can bs procured from
most of the metal-shops in Clerkenwell, where they are
known under the name of " metal beads." A female
screw can be made in the orifice of the brass ball, and
a thread put on a short length (say 6 in.) of stout brass
wire to fit. This "wire, o r rod, with its accompanying
ball is now fitted to the bung. This is effected by
making a small central hole in the bung with a bradawl,
considerably smaller than the rod. This latter is then
forced in until about two-thirds of its length has been
pushed through the bung. When this has been done,
the upper surface of the bung (that nearer the brass
ball) should receive its coating of sealing-wax. Should
there be any difficulty in procuring a brass ball, a
substitute, quite as efficient, though not so imposing
in appearance, will be found in a leaden bullet, cast
upon the end of a brass or copper rod of the desired
diameter and length. The ne:ct step is to coat the jar
with tinfoil. It is better to begin with the inside, as
when the outside is coated it b not easy to see what is
going on inside. A piece of good tinfoil is cut intD
a circle, a trifle narrower than the outside diameter of
the bottom of the jar. One side of this is carefully
rubbed over (\yith the finger) with thin hot glue. It
THE LEYDEN JAR. 73
is then allowed to fall in the jar, glue side downwards,
und is pressed into its place at the bottom by means
of a small mop (similar to those used to clean lamp-
glasses). The jar is now measured from the bottom to
the bung. Three-quarters of this are marked off as the
height to which the tinfoil coating may reach. A strip
of tinfoil of this width, and three times the diameter of
the jar, is now cut. For convenience of getting in the
jar, especially if this latter be narrow, it will be well to
divide the strip of tinfoil into two halves, and having
glued one half, slip it in the jar, having previously given
it a little bend, push it into its place, and smooth it;
'down by means of a tooth brush, then insert
the other half in a precisely similar manner.
Care must be taken that these side pieces
come into actual contact with the bottom
circlet. The inside being thus coated, the
outside may be proceeded with in precisely
similar manner ; only the sides may be
covered by one entire piece of tinfoil, and
this should lap somewhat over the bottom.
The coatings should be allowed to dry
thoroughly before the jar is used or touched,
as otherwise the outer coating will be wrinkled
up and spoiled. Very charming effects of
a multiplicity of sparks duripg discharge, may .be
obtained by coating the inside of the jar with a number
of small diamonds of tinfoil, almost, but not quite
touching, instead of with one continuous layer. The
outside should, in this case, be covered with similar
I^LECTRICAL INSTR UMENT MAKING FOR AM A TE URS.
diamond -shaped pieces, and this latter should have a
circular hole cut in each, so that each hole should stand
over the corners of the inside diamonds. By this means
not only are the sparks betv/een the outer diamonds
seen, but also those between the. inside ones. The jar
being coated and dry, the bung, with its rod and ball,
is fitted to it. It is well to attach a small piece of metal
chain to the end of the rod '(by spidering), to insure
contact between the rod and the inner coating. The
absolute size of the jar and its parts may be varied at
Fns. 22.
will ; but, to obtain the best results, the following
relative dimensions v/ill bs found useful : Let the height
of the jar be three times its diameter ; the length of
rod and ball four times the diameter. With these pro-
portions, and especially if the coatings do not extend
bsyond thres-fourths of the height to the bung, the jar
v/ill be found to hold a good charge for some time.
A lesser charge, with longer retention, may be obtained
by using less coated surface say, two-thirds, or even a
half, of the total height, instead of three-fourths. Fig. 21
FULMINATING PANES. 75
shows the relative sizes and shapes of the different
portions of the jar ; Fig. 22 shows the inner and" outer
diamond-shaped pieces of tinfoil to be used if a "spangle
jar" be desired.
45. FULMINATING PANES, or"" Franklin's plates" as
they are also called, are easily made by coating both sides
of a sheet of glass with tinfoil, to the extent of half of the
entire surface, leaving the margins all round clear glass.
The glass should be chosen with the same precautions
as to insulating power as in the jcase of Leyden jars
FIG 23.
( 44), and the corners of the tinfoil should be rounded
as rounded ones dissipate electricity less than pointed
ones. Good paste or glue (thin) will do : very well to
stick 'the foil down to the glass.^ When quite dry, any
superfluous glue or pasfe may be removed from the
edges of the foil and glass by means of a slightly
damped rag. The tinfoil should be lightly burnished
with a bone or ivory knife handle. As a further
protection against sparking over, it is well" to varnish
round the edges of the tinfoil with a coating, of good
shellac varnish. In charging this condenser, it must be
borne in mind that one surface must be connected to
earth or other large conductor, while the other is.
?6 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
receiving the charge ; otherwise its capacity is very
limited. A very convenient size and form of the
.Franklin plate is shown at Fig. 23.
46. Owing to the facility with which these glass
plates are broken, they are not much used at present as
condensers. Other insulators, having at once the
advantages of being lighter, less fragile, more flexible,
and of higher specific inductive capacity than glass, are
now used in all instruments intended for- practical work.
Of these, good paper, free from holes, soaked in melted
paraffin, stands, if not alone as the first, in the very first
rank. Next comes paper soaked in good shellac
varnish ; then thin sheet ebonite, which can be bent by
heat to any shape ; and, lastly, good indiarubber cloth.
The mode of making a condenser, with either of these
insulators, will be the same in each case, so the student
will do well to try his hand at a Fizeau's condenser, as
being at once, the most useful and least expensive.
47. FIZEAU'S CONDENSERS. These are employed foi
two purposes : (a) to increase the efficiency of induction
coils, by taking up the " extra " current induced in
the primary ; (b) to measure electrpstatic capacity.
Condensers for coils may be of any dimensions between
9 in. by 7 in. and 2 in. by 4 in., according to the power
of the instrument with which they are intended to be
used ; the former size for coils giving from I in. to
3 in. spark, the latter for coils giving sparks of J in. or
less. In the former case as many as 150 alternations
of tinfoil will be needed; in the latter, from 25 to 30
v/ill be ample. Having decided upon the size to be
FIZEAU'S CONDENSERS.
77
employed, the student will select, at any photographic
stores, some sheets of plain paper (not salted or
albumenised) known as Fapier Rive. These sheets run
about 22* in. xi8 in. so that each sheet, if cut in half
lengthwise, and in three across, will give six sheets
7 in. x 9 in. In selecting this paper it should be held
between the eye and the light, so that any holes or
imperfections may at once be seen ; and any sheet which
is faulty in this regard at once rejected. The paper
having thus been chosen, and cut to the desired size,
should be placed in a square flat dish, a trifle larger than
the 'sheets themselves. The best dish to use for this
FIG. 24.
purpose is undoubtedly one of the square white earthen-
ware dishes in which photographers sensitise their
papers ; but if one of these is not procurable, a very fair
substitute may be found in a square, flat, tin baking
dish made scrupulously clean. The sheets having been
placed in the dish, several lumps of good paraffin
wax (one requiring a very high temperature for
fusion is the best for this purpose) are scattered
over the paper, and the whole stood in a gentle oven
.until the wax is thoroughly melted, and the paper
78 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
has become semi-transparent by having absorbed
it. If there are any patches of unsaturated paper
more paraffin -wax must "be added, until the paper
is equally imbued with the melted paraffin. When
this is the case, the sheets of paper should be
removed from the oven one by one, allowing each one
to drain until it drips no longer. The paraffin will set
almost as soon as each sheet is removed from the oven.
The next operation consists in placing leaves of tinfoil
in alternation with the paraffined paper. Supposing we
have cut our paper sheets to the size 9 in. by 7 in., we
shall require about 1 50 sheets of tinfoil 7 in. by 5 in. ;
or, in other words, we must allow a clear margin of I ?n.
of paper all round the squares of tinfoil We shall also
need 150 strips of the same tinfoil about I in. wide byj
3 in. long.
Having procured two thJn pieces of board (common
deal will do) of the same sizes aa the paper, but only in.
thick, we proceed to lay upon it a. sheet of paraffined
paper. In the centre of this v/e lay a sheet of tinfoil,
and on this sheet of tinfoil we place one of the 3 in. strips
to the right hand, so that the strip projects beyond the
paper about I in. We now place another sheet Ot
paraffined paper squarely over the first sheet, and then
a sheet of tinfoil as before. On this second sheet of
tinfoil we put a projecting strip of tinfoil, but this time
to the left. In this manner the condenser is built up of
alternate sheets of paper and tinfoil, with strips of tin-
foil projecting alternately in the right and left. In our
case, all the odd numbers, I, 3, 5, 7, etc,, being to the
MICROFARAD CONDENSER 79
right, and all the evens, 2, 4, 6, 8, etc., being to the left.
Care must be taken to end the condenser with a sheet
or two of paraffined paper. The other board should
now be placed over all, and the whole bound neatly
and tightly together by Wrapping round crosswise (not
lengthwise because of the projecting strips) with rather
wide tape from end to end, and then stitching down the
ends of the tape. This being done, the projecting strips
on each side are folded tightly together, so that the
whole 150 strips come into absolute contact. It is to
these two projecting lugs that remain after this opera-
tion, that the contact pillar and spring of the coil are
attached. Fig. 24 shows the disposition of the sheets of
FIG. 25.
paraffined paper and tinfoil, where pp' are the sheets of
paper, and TT 7 the tinfoil. At Fig. 25 is shown the
complete condenser, with the projecting tinfoil strips
turned up and compressed together.
48. MICROFARAD CONDENSER. Take 37 sheets of
good tinfoil, 7 in. by 6 in., with an equal number of strips
3 in. by \ in. Procure about 76 sheets of very thin, hot-
pressed paper 9 in. by 8 in., such as is used for bank-
notes, or similar. Choose it carefully, leaf by leaf, free
from holes or blemishes. Soak it as above described in
hot melted paraffin, drain, and blot off each individual
8o ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
sheet between good blotting paper. A hot iron must
be used to do this effectually. No/w lay on a perfectly
smooth and flat iron plate two sheets of this paper (as
shown at P, Fig 24), then a sheet of tinfoil, quite in the
middle, then a strip, projecting to the right. Over this
are placed two sheets of paraffined paper (as shown at
P'), then a sheet of tinfoil with the projecting strip to the
left (as shown at T"). Proceed in this manner, laying
the sheets of tinfoil over every two sheets of paper until
the 37 sheets have been used up. Now, subject the
whole in a press between two iron plates to a pressure
of half a ton. Solder lightly all the 19 ends on the
right-hand side together for attachment to one terminal,
and the 18 ends on the left-hand side to attach to the
other terminal ; place the condenser between two stout
boards clamped together by screws, so as to maintain
one constant and invariable pressure as the capacity
increases by pressure. Two binding screws must bs
attached to the two soldered ends of the tinfoil strips
to serve as terminals. By using two sheets of paper
between each pair of tinfoils, any liability to leakage
through the holes in the paper is minimised ; of course,
the instruments should be tested against one of known
capacity if great exactitude be required. If too small,
increase the pressure, or the number of tinfoil and paper
leaves ; if too great, remove a leaf or two.
PART II.
DYNAMIC OR CURRENT INSTRUMENTS.
49. THE MEDICAL COIL. Procure a well-seasoned
board of walnut about 21$ in. in length, 3 in. wide, and
f in. thick. From this cut one length 12 in. long for
the base board (Fig. 30), and three pieces 3 in. square
(like Fig. 29), for the coil heads; when cut, a fillet 8 in.
long must be nailed or screwed on the two sides of the
base board (as shown at Fig. 30) ; these fillets should be
in. square section. Corresponding square nicks must
be cut in two of the square heads (as shown at a } a, a, a,
Fig- 3 1 )- All the woodwork when thus squared and
finished, should be soaked for a quarter of an hour in
melted paraffin wax, and then rubbed dry while still
warm.
Procure a thin brass tube (knovrn in the trade as
" triblet tubing ") about $ in. diameter, 4^ in. long ; turn
up a short plug and button to fit one end of this tube
and serve as a handle (see Fig. 26). This may be
fastened to the tube by driving in three fine brass brads,
and filing off the heads flush with the tube.
Now cut up about IOO lengths of straight iron wiro
o
8a ELECTRICAL INSTRUMENT MAKING FOR 'AMATEURS.
(best soft annealed) No. 22 gauge, say, about 4$- in.
in length ; fill the brass tube with them as tight as you
can fit them; cut them all to the same length (they must
protrude a little beyond the tube). Now draw out
about a couple cf inches of the iron bundle, and wrap
it tightly round with twine, leaving about $ in. free.
Draw more out,. and continue v/rapping until you have
wrapped to within \ in. at each end of the bundle.
Tie the string, and withdraw the bundle from the brass
tube. Melt a little solder in a ladle, dip the ends of the
iron bundle into soldering fluid (zinc dissolved in hydro-
chloric acid), and then at once into the melted solder.
Allow the bundle to cool ; file off the superfluous colder,
so that the bundle will just enter freely into the tube.
It should appear like Fig. 27 when the string has been
removed.
50. The next operation is to make a good stout
paper tube, also about 4^ in. in length, into which the
brass tube (Fig. 26) can slide easily. To make this, put a
few turns of soaped writing paper round the tube Fig. 26,
then roll and glue .seven turns of good, stout brown
paper, 4$ in. in length, round this writing paper, or else
it will be difficult to draw out of the tube. This paper
tube (Fig. 28) must be allowed to dry thoroughly while
still on the brass tube (Fig. 26). When quite dry, it must
be slipped off, the writing-paper lining drawn out, and
then it must be soaked for a few minutes in melted
paraffin wax.
51. The iron bundle should also be allowed to stand
in melted paraffin v/ax for some time, and then stood
THE MEDICAL COIL. 83
up to dram in a warm place. This will prevent rusting.
When quite cold, all superfluous paraffin having been
removed, .a strip of brown paper, \ in. wide, is rolled
round one extremity of the iron bundle, until it is of
such a diameter as to fit tightly into the paper tube
(Fig. 28). This paper strip must be cut off at this point,
and glued tightly round the end of the iron bundle.
The brass tube (Fig. 26) is then slipped over the iron
bundle, until it just reaches the little paper collar just
made. The brass tube 1 and bundle together are pushed,
button end first, into the paper tube (Fig. 28), and when
the paper collar around- the iron bundle is just about to
enter the paper tube, it is to be well served with, hot
glue and forced into the tube. The whole must now be
allowed to dry and set thoroughly.
52. Taking one of the 3 in. heads (the one which
has not any nicks in the sides) we bore a central hole
with a brace and centre bit, just large enough for the
paper tube (Fig. 28), with its iron core, to fit tightly
(see Fig. 29). Putting a little thin good hot glue round
the free extremity (the end opposite that at which the
brass enters), we push it into the hole in the square
head, until it projects about i in. on the other side.
This must be allowed to dry thoroughly before pro-
ceeding to the next operation.
53. We may now proceed to wind the primary coil.
To this end, we take about Ib. of No. 24 silk-coVered
copper wire, and wind it round the tube (as shown at
Fig. 33), from end to end in continuous layers, taking
care to put a sheet of paraffined paper between each
8 4 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
layer, and also to baste each layer with melted paraffin
wax- before winding on another. About four layers will
thus be got on, arid an even number of layers must be
aimed at, so as to get the two ends of the wire at the
same extremity, thus rendering it easy to fasten them
under the binding screws A A (Fig. 33). To effect this,
before screwing down the said screws, the ends of the
copper wire are stripped of their covering and wound
once round the screw of the binder. Free ends of wire,
at least 6 in. in length, must be left for attachments,
etc. This is shown at Fig. 34.
54. This primary coil, with its iron core, sliding
brass tube regulator, &c., may now be' fastened to- the
base board by means of two screws from underneath, as
shown at Fig. 32, at 4 in. from one end, and therefore
8 in. from the other. One of the free ends of .the
primary wire is brought to one of the binding screws
A, while the other connects to the clapper, B. A short
piece of wire connects the platinum screw pillar c to
the other binding screw, which is not visible, as it is
behind the platinum pillar. At this point it will be well
to try the working of the primary coil. For this purpose
couple up the two binding screws on the base D9ard with
a good bichromate cell. Connect the two binding
screws D D (Fig. 32) with the two brass handles intended
for use. Screw up the platinum screw, C, until the
clapper, B, begins to vibrate. Now hold the handles in
your hand. As long as the brass tube, E, is entirely
over the iron core little or no sensation is perceptible.
If an assistant pull out the tube, little by little, the
THE MEDICAL COIL. ^ 85
current will be found to increase in strength until the
regulator tube is quite out
55. The secondary coil now demands our attention.
A papef tube, precisely similar to Fig. 28, brit of such a
size as to slide easily over the primary coil E (Fig. 32),
is prepared, and paraffined. This must be cut exactly
the length of the coil F, leaving the knob E projecting.
The two square pieces of board in which the nicks were
cut (Fig. 31) must have central holes cut in them to take
this paper tube, and then be glued, one at each end of
the said tube, as shown at Fig. 31. Two small binding-
screws are then to be inserted in the centre of the upper
edge of each square. A bung is now placed in each
end of the tube, and a \ in. iron rod pushed through
both, to serve as an axle. This is then mounted on two
standards, as shown on Fig. 35, and beginning by
attaching one end of the uncovered wire to the binding-
screw A, about \ Ib. No. 36 silk- covered copper wire is
now coiled pn, being most diligent in avoiding kinks,
breaks, or flaws of every description. Each layer must
be paraffined and separated from its neighbour by
paraffined paper. When the quantum of wire has been
laid on, the finishing end is connected to the binding
screw A 1 , Fig. 35, The last coil should be covered with
paraffin paper, and finally covered with a jacket of good
silk velvet.' The secondary coil is then complete, and
may be slid in its place over the primary coil (see Fig.
S<?).. When it is quite over the primary, the secondary
current will be at its strongest, if the metal tube regulator
is drawn out; it v.'ill be weaker as the metal tube
86 ELECTRICAL INSTRUMENT MAKING FOR AMATEVRSi
,*
FIG. 29.
FIG.
W
FIG. 31,
FIG. yt.
THE INDUCTION COIL,
s
Fio. 33.
FIG. 34.
A
\ \
flL
FIG. 35.
FIG. 36
88 ELECTRICAL INSTRUMENT MAKING FOR AMA TEURS.
regulator is more and more inserted ; or may be even
more delicately regulated by sliding the secondary coil
itself more or less over the primary. The secondary
coil, while $he primary is- being excited with a freshly
made pint bichromate, will give a \ in. spark, when the
regulator is out, and the secondary coil right over the
primary. This will pass easily through a dozen persons.
'56. THE INDUCTION COIL. If, in the construction
of the coil described in the last eight sections, the
following modifications be made, the result v/ill be a coil
Capable of giving with 6 chromic acid, or Bunsen cells, a
good inch spark.
'In the first place, a brown paper tube should be made,
ins. long, f in. in internal diameter, and about A in.
thick by rolling and gluing the brown paper as recom-
mended at 50, round any cylindrical rod of the desired
fliameter. r
This tube should be treated with melted paraffin wax
as described at 50, and then filled with straight iron
wires, No. 18 gauge, as tight as they can be made to fit.
(These wires will not require soldering, &c., as they are to
remain permanently in the paper tube.) The wires
should protrude about in. on end side of the paper tube.
57 The wooden heads should be only two in
number, and 4 ins. square, instead of 3 ins. as directed
at 49. In one of these heads (the one intended to be
near the vibrating hammer), two -h in. holes'are drilled,
close to the central hole through which the paper tube
is passed. The two heads are to be glued on to the
paper tube, precisely as recommended for the secondary
TItM INDUCTION COIL. 89
coil tube at 49. When dry, the whole affair, wires
and all, should be soaked in melted paraffin wax.
It should be then set up between standards, and
wound carefully with four layers No. 18 double silk
covered wire. The ends of this wire should be brought
out, at the hole just made in the wooden head. When
this, the primary wire has been well, and evenly laid on,
it should be well basted with hot melted paraffin, and
then surrounded by a layer of paraffined brown paper,
pulled very tight and smooth, and made to adhere by
means of melted paraffin.
Over this layer of paper may now be wound the
secondary wire. This should be I Ib. of No. 39, in one
continuous length : and should be tested electrically for
continuity before laying on. The whole reel of wire
should be soaked in melted paraffin wax, and allowed to
drain while still hot, before being wound on ; each
separate layer should be basted with hot paraffin wax
before being covered with the next layer of paraffined
paper. The ends cf the secondary should be attached
to the binding screws marked A and A 1 at Fig. 35, 55.
The coil may now be fitted to a stand, with the vibrating
hammer and platinum screw, as described at 54; but
the stand chculd take the fcrm of a shallow box inverted,
and in the bottom muctbs fastened, by means of catches,
a Fizeau's condenser 47, and the hammer pillar and the
platinum screw pillar, each separately connected to one
of the projecting lugs. A falsa bottom is now put in the
stand, to hold all in its place. The condenser is essential to
Obtaining a good long and dense spark Careful insulation
90 ELECTRICAL INSTRUMENT MAKING FOX AMATEURS.
is also another important, if not the most important
item, in a successful coil.
58. THE MAGNETO-ELECTRIC MACHINE. This
is the first form of dynamo which the inventive genius
of Faraday placed before the scientific world. It had
its origin in the discovery "that a conductor moving
before the poles of a magnet, in such a direction as to
cut the lines of force of the said magnet, had its
electrical condition upset, so that a flow of electricity
was produced within it " ; and this could be rendered
evident by suitable means. Two forms of magneto-
electric machines will be described : firstly, the ordinary
" medical " or " shocking " machine ; and secondly, a
rather more scientific instrument, which can be used"
for many experiments, for which the former wo\ild be
totally useless.
59. THE "SHOCKING" MACHINE. The amateur
will need a pretty powerful horseshoe magnet, as shown
at Fig. 37 M, about 8 in. long, made of in. steel.
Each limb should be about l in. wide, and should at
the polar extremities stand about $ in. from the other.
If the amateur is able to work in steel, he may make
these magnets himself, and magnetise them by passing
magnetising coils of wire round each limb, and sending
a powerful current of electricity through the coils, until
the magnet is sufficiently strong. For the purpose
required, the magnets should pull easily a weight of
10 Ib. ; but it will, in most cases, be found cheaper to
buy these magnets ready made. The armature can be
readily constructed at home. It consists in two iron
THE "SHOCKING" MACHINE. 51
bobbins, F F*, screwed on to an iron yoke, or cross-
piece, Y ; these bobbins should be turned up out of the
very best soft iron circular rod, about if in. diameter.
.-*-- *
FIG. 37.
The centres, or cores, should be turned down to about
\ in. in diameter, a flange about rs in. in thickness being
left at both erds. A hole must be drilled up the centre
92 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS, j
of each bobbin, and this hole must be tapped to receive
an iron screw about I in. long, A in. diameter. These
screws serve to clamp the bobbins to the yoke. Owing
to the weight of the coiled bobbins, and the high
speed at which they mUst be driven, it is needful that
these screws should be strong to resist- the centrifugal
tendency. The length of the bobbins, including flanges,
is I in. If the operator has not a lathe, he may make
a fair substitute for the turned bobbins by putting a
screw-thread on the top and bottom of two pieces of
\ in. iron rod, ig in. in length, and screwing thereto
discs of soft iron, -rs in. in thickness, if in. in diameter,
having holes drilled and tapped in their centres to take
the extremities of the rods. The yoke Y has in its
centre a I in. hole, which must be carefully bushed with
some good, hard insulator, such as ebonite, ivory, or
boxwood, soaked in paraffin wax.
6cx The shaft or spindle on which the armature
revolves constitutes the distinguishing feature of this
particular form of magneto machine. It must be com-
pact, easy to construct, strong, and well insulated in' its
two halves. It must also have some device whereby
contact can be made and broken two or three times
during each revolution. To this end an iron rod, about
5 in. long and in. in diameter, is turned down at both
ends for a length of \ in., to about A in. in diameter.
This is to produce a shoulder at each/ end for the
spindle to rest in its bearings. These thinner ends are
shown at A and A'. Another, A in., is taken off at one,
end for about \ in., say, at the end A'. Then a screw-!
THE "SHOCKING" MACHINE. 93
thread of about \ in. in length is run on at D'. The
rod is then cut in two at D, and a length of about
\ in. is turned down to about i in. in diameter. A
screw must be formed oh this thinner portion, as shown
at D. The portion B' has then a hole about | in. in
depth drilled in it, and about -is in. in diameter. A
female screw must be cut in this, and the. hole bushed
with ivory or ebonite, as shown at C. Great care must
be taken in tapping this to receive the screwed end
of D, that the two halves of the divided rod are perfectly
insulated from one another. To this end an ebonite
washer is placed at w. It is well to test for insulation
by inserting the spindle at this point, in the circuit
between a galvanometer and a Battery. If any current
passes so as to produce a deflection, the -hole C must be
cleaned out and replugged, until perfect insulation has
been effected. Two small holes, to take short screws
about tk in. diameter by J in. in length, must be drilled
and tapped, near B and B'. These are intended to
make connection with the two ends of the wire coming
from the bobbins, as shown at Q. The next operation
is to turn up a small brass pulley, about A in. in
thickness, by about $ in. in diameter, which must have
a female screw put in it to fit over D', as illustrated
at P. This pulley serves at one time to clamp the yoke
Y in its place on the spindle, and to communicate tho
motion from the little driving-v/heel to the armature
spindle. All that now remains to be done to the
spindle is to file about J in. of its length, just beyond
the screw-thread D', into a triangular form, like the
94 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS
Grecian letter A, as shown at E' and E. The length of
the finished spindle should be 3^ in.
61. A brass frame, of the form "and dimensions
figured at Fig. 38 A, must now be provided. In
substance it should be in. wide by \ in. deep. At the
point d the frame expands both above and below, so as
to form -a lug, projecting about \ in. on either side,
against which can be clamped the magnet, M (Fig. 37).
The amateur should construct a pattern of this frame
in wood a trifle larger than it is intended to be (to allow
for shrinkage), and after having carefully smoothed and
bevelled the edges, send it to the brass founder's to get a
similar one cast in brass. When cast, the frame will
require careful cleaning and trueing up with a file.
Holes to take -ft in. diameter screws must then be
drilled and tapped at c c. These serve to fasten the
frame into its box. At B and B' rather larger holes
must be bored, the former being carefully bushed with
ivory or hard ebonite, the latter fitted with a metal
screw, through both of which a hole is put, sufficiently
large to serve as bearings for the end, A and A', of the
spindle (see Fig. 37-. AA*). At e and e\ precisely
opposite one another, are drilled two A holes, which
serve as bearings for the driving-wheel. At /, a small
hole about \ in. and i in. deep, is drilled and tapped, to
receive a small screw. This js intended to receive the
small contact spring R (Fig. 37.)
62. The next tiling needed is a small brass driving-
wheel about 4 in. in diameter and J in. thick, with a
grooye cut in the periphery, to take a gut band. The
THE "SHOCKING MACHI 93
tyro need not make a pattern for this, for nearly e^ery
toy-engine shop keeps brass flywheels of about this size.
If it have not a groove when bought, one can easily be
U - //---*
f 4% */^\ :;'
;;
i. ' e
.-. c
e'
rl
' ' Ml - = ^* " " L "V
"TV
FIG. 38.
put in it on the lathe. This wheel fe shown at Fig. 38,
G, mounted on an iron shaft, 3 in. long, H. This fly-
wheel may be keyed or brazed to the shaft Just
beyond the fly-wheel at I, the shaft is turned down
96 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
so as just to enter freely into the holes e and ^.
At the extremity, G, a screw thread is put on the
projecting end of the rod, to take the female screw of
the driving handle, shown at J. At the opposite
extremity, H, the shaft is drilled and tapped to take a
rather large-headed screw, which serves to prevent the
shaft from riding to and fro.
63. The operator may now proceed to wind the
bobbins. For this purpose he will need about I Ib. of
No. 26 or 28 double silk-covered wire. Having ascer-
tained, by testing with a battery and galvanometer, that
the wire is continuous, it will be well to soak the coil of
wire for a few minutes in hot melted paraffin wax,
allowing the paraffin afterwards to drain off, by suspend-
ing the coil for a few minutes in a rather warm place. The
iron bobbins, F F' (Fig. 37), are then wrapped round with
one layer of brown paper, which is fastened down to the
cores with good glue. Two semicircles, with hole cut in
the interior to fit the iron cores, are also glued inside
the flanges, so that the whole of the inside of the
bobbins is entirely covered with brown paper. When
the glue is quite </>j, the bobbins maybe warmed, and
plunged for a few seconds in hot melted paraffin. Each
bobbin is then to be wound, in the same direction^ with
the wire above specified, until the bobbin is filled to the
top of the flange. To prevent unwinding the finished
ends may be tied down to the bobbin with a bit of silk,
leaving, however, about 2 in. of the wire free for attach-
ment to the spindle. The commencing ends of the
wire must also be allowed to protrude beyond the flange
THE "SHOCKING" MACHINE. 57
for 2 in. or 3 in. fat connection. The two bobbins are
then screwed firmly to the yoke, the yoke afterwards
being put on the spindle. When this has been done the
four ends of the wires are joined, as shown at Q (Fig. 37) ;
that is to say, the two lower ends (after their covering
has been removed), are twisted together, soldered, and
rubbed over with Prout's elastic glue, to insure insula-
tion ; while the two top ends are screwed each one
down to the shaft, one at B, and" the other at B', the
covering having, been previously removed from the wire
at these points to. insure perfect electric contact with the
two halves of the shaft. In order that the machine
may work, the relation between the winding and connec-
tions of the lower ends of the bobbin wires (as seen
from the yoke end), must be like this CO. A thin
ebonite washer is now placed over the yoke, and the
little pulley, P, screwed tightly in its place. This must
be screwed down very firmly, as upon this depends the
stability of the armature. The armature is noc
finished, as far as actual work goes, but the bobolns may
be covered with any pretty-coloured silk velvet, if
appearance be studied.
64. The brass frame is now clamped by means of
a stout brass cross-piece K (Fig. 38) to the magnet; and
if necessary a second cross-piece (also of brass) is screwed
to the frame at L. The back end of the armature
spindle (the end farthest from the pulley) is passed into
the larger hole B, of the frame A. This will allow the
other end of the spindle being inserted into the opposite
hole B, without strain. When the spindle is in its place
$8 ELECTRICAL INSTRUMENT MAKING FOR AMA TEURS.
the hollow screw is place in B 1 , and screwed home, until
the armature spindle can just turn freely, without too
much play, before the poles of the magnet In like
manner the driving-wheel G is put in its place by passing
the long end of spindle through the holes e and e, and
then fixing it in position by means of the larger headed
screw already mentioned at the end of 62.
The small spring R is then screwed down to the frame
at/! It mustjusf rest on the projecting corners of the
triangle E 1 , during rotation, and just clear the flattened
portions. On careful attention to this point a great
deal of the efficiency of the machine depends. The
shock is felt, not while the current is continuous, but at
the instants of breaking and making contact. The
instrument can now be placed in any suitable box, which
must be sufficiently long to allow the armature to rotate
without striking against the ends, and just a trifle say
$ in. wider than the frame. The frame is attached to
the box by means of screws which pass through the
box into the holes c and c 1 of the frame. The screw
at c "should be hollow, so as to take a small pin, or
metal hook, which is intended to make connection
with the metallic cords and handles, that are used for
giving shocks. Another similar screw is put in the side
of the box almost opposite the armature, and to this
screw, in the inside of the box,, is fastened a rather stiff
brass spring about \ in. wide, bent into\he shape of L,
the longer end of which must press firmly against the end
of the armature spindle which projects through the hole B
Fig. 38 . This latter screw forms the other terminal of the
THE UNI-DIRECTION CURRENT MACHINE. 99
machine, the other cord, etc., being affixed thereto. It
is usual, though not essential, to place a soft iron keeper
at the back of the magnet's poles, which may be partially
or wholly removed at will This enables the operator
to regulate somewhat the strength of the shocks by
increasing the inductive effect of the magnet on the
armature, consequent on the removal of keeper from near
its poles. There must also be a hole in the box, to
allow of the insertion of the driving- handle, J.
65. THE UNI-DIRECTION CURRENT MACHINE.
r At 59 we studied the construction of the magneto
specially designed for physiological effects. Here we
shall direct our attention to one that may be used for
heating, lighting, chemical, and electro- magnetic experi-
ments generally.
The first thing to be procured or made, as described
at 59, is a horse-shoe magnet, having a clear space of
I in. between the poles, 8 in. long, \ in. thick, and it in.
wide. This should be capable of lifting at least IO Ib.
An armature, or iron core, 3 in. long, ^ in. diameter, 1
with a channel all round its length, I in. wide, | in. deep
at the sides, j in. deep at the ends, as shown at Fig. 39, must
next be made. It may be cast in good malleable iron,
and carefully annealed ; but it will give better results if
cut out of a soft piece of circular wrought-iron rod, which,
with a good file and a little patience, may be filed up to
the desired shape. Should, however, a casting be pre-
ferred, the pattern for the purpose may be cut out of a
common broom stick, nicely sand-papered to the desired
size, the channel being cut out with a sharp chisel.
loo ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
Whichever plan may be adopted, the armature, pre-
vious to windiftg> must be fitted with two brass or gun-
metal heads, wherein the spindle is affixed. For this
purpose, the amateur who has no lathe must strike two
circles }g in. diameter on a piece of hard sheet brass, J in.
thick, great care being taken to get the circles true, and
the centres distinctly marked. These circles can be cut
out roughly with a hack-saw, and finished up carefully
to the line with a file. A perfectly central hole is to be
drilled in each disc with an Archimedean drill, to take
circular iron rod A in. diameter. Two smaller holes,
FIG. 39.
one on each side of the central aperture, are also drilled
through the brass circlets. These are to take the screws
which hold the heads on to the armature. Holes cor-
responding exactly to these in position, must now be
drilled in the two extremities of the armature, the
greatest care being taken that the heads are fitted truly,
so that the armature may run without wobbling. These
holes in the armature must be tapped so as to take the
screws.
On the brass circlet which will afterwards be placed
nearer to the commutator, must be drilled yet two more
holes, through which the ends of the wire, with which
THE VNI-DIRECTION CURRENT MACHINE. 10!
the armature will be wound, must protrude. When
the armature and heads have thus been fitted, two
pieces of the V in. iron rod before-mentioned should be
cut, one for the back and the other for the front or com-
mutator end, the former being about li in. long, the
latter 2 in. A thread must be put on the end of each
of these rods where they enter the brass circlets, and
after being screwed in, the end must be burred over by
hammering, so as to prevent the rods (which serve as
the spindle for the armature) from working loose during
rotation./
66. A small piece of boxwood about I in. long is
now to be cut or turned into a perfect cylinder about
} in. diameter. A hole & in. diameter, is put centrally
through this, so that it can be made to fit tightly on the
spindle. A piece of brass tubing, about T V in. thick, i
in. long, and I in. in internal diameter, is now driven on
to the boxwood cylinder. Two short brass screws, not
reaching to the iron spindle, must now be procured, and,
holes having been drilled at two opposite points in the
diameter of the ring, and countersunk, the screws are in-
serted and tightly screwed down so as to hold the ring
in its place. The brass tube or ring is now cut into two
102 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
halves by giving two fine saw cuts across the ring at
two points equi-distant from the screws just inserted.
This completes the commutator, which is now ready
to be slipped on the longest end of the spindle when
required.
67. The next operation is to wind the armature ;
and here the amateur must use his own discretion as to
whether he will wind it for a large current of low E.M.F.,
or for a high E.M.F., but little current If he desires
to have both he will do well to make two armatures
precisely as described, and wind the one with coarse
wire, say No. 14, for large currents, and the other with
No. 22, for smaller currents of higher E.M.F. Previous
to winding, however, the armature must be taped, as it
is technically called, to insure perfect insulation.
To this end the brass heads, with the spindle, etc., are
removed, and a piece of silk ribbon, i in. wide, glued
right round the central portion of the channel of the
armature so as to entirely cover the iron. In like
manner, the sides of the channel are also covered with
silk. When the glue is quite dry, the armature may be
immersed bodily in melted paraffin wax, so as to satu-
rate the silk thoroughly with paraffin. It should then
be taken out, allowed to drain, and set
N 68. The armature may now be wound. The
operator takes the armature in his left hand, with that
end which is to be against the commutator nearest to
him. Holding in his left hand also, about 6 in. of spare
end of the wire with which he intends to wind the
armature, he grasps the reel or hank in his right, and.
THE UN1-DIRECT10N CURRENT MACHINE. 103
winding always in the same direction, fills up the channel
with the wire, taking care to wind tightly, smoothly, and
evenly, and being most careful not to abrade the covering
of the wire, so as to produce leakage. About 3 oz. can
be laid on without extending beyond the sides of the
channel. (See Fig. 40'.)
It will be well, before finishing the armature to test
the wire for insulation. This can be done by connecting
one end of the wire to one pole of a single-cell chromic
acid battery, the other pole of which is connected to
one terminal of a galvanometer. A short length of wire
is attached to the other terminal of the galvanometer.
If with this wire the iron part of the armature be
touched no deflection will ensue if the insulation between
the coiled wire on the armature and the iron body of
the armature is good: but if it be defective, the needle
will swing round and indicate the fact. It is needless
to point out that the iron of the armature should be
sand-papered at the spot where it is to be touched with
the wire, as otherwise the paraffin wax adhering to it
might falsify the results of this trial.
Both the beginning and ending portions of the coiled
wire must be brought out at the commutator end of the
armature. The heads of the armature can now be
permanently screwed on, the ends of the coiled wire
pushed through the two holes left in the head, the wire
pulled very tight, the holes bushed with melted gutta-
percha to prevent any chance contact of the wires with
the brass head, then the commutator pushed on to the
spindle (a drop of while hard varnish having been pre-
104 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS]
viously applied thereto to insure adhesion) ; and lastly
the two projecting ends of the wire, cleaned from their
covering, and soldered, one to each brass cheek of the
commutator, as shown in the cut at Fig. 40, which also
shows the position the slits on the commutator should
occupy with relation to the channel of the armature.
N.B. Only one slit is shown, as also only one wire,
since the other is hidden by the spindle.)
69. The magnet should now be mounted on a stained
wood or mahogany base board, about 14 in. long, by 8 in.
wide, and I in. thick. Two pieces of well-seasoned wood
(deal will do) i in. in square section and about 6 in.
long are to be glued one each side of the centre of the
base board, so that the magnet can lie with each pole
supported on one of these strips, leaving a clear place
for the armature to lie between without touching any-
thing.
A piece of square brass rod 1 in. in "section and 4! in.
long is next procured. A i in. hole is drilled at each
extremity to take a long screw. At the centre, another
piece of similar brass, $ in. square, but**only j in. thick,
is soldered, and through this is drilled a iV hole to
serve as a bearing for the, back spindle of the armature.
This piece of brass serves a double purpose. It acts as a
strap td hold the magnet in its place when screwed down
by the two side screws ; it also serves to carry the bear-
ing. The exact distance of this brass strap, from the
poles of the magnets, depends on the length of the arma-
ture ; it should be so placed that when the spindle is in
the bearing up to the brass head, the front head should
THE UNI-DIRECriON CURRENT MACHINE. 10$
just be flush with the poles of the magnet. The
appearance of the magnet, supported on the two wooden
blocks and held down with the straps, is well shown
at Fig. 41.
For the front bearing, a piece of stout sheet brass
about | in. in substance is cut into the shape of an angle
about li in. wide at the base, 2 in. high, rounded at the
apex. The base of this triangle is bent at right angles
to the rest to the depth of about 1 in., in order to form a
FIG. 41.
foot Two holes must be drilled into this reversed
portion, into which are passed two screws to attach it to
the base board. The armature having been placed in
the back bearing, is supported by a cork and side
wedges, so that it stands equi-distant from i either
pole of the magnet, touching at no part. The front
bearing is then rested on the base board, and pushed
against the front spindle. A mark is made where, it
touches, and a A hole drilled through the bearing at this
sj)ot This must be cleared out, so that the armature can
106 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
rotate freely between the poles of the magnet, supported
on the back and front bearing, without any strain, twist
or bind.
A keyway must be filed on the back spindle, and a
small $ in. grooved pulley keyed thereon.
70. In order to collect the electricity set up by the
rotation of the armature, two " brushes " are needed
These consist in two strips of sheet copper, about S 1 o in.
thick, hammered until quite springy, and then cut about
\ in. wide, by i\ in. long. At about \ in. from one end
of each strip is punched a circular hole, wide enough to
take the tang of a binding screw. Two little blocks of
boxwood, about \ in. in square section, one being f in.
high, the other about i\ in. in height, are now to be glued
on to the base board, one on each side of the commutator
( 66) at a distance of about if in. from it To insure
rigidity and strength, these two blocks should be made
longer than they are intended to stand above the level of
the base board, a corresponding square hole being cut
into the base board with a chisel. On gluing these
blocks in their places, there will be no fear of there
becoming detached.
When quite dry, a small hole (a little less than the
tang of the binding screws which are to be used) is
drilled centrally and perpendicularly in these blocks.
Each copper strip or "brush" is now bent, so that when
lying on its block it j**st touches the commutator ring ;
that on the higher block resting on the commutator
above, while the one on the lower block can be just
caused to touch the commutator underneath Th,2
THE UNI-DIRECTION CURRENT MACHINE. 107
binding screws are then inserted in the holes through
the brushes into those in the blocks, and then screwed
down tightly, so as to hold the brushes firmly in their
place, pressing lightly against the commutator above and
below. These two binding screws, as shown at Fig. 42,
serve as the/^/?j or terminals of the machine, and to
these the 'wires, etc., may be attached to lead the current
where desired.
FIG. 42.
71. The only thing now needed is a bracket and
hand-wheel, wherewith to drive the armature. The
simplest, and at the same time the strongest mode of
affixing these, is to have a A-shaped bracket cast in iron
at the nearest foundry. This should be 9 in. in height,
and should stride over the magnet)* clearing it on each
side by about I in. Holes must be drilled in the feet to
io8 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS!
screw it to the base board, and at the top a hole must be
put through it to take a bolt and nut. A 12 in. wheel,
either of iron or wood, and furnished with a wooden
handle, is put on this bolt, and a back-nut screwed on to
keep it against the bracket. A gut or leather band
connects the hand-wheel to the pulley at the rear of the
armature. The complete machine is shown at Fig. 42.
With this machine a variety of instructive experi-
ments can be performed. Small lamps can be lit, an
arc light can be shown, coils worked, water decomposed,
etc. The direction of the current can be changed by
changing the direction of rotation ; and by sending the
current from a battery into the armature the machine
gives a proof of its reversibility by running as a
motor.
$ 72. THE DYNAMO. In another work* the author
has gone pretty fully into the theory and practice of the
construction of dynamos of the Siemens' type. It is
proposed here to give a general outline of the mode of
calculating the winding of dynamos of the ring arma-
ture type, along with such instructions as will enable the
amateur to decide upon the size of the armature and
field magnets required to produce a given effect.
Machines of the "ring" class may be conveniently
constructed either of the A Gramme form, as shown in
the annexed Fig. 43, or of the form Fig. 44, now generally
known as the " Manchester " type, due to Mather,
Hopkinson, and Platt. Wooden patterns should be
made of the fields, from which good malleable iron cast-
* The Dynamo : How made and how used.
THE DYNAMO.
109
ings can be obtained from any founder who knows his
work. The castings must be soft, or else quite 40 per
cent, of the efficiency of the machine will be lost
The ring (which should be of the toothed or Pacinotti
form shown at Fig. 44, may be also cast, but will be better
in every respect if built up of laminations in sheet iron,
of the same shape as shown, and strung together by
pins running through every alternate tooth, which pins,
if screw-headed, serve to bolt the armature firmly to
FIG. 43.
the brass star-wheel, or " spider," by means of which it is
affixed to the shaft. As these laminations or " punch-
ings " can be had ready made in many convenient sizes
of the makers of electrical instruments, the manufacture
of such is not advised here, as a punching tool costs from
.3 to 4. to make ; whereas the punchings can be had
for a few shillings per gross.
The amateur, having decided on which patterns he
intends to employ for the fields, has next to consider the
I to ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
purpose to which he intends to put the machine when
complete, and the power he has to drive it. It is really
the former of these data which will decide the size
of the armature, and, consequently, of the machine
itself.
First then, as to power. The best modern dynamos
return about 95 per cent, of the energy spent upon
them, as electricity. Theoretically, I h.p. is equal
to 746 watts. The watt is I arnp^re multiplied by
I volt, so that we are at liberty to take our " watts " out
I
PIG. 44.
of our dynamos as we please. We can have, for
instance, a low tension machine of only 2 volts E.M.F.,
but giving a current of 373 amperes, or we may have a
small current, of 2 amperes only, with an E.M.F. of 373
volts ; in either case 746 watts would be generated by
the dynamo, and rather more than I h.p. needed for
its evolution.
The next question to be considered is, what number of
volts is the machine to give ? Having decided this point,
the length of the wire to be employed is immediately found j
THE DYNAMO. UI
since large, well-constructed machines, give on an average
I volt for each yard of copper coil in the armature.
Therefore, so many volts as are desired as the E.M.F. of
the finished machine, so many yards of wire on the
armature. If the machine is a small one, i.e., does not
greatly exceed 2 cwt. in weight, it will be well to allow
more than a yard per volt, as small machines are not so
efficient as large ones. In calculating the voltage, the
desired current must also be considered, and allowance
made both for the internal and external resistances, as
also for the amount of current required to keep up the
requiske intensity of the magnetic field. Remembering
Ohm's law that the current will be equal to the E.M.F.
in volts, divided by the R in ohms, we can easily (know-
ing what resistance we are going to have in the outer
circuit) calculate from a table of wire resistances, what
gauge copper wire we may use on the armature, so that
the length desired to produce the desired number ol
volts shall not exceed in resistance the margin left from
the outer circuit, to allow of the passage of the desired
number of amperes. In making this calculation, it must
always be borne in mind, that even in the best made
shunt dynamos, it is usual to allow 4 to 5 per cent, of the
current to be shunted off through the field magnet coils.
The annexed table of sizes, weights, lengths, and resis-
tances of the usual covered copper wire found in
commerce, will enable the amateur to calculate at once
the desired gauge and length of wire to be used to
furnish a given E.M.F. and current :
73. Table showing relative resistances/ weights
ii2 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
and length, of different gauges of covered copper
wire.
B. W. G.
Decimal
of an
inch.
Equiva-
lent in
m/m.
Yards to the lb.,
approximate.
Ohms per Ih.
approximate.
Copper.
Silk
covered.
Cotton
covered.
IO
134
3'44
6
6
01058
12
ICQ
2770
9
9
02416
14
083
2-108
15
15
07186
16
06 5
I '65 1
25
24
igiO
18
049
1-245
45
41
5916
20
035
8890
64
59
2-273
22
028
7112
119
109
5*548
24
X>22
' -5588
167
156
I4"?6
26
018
4571
266
244
32'49
28
014
3556
414
383
88-77
30
'012
3048
542
500
it>4' 5
32
009
2286
870
800
521-19
34
007
1778
1,170
1, 080
1421-98
35
005
'13
1,475
1,360
5473-332
36
004
1016
i ,660
1,530
13341.014
In calculating from this table, it must be remembered
that the resistance of any wire coiled on a Siemen's n:
girder form of armature, is proportional to the length of
wire used: but in the ring armature form where the
winding is continuous, the resistance is only that of
the entire length of wire employed. There is still one
consideration to be made in deciding upon the gauge of
wire to be used in the armature, and that is, that the cur-
rent carried must in no case be so great as to heat the
7 HE DYNAMO. 113
wire. It is usual to allow I square inch section for each
2,000 amperes to be carried ; and this rule holds good
in the smaller diameters.
Based on these rules, the armature may be constructed
as small as will hold the requisite amount of wire. The
dynamo figured at 43, is capable of working up to 120
c.p., say six lamps of 50 ohm's, cold resistance, requiring
one ampere each == 6 amperes. The E.M.F. of this
machine should be about 50 volts. In Fig. 44, owing to
the fact that the armature is made deeper, as many as
ten such lamps may be put on. In winding as a shunt
machine, it will be well to put the following rules into
practice. 1st. Let the resistance of the armature coils
( the entire length of wire) be -fa of the resistance of the
outer circuit, including, lamps, leads, baths, accumulators,
etc. 2nd. Let the resistance of the field magnet coils be
2O times that of the outer circuit 3rd. Let the diameter of
the wire chosen to fulfil this latter condition be such, that
the diameter of the wound fisld magnets does not exceed^
nor fall greatly short of, twice the diameter of the bare
iron cores.
One example, worked out in full, is here give* to
show how these data are to be employed in practise.
Let it be desired to construct a gramme dynamo to
light five twenty c.p. lamps of 50 ohms cold resistance,
each requiring about one ampere to light it to the
full. Supposing the resistance of the " leads " (cables,
connections etc.) to be one ohm. The highest resis-
tance we shall have lo overcome in the outer circuit will
be. when only one lamp is used, and this will not exceed
114 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
50 ohms for the lamp, and one ohm for the leads
= 51 ohms. In this case one ampere only will be
needed. When all the lamps are on, in parallel circuit,
the resistance falls to of 50 for the lamps = 10 ohms,
and one ohm for the leads = 1 1 ohms. In this latter
case, however, we shall need five amperes to feed the
five lamps. Hence in the former case, if the armature
be wound so as to give 5 1 volts, we shall get
E = 5 1 volts <
v, ,- = I ampere :
R = 5 1 ohms
and in the latter, if the armature be wound to give
55 volts, we shall have
E = 55 volts
_ r- =5 amperes.
R = 1 1 ohms
Hence," as far as the outer circuit is concerned, an
E.M.F. of 55 volts will be ample. It will be well to allow
a trifle more than this, because of the resistance of the
armature. Therefore, allowing one yard per volt, 56 or 58
yards of No. 18 wire will be sufficient on the armature.
As the current will never exceed five amperes, the
diameter of the wire need not exceed No. 18 gauge, as
this*will carry safely five amperes of current. Hence
56 to 58 yards of No. 18 wire will suit our purpose very
well. On measurement it will be found that a ring of
about 5 in. diameter will take this amount of wire, the
resistance of which ( of 56 yards = 14 yards = 0*25 of
an ohm) falls considerably within the limit of $, of the
outer circuit resistance, An armature of this size will
admit of field magnets having cores presenting 12 inches
length by 2 inches diameter, on which the magnetising
THE DYNAMO. 115
coils may be \vound. Since the armature resistance is
only 0*25 cf an ohm, we muct put on a v/ire having 400
timecthic, or 400 X '25 = 100 ohms ; and yet shall not,
when wound on, greatly exceed the original core diameter.
It will be found that 13 t' 14 Ibs. of No. 24 gauge v/ill
fulfil the requirements. In the above example, the very
best soft iron is supposed to be employed. Any devia-
tion from this will entail the employment of more wire,
both in the armature and in the field magnets.
74. As to the mechanical part of winding the cores
and armature, the following points must be observed
to obtain the best results. 1st. The cores of the F.M.
must be carefully taped and varnished to insure insula-
tion. 2nd. The armature must* also be most caref-ully
taped and varnished. No part of the iron, where the
wire has to bo wound, should be left uncovered. 3rd
Plaving decided into how many sections the armature is
going to be wound, the wire needed to go round one
section must be measured off, and the whole series of
sections cut off to the same length. 4th. The wire, both
on the F.M's. and on the armature, must be wound as
tightly and smoothly as is possible. 5th. After winding,
the armature coils should be soaked in a thin shellac
varnish (French polish does admirably), and then baked
in "a. slow oven at a heat not exceeding 212, until the
varnish is quite hard and dry.
In both the Gramme and the Mather-Hopkinson
type of dynamo, the field magnet bars must be wound
so as to produce consecutive poles at the top, and
consecutive poles at the bottom of the machine.
Ilfi ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
The commutator must have as many divisions as there
are sections in the armature.
The brushes should be self-regulating by means of
spring and swivel, for pressure ; and should be capable
of adjustment by means of a set screw, for the best
angle of commutation
In the shunt wound machine, the two_ends of the
field magnet coils are each connected to one of the
brushes, and these latter again connected to the two
binding screws, which form the terminals of the
dynamo.
On no account should the armature be mounted on a
wooden hub. This was a favourite mode with the older
makers, but it is to be condemned, in view of its
mechanical inefficiency, and more so, because of the
resistance it opposes to the escape of the heat generated
by the Foucault's currents. Brass or gun metal spiders
only are admissible. Iron or any other magnetic metal
must not be used, either for the spiders or for the bearings
of the shaft, as they short-circuit the magnetic field, to
the great detriment of the inductive effect of the field
magnets or the armature coils. The collector bars of
the commutator should be as massive as possible, and
separated from each other by air spaces, as in the
Hochhausen form.
75. AMMETERS. These instruments are intended
to measure the amount of current in amperes passing
through any given circuit. One ampere of current,
parsing through a solution of sulphate of copper, is
capable of depositing 18-35 grains of copper per hour.
AMMETERS. 117
To make such an instrument, suitable for the general
purposes of the amateur, and capable of reading from
I up to 5 or 6 amperes, the following mode of procedure
may be adopted :
76. Procure a square piece of deal 4 in. x 4 in.
x j in., smoothly planed up ; also a glass-capped circular
cardboard box, 3 in. diameter by ii in. deep, inside
measure. (Such boxes are sold by mineralogists, etc.,
for preserving specimens.) Push^the cardboard bottom
out of the box. This bottom may be used as the
" dial ',' \vhereori to inscribe the degrees of current.
Stain the wood black, or any desired colour, and varnish
jt. Now trim the edges of the bottom of the box into
an exact circle, capable of entering easily into the box
from which it was removed. Describe nar its circum-
ference three circles in ink, with a pair of compasses.
Let these circles stand about in. from one another.
Describe also a small circle about a \ in. diameter in the
centre of the card, to serve as a guide for a central hole,
which must be punched out with a \ in. circular punch.
77. Now make a small glass pivot, as described at
5 and" 6, about in. long, and not more than \ in
diameter at its base. When made, test it for freedom
and absence of friction by poising it on the point of a
very fine needle, the head of which is stuck in a bung.
If it spins round quite freely without any hitch, well and
good ; if not, make others until you get one perfectly
freely suspended. This is most essential.
Procure a strip of copper sheet, about ,V in. thick,
4 in. long, and \ in. wide. Punch three \ in. holes in it.
1 18 ELECTRICAL INSTRUMENT MAKING FOX AAfA TEUKS.
one at each extremity, wherein to pass the binding
screws, and one in the centre. Lay this strip on the
little wooden square, across its centre, and screw it down
to the square by means of two small binding screws,
taking care that there is space left for the cardboard
box to stand between them without touching either.
Now stick a fine, bright, and sharp No. 8 needle, point
upwards, into the centre of the square, just where you
have punched the central hole in the copper slip. Care
must be taken that this be perfectly perpendicular, and
clears the glass top of the capped box by about in.
when this latter is placed over it.
45-
$ 78. With a pair of round-nosed pliers, make, from
a short piece of No. 16 pianoforte wire (steel), a single
coil to fit tightly round the base of the glass pivot,
having a straight \ in. prolongation on either side of
the central loop or coil, as shown at Fig. 45 , A. This must
be thoroughly magnetised by rubbing with a good horse-
shoe magnet, and then fastened with a drop of glue to
the base of the pivot, extreme care being taken that no
gjue touches the rim of the pivot, otherwise it will not
swing freely. The pivot must now be placed en the
needle in the centre of the square ; and the punched
AMMETERS. 119
circle, which is to serve as a dial, held over it, and
lowered down over it until the point of the pivot protrudes
about J in. above the central aperture, and the magne-
tised needle below clears the card also by about i in.
The dial is to be fixed -in this position by gluing three
corks of the right height to the square, and then gluing
the card on to them. Here, again, care must be taken
that the pivot does not touch the card dial at any point,
and also that the magnetic wire needle below is clear of
the dial.
A small pointer in black paper, straw, or any other
very light rigid material, about ij in. long, is to be
lightly glued to the top of the pivot at 'right angles to
the needle below. The glass-capped box may now be
placed over all, and glued to the square by running a
lit'tle thin glue round its bottom edge. When quite dry,
the grading maybe proceeded with.
79. To grade correctly, at least 10 quart Daniell
cells will be needed. These must be freshly made up,
and filled to about half their capacity. A decomposi-
tion cell (a stoneware basin or foot-bath will 'do),
capable of containing two squares of copper sheet,
facing one another, at least 6 in. X 6 in., must
next be provided, and filled with a strong solution of
copper sulphate slightly acidulated with sulphuric
acid.
The copper plate which it is proposed to attach to
the negative pole of the battery (the zinc end), having
been caref illy weighed in a balance capable of turning
to the iic of a grain, and its exact weight noted, a
120 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
Daniell cell, the ammeter, and the decomposition cell
are arranged in series, as shown at B, and allowed to
work for one hour exactly. At the end of that time
the copper plate is withdrawn, washed (not rubbed), and
dried. When dry, it is weighed, and the increase noted.
If exactly 18*35 grains, well and good ; if not, the
following alterations must be made until the deposit
reaches exactly this amount.
Should the deposit be less, approach the plates
in the decomposition cells, and add more fluid to the
battery cell, until the deposit reaches exactly the
18*35 grains per hour. If the batteries are less than
quart cells, two, coupled for quantity, may be needed
to produce the desired result. Should the deposit
be greater, remove the plates in the decomposition
cell farther apart, in either case carefully noting
the distance. Once the desired result attained, the
greatest care must be taken to maintain all in statu
quo, and the exact position of the two copper plates in
the decomposition cell being marked, the temperature of
room taken, the height of the fluids in the battery cell,
or cells noted, etc. One pole of battery is now detached
from the ammeter, and this latter moved round until the
magnetic needle lies quite parallel with the copper strip,
hence the pointer exactly at right angles to it The
spot at which the pointer stands is marked with O. The
battery is again attached to the ammeter, and the
deflection, when the needle comes to rest, noted. Thfs
deflection corresponds to one ampere of current, and
Should -be .marked as such on the dial. To get the
THE VOLTMETER. 121
corresponding deflection on the other side, the poles of
the battery must be reversed.'
All the other degrees may be got in precisely similar
manner, multiples of i8'35 gr. of copper being considered
and counted as so many amperes, more cells being
connected up, in parallel, to obtain the desired weight
of deposit per hour.
80. THE VOLTMETER. This instrument serves, as
its name implies, to .measure the voltage, or electro-
motive force , of any source of electricity. Perhips there
is no subject so puzzling to the electrical student as the
difference between electro-motive force (E.M.F. as it is
generally abbreviated) and current. As it is essential
to have clear ideas on these points, in order to grasp the
principles on which the construction of the voltmeter is
based, I shall deviate somewhat from the plan hitherto
followed in these pages, and devote a few lines to the
consideration of what E.M.F. really is.
8 1. According to the present state of our know-
ledge, the phenomena which we group together under
the name of " electricity " are simply manifestations of
a peculiar mode of motion in the ultimate particles ol
bodies, called atoms. What the nature of this motion
may be, whether rotary, undulatory, vibratory, etc., we
are as yet utterly unable to say. We have, however,
at our disposal, many means of setting up this motion,
such as, for in stance, friction (and many other mechani-
cal movements), cJiemical action, the application or ab-
straction of heat (itself a mode of motion), or of .light
(another form of motion), etc. I purp sely leave out
122 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
magnetism as a source of electricity, since magnetism is
itself but a manifestation of electricity.
Whichever of these means be employed ' as sources of
electricity, we find (other things remaining Jlie same)
that an increase in the means employed is followed by
an increase in effect. Now the means employed, whether
they be friction, motion in the field of a magnet, chemi-
cal -action, or heat, are called, when viewed under this
aspect, " electro-motive force," or that which sets up an
electrical condition. In the same way as we can only
measure the power of a man/ or of a steam-engine, by
the work performed by them when they exert their
force, so we can only measure the E.M.F. of any given
battery, dynamo, frictional or induction machine by the
electrical work it can perform. It is optional whether
we use as measures the chemical, the magnetic, or even
the mechanical work which the electricity set up by the
given means can perform. As a matter of convenience,
the magnetic work performed is generally preferred as
? means of measurement.
$ 2. From a study of Ohm's law, we learn that the
amount of current set up in any given circuit is equal
to the " electro-motive force divided by the resistance in
that circuit." This simply means that, with any given
force setting up electricity, the current will be greater
as the resistance is less. This i is usually expressed
p = C. ' From this it is equally evident that if we know
the value~of R (the resistance), and of C (the current),
iwe can always calculate the value of E, by multiplying
THE VOLTMETER. 123
R by C. Moreover, it will be equally evident that, if
R be constant, E will always be directly proportional
to C. To prove this, let us give an actual value in
figures to the E.M.F., the R and the C. Let us sup-
pose, for example, that we are working with a battery
having a known E.M.F. of two volts, and we get a cur-
rent of one ampere when working through a total resis-
tance of 2 ohms. This would actually be the case,
Now, let us imagine that
using another source of electricity, pitted against a pre-
cisely similar total resistance, we found we obtained a
current of 10 amperes. What would E.M.F. be ?
Simply-- (C = 10) x (R = 2) = (E.M.F. 20).
Here we see that as the current Is ten times as large as
in the first instance, so also is the electro -motive force.
But it will be perfectly evident, that as the current would
be altered in amount by any alteration in the resistance,
it is essential that any resistance in circuit should be
"kept as nearly as possible invariable, if we desire to have
results that are ev/en approximately correct.
83. Unfortunately, no two batteries or sources of
electricity have precisely the same resistance ; and even
if they had at one instant, this would be found to vary
during work. On the other hand, the internal resistance
of most batteries does not often fall below 0*0016 of an
ohm, 'nor rise above 2 ohms per cell It follows from
this, that if we make the resistance of the voltmetei;
very large in proportion to the possible resistance of the
battery, we can reduce the possible error to within any
4 ELECTRICAL 1NSTR UMENT MAKING FOR A MA TEURS.
desired limits. In practice, it is usual to allow a resis-
tance of 50 ohms for every volt to be measured. This
gives sufficient accuracy for most purposes, as the error
is theo-^vithin 3 per cent, of the total readings.
84. From the above, it follows that a galvanometer
having a resistance large in proportion to the current
to be measured, if graded to indicate the current deflec-
tion which corresponds to a given voltage, can be used
as a voltmeter. Certain precautions are necessary in
order to obtain deflections at all parts of the scale, suffi-
ciently distant from one another to be easily read. The
following instructions will enable the amateur to con-
struct an instrument, which, if used intelligently, and
far from masses of iron or other magnetic bodies, will
give results which for the measurement of low E.M.F.,
say from I to 5 volts, far exceed in accuracy those given
by m,ore pretentious apparatus. As there are several
points of resemblance between the ammeter and the
voltmeter, the student will do well to refer to 75, pre-
vious to making the voltmeter.
85. The first thing needed is a light cardboard 01
chip box, 2j in. long, by \\ in. wide, and in. in
height. There must be no ends to this, which exactly
resembles in size and shape the 'sliding cover of a
Swedish " Tandstick " box. Indeed, such a cover may
be used for the purpose with good results. If the
amateur desires to make this himself he need only take
a strip of stoutish millboard, 4$ in Jong, by 2\ in. wide,
fold it three times on itself, and gJue the top and
side edge as shown in figure 46.^ A circular central
THE VOLTMETER.
12$
aperture, -about J in. in diameter, is to be cut in the
bottom of the box with a sharp penknife, while a central
strip, also about \ in. wide, but extending the whole
width of the box, is cut away from its upper portion.
This being done, the box is slightly moistened with thin
glue, and then quickly, but carefully, wound with about
j oz. of No. 40 silk-covered German silver wire. This
should be about '0058 of an inch in diameter, and have
Fio. 46.
a resistance of not less than 2570 ohms to the ounce.
Care should be taken to wind this wire evenly, closely,
and without kinks, leaving about 3 in. wire free at each
end. Having started winding at one end, the operator
proceeds to wind continuously across the box, coil after
coil, till he reaches the central hole and slit ; these he
leaves open, crossing over to the other half by letting the
wire pass diagonally across one side. He continues the
winding on to the othehside, in the same direction, tiU
126 ELECTRICAL INSTRUMENT MAKING FOR AMAT&VRfy
he reaches the end of the box. If the glue gets dry
during the winding he may apply a little more fresh glue
as he nears the end, so as to ensure that the wire should
not uncoil when released. When the box has been
wound from end to end, as shown fit Fig. 47, it must be
allowed to dry thoroughly in a warm dry place, and
when quite dry soaked for a few minutes in hot melted
paraffin, until permeated with it, and then hung up to
Fio. 47.
'drain and set. While this is taking place, a small base
;board of \ in. deal or mahogany is planed up and stained
or polished as desired. This piece should be 4 in.
square. The exact centre of this having been found, a
needle l-r in. in length is forced head downwards in the
iboarJ at this spot so that the pointed end stands
perpendicularly upwards at a height of i in. from the
surface of the base board. A small I in. magnetic
needle is now made out of a short length of piano wire,
THE VOLTMETER. 127
and fastened to a glass pivot precisely as directed in the
case of the ammeter (see 77 and 78). This is carefully
tried for- freedom of motion, equality of magnetism, etc.
The box, coiled with wire, is now placed over the upright
needles on the board (this latter being made to pass
through the central apertures, without touching them,
so as not to blunt or grease the needle point, which
would be fatal to free movement), and while in this
position, the pivoted magnetised needle placed on the
point, so as to see whether the magnet swings freely
without touching any part of the inside of the box, of
which, in fact, it ought to stand in the centre. It may
require raising a trifle, which may be done by means of
thin slices of cork. When this has been duly adjusted,
the cork slices (if any) and the box are fastened down to
the base board with a little good, hot glue. Care must
be taken that the coiled box stands squarely on the
base, with its longest edge parallel with one side of
the base board, and with the poising needle passing up
the middle of the' central circular hole. When this has
been arranged, the glue is allowed to set firmly. The
next operation is to bore two small holes right through
the base board one at each extremity of the coiled
box to allow the projecting ends of wire to pass through.
These holes, as v/ell as one at each of two corners of the
base board in which are to be inserted the terminal
binding screws, are best made with a small Archimedean
drill. With a sharp penknife, two little channels are cut
in the under surface of the base board leading from the
first two holes to the latter two. The ends of the wire
128 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS*.
on the coiled box are then passed through the more
central hole, and the extremities, having been stripped
onXiheir silken covering, pushed through the corner holes
so as to return to the upper surface of the board. They
are held in this position v/hile a binding screw is screwed
in each of the two corner holes. This having been
effected, any excess of wire above is cut off, and the
portion below caused to, make perfect contact with the
tang of the binding screw by a touch with a drop of*
solder on^ a <hot iron.i The .channel in which the wire
ends lie,junder^the.base board, ^should jiow- beJUledJaj
PIG. 48.
with a-little hot guttapercha, or Prout's glue (see Fig. 48)}
The pivoted needle may now be definitely placed on thq
point. A circle of about 3 in. in diameter is now cut
out of a clean stout white card, and two inner circles
described with the compasses and good black inlc,
within. One should be \ in. less all round than the
card, and the other \ in. all round less than the
former. For convenience of grading, a central line
should be drawn across the card, so as to divide it into!
two equal semicircles. One end of this line should bej
marked o.
THE VOLTMETER.
129
A perfectly round central \ in. hole should now be
punched in this card, which must then be glued on to
the coiled box, great care being taken to have the central
O line parallel with the wire coiled round the box, and
therefore parallel with the centre slit. Care also must
be taken that no glue, etc., be allowed to touch the glass
pivot, the apex of which must protrude centrally through
the hole in the card, and stand above about \ in. To
insure the card drying flat, it is well to place two or
three half bullets, or similar weights, on it until dry.
A small pointer, made either of stout glazed black
paper or of blackened straw, about ij in. in length, is
now glued to the top of the pivot, care being taken that
it is exactly parallel with the magnetised needle below.
It is not difficult to secure this parallelism if the
magnetised needle be held perfectly at right angles with
the zero line by means of a bar magnet inserted inside
the coiled box, while the pointer is being glued on
outside, also at right angles to this zero line.
The pointer must be allowed to dry thoroughly before
any attempt at grading, etc., be made. When quite
firm in its position, a 3 in. glass-capped box ( 76)
about l in. deep, must be selected, and glued on to
the base board, the bottom of the box having previously
been removed, as described in the case of the ammeter.
This box, with its movable glass-capped lid, serves to
keep the instrument free from dust, and unaffected by
currents of air.
To grade this instrument, it is necessary to have access
to five freshly- charged clean Daniell cells. (See 79;)
K
130 EtECTRICAL INSfR UMENT MAKING FOR AMA TE U$S.
These give a current of about I '079 volt per cell, and
before using them definitely they should be separately
coupled up to the voltmeter, to see if the deflection
given by each cell is sensibly the same. Supposing
this to be the case, the voltmeter is allowed to come
to rest, turning the instrument until the pointer stands
exactly at zero. (The instrument should be held firmly
in this position during the following trials.) When this
takes place one cell of the Daniells is to be coupled
up to the terminals of the voltmeter, and the deflection
noted. With a sharp pointed HB pencil (the glass
cover having been previously removed), a dot is made
on the end at the spot where the pointer indicates.
This is one volt, and very nearly i-i2th over; more
exactly, ixftfo, if the battery be in perfect condition ;
but it may fall as low as irgfo when the copper solution
works through the porous cell. Having made this mark,
the battery is uncoupled from the terminals, and the
C9nnections reversed, so as to get a deflection in the
opposite direction. When the needle has come to rest
(with the glass cover over), the deflection is noted as
before, and, as before, a dot made on the card where
the pointer stands. Two cells are now coupled in
scrie ( s, and the same operations gone through to get
a second set of dots, which stand for 2 volts and, say, .
In like manner, one -after the other, the whole 5 cells
are coupled in series, and connected to the voltmeter,
and the relative deflections noted. Lines can then be
ruled from the circumference towards the centre of the
card, In ink, allowing for the one- twelfth excess on each
GALVANOMETERS. I 3 I
dot, and these lines may be numbered I, 2, 3, 4, 5, and
they will indicate, with very fair accuracy, the deflection
given by, a corresponding number of volts. At Fig 1 . 49 is
shown the mode in which the card dial is divided,
centred, and finally graded. Each instrument will vary
a little in its degrees owing to the different resistance
of the wires, their -distance from the magnetic needle, etc.;
Fia. 49.
but once graded, they will be found to remain very
fairly constant in their indications as long as the pivot
remains uninjured.
86. GALVANOMETERS. Besides the ammeter and
voltmeter described in our last sections, there are two
other forms of galvanometer which, although giving no
absolute measure in currentror voltage (unless calibrated)
are extremely useful for the purposes of detecting
132 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
weak currents, or comparting their forces. These are
the ordinary single and double needle galvanometer
(also called multiplier and galvanoscope), and the
tangent galvanometer.
87. A very convenient form, of single needle gal-
vanometer may be constructed on precisely the same
lines as given at $ 85 for the voltmeter ; the only
difference being, that >silk covered copper wire should
be substituted *for the German silver wire, therein
suggested. The diameter of the wire, and the quantity
to be employed; will depend entirely on the use to
which it is intended to put the galvanometer when
completed. I/ it is to be used for large currents of
very low tension (such as thermo currents, etc.), it will
be better to use a few turns of No. 16 wire. If on the
contrary small currents of higher tension are to be
detected, then wire as fine as No. 40, may be coiled
round the frame br box. A very useful all round
instrument may be constructed, by winding about 200
turns of No. 36 silk covered copper wire round the
frame, or box. Th,is will produce a galvanometer that
will give a deflection of about 20, with the current set
up by heating the junction of a 6" length of copper
wire, twisted to a similar piece of iron wire. Of course,
instead of using & card dial as shown at 85, graded
to volts, the divisions of the circle should be in 360,
numbered from o to 90 on either side of the zero
line.
$ 88. If, instead of poising the needle on a point and
pivot, it be suspended by a single fibre of silk, depend-
GALVANOMETERS. 133
ing from a brass rod, bent at right angles like an P, the
short arm standing about 6 in. above the centre of the
coiled box, a very much mere cencitive instrument is the
result. In this case the magnetised neddle had better
be a well magnetised sewing-needle, broken off at its
point to the length required. This should be thrust
half through a fine straw, about I in. long, at right
angles to the length of the straw, and nearly at one
extremity of it, similar to a . The silken fibre can be
attached to the opposite end of the straw by means of a
drop of sealing-wax. A similar needle (not magnetised)
is now thrust through the opposite end of the straw,
perfectly in the came plane, and parallel to the lower
needle, so that the combination presents the appearance
of a letter X . This arrangement, hanging from the bent
brass rod over the central clot in the coilec^box, must be
fixed to the brass rod by the silken fibre, at such a height
that the magnetised needle enters the slot, and hangs
equi-distant from the top and bottom cf the box. The
upper needle, or " pointer," should clear the upper sur-
face of the coiled box by nearly a half inch. Since it
would be impossible to fasten the graded circle on the
box when the needles are in position if the circle is
entire, we "must cut the circle into two halves down the
zero line, and glue the two halves together again on the
box by the under side. A glass shade placed over the
whole will protect the needles from air currents.
89. If in either of the two last-described instruments
the " pointer ' ' be replaced by a carefully magnetised
needle, of the same size and "moment " as the under
134 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
one, placed with its north pole over the south pole of the
lower needle, and vice versd, we have the "astatic
galvanometer." Owing to the (almost) complete neutral-
isation 'of the earth's pull on the magnets, and to the
increased effect of both upper and under surface of coiled
box on the two magnets, the sensitiveness of this in-
strument is much greater than that of any of the
preceding.
$ 90. The tangent galvanometer presents no difficulty
in construction. A small lozenge-shaped "needle "is
made from a thin piece of watch spring, about I in. long
and j in. wide. This is " let down," or softened, by
being held over the flame of a spirit lamp until of a dull
red, and allowed to cool gradually. When quite cold
a small hole A in. in diameter is drilled through the
centre. The * needle" is then straightened out, and
tested for centrality; and, if defective, filed until the
hole corresponds with the centre of gravity. It is then
hardened by being made nearly red hot over the flame
of a spirit lamp, and being dropped into cold water. It
must then be carefully magnetised by being rubbed at
each extremity with the opposite poles of a good horse-
shoe magnet. When fully magnetised it must be fitted
with a small glass pivot, made as described at 6, small
enough to enter the A in. hole in the needle, and about
J in. in length. Great care must be exercised in the
choice of a pivot, which must be very perfectly shaped,
so as to allow great freedom of motion in the poised
needle. This point being settled, the pivot is attached
to the needle by means of a mere trace of good glue,
GALVANOMETERS. 135
applied to the hole in the needle only. The needle
must now be poised by its pivot on a fine steel sewing
needle (No. IO will do), and any want of perfect hori-
zontality must be remedied while the glue is still moist.
When the above is quite dry, a very fine straw, about
i\ in. long, has a small hole made in its centre (half
way between its two extremities) with a rather coarse
pin ; then the head of the pivot is pushed through this
hole in the straw, so as to cause the straw to lie exactly
at right angles over the needle. The merest trace of
glue will now cause the straw to adhere to and retain its
position on the glass pivot. Thjs can now be set aside
to dry. While this is drying the base board and
standard can be fitted up. A piece of good dry
wood, either deal or mahogany, is planed up and cut
into a slab 6 in. x 6 in. x in. In the centre of this
is inserted a circular wooden rod, I in. in diameter, and
6 in. in length. To the top of this rod is glued one of
the glass-capped boxes mentioned at 76;* which in
this case should be at least 3 in. in diameter, but need
not exceed in. in depth. This should stand squarely
and centrally on the top of the rod, like the cap ol a
mushroom on its stalk. When the glue has dried, and
the box is firm in it? place, the exact centre of the
bottom of the box is found by a pair of compasses, a
small hole being made with a sharp needle to mark the
spot. A No. 10 needle is then driven (head down-
wards) through the centre of the bottom of the box into
the wooden rod below, until the point of the nfeedle is
out \ in. below the glass of the cover when the cover
136 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
is put on ; more exactly, the point of the needle must
stand at such a height that when the pivoted magnetic
needle and straw pointer is placed on it the whole
system shall ride clear of the glass cover when this is
placed on the box. Before placing the magnetised needle
permanently in position, a white paper circle of the
same diameter as the interior of the box, and graded
from o to 900 on either side of the zero line, should be
pushed over the needle point and glued to the bottom
of the box. (The greatest care must be taken that no
glue gets on the needle or pivot, as all freedom of
motion would thereby be destroyed.)
91. A band or strip of copper, about I in. wide by
A in. thick, and 3 ft. 4 in. Jong, is bent so as to form a
circle, I ft. in diameter, with side extensions, like the
Greek letter Q. Two holes must be put through the
extended extremities, to allow 01" two binding screws
being inserted, which binding screws serve at one and
the same time to make contact with the copper ring, and
to hold this latter to its place on the base board. The
position of the ring, with regard to the graded dial at
its centre, should be such that the 90 line lies parallel
to the plane of the ring; so that, in fact, when the
magnetised needle is parallel with the ring, the straw
pointer s'tattds at zero. This constitutes all that is
necessary to the efficient action of the tangent galvano-
meter. Of course, the stand may be polished, or stained
and varnished ; if required more sensitive, a few turns of
No. 16 copper wire (single silk covered), maybe substi-
tuted lor the copper ring. If currents of high tension
GAL VANOME TERS.
137
are to be measured, the copper band must be replaced
by a light wooden frame (like a tambourine frame), on
which must be wound several hundred coils of fine
insulated copper, or even German silver wire. When
well made, the tangent galvanometer indicates the force
of the current, as the " tangent of the angles of deflec-
tion." As this may not be quite clear to the non-
mathematical amateur, I append a table, in which the
relative values of the angular deflections are approxi-
mately expressed in degrees of force :
Amount of Deflection in degrees
of Arc.
Relative forces of Currents producing
deflections.
I
rooo
5
5-012
10
10-118
IS
15-347
20
20-847
25
26-814
30
33-076
35
40-114
40
48-066
45
57-290
50
68-275
u
81-818
99-220
65
122857
70
157-398
75
213-836
80
324-867
I 3
654-824
89
3281-871
9
Infinite.
If one semi-diameter of the graded circle be divided
into degrees of arc from o to 90 on either side of the
1*38 ELECTRICAL INSTRUMENT MAKING FOR AMA TEURS.
zero line, and the other semi-diameter divided in the
same manner, but numbered with the integers showing
the corresponding amounts of force, the readings may be
taken from this latter scale, without appreciable error,
and without necessitating a fresh calculation for each
experiment. When absolute accuracy is required,
reference must be made to a table of "Natural
Tangents." A glass shade to cover the whole is essen-
tial, if the glass capped box be not used.
92. The previously described galvanometers are
frequently required in testing the resistances of different
lengths of line, or of different samples of wire, lamps,
etc., in which case some means by which these unknown
resistances may be compared to a known one must be
adopted. The instrument usually employed for this
purpose is the " Wheatstone Bridge " or balance.
The Wheatstone Bridge consists in a dry seasoned
piece of wood, nicely planed, stained, and varnished, 2 ft.
long, 4 in. wide, by j in. thick, as shown at Fig. 50. Two
strips of thin copper, B B, of shape of the letter -j, are
fastened, one at each end of the base board. These
strips are cut J in. wide, and are si in. bng in their
TTHE THERMOPILE. 139
longest arm, and 2 in. in their' shortest. Another
straight strip, of the same width, but 1 6 in. long, is
placed between these two short arms, leaving 2 in. gap
on either side. These strips are fastened down in their
places by nine telephone binding screws, as shown at CC f t
E, F F', G G', and H H'. A thin German silver wire
(No. 30) connects the binder C, to c', and a small
wooden block, carrying a spring contact piece, runs
along the v/ire. A tenth binding screw, D, serves to
connect this to one pole of the battery, E being
connected to the other. The galvanometer is con-i
nected to F and F*, while the known and uflknown
resistances are placed betv/cen G G' and H H'. Just
below the wire which is stretched between the two
binding screws is gummed a piece of white paper about
^ in. or i in. wide. In the exact centre is marked the.
zero line, and the portions to the right and left of this
are divided into 10, 20, 100, or 500 divisions, according
to delicacy required.
93. THE THERMOPILE. This instrument depends
in its action on the fact that when the point of junction
between two pieces of metal is heated, or cooled, a
current of electricity is set up. It is not necessary to
this effect that the metals should be of different nature ;
but they must bs in different molecular states, so that
they oppose different resistance to the passage of heat
and electricity. As, hov/ever, the greater the difference
in these respects the greater the force called into play,
it is usual to employ dissimilar metals, or metallic alloys^
etc., in which these differences are highly accentuated*
!40 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
$ 94. A very convenient form of thermopile for the
imateur, and one which, with a little care in the manage-
ment of the heat, gives a very constant current, adapted to
delicate and long-continued experiments, such as grading
delicate galvanometers, etc., may be constructed as
follows : Cut 25 or 30 pieces of No. 16 German silver
wire into 6 in. lengths ; do likewise with a similar
number of pieces of No. 16 copper wire. With a pair
of pliers twist tightly about I in. of the extremity of one
copper wire to the extremity of one German silver wire,
FIG. 51.
so as to join them firmly together, as shown at Fig. 51 A
In a similar manner join the remaining pairs of copper
and German silver wires ; clean the junctions with a bit
of sandpaper, and solder the junctions with as little hard
solder as you possibly can. Now twist and solder the
opposite extremities of the joined wires, each German silver
to the copper of the next pair (not its own), leaving the
first copper and last German silver as terminals (see
Fig. 51 B). Now procure two wooden cornice-pole rings,
about 3 in. in diameter (such as are used to suspend
burtains from the pole), and with a coarse rasp, file one
THE THERMOPILE. I 4 i
surface of each ringjfo/,so that if laid upon one another,
after filing down, they form one thick ring, .is sfcown at
Fig. 52. To the lower ring, at equi-distant points,
should be affixed three wooden rods, about 6 in. long,
spreading out somewhat, so as to admit of the insertion
of a spirit lamp between them. For the sake of stability,
these rods, which form the feet on which the thermopile
rests, may be inserted into a heavy wooden or leaden
base as shown at D in Fig. 53. The next step is to
place the joined wires in a radiating fashion on the ring,
Fid. 52..
and clamp'them in their place by covering them with
the upper, ring, which must be screwed tightly down to
the lower one, care being taken that the screws do not
tpuch the wires, and also that the wires do not touch
each other, except at the soldered junction. The inner
circle of junctions^must not touch^but must stand at
equal distances all round the centre of the circle described
oy the ring, leaving a space of about \ in., in which the
flame of the spirit lamp can burn and heat these
junctions simultaneously. The two unsoldered ends (of
f copper and German, silver wires) are then to be attached
t4* ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
to binding screws, which serve as the poles or terminals
of the thermopile. Fig. 54 illustrates the appearance of
the finished instrument On plating a lighted spirit
lamp on the base, so that the flame plays against the
junctions pointing to the centre, a current will be found
to flow from the two terminals. The electro-motive force
of these little instruments is about one-twelfth of a volt
D
FIG. 53.
for each pair of wires, so that a dozen pairs of wires
give very nearly one volt. On the short circuit each
fair is capable of giving about one-third of an ampere.
Coupled up in series (as shown), this remains, of course,
the same. To increase the current, the elements must
be made larger, or, what amounts to the same thing,
coupled up in parallel instead of in series. For the
TffE THERMOPILE. M3
benefit of those amateurs v/ho may be desirous of
attempting something on a larger scale*, the following
account of a thermopile which was made on the plan of
Clamond's pile, with some slight modifications, is sub-
joined: 288 strips of tinned iron, I in. wide by 5 in. long,
were cut out. A mould ofplaster-of-Paris was prepared
Pro. 54.
in which could be cast oblong squares, 2^ in long by I in.
in square section. At end of the two extremities of the
mould were placed, standing upright, a tinned iron strip.
The mould was then filled with a molten alloy, consisting
of two parts of antimony, melted with one part of zinc
(as this alloy expands considerably on cooling, it must
144 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
be withdrawn from the mould as soon as set). B^ this
means were obtained 144 castings, having a tinned iron
lug at each end, looking something like a letter E, withr
out the central stroke. . An "iron ring, about I in. wide,
\ in. thick, and I ft iri internal diameter, was supported on
four stout iron rods, screwed to it, at a distance of about
10 ins. frorri the ground. On this ring was placed a
coating of good Portland cement, about \ in. thick. The
(castings, with their lugs previously bent, as shown at
FIG. 55.
Fig. 55, and having a thin piece of mica (talc) inserted
between the inner strips and the alloy, to prevent
contact, were then arranged in a circle on the iron ring,
each one being separated from its neighbour by a thin
piece of mica, and the whole held together by means of
a little Portland cement put in between as each casting
was placed in position. Several such layers of circles of
castings were thus arranged, care being taken that the
faces of the castings projected about \ in. inward beyond
fine iron ring. When the entire series of castings had
THE THERMOPILE. 145
been thus arranged in circles, and set in cement, a final
layer of cement, about \ in. thick, was spread over the
upper layer, care being taken in each layer to leave
quite I in. of the castings projecting outwardly, free
from cement. A second iron ring, precisely similar to
the first, was now laid over the top, and when the
cement had set, clamped to the lower one by means of
.three long screw clamps. The iron strip attached to the
inner end of one casting was then soldered to the iron
strip attached to the outer end of its neighbour (previously
cut to the required length), and so on all round each
circle, except at one point of each circle only, where, of
14$ ELECTRICAL INSTRUMENT MAKING FOR AHA TEUR3.
course, the projecting strips were left free, to be afterwards
attached to binding screws as terminals. Each circle had
its own pair of terminals, and these could be coupled up
to the neighbouring circles either in parallel or in series^
as the case might demand. This pile was used with a
large paraffin burner having an iron chimney nearly
couching the interior ends of elements. It has been
also tried with a charcoal brazier (with a similar iron
chimney), with gas, and with a small coke stove. For
steady currents of lowE,M.F., but considerable quantity,
the 6 in. paraffin burner answered admirably.
The following table will give a fair idea of the force
developed with the said paraffin burner, the elements
being in all ttjese cases coupled up for tension *>., in
scries :
No. of Amperes on 8
Elements. E.M.F. short circuit.
36 2 0*6
72 4 0-6
'108 6 o - 6
144 8 0-6
p1gr~"56 is reproduced from 'a photograph of the
identical thermopile (of four circles of 36' elements)
with which the above trials were made. By using
more powerful sources of heat (up to a certain point)
correspondingly more powerful effects were obtained.
S 95. BATTERIES. For the convenience of classifica-
tion, batteries may be divided into two great families
viz., single fluid and double fluid. To the former class
belong all such as do not require a partition of any kind
BATTERIES. 147
(whether porous cell, septa, sa\vdust, sand, difference of
specific gravity, etc.) betv/een the fluid surrounding the
negative and the positive plate or element
To the latter class belong all those, in which, either
for the sake of obtaining constancy of effect, overcoming
polarisation, etc., the fluid or fluids surrounding the
negative and positive elements respectively, are kept
from mixing, by any of the means just mentioned.
Few amateurs would care to manufacture their o'vn
jars, or porous cells, so no attempt will be made here to
describe the construction of such, except to point out
that where great lightness and strength is required, as
in the case of small batteries to drive model yacht motor,
or pocket coils, very efficient and perfectly acid proof
cells, of any shape, may be constructed by gluing to-
gether with good tape, stout brown pasteboard, of the
size and form required. The cells thus formed, after
being allowed to dry thoroughly, must be immersed fot
a few minutes in hot melted paraffin wax until thoroughly
permeated, and then allov/ed to dry and set. Cells of
this kind will stand any acid, and even a solution of sul-
phate of copper.
Small porous cells may be made out of bowls of to-
bacco pipes, the small hole being stopped with Prout's
elastic glue. Larger ones for any experimental purposes,
had much better be bought, but can be made by the
amateur from any good clean yellow clay kneaded so as
to free it from stones, etc. This may be moulded of the
desired shape, allo\yed to dry perfectly and then gradually
heated to redness in any ordinary fire. Greater porosity
148 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS,
may be imparted to the clay by the addition of powdered
graphite or even charcoal.
Zincs may be cut to any shape by making a pretty
deep line with a file at the spot where it is desired to
divide, and then running a little quicksilver in the furrow
thus produced. In a few seconds this permeates through
the zinc at this place, rendering it brittle and rotten, so
that the least pressure suffices to cause the zinc to break
at the line.
Amalgamation is best effected by making up a mix-
ture of I part oil of vitriol with 19 parts of water, placing
this in a large flat shallow dish, in which a little mercury
is also placed. The fingers having been rubbed with a
greasy rag to: prevent the acid affecting the skin, the
zinc plates or rods are one by ofie immersed in the acid
and quickly rubbed over with an old tooth brush so as
to carry the mercury all over the surface. The excess
of mercury should be allowed to drain off by rearing the
plates on end, in a plate or other earthenware vessel.
In separating the positive and negative elements from
each other, ebonite will be found of the highest value for
small batteries. In larger ones, teak, mahogany, or box-
wood strips, previously boiled in hot melted paraffin wax,
give excellent results, and are impermeable to the acid
used.
Binding' screws y though ' very convenient, are not
absolute necessities. In many cases the negative plates
(even if of graphite) can be held in their places by
ordinary wood screws, by being screwed to the wooden
bar which separates them from the zinc, Connecringr
TEE SINGLE FLUID CELL. 149
.wires can be soldered to the zinc, or twisted tightly
found the shoulders of the screws. For coupling up a
number of elements or circuits, strips of copper, about
I in. wide, i\ in. long, about sfo in. thick, made perfectly
clean and bright, and then rolled contrariwise at each
end, so as to present the aspect (in section) of an CO , will
be found very convenient. It is not proposed here to
give details for the construction of all the batteries
which have been from time to time " invented," patented,
or described, for their name is legion, and their utility
in many cases, highly problematic. General outlines
will be given for the construction of a single fluid, and
double fluid battery ; the student can then use what
excitant he may fancy, or circumstances dictate.
96. THE SINGLE FLUID CELL. For the contain-
ing vessel, a Westall salt jar, or one of the 2 Ib. plum
bottles, wiU do very well. For very small cells, the
s mailer sizes of Liebig's " extract of beef " pots, answer
admirably. The zinc and copper (or graphite) elements
having been cut of the right size to enter the vessel, and
yet leave a good in. clear between the zinc and nega-
tive element, two strips of paraffined wood, in. thick,
and from \ in. to I in. wide, according to the size of the
battery, are cut, a little longer than the diameter of the
containing cell. A small strip of clean sheet copper, to
one end of which the wire which is to form one pole of
of the battery is soldered, is placed in contact with the
upper end of the zinc plate. Over this is to be placed
one of the paraffined wooden strips : then the negative
element. If the amateur is content with a single
ISO ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
negative element (be it copper, graphite, silver, or
platinum), he need now only place the second strip of
paraffined wood, over the top edge of the negative plate
with a little copper strip and wire ( as before, to form
the other pole of the battery) between the negative
plate and wooden strip. The whole is now clamped
together between the jaws of a clamping binding screw,
or, if the larger elements are used, between the jaws of
a sewing machine clamp (these can be got at id. and
lid. each).- -Care must be taken, that the binding screw
or clamp, does not make contact between the negative and
positive elements. As much better results' are obtained
when the negative elements are double, the amateur will
probably prefer to have two carbons, or coppers, to each
zinc. In this case, after having placed the copper piece,
to which the wire is attached, against the top end of the
zinc, he will put a paraffined wooden 1 strip on each side
of the top end of the zinc, then a negative element on
each side of the zinc. If the negatives are graphites, a
wide copper band should ' encircle both , graphites, and
the c 1 amp should grip the band against the graphites ;
care being taken as before, that the graphites, neither
through the clamp, nor through the copper band at any
place make contact with the zinc.
97.' MOUNTING GRAPHITE RODS AND PLATES.
In cases where the negative elements are graphite, and
more especially if the plates are to stand long in the
fluid, as in the Leclanche, the bottle bichromate, etc., it
is advisable, owing to the porosity of the graphite, to
adopt some other means of making connection with the
MOUNTING GRAPHITE RODS AND PLATES. 151
terminal. It is usually recommended to electroplate the
upper end of the carbon with copper, and then solder
connection to it, but a better plan, is to make several
nicks round the plate or rod, with a file, or even to drill
a few holes in with a screw drill, and then to cast a
leaden cap round the top end. If the lead have a lit'tle
antimony added to it during fusion, it will set much
harder, and fit closer. This is hardly acted on by the
usual acids ( of the battery. To prevent any chance
access of acid, the top end of the -graphite may be
painted round with hot paraffin wax. This treatment is
specially useful in the case of bichromate or chromic
acid cells.
98. It is a well-known fact that the remarkable fall
in current strength which takes place in the single fluid
batteries of this class depends to a great extent on the
absolute immobility of the exciting fluid. Thi ; is due
to the fact that as the hydrogen is absorbed by the
oxidant (chromic acid, bichromate of potash) as fast as
it is generated, no mechanical movement is produced
in the mass of the liquid, so that the liquid
near the zinc plate becomes quickly charged with
sulphate of zinc, thus protecting the plate from the
farther action of the acid. Many schemes have been
proposed to avoid this, such as setting up circulatory
currents in the fluid by the external application of heat
(Sprague), or by means of an aspirator (Courtenay), etc.,
etc. These methods are excellent in their way, but are
rather inconvenient of application by the amateur, who
has only to deal with a few cells and fewer shillings.
152 ELECTRICAL INSTRUMENT MAKING FOR AMATEVRS.
A very simple mode, which is quite effective, which
gives no more trouble than snuffing a candle, and which
could be made automatic if desired, is one which the
author adopted in his own 4-coll batteries for temporary
lighting purposes, etc.
The battery (shown below, Fig. 5) consists essentially
of foitfjjlass cells, -A A A A, about 2\ in. in diameter,
Fio. 57.
standing on a tray, T, from the centre of which
rises a screwed and jointed rod, R, by means of which
it can be raised or lowered, along with the four
cells in the box B. These cells are filled to about
two-thirds of their height with the excitant (chromic
acid 3 parts, sulphuric acid 3 parts, water 17 parts).
The zincs and carbons are attached, by means of long
THE DOUBLE FLUID CELL. 153
binding- screws, to the lid L, and each element is
connected in series to its neighbour by means of metal
straps, the first and last, of course, forming the electrodes.
The rod R, passing through the lid, enables the operator
to raise the cells to the plates ; and this in practice will be
found a great advantage over lowering the plates into
the cells. The arrangement for setting, up movement
in the fluid consists simply in ebonite rings, E E E E,
which encircle the plates, and which -are attached to the
ends of guttapercha-covered wires, \v\v w W, the upper
extremities of which pass through the lid of the box, and
are soldered to the four corners of a flat square of wire,
F, which, on being raised and depressed, agitates the
fluid in the cells, and thus prevents the accumulation of
zinc sulphate round the plates. For the convenience of
carriage etc., the lid L is^ fastened to the box by means
of two rings and catches, and is furnished_with_a,central
handle, not shown in the sketch.*^
99. THE DOUBLE FLUID CELL. Differs from" the
single fluid cell only inasmuch as a porous cell, or
substitute, is employed to separate thq fluid acting on
the zinc from that in contact with the negative element.
As before, the containing vessel may be a stoneware
jar or glass wide-mouthed bottled A porous cell is
chosen, a little taller than the containing or outer cell,
^he bottom of this porous cell, and also in. round the
top, should be immersed in hot melted paraffin wax.
This prevents "creeping" and is specially serviceable
in the case of the Daniell cell. The zinc may take the
form of a casfrod, and this obviates the necessity of a
154 ELECTRICAL INSTRUMENT UAK1NG FOR AMATEURS.
binding screw, as a copper wire may be cast in as a
terminal. At one time " cast " zinc was regarded with
suspicion ; but now it has been pretty well proved that
cast zinc, even if it contain a certain amount of tip and
lead, is quite as efficient, if not even more so than
the rolled metal. But in any case it must be well
amalgamated. A little vooden lid or cover should be
fitted to the porous cell, through the centre of which
should project the wire coming from the zinc. The
negative element, if copper, may be bent into the form
of a circle to fit the inside of the jar, and the other
terminal soldered to it. Also a little shelf or ledge
should be soldered inside Hie coppe/, at the top edge, to
hold crystals of copper sulphate, if the Daniell form be
preferred. If carbon be chosen as the negative, then it
will be- well to take a sufficient number of carbon
(graphite) pencils, such as are used for electric lights,
about i in. thick and a little longer than the outer cell,
and having tied them all round a bottle or other
cylindrical body, cf such a size that they will then freely
enter the outer vessel, cast a leaden ring round one
extremity, to which the other electrode can be attached.
This forms a very excellent negative, presenting a very
large surface. Annexed is a table of- the names,
elements, fluids, and E.M.F., etc., of the most useful
batteries ;
THE DOUBLE FLUID CELL.
155
NAME OF
CELL.
POSITIVE
ELEMENT.
NEGATIVE
ELEMENT.
EXCITING
FLUID:
DEPOLARIS-
ING FLUID.
E.M.F. ra
VOLTS.
INTERNAL
RESISTANCE
IN OHMS.*
Bunscn
Zinc
Graphite
Sulphuric
Acid dilute
Nitric acid
i'734
08 to -ii
Do.
"
M
"
Chromic
acid
'734
'1 tO '13
Chromic
Acid, single
fluid
-
Sulphuric
acid and
chromic
acid, dilute
None
separate
3-3
001 to -08
Daniell
"
Copper
Zinc sul-
phate solutn
Copper sul-
phate sol.
1-079
2- to 5
Fuller
Graphite
Chloride of
zinc
solution
Potash bi-
chromate
and hydro-
chloric acid
*'S
0-5 to 0-7
Gaiffe
"
Silver
Zinc
chloride
Silver
chloride
I '02
0-5 to 06
Grove
*
Platinum
Sulphuric
acid dilute
Nitric acid
I- 9 6
'I tO '13
Lalande
Chaperon
"
Copper or
iron
Caustic pot-
ash sjlution
Oxide of
copper
0- 9 8
1- 3
Latimer
Clark
"
Pure
mercury
Sulphate of
mercury
None
separate
''457
o'j to o's
Leclanche"
"
Graphite
Ammonium
chloride sol.
Manganese
dioxide
r 4 8
1-13 to 1-15
Maiche
Zinc scraps,
in bath of
mercury
Platinized
carbon
Common salt
solution
None
separate
i'2S
i- to 'a
Marie Davy
Zinc
Graphite
Sulphuric
acid dimte
Paste of
sulphate of
mercury
'524
75 to i'
Niaudet
"
"
Common sait
solution
Chloride
of lime
j-5 to 1-6-
5 to6'
Poggendcrf
-
Saturated
sol of p -tash.
bichromate,
and sul-
phuric acid
None
separate
1-98
ooi to -08
Schansch.eff
"
"
Mercurial
solution
None
separate
i- 5 6
05 to 0-75
Skrivanow
"
Silver
Caustic
potash
Chloride of
silver
i'S
i'S
Smee
Platinized
silver
Sulphuric
acid dilute
None
o- 47 toi-i
'S
Walk r
"
Platinized
graphite
"
'
o-ee
o'4
Warren de la
Hue
"
Silver
Sal ammo-
niac solution
Silver
chloride
i*
0.4 to o'6
: The resistances were measured in cells standing 6" X 4'
156 ELECTRICAL INSTRUMENT MAKING FOR AMA TEURS.
ioo. THE TELEPHONE. Although the effects pro-
duced by this instrument are at once among the most
beautiful and astounding in the whole rahge of physics,
nevertheless the apparatus necessary to their production
is of the simplest description. In the form patented by
Graham Bell (see fig. 60), which embodies all the essen-
tial points of a serviceable working instrument, we have
a bar-magnet, around one pole of which is coiled about
a hundred feet of fine insulated copper wire. The ex-
tremities of this coil of wire are attached to two binding
screws, by means of which connection can be made to
the transmitting lines, etc. In front of the coiled pole
of the bar-magne't, but not in actual contact, is a
circular plate of very thin sheet iron, gripped at its
edges, but free to vibrate centrally. This arrangement
is ail that is really necessary in the construction of the
telephone. In order to carry on a conversation by the
aid of the arrangement described, two precisely similar
instruments are employed, one at the speaking or
"transmitting" end, and the other at the hearing or
" receiving " end. The two binding screws belonging to
each instrument are connected together by means of
separate insulated wires. An individual speaking near
the thin iron disc of one of the telephones, causes the
air to enter into vibration. These vibrations are taken
up by the thin iron disc, which performs, in consequence,
oscillations, " excursion and incursions " to and from
the pole of the magnet. By virtue of the well-known
law, that " motion before the poles of a magnet sets
up currents of electricity " (see 58), and that the
THE TELEPHONE. 157
Currents flow in one direction when the movement is
one of approach, and in the contrary when the motion
is one of recession, it follows that during speech a
number of waves of electricity, now in one direction,
now in another, flow around the wire encircling the
coiled pole of the magnet, and, traversing the lines
leading to the farther instrument, flow round the coiled
pole of that magnet. When these flow in one direction,
the magnet is strengthened by their advent, and pulls
down the disc before it more forcibly. When the flow
is in the opposite direction,J:he magnet is correspond-
ingly weakened, its pull on the disc is lessened, and
consequently the disc performs an excursion from the
magnet by virtue of its elasticity ; and every trifling
modification in the strength and direction of current
set up by the vibrations in the disc moved by the
speaker's voice at the transmitting end, is faithfully
reproduced in vibrations of corresponding amplitude
and strength, in the disc at the receiving or hearing end.
To construct a pair of telephones for experimental
purposes, capable of transmitting to a distance of two
or three miles, the following materials will be needed :
1st. A pair .of square bar-magnets, about \ in. by 3 in.
such as may be procured from the ironmongers from
ld. 01; 2d. each. These should be capable of sustaining
eacho':her if the marked end of the one be presented
to the unmarked end of the other.
2nd. A pair of wooden tooth-powder boxes, 2^in.
diameter by about I Jin. deep. These may be obtained
at any chemist's shop.
158 ELECTRICAL INSTRUMENT MAKING FOP AMATEURS.
3rd. Axpair of ferrotype plates, 4^ in. by 3^ in. These
-are to be bought from the dealers in photographic
goods, being the thin iron plates on which the cheap
" ferrotype " pictures are taken.
T 4th. Two pairs of small binding screws/ of any
pattern, so long as they are small.
5th. A piece of cylindrical white wood, similar to a
broom -handle, about I in. dr meter, and 12 in. long.
6th. A quarter of an ounce of No. 36 silk covered
copper wire.
101. The operator begins by cutting off two pieces of
the cylindrical wooden rod, 4^ in. in length, and, with a
sharp knife, splits each one down the middle, into two
semi-cylindrical halves. Taking care to keep each
pair of halves so as to v be able to fit them together
again when required, he cuts, with a -in. chisel, a
channel in the flat face of each half-cylinder, to such a
depth that the bar-magnets can lie between the two
halves when the'se are fitted together. The channels
should extend from end to end of the cylinders, and
should just allow the magnets to slide in them, without
any lateral play. At one end, for a depth of about
2 in., the channels should be made a trifle wider than
the bar-magnets, say ^ in. When the channels have
been thus satisfactorily cut, the half cylinders must be
glued together, so as to form two cylinders with a
square channel running up the middle. The next step
consists in cutting a circular hole (of exactly the same
diameter as the wooden cylinders just prepared) in the
centre of the bottom of each _ tooth -powder box. A
THE TELEPHOKn. 159
little good glue is now rubbed round the edges of this
hole, and also round the top end of each cylinder.
(The top end is the end at which the channel is
smallest). This end is then thrust into the bottom of
the box, from the outside, until it is just flush with the
inside of the bottom of the box. The boxes, with their
cylindrical handles, are now set aside to dry. Whilst
these are drying, the student may cut a circular aperture
l| in. in diameter, in the centre of the lid of each tooth-
powder box. He then proceeds to glue a cone of stiff
pasteboard, shaped something like the bell of a
clarionet, into this hole. This cone, or bell, must open
and extend outwards for about I in., and be cue
off flush with the inside of the lid of the box. Its
only service is to collect and re-enforce the sounds
which are to fall on the vibrating plate. The next
step is to cut two circles out of the ferrotype plates
of such a diameter as to fit exactly, without binding,
or without any shake, into the inside of the lids of the
boxes. As any dent or buckling in the ferrotype
plates would prove fatal to their action in the telephone,
it is rtot permissible to use a compass, or any similar
instrument, wherewith to strike the circles on them; but
the following mode of procedure must be adopted :
Having measured the exact diameter of the inside of
the lids, the operator strikes out, with the compasses, a
corresponding circle on a stout piece of cardboard ;
cuts out this circlet with the scissors, and then, laying
this v as a template over the ferrotype plate, scratches
lightly a line all round with the point of a pin. It is
iCo ELECTRICAL INSTRUMENT MAKING FOR AMA TEURS'^
easy then, with a sharp pair of scissors/" to cut a 1
circle of the desired size in the ferrotype^ plates, ^without
any buckling or denting.
Two pieces of brass wire, about in/ in diameter and
2 in. long, are now run through a screw plate, so as to
produce a thread on them for their entire length. Two
small hexagonal nuts are cut out of a piece of sheet brass
\ in. thick, and a corresponding female screw produced
in them.
A piece of brown paper, \ in. wide, is glued around one
end of these screwed wires, until this end is of the same
thickness as the sealing-waxed end of the bar-magnets.
When this is the case, the screw wires are each respec-
tively glued, as prolongation, to the sealing-waxed ends
of the bar-magnets, one to each, and further strengthened
in their attachment by having a roll or two of brown
paper glued tightly round, of such a length as to embrace'
about fin. up the bar magnet and in. of the screw. The
thickness of this paper must not, however, exceed the
width of the channel at the lower end of the cylinder.
A small bobbin of cardboard is now made to fit the
polished ends of the bar-magnets. These bobbins should
be about I in. in diameter by about I in. wide in the
channels ; they should be made to fit pretty tightly on to
the poles of the magnets. When made and glued
together, they should be allowed to dry, and then soaked
for a minute, or two in melted paraffin wax. After this
they may be wound with about I drachm (60 grains) of
No. 36 silk-covered copper wire, particular care being
taken to wind in one continuous direction only, and to
THE TELEPHONE. ifi
avoid all breakages or even kinks in the wire, as being
fatal to success. Each bobbin, after being wound (and it
should be wound while on the pole of the magnet), should
be removed from off the pole of the magnet and dipped
for an instant in melted paraffin wax. About 3 in. of
each end of the wires should be left free, for attachment
to the binding screws.
This being done, the nut is removed from the screwed
tailpiece of each magnet. Two small circlets are cut
out of thin sheet brass or zinc, of the same diameter as
the lower ends of the cylinders. A central hole Is bored
in these circlets, to admit of the passage of the screwed
tailpieces of the magnets ; and two lateral holes, by means
of which they can be fastened to the ends of the cylinders
with two screws. The bar-magnets are then pushed up
the central channels until their polished ends are very
nearly flush with the edges of the boxes, before the lids
are on. The little brass circlets just prepared are then
screwed on to the ends of the cylinders, leaving about
\ in. of the screwed tailpieces projecting. (Should the
magnets play too loosely in the channels, a thin sheet of
paper maybe wrapped round them, to increase the fric-
tion). The nuts may now be replaced on the tailpieces
Each wound bobbin is now to be slipped in its place
over the pole of its respective magnet, and, if too loose,
retained there by touching the inside of the bobbin with
a brush lightly dipped in white hard varnish. The free
ends of the wire proceeding from each bobbin are soldered
to the prongs of the binding screws, which are driven into
the bottom of the boxes, at opposite points in its diameter,
162 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
near the edges. The ferrotype plates are now placed
inside the lids, with the glossy side outwards, and the
dids pressed firmly on to the boxes, The lids should
THE TELEPHONE. l6j
fit tightly ; if they do not, they should be made to do so
by gluing a strip of paper round the edge of the rim of
the box. The boxes, handles, and cardboard bells may
be stained and varnished if desired.
The distance of the magnets from the ferrotype plates
can be regulated to a nicety by means of the nuts and
screwed tailpieces of the magnets. In practice, I find
that the best effect is obtained when the magnets are
only just clear of the plates. To secure this result, it is
only necessary to screw up until the magnet just touches
the plate. This can be told by the dull blocked sound
which -is given on tapping lightly the ferrotype plate
with the finger nail ; then, if the nut be turned gradually
in the opposite direction until the tapping gives a clear
sound instead of a dull thud, the magnet will be just clear
of the diaphragm. The annexed cut will make the
general disposition of the parts clear: A is the bar
magnet, attached by means of the brown paper roll to tho
regulating screw H ; this is controlled by the nut I
which is rotated against the circlet J. The coiled bobbin
is shown at C, with its free ends of wire attached by a
drop of solder to the binding screws G G. At F F, we
have the channelled cylinder, in which lies the magnet.
D D., represents the tooth powder box, which forms the
case and resonant box ; B is the ferrotype plate, and
E E tne conical mouthpiece made of stout cardboard.
164 ELECTRICAL INSTRUMENT WAKING FOR AMATEURS.
$ 102. ELECTRO-MOTORS.
A very efficient little motor for driving model boats
!or locomotives, and for producing rotation of vacuum
tubes.etc., may be constructed in the following manner :
Procure a piece of soft hoop iron about \ in. thick, \ in.
wide, and I ft. long; cut this into two equal pieces 6 in.
in length ; drill a t^in. hole through the centre of each
one ; then bend each piece of the iron into the shape
of the letter U, having the limbs if in. apart, and
therefore about 2\ in. in length from the middle of
bend to the end of limb. In one of these pieces, which
is to be the fixed magnet, two other holes, in a line
with the first, but near the edges of the iron, should be
drilled, in order to enable the operator to fasten it to
the base board when finished. A piece of sound
mahogany or well-seasoned pine about 3 in. wide, $ in.
thick, and loin. long,-should now be procured; and a
'piece about 3 in. square cut off one end and glued and
screwed to the remaining 7 in. at right angles to it,
'similar to a letter L. This piece, which forms the base
board, may be stained, varnished, or polished, at the
option of the maker. The next step is to wind the two
U-shaped electro-magnets previously produced ; great
care, of course, being taken to cover the iron first with
ja dressing of paper or tape, as described at H 67 and
'74, to insure that no electricity shall leak through from
the wire, with which the magnets are wound, to the'
iron below. Each magnet will require to be wound
ELECTRO-MOTORS. 16$
with 6 layers of No. 24 silk-covered wire ; that is to
say, between 6 and 9 oz. of the said wire; 'the exact
amount got on depending on the skill and neatness of
the operator. The connection between the windings
must be in the direction shown in our last paragraph,
viz., like a letter (. It will.be well, in order to avoid
joins, to measure off one-half of the wire intended to be
laid on one electro-magnet, and without cutting it off,
from the remaining half, to wind one limb with the first
half, the other limb being wound with the other half.
In both electro-magnets, the winding should be begun
near the bend; just at those portions where the limbs
of the U begin to straighten. If, as directed, 6 layers
of wire are got on, the finishing, or free extremities of
the wires, will be found near the bend of the U's. At
this point they should be carefully tied, with silk of the
same colour, to prevent uncoiling. If the operator pre-
fers appearance to efficiency, nothing further need be
clone to the coils ; but if, on the contrary, efficiency be
the first consideration, it will be well to soak the coils
in white hard varnish, and let them dry in a warm
"place. Of course, in either case, the electro-magnets
should be tested for insulation, before anything else
be done as described at 68. The next step is, to
fasten one of these electro-magnets (the one with s the
three holes drilled at the bend) to the upright piece of
the base board, with the limbs parallel to the base
itself, and at such a height that the other electro-
.magnet shall be able to rotate freely in front of it
Svithout touching the base ; that is to say the height
iG6 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
from the base to the central hole of the fixed electro-
magnet must not be less than \\ in The fixed electro-
magnet must be fastened to the upright of the base-
board by means of two screws, one in each of the lateral
holes previously, drilled in the electro-magnet. In the
central hole of this same electro-magnet is passed a
piece of stout brass wire about \ in. diameter and f in.
long, and which has been filed down for about f of
its length to such a size that it can enter the hole
in the centre of the electro-magnet, leaving a head
about ^ m - protruding from this hole. In the centre
of the head of this piece of brass is drilled a conical
depression by means of the Archimedean drill. This
depression is to form one of the bearings in which the
shaft or spindle which carries the movable electro-
magnet will rotate. We will designate this the back
bearing. The next operation consists in making a little
pillar about 2 in. in height, from a piece of brass wire
about \ in. in diameter. About Jin. of one extremity
of this pillar should be reduced by filing to \ in. in
diameter, and a thread put on it by means of a screw
plate. At the upper extremity a hole must be drilled
and tapped ; at right angles to the length of the pillar,
and at such a height that when the pillar is screwed
into the base-board the said hole shall be exactly in a
line with the conical depression in the back bearing.
This hole should be about |in. in diameter, and be
fitted with a short length (about I in.) of brass screw, to
serve as the front bearing. The face of this screw must
also have a conical depression drilled, corresponding to,
ELECTRO-MOTORS. 167
and facing the one in the back bearing. The pillar
may now be screwed into the centre of the base-board
at a distance of about 6\ in. from the back bearing, with
the screw which forms the front bearing in a line with
and perpendicular to, the back bearing. If necessary, a
small nut may be put at the lower extremity of the
pillar" screw which passes into the base-board, so as to
insure rigidity. In this case a hole must be counter-
sunk at the under surface of tlie board, so as to allow
the nut to lie flush with the board. A piece of steel
rod, such as a stout knitting needle, is now procured,
and cut so as to be a little longer than the distance
between the back and front bearings. This is to form
the shaft or spindle of the motor, and must be lowered
in temper by holding over the flame of a spirit lamp at
its two extremities, which are then to be filed to fine
conical points, so as to run freely in the conical depres-
sion of the front and back bearings. These points can
then be hardened again by making red-hot and plung-
ing in cold water. The free electro-magnet is now to be
fitted to this spindle. For this purpose the spindle is
pushed through the central hole at the back of the
bend ; should the existing hole not be large enough, it
must be rimed out until the spindle will just enter.
The spindle with the electro-magnet on it is then placed
between the bearings, the screw of the front bearing
being then tightened up to hold the spindle immov-
able. The movable electro-magnet is then placed
with its poles facing, but not touching those of the fixed
electro -magnet, a piece of stout cardboard, ^in. thick,
168 ELECTRICAL INSTRUMENT MAKING FOR AMA TEURS.
being placed between the poles to prevent actual con-
tact. The magnet being held firmly in this position
by the left hand, a fik-mark is made with a sharp
triangular file, at the point at which the bend of the
electro-magnet touches the spindle. The spindle with
the electro-magnet is now removed from between the
centres, and the electro-magnet soldered to the spindle
at the point just marked with the file. In order to
solder effectually, the back of the bend of the electro-
magnet, as also the inside of the bend, must be filed
perfectly clean, and run over with the tinned soldering-
iron, so as to get a coating of solder, before attempting
to solder the spindle thereto. When this has been
done, the movable electro-magnet should be again
placed between its centres to see whether it runs freely
before the poles of the fixed magnet, without cither
touching it on the one hand, or being more than -^ in.
away from it on the other. If it should not do so, the
soldering must be repeated until this result has been
attained. A commutator, precisely similar in form,
but somewhat smaller than that described in 66, is
now made, and fitted to the spindle. It should be i in.
long, and \ in. in diameter, the central hole being a tight
fit on the spindle. As the brass cheeks of this commu-
tator must be carefully insulated from each other, from
the spindle, and from the iron of the electro-magnet,
very short screws must be used ; and a small paraffined
paper washer put over the spindle, against the electro-
magnet, before the commutator is put on the spindle,
and pushed up into its place, which is close against
'ELECTRO-MOTORS:^
169
the outside of the bend of the movable electro-
magnet/
A drop of white hard varnish, painted over the
spindle where the commutator is finally to remain, will
prevent the '\ commutator from slipping round. The
commutator must be so placed on the spindle ; that when
the limbs of the movable eleclro-mag net are exactly op-
posite the limbs of the fixed electro-magnet > the slits of 'the
commutator are in a line with the limbs ; that is, they
should find themselves at the, two "sides, of. the j:om-
FIG. 61.
mutator, and not above and below. A small pulley,
either of brass or wood, may be keyed or soldered to
the spindle, to serve as a driving wheel, wherewith to
communicate the motion wherever desired. The free
ends of the movable electro-magnet wires should now be
cut a convenient length and soldered, one to each cheek,
of the com mutator. Two L shaped springs (to serve as
brushes) are now to be made out of a piece of very thin
and springy brass, about li inches in length beyond the.
bend, and \ inch wide. These are to be screwed dovyn.
170 ELECTRICAL INSTRUMENT MAKlNQ FOR AMATEURS.
to the base-board in such a position that they press
squarely, firmly, yet not too heavily on the opposite
side of the commutator. Finally,*one of the ends of the
wires coming from the fixed electro-magnet E (see
fig. 6 1) is connected to a binding screw A. The other
end D is carried in a groove under the base-board to the
screw of the brush C, to which it is soldered, or other-
wise electrically connected. The other brush C is con-
nected by another wire passing under the board (and
shown at B) to the terminal A. The spindle carrying
the movable electro-magnet and its attachment may
now be put in its bearings, the brushes carefully ad-
justed, and the back screw N screwed up until the spindle
can rotate freely, but without too much play on .its two
centres, which should be kept oiled. If well made, this
little motor will run well with a single Leclanchk, better
with a pint bichromate, or chromic acid cell, and at a
furious speed with the .four cells described and figured
at 98.
103. THE PHONOGRAPH.
Although the phonograph cannot by any stretch of
the imagination be called an electrical instrument, yet
it is so closely allied to the telephone in its mode of
acting that a short description of the manner of making
a simple form may not be out of place here. The
following 'directions are due to Mr. Shelford Bidwell,
and were originally published in the English Mechanic ;
" The most important part is the cylinder. This in
my phonograph is a hollow brass casting, 4^ in. long and
TffE PHONOGRAPH. lit
4^ in. in diameter. It is mounted upon an iron spindle f in.
in diameter and 16 in. long, atone end of which is a
winch-handle. Upon that part of the spindle which
lies between the handle and the cylinder a screw is cut,
having eight threads to the inch. The other end of the
spindle is left plain. The cylinder having been turned
perfectly true, a screw is cut upon its surface of exactly
the same pitch as the screw upon the spindle, i.e., eight
threads to the inch. The depth of the spiral groove thus
formed is T V in., and its breadth is T \ in. It is better to
cut it square, and not V shaped. Two brass bearings
for the spindle are made of the following dimensions :
length, 2\ in. ; thickness, i in. ; height, i in. One of
these has an inside screw corresponding to the screw
upon the spindle. Each bearing .has two holes for
FIG. 62.
screwing it to the support, as may be seen in Fig. 62,
which is engraved from a photograph of the instrument.
The cylinder, spindle, and bearings being completed,
ten pieces of wood must be prepared as follows :
A is 12 in. x 9! in. x i in.
B is 3 in. x 3 in. x i in.
C is similar to B.
l;* ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
D is 5 in. x 5^ in. x \ in., and has a circular hole
2f in. in diameter, cut in its centre.
E is similar to D.
F is 5 in. x 5J in. x \ in., and has a hole, i in. in
diameter, in its centre.
G is 5 in. x 2^ in. X \ in.j
H is similar to G.
l is 8 in. x -2\ in. x f in.,
K is similar to I.
B and C are the upright supports for the bearings,
^one of them being shown in Fig. 62. The position of the
others is indicated by, the letters corresponding to them
FIG. 63. FIG. 64.
in Figs. 63 and 64. The uprights B and C are fixed near
the ends of the base board, A, by means of 2\ in, screws.
D and F are screwed together (see Fig. 63) and the two
are then fixed perpendicularly upon G. G is joined to
I by a pair of hinges. The two ends of I are screwed
to the base-board, but two holes in I are \ in. larger in
diameter than the screws which pass through them.
The heads of the screws are effectively enlarged by iron
THE PHONOGRAPH 17$
washers f in. in outside diameter. The object in" this
arrangement is to allow a certain amount of play in the
board I for purposes of adjustment. When properly
adjusted the screws may be tightened, and the board
firmly fixed in position. E is attached perpendicularly
to H, and made rigid with two small triangular pieces
which are not shown in the figure. H is hinged to' K,
and K is fixed to the base in exactly the same manner
as I. Through the middle of I is passed a brass screw
bolt, the square head of which is fixed in I. The screw
goes through an elongated hole in G and is fitted with
a round milled brass nut It is well to plaqe a washer
under the nut. Screw-bolts of this description are used
for fixing the expanding bodies of ordinary photo-
graphic cameras, and may be had of any optician. H
and K are fitted with a similar bolt. Two rather stiff
pieces of steel spring are attached to the ends of I and
extend for a little distance underneath G. These
springs tend to separate G and I, or rather to cause G
to turn backwards, like the lid of a box when opened.,
One of the springs may be seen in Fig. 63.
11 The nut, of course, works against the springs.
When the nut is screwed up tight G and I approach,
and may be made almost to touch each other. When
the nut is loosened the spring causes G to rise. Very
delicate adjustment is thus rendered possible. H and
K are fitted with similar springs, for a similar purpose.
We come now to the diaphragm and points. The
diaphragm, which receives the voice, is fixed over the
circular hole in D, as shown in Fig. 63. It consists of a
circular plate of very thin iron 4 in. diameter. Ferro-
174 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
type plate will answer the purpose very well, but thin
charcoal iron is better. It is, however, possible to have
the iron too thin. I should say that about two-thirds
the thickness of an ordinary ferrotype plate is the best.
The point is made from a knitting needle about -^th
of an inch in diameter, which must be very hard one
which can be bent is no use. The original point had
better be broken off, and a new one ground upon an
oilstone. For this purpose, the needle is held at an
angle of about 30 degrees with the stone, and is con-
stantly turned round. The point, having been made
tolerably sharp, is polished and cut off with a file. The
part so cut off is T \ths of an in. long. This has now
to be attached perpendicularly to the centre of the
diaphragm, and the method of doing so is as follows :
The diaphragm is laid upon a sheet of ^lass, and a
little spot in its centre about \ in. in diameter is
scraped clean with a knife. This must be done care-
fully and gently, or a bulge will be produced. The
fragment of knitting needle is then taken up with pliers,
and its blunt end, having been moistened with solder-
ing fluid, is held above the flame of a spirit lamp, and
touched with a piece of tinfoil. With a little manipu-
lation a small bead or globule of tin may thus be made
to adhere to the end. The scraped spot on the dia-
phragm is now moistened with soldering fluid, and the
diaphragm is supported at some distance above a small
spirit flame. The ring of a retort stand forms a con-
venient support. The butt end of the point, with f.in
globule attached, is then applied to the scraped spot
ELECTRO-MOTORS. 175
with pliers. In a few seconds the globule melts, when
the lamp is -instantly removed, and the point manipu-
lated with the pliers, so as to be perfectly upright when
the tin hardens, which will take place in a few seconds
more. The point will then be found to be firmly at-
tached. The diaphragm and point must, after this
operation, be thoroughly washed with soap and water,
and slightly oiled, otherwise they will rust. The only
precaution to be observed is to apply no more heat than
is just necessary for melting the tin. Too much heat
wiil warp the disc and, if it is a ferrotype, blister the
japan. The soldering fluid consists of equal parts of
hydrochloric acid and water, in which is dissolved as
much zinc as possible. A pile of books will be found
useful for steadying the arm while manipulating the
point. The diaphragm is fixed in its place by means of
a brass flange (like a camera flange), 4 in. in outside
diameter, with a 2| in. opening. Four screws are used.
The second diaphragm is made of parchment paper,
like that used for covering jam pots. It is 4 in. in
diameter, and is gummed over the hole in E on the side
remote from the cylinder (see Fig. 64). When the gum
is dry the diaphragm is moistened, and again allowed
to dry, when it will be found to be as tight as a drum.
The second point is exactly like the first, though it may,
with advantage, be a trifle sharper. It is not attached
directly to the paper diaphragm, but to a steel spring,
which may be seen in Figs. 63 and 64, This is a piece of
main spring ^ in, wide and 2| in. long. It is fixed
above the hole in E, by means of two strips of brass, as
176 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
shown in Fig. 64, and is just so much, bent that the end
of it, when free, is f in. distant from the plane of E.
The power of this spring may, however, be varied
within considerable limits without appreciable diffe-
rence in its performance. The point is fixed to the
spring in the same manner as to the iron disc, but the
same care as to overheating is not requisite, and the
operation is consequently easier. Before the spring is
screwed in its placed, a loop of sewing silk is attached,
FIG. 65.
^ to the centre of paper diaphragm by means of a piece"
of court plaister, \ in. square (see Fig. 65, where A is the
piece of plaister, B the loop). The length of the loop
must be such that when it is drawn over the spring just
above the point, the end of the spring may be nearly in
the plane of E. The spring is thus caused to draw the
paper drum even tighter than before, and its inner sur-
face is rendered slightly convex. Another flange carry-
ing a short rim or spout is now screwed round the paper
drum. A paper resonator is made to slip over the
short rim or spout. It is a cone made of two or three
thicknesses of stout drawing paper. Its length is 18 in. ;
diameter at small end 2 in., and at large end 7 in.
The resonator greatly reinforces the sound v/hen the
phonograph is speaking. A w.ooden mouthpiece, like
those used for speaking tubes, is inserted into the hole,
THE PHONOGRAPH. 177
F (see Figs. 63 and 64). The instrument is now complete,
but it will require careful adjustment before it can be
used. In the first place the screws which attach it
to the base must be loosened. The milled nut on A
screwed up tight, and the piece I, shifted about until
the point on the iron disc is exactly in the middle of
one of the grooves on the cylinder, and barely touches
the bottom of it. Then the screws must be tightened,
and this part of the apparatus finally adjusted. The
same process is repeated on the other side ; but in this
case the adjustment is not quite final, as will hereafter
be seen. The next thing is to procure suitable tinfoil.
This should be rather stout about 15 square fe.et to
the Ib. and should be cut into pieces 14! in. by 4^ in.
Before putting a tinfoil on the cylinder the two nuts
are removed, and the diaphragms turned back out of
the way. A little gum brushed along one end of the
tinfoil will be sufficient to keep it firmly in its place ;
the join must be carefully smoothed.
"The diaphragms are then turned back to their places,
and the nuts screwed on. The nut on G is screwed up^
just far enough to cause the point on the iron diaphragm
to touch the tinfoil very lightly. The handle is then
turned about a quarter of a revolution, causing the
point to make a short scratch on the tinfoil. The nut
on G is thereupon loosened, withdrawing the point
from the tinfoil, and the nut on H being screwed up,
another turn is given to the handle. If the scratch
thus produced exactly coincides with the former one,
well and good ; if not, the screws attaching K to the
178 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
base must be loosened, and K shifted about until
absolute coincidence is attained. The utmost accuracy
on this point is essential. The instrument may now be
considered fit for use. Loosen the nut on H, so that
the point on the spring may be well away from the
tinfoil, and screw up the nut on G, so far that the point
on the iron diaphragm may score a well-defined furrow-
on , the tinfoil when the handle is turned. Turn the
han'dle with as great regularity as possible, at the rate
of about one turn per second, or a little slower. Speak
loudly and distinctly into the mouthpiece, putting the
mouth as near as possible to it, without actually,
touching it. When you have finished, ^withdraw the
point by loosening the nut, turn he handle backwards
until the cylinder is in its original position, and screw,
up the second nut until the second point presses lightly
but steadily upon the bottom of the furrow. Then put
on the paper resonator, and turn the handle at the same
speed as before. If the adjustments are perfect, the
result will be astonishing. I will conclude with a few
general remarks and hints. It will be observed that
this instrument has two diaphragms, whereas Edison's
latest have only one, which does both the receiving and
the speaking. I have made many experiments with
the object of dispensing with one of the diaphragms,
but I have never, under any circumstances, obtained so
good results with one as with two. Mr. Preece told the
Physical Society that the employment of only one was
a ' retrograde step.' Reason and experience led me to
concur in his opinion. I do not believe that iron and
THE PHONOGRAPH. 179
parchment paper are the best possible materials for the
diaphragms, though they are better than any others
which I have hitherto tried. The great fault in the iron
appears to be in its tendency to resound forcibly to
cerlain overtones, in certain vowel sounds. I have
tried to overcome this with coating the diaphragm with
indiarubber, but with no great success. I think, how-
ever, that a ring of indiarubber between the diaphragm
and the flange has amundoubted effect in diminishing
the nuisance. The steel spring is subject to inde-
pendent vibrations of a similar nature. These may be
damped by causing a piece of soft indiarubber to press
lightly upon it at a point about \ in. below the lower
strip of brass. I have also found it an advantage to
wrap indiarubber round the top of the spring before
screwing it on.
" I believe that the mouthpiece of a telephone would
give better results than that of a speaking-tube. A
long resonating mouthpiece like that which Edison first
used is worse than useless. The point on the steel
spring should be made to turn very slightly upwards,
instead of being perpendicular. In the latter case, it is
liable to produce a squeak something like that of a
pencil when drawn up a slate. If the points are too
sharp, they will cut and scrape the tinfoil ; if too blunt,
the articulation will be muffled. After the points have
traversed the cylinder 200 or 30x3 times, they will
require sharpening. This can be done with a small
oil-stone, without removing them."
I8o ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
$ 104. THE MICROPHONE.
A simple and yet sensitive form of this instrument
may be made as follows :
Procure a piece of good graphite from the gasworks,
and cut from it with an old saw three rods about f in-
square by 2 in." long. If these are roughly cut out
with the saw, they^can be finished up beautifully by
rubbing on a flat stone, with a little fine sand and
water. When this has Been done, they should be put
in an oven to dry. A piece of planed deal 8 in. by
3i in. should now be procured, and a length of 3 in.
cut off one end, and glued and screwed to the longer
piece so as to form a letter L. A line is now drawn
down the centre of the longer piece ; along this line, at
equal distances from the two ends of the piece, and
about i| in. from each other, two square holes are
cut into the board, of such a size as to admit
ihe square ends of two out of the three graphite
rods just prepared. Previous to inserting the carbon-
rods in these holes permanently, these holes must be
bushed with thin sheet copper (the same as is used for
dynamo brushes). This bushing serves to make a
good connection between the carbon-rods, and two
binding screws inserted into the shorter board which
forms the base of the instrument. This is effected by
soldering a piece of No. 20 copper wire to each bushing,
and bring a wire from each to its respective binding-
screw. For the sake of neatness, the wires may be
taken, out at the back of the L, and brought round under-
THE MICROPHONE. '181
neath to the two binding screws, a channel being cut
with a penknife to allow the wires to He in. The third
piece of graphite-rod, which has not hitherto been
brought into use, is now sharpened at both extremities
(by rubbing on a flat stone with sand), so as to produce
conical pyramids, with sharp points at both ends of the
rod. By" means of a \ in. bit, put in the Archimedean
drill, a slight depression is made at about J in. from the
edge of each of the carbons which have been fitted to
the holes in the upright backboard of the L. These
depressions must be in the inside (on the upper surface
of the lower carbon, and on the lower surface of the
upper carboti), and serve to support the pyramidical
graphite by its two opposite points. To adjust this
properly, the upper carbon is removed bodily ; the
lower carbon pushed into the lower bushed hole until
it is flush with the back of the board. If it is not quite
firm in this position, small pieces of copper may be
wedged in, until it is quite firm. The double-pointed
carbon-rod is now placed with its lower point resting in
the depression in the lower rod, and is held thus while
the upper carbon is being inserted into the upper
bushed hole, care being taken that the upper pointed
end of the loose carbon-rod is resting in the depression,
on the under surface of the upper carbon-rod. The
upper carbon is now pushed into the upper hole until
flush with the back of board. Care must be taken that
there must be sufficient space between the upper and
lower depressions to allow the central carbon-rod tc
turn quite freely on its axis, but not so much as to le|
X82 ELECTRICAL INSTRUMENT MAKING FOR AMATEURS.
it fall out. This microphone, if glued by its shortest
base to the bottom of an empty cigar-box (turned
upside down), from which the lid has been removed,
will be found extremely sensitive. For use, connect
one binding-screw to one. pole of a small Leclanche.
Connect the other binding screw, by means of a long
wire, to a telephone, and bring a return wire from the
telephone to the other pole of battery. The tick of
a watch, the sound made by the walking of a fly can be
distinctly heard at the telephone end, if the fly or the
watch is placed on the cigar-box forming the sounding-
board of the instrument.
WATER REGULATOR TOR INDUCTION COILS.
Instead of the sliding tube arrangement described
at 49, it is sometimes convenient to have a
separate regulator of the strength of shocks, etc. The
following will be found very easy to make. Procure a
stout glass tube i in. bore,. 1 2 in. long. Fit it with a sound
FIG. 66.
cork at each end, run a short length of No. 16 copper-
wire through the centre of one cork, and a longt length
(about II in.) through the other. Make a ring of
the wire at each outer extremity. Put one cork in, fill
the tube with water, then insert the other cork. Wipe
dry, then sealing-wax the outsides of corks. The more
the long wire is drawn out, the weaker will be the shock
from the coil, when this regulator forms part of the
circuit.
APPENDIX TO TABLE OF WIRE
RESISTANCES, ETC., 73.
It is frequently necessary to know what diameter of wire
must be used to carry a current safely ; id esf, without
injurious heating. The following table gives approximately
the ratio between the gauge of the wire, and the safe current
in amperes, which the wire can carry.
B W G
6
9
10
ii
12
13
14
!i
17
18
19
20
22
SAFE CURRENT IN AMPERES.
60
43
8
21
IS-
13
IO
5
3
2'5
2
The above table refers to ordinary commercial copper
wire only ; it is NOT applicable to any wire of different
material, such as brass, lead, tin, platinum, German
silver, &c. Even pure copper differs considerably from
the ordinary commercial article, in carrying power; somfe
samples tried by me having a " safe carrying capacity "
of nearly double that given in the table.
APPENDIX TO GALVANOMETERS, 86.
PREVIOUS to publication in a complete form, the fore-
going chapters on *' Electrical Instrument-making for
Amateurs" have elicited a considerable amount of corre-
spondence from the readers thereof and many kind sugges-
tions from friends of the author.- Foremost among these, and
replete with valuable hints as to the desirability of encourag-
ing pupils and teachers to make their own instruments, is
one by Mr. Wm. Robertson, of Castle Douglas, N.B. Him-
self a teacher, he has had practical experience of the
thoroughness of the knowledge acquired by students who
learn to make as well as to use their own instruments. In
these remarks the author fully concurs. During the ten
years in which he was engaged in teaching the science of
electricity at the Carshalton House School, he invariably
adopted the plan of causing the pupils to construct the
apparatus which formed the substance of each lesson ; and
the result has been that, although the apparatus turned out
may have been of the roughest description, yet the insight
obtained by the students has been so thorough as to enable
them to pass examinations, etc., much more creditably than
in cases where book knowledge alone was imparted.
Feeling that the subject may interest others, he ventures
to reproduce here a portion of Mr. Robertson's letter, referring
APPENDIX. tt$
more especially to the construction of two novel forms of
galvanoscspe, eminently adapted to show the laws which
regulate the action of currents on magnets, etc.
" Permit me," says Mr. Robertson, " to make a suggestion.
I have found that in teaching no apparatus is half so valuabU
as that which is home-made. I have constructed several
simple galvanometers (or rather galvanoscopes) which have
proved very useful. Even if the teacher himself has learned
to make a ' fine job ' he will find it of advantage to use very
plain and simple apparatus in his class work. In this way
the students are encouraged to try their own hands at ap
Fio. 58.
paratus making. Occasionally my pupils have made fairly
good ' galvanoscopes ' at home, even in spite of their
mammas, and their dislike to a mess at the fireside.
" Inclosed you will find sketches of two novel forms of
'galvanoscopes,' by means of which action of currents on
magnets can be easily and accurately studied."
The first instrument is figured at Fig. 58. It consists of a
base board, about 8 in. by 5 in. by in., shown at b. In the
middle pf this is placed a stout brass wire, bent into the
form of a ring, with the ends prolonged at e } e' } where they
|g APPENDIX.
are held down to tho base board by tne two binding screws,
*, y. At the point where the r/ire crosses itself to form the
hoop (which should be about 4 in. in diameter), the wire is
wrapped round, on both portions, with a layer or two of silk
ribbon, which serves at once to separate them from electrical
contact witV each other, and to retain the wire in the shape
of a ring. At the upper portion of the ring is fastened a
little hook, , which is held in position by being neatly
bound round with a little silk twist.
At each extremity of the stand is a wide strip of sheet
brass, bent at right angles x *', of the shape figured. Each
strip is secured to the base-board by two small screws, c c',
and furnished with a binding-screw, K. K', to make connection
with a battery if required. Each of these strips (the centre
of which should coincide with the central plane of the ring)
is perforated with four equidistant holes, one on each- side
of the centre and one above and below. These holes are
to permit the introduction of a straight wire or wires, above,
below, or on either side of the centre of the ring. These
wires serve to conduct the current from one strip to the
Dther. Two magnetic needles may be used with this in-
strument namely, one free to move vertically only, as
figured at M ; and another so suspended as to swing
horizontally. The former is furnished with two hooks, by
which it may be temporarily held by the ring ; the latter is
provided with a fine silk suspension, which can be slung on
to the hook h in the ring. This instrument, which Mr.
Robertson calls the " Deflector," may be used in the following
mode:
I. Having placed the "deflector" with the brass strips
pointing north and couth, hang the "vertical-" needle M
from the top cf the ring. Now insert a brass or copyer wire
APPENDIX. 187
through the upper holes in the strips, and couple up the
binding-screws on the strips with a battery. No Deflection
ensues.
2. Place the wire in the lower pair of holes, and again
Couple up. Still no deflection.
3. Now place the connecting wire in the right-hand pair
Of holes. Deflection takes place.
4. Place the connecting-wire in the left-hand pair of holes.
Deflection ensues in the opposite sense.
5. Remove the " vertical " needle, and substitute t'ie
" horizontal." Now place the wire connector in the pair of
holes ovef the needle ; couple to battery. Deflection ensues
in one direction.
6. Place the wire connector in the pair r,f holes under the
needle. Deflection is obtained in the opposite direction.
7. Place two exactly similar wires, one in the pair of holes
above and the other in the pair of holes bslow the " horizontal "
needle. On coupling up to battery no deflection occurs, if
the two wires are equidistant from needle, and if both be of
equal conductivity. This is, however, seldom the case ; and
then the nearer, or better conducting wire, rules the
deflection.
8. Wires of different gauge, but of same material.
9. Wires of same gauge, but of different material. In
these two latter cases the needle obeys the better conductor,
provided the distances be equal.
10. Change the position of the instrument, place the
plane of the ring in the magnetic meridian, remove all the
wires out of tho hole in the strips, suspend the horizontal
needle in the ring, and when it is at rest in the plane of the
ring, pass current through the binding screws connectod
v/ith the ring s /. Defoctivn ensues*
l88 APPENDIX.
ii. Reverse the direction of the current. Deflection in
Apposite direction.
The other instrument, or "differential galvanoscope,"
consists simply in a base board, as shown at Fig. 59. B, to
which is fastened by four binding screws, A A and c c', a
ring R made of two insulated wires lying side by side, ths
extremities of which are connected to the four binding screws.
At right angles to the plane of this ring rises a light rectan-
gular frame of stout brass wire F, on the top of which slides
with friction a small spiral of wire, terminating in a hook
Q. (This serves to suspend the magnetic needle.)
A short piece of brass wire, bent twice at right angles, \V,
called the "bridge," serves to connect the nearer pair of
binding-screws, c c', when required. A " horizontal " mag-
netised needle, suspended by silk, may be hung in the middle
of the ring, or to one or other side of it, by means of the
hook Q.
This instrument can be used to prove the counteracting
effect of currents in opposite directions ; since, if the bridge
W be inserted in the- binding-screws c c', and a current be
sent round by the binding-screws A A', it traverses the two
APPENDIX. 183
rings in opposite directions. Hence the needle is not deflected
if perfectly central ; but if the needle be to one side, the
nearer circle governs the needle. Having adjusted the
needle to perfect centrality by sliding the spiral Q until the
needle is unaffected by any current passing, the instrument
rtlay be used for comparing the current given by different
cells. To this end it is only necessary to remove the bridge
arid couple up one cell to A c and the other to A' c\ care
being taken, of course, to send the currents in fr-he same
direction.
APPENDIX TO BATTERIES 95.
/ACCUMULATORS, OR STORAGE BATTERIES. If a current
of electricity traverse an electrolyte (any compound body
capable of allowing the passage of electricity) decomposition
of that body takes places. The surface of the conductor
by which the current enters (the positive electrode or anode)
attracts to itself the more negatii^e constituents of the said
body ; while the conductor by which the current leaves the
decomposition cell (the negative electrode or kathode) in like
manner attracts to itself the more positive constituents of
the compound. As this decomposition is only effected by
means of the electric strain put upon the constituents of the
compound, it is evident that if the strain (the original
electric current) cease, the effect will also cease, and recorn-
NOTE. For a more exhaustive work on this subject, the rear!**
referred to Sir D. Salomon's practical handbook, " Maiucemeni of
Accumulators and Private Electric Light Installation " (Whitiiker a*>d
Co.),. 1 5 88.
190 APPENDIX.
position will be effected ; in other words, the original elec-
trolyte will be reformed, and a current of electricity set up
in the opposite direction. (See 81). We may liken the
effect of the current to a force exerted against a spring, and
the electrolyte to the spring. When the force exerted is
removed, the spring, in recovering itself from the strain,
exerts a force in the opposite direction to that originally
employed.
Based on a knowledge of these facts, and an application of
Ohm's law ( 82), we are able to construct an apparatus
which shall return to us as electrical energy a considerable 1
percentage of. the current passed through it, and at will to
cause the energy to manifest itself either as a large current
at a low pressure, by diminishing the internal resistance ;
or as a smaller current at a high pressure, by arranging the
elements in series, so as to get cumulative effects.
The first accumulator ever made was, perhaps, that of
Ritter, who in 1803 constructed a secondary pile of a number
of discs of similar metal, separated by pieces of moistened
cloth. On connecting for a few seconds the opposite extre-
mities of this pile with the poles of a battery, the pile will
be found to have acquired the power of producing, for a shor
time, a current opposite to that of the battery.
The next to utilize the polarization current was Grove,
who devised what is known as the " gas battery." This
consists of two platinum plates, standing upright in a vessel
of water, each plate being surrounded by a glass tube closed
at the top with a platinum wire in connection with each
plate sealed in the top of each tube, and projecting out of it.
On passing a current through the water by means of the
projecting wires, the water is decomposed, the oxygen col-
lecting in the tube at which the current enters, and the
APPENDIX.
191
hydrogen in that at which it leaves. On Interrupting the
main current, and connecting up the two platinum wires, a
current is set up in the opposite direction, while the hydrogen
and oxygen recombine to form water.
To Gaston Plant is, however, due the honour of having
made the accumulator a practically useful instrument.
THE PLANT^ ACCUMULATOR, which for efficiency has not
yet been surpassed, may be made by taking two sheets of
r \- in. lead, each about 6 in. wjide by 3 ft. long, placing one
on a flat table, then placing lengthwise on this sheet three
strips of india-rubber, a trifle longer than the leaden sheet,
but only \ in. wide, at equal distances from each other
Over these is to be laid the second leaden sheet, and over
this latter again, three india-rubber strips, similar to the first
three. (These serve only to keep the sheets from contact
with each other, and may be replaced by asbestos cloth or
any insulator not acted on by acids). The two sheets are
then to be rolled into a tight spiral on a wooden cylinder.
A leaden strip or lug is soldered to one end of each sheet to
serve as terminals, for connection.
A cylindrical glass or earthenware vessel (glazed), of
sufficient size to contain the spiral, but leaving the lugs pro-
jecting above the upper extremity, is now fitted with a
paraffined wood, or ebonite cover, having two apertures to
admit of the passage of the lugs. The spiral having been
tied together on the outside with a gutta-percha or india-
rubber band, may now be inserted in the containing vessel,
into which must have previously been poured the follow-
ing mixture, to reach nearly to the top of the spiral, when
this latter is inserted into the jar, viz. :
Water ... ic parts 1
Sulphuric Acid i part J b ? measur *
I 9i APPENDIX.
N.B. In mixing sulphuric acid with water, it ^nust he
borne in mind that the acid must be added in a fine stream
to the water, stirring with a glass rod in the meantime, and
not the water to the acid ; otherwise the violent reaction
may cause a dangerous accident. . The Plante cell thus
constructed has to be " formed," that is to say a current of
electricity has to be passed into it until small bubbles of
oxygen gas show themselves at the anode, then discharged,
then again charged in the opposite direction, and again dis-
charged, and so on for a fortnight or more, until the surfaces
of the leaden sheets, by continual oxidation and deoxidation
have become sufficiently spongy to retain a considerable
charge. It is this " forming " that constitutes the great,
and, indeed, the only objection to the Plante accumulator.
The time necessary to effect the " formation " may be
greatly shortened, as shown by Plante in 1883, by immers-
ing the leaden sheets in a 20 per cent, solution, of nitric
acid, previous to subjecting them to the action of he charg-
ing current. Faure, later on, showed that the long and
tedious process of " forming " the plates by the influence of
the current, might be still farther abbreviated, by mechani^
cally dressing the surfaces of the plates with lead in an
oxidised condition. This is effected by making a paste of
red lead and sulphuric acid, and smearing the surfaces with
this paste, previous to rolling up the sheets. This enables
the accumulator to be used, after it has been charged two
or three times only. Innumerable "patents" and improve-
ments have been made on this device, such as punching,
honeycombing, or gridding the leaden plates, to enable
them to retain the paste, which is very apt to fall away
from the surface during action.
APPENDIX. 193
The following data as to the E.M.F. and capacity of
accumulators may be of interest :
ist. Each cell, irrespective of size and number of plates it
may contain, provided these are connected to form virtu-
ally but TWO plates, will have an E.M.F. of about 2-25 volts
when first charged, quickly falling to about 2 volts, at which
it remains steady until nearly exhausted. When the
E.M.F. falls to i'9 volt, it is a sign that the discharge should
be stopped, as it is not advisable to completely discharge the
cell.
2nd. Every square foot of surface of the positive plate in a
well-made accumulator (say the E.P.S. type) is capable of dis-
charging at the rate of six amperes per hc-ur, or one ampere
for six hours (six ampere hours). This is only true if the
plates are arranged in parallel, if they are arranged in
series, so as to obtain a higher E.M.F., then the quantity to
be discharged is equal to the surface of the one element only
at the same rate.
3rd. The charging current, in amperes should be some-
what less (say less) than that of discharge.
As a practical illustration cf the application of thd two
first rules : suppose v/e desire to light a 5 c.p. lamp, of 8
hours' resistance, for one hour. Such a lamp will require
about I ampere of current to flow through it to light it
properly.
Hence to drive I ampere through 8 ohms we shall
p-
need an E.M.F. of 8 volts, since ~ = C. Therefore 4 cells
R
will be needed, coupled in series. As I square foot of posi-
tive surface can furnish 6 amperes per hour, ^ of a square
toot will be sufficient to furnish the I ampere needed foi I
hour. Hence each csll (as these are coupled in series) must
194 APPENDIX.,
present at least this amount of positive surface. Therefore
6 '"cells, coupled in series, each cell containing a pair of
plates i foot long by 2 inches wide, would just do the work
desired.
POCKET ACCUMULATOR. Many amateurs are desirous of
constructing a small pocket battery, capable of lighting the
little 4 volt " fairy " lamps, used for scarf pins or head
decorations. The following will be found easy to make, and
effective in action : Having procured some good sheet gutta-
percha, \ in. in thickness, let three squares be cut from it
3$- in. x 5" in. each, two others 5 in. x i in., and one
3f in. X i in. This latter having been laid on a smooth
elate (previously moistened on the surface to prevent adhe-
sion) a rather hot poker is passed quickly once or twice over
its surface, so as to render it soft and sticky. The narrowest
edges (\\ in.) of the three larger squares (those 5 in. X 3^- in,)
are then lightly rubbed over with the hot poker, and imme-
diately pressed down upon the strip on the slate, so as to
divide it into two equal spaces, or little over \ in. wide, by
3^ in. long. Special care must be taken that the three up-
right squares which) are to form the sides and central parti-
tion of the accumulator box) adhere firmly and at all parts
to the bottom strip. An assistant should hold these square
upright and parallel, while the operator proceeds to heat
with the poker the two remaining side strips (5 in.. by ij in.)
which he applies while hot to the sides o the cell. The
same care must be taken that perfect adherence takes place
with these strips and the three squares, as in the case above
with these latter and the bottom. If this be not attended
to, either the whole cell will leak into the carrier's pocket
or the internal divison will allow the fluid to pass from one
side to the other. This latter defect would be fatal to the
APPENDIX.
'9.S
due action of the accumulator^ To test this, when the cell
is quite cold and hard, it will be well to fill one side care-
fully with water, and notice whether there be any leakage,
cither through the sides of the central- partition, or to the
outside. Should there be any leakage externally, the water
should be emptied out, the cell dried, and the defective joint
rubbed over externall) with the hot poker, until the joint is
made sound. Should the defect be internal, a piece of sfouf
wire bent into the shape of L should be heated at one limb,
and this rubbed over the defective portion, so as to secure a
perfect water-tight compartment.
Four plates, 4^ in. x 3 in., with an ear or lug about one
in. long by one in. wide, must now be cut out of some -fa
in. sheet lead. With a small punch or bradawl, these
plates must be perforated all over, as thickly as possible
to within about ^ in. of the top, or lug end. A thick paste
must now be made by mixing some good red lead with
equal parts of oil of vitriol and water. The holes which
have been punched in the four plates, must now be filled
in with this thick paste, by means of a flat wooden stick
or spatula. The surface also of the plates must be liberally
smeared over with the paste, the lugs only excepted. The
plates should now beset aside for a short time, to allow
the paste to harden. While this is taking place, a piece of
cigar box should be qut of- the exact size to fit the top of
the gutta-nercha box previously made, viz., 3|in. X if in.
A central line having been drawn lengthwise down this,
two slots ^ in. wide by I in. long are cut on each side of
the said line. The use of these slots is to admit the pas-
sage of the lugs or ears of the leaden plates. They should
be all parallel, and stand about J in. apart. Two cir-
cular holes, nearly ^in. in diameter must also be drilled
1 96 APPENDIX.
through this cover, one on each side of the central line, and
equidistant from the slots! These holes serve to take short
lengths of glass tube, similar' to that used in feeding bottles.
The wooden cover must now be soaked in hot melted paraffin
until thoroughly permeated with it. The lugs of the leaden
plates are now passed through the slots'(which they ought
to fit tightly), the two central ones bent towards each other
till they meet, cut off at the point thy meet, and soldered
together. To each of the two outer lugs is spidered a small
binding screw, to serve as terminals. Any excess of lead
on the lugs may now be cut off. The gutta-percha cells
should now be nearly three parts filled with dilute sulphuric
acid (r part acid to 4 parts water) and the plates im-
mersed therein The acid should not reach the top of the
cells when the plates are in, the lid or cover should rest on the
top of the cells all round. Should this not be the case, the
leaden plates must be cut a trifle shorter. Two short lengths
of tube are now inserted into the holes left for that purpose,
and cemented into position with a cement made of one part
melted pilch and two of gutta-percha, applied hot. With the
same cement, the top or cover must be cemented down to
the outer cell ; if the cover be cemented all over (except
the mouth of the tubes), it will prevent any chance leakage.
A little soft india-rubber stopper should now be made to
fit the orifice of each tube, removable at will.-'
To charge this accumulator it should be connected to
a small dynamo, or to a four-cell bichromate or chromic acid
battery ( 98) for three hours, then the terminals connected,
so as to allow it to discharge itself, then charged in the
reverse direction, and so on for several days in succession, or
until it is found that the accumulator will ring an electric
bell for fifteen minutes, after being charged only ten minutes.
APPENDIX. 197
When this occurs, the cell must be charged in one direction
only, the terminals being marked in order to know where to
connect for the next time of charging. When complete, this
accumulator will light a 3 or 4 volt lamp well for about
two consecutive hours.
It is advisable to charge the accumulator within about an
hour or so of being wanted for use, so as to incur as little
loss of power as possible, due to the unavoidable short-
circuiting which takes place in the inside, through the
damp lid. Previous to permanently sealing up the cell, it
will be necessary to place a small piece of india-rubber
between each pair of plates, to prevent any accidental
contact or " short circuiting."
APPENDIX TO DYNAMO, 74:
ELECTRO-MAGNETS. Many amateurs are puzzled how to
wind tho iron cores of dynamos, electric bells, &c., in order
to obtain desired results.
The first point to be noted is the amount of wire to be
got on the core. Since the magnetizing effect of the current
decreases as the distance from the iron core increases, in
proportion to the square of the distance, it follows that we
do not gain power if we add on layers of coils beyond
a certain point. In practice, it will be found that no
advantage is gained in putting on more wire than will
increase the section of the completed electro-magnet to
three times that of the bare iron core. Let us suppose we
had a core one inch in diameter : the diameter of the
wound electro-magnet, including the wire coils, should not
exceed i hue inches,
I9 APPENDIX.
The second point is, What gauge of wire must be used
wherewith to coil the core ?
By experiment it has been ascertained that the magne-
tizing effect is the same, whether 1,000 amperes are sent
once round an iron core, or whether one ampere is sent
1,000 times round the same core ; or, in other words, " the
magnetising effect is proportional to the number of ampere-
turns of coil." If, therefore, we have to wind an electro-
magnet to be used with a large current of low E.M.F. we
must wind our electro-magnet with few coils of thick wire,
both for the sake of carrying this large current, and of
avoiding resistance. If, however, the current at our dis-
position is small, but delivered by a high E.M.F. then we
select a finer wire.
The third point is the direction of winding.
To produce a magnet with normal poles (one north and
the other south), the direction of winding must always be
the same. Let us suppose we start winding in the direction
of the motion of the hands of a clock : then, although we
are at liberty to coil over the first layer, any number of
times, yet we must always wind in that same direction, viz.,
from left over to right. This is true, whatever be the form
of the magnet core. It is easily seen, that if (as in bell
magnets, horseshoe magnets) the winding is not carried on
right round the bend or " yoke," but the wire made to cross
over to the other limb, the winding will apparently be in the
opposite sense, that is, if viewed from the poles, will pass
over one core to the other, thus : CD. Lastly, it is some-
times necessary, as in the Gramme machine, 72 Fig 43,
to produce one pole in the middle of a bar, and two opposite
poles at the extremities. In a Gramme machine, the upper
ole-piece is to be, say north. Hence, the two extrmities
APPENDIX. 199
of the upper bar or core must be south. To obtain this
result (consecutive poles) the direction of winding must be
reversed when we cross over from the left-hand bar or core,
to the right-hand one. Suppose we start winding to the
loft-hand of the upper pole-piece, and begin winding in a
direction Apposite to the motion of clock-hands. When we
have coiled the bar with the desired amount of wire (always
in the same direction) we carry the wire over the pole-piece
and commence winding the right-hand core or bar in the
opposite direction, ;',., the same as the motion of the hands
of a clock.
INDEX.
Accumulators, see Appendix
AJloy for Thermopile, 143
Amalgamation, 148
Ammeter, 116
Ampere, 116
Apparatus necessary, 1
Armature, Insulation of, 103
Pacinotti, 109
-^ ^ Siemens, 99
Winding, 102, ill
Batteries, 146, 189
Chromic Acid, 151
Daniell, 154
Double fluid, 153
Secondary, Appendix
Single fluid, 149
Table of E.M.F., &c., 155
Battery for coils, 88
Bertsch's Machine, 28
Bichromated Paste, 35
Binding Screws (substitute^), 148
Bobbins, winding, 96
Boxes, G'ass-capped, 117
Brass Pivots, 9
Brushes, Wimshurst, 6$
Dynamo, 106 and 116
Canada Bajsam, 56
Carre's Dielectric Machine, 49
Cells, 140
Creeping in, 153
Clarnond's Thermopile, 143
Coating LeyttSn Jar, 72
Coils, Induction, 88
Medical, 81
Primary, 83
Secondary, 85
Collectors, 69
Combs, 49 and 55
Commutator, IOI, Il6
Condensers, 70
Fizeau's, 76
Microfarad, 79
Consecutive Poles, 115
Contact Breaker, for coil, 4
for magneto, 98
Core, iron, 83
Coulomb's Torsion balance, 20
Creeping, in cells, to prevent, 153
Cushions, 66
Cutting Zincs, 148
Dynamic Instrument. 8t
Dynamo Brushes, 1 06 and I ft
Gramme, loS
Manchester, 109
Magneto, 99
Volts in a, no
E.M.F., what it is, 121
Electro-motors, 164
magnets, 197
Electrophorus, Volta's, 23
Composition for, 27
Electroscopes, pith ball, 12
Gold leaf, 14
INDEX.
Faraday, 90
Fizeau's Condenser, 76
Foucault's Currents, 116
Franklin's Plate, 75
Frictional Instruments, I
Fulminating Panes, 75
Galvanometers, 131, 184.
Astatic, 133
Deflections, table of, 137
Single needle, 132
Tangent, 134
Glass-capped boxes, 117
Cutting, 29
' Drilling holes in, 31
Plate, mounting, 29 and 34
Pivots, 9
Threads, II
Testing for insulation, 41
and 71
Glue, acetic, 32
; Prout's Elastic, 3
Gold-leaf electroscope, 14
Gold-leaf, to cut, 18
Grading, 119
Graphite, mounting, 150
Hochhausen Commutator, 116
Holtz Machine, 47
Hubs, condemned, 116
Induction Coils, 88
Inductor, 54
Influence Machine (Wimshurst), 57
{Holtz), 47
(Carre), 49
(Bertsch), 28
(Volta), 23
Iron, core, 83
Iron, soft, importance of, 115
Jar, spangle, 73
Kay's Coaguline, receipt for, 33
Knobs or Balls, size of, 63
Laminated Armatures, 109
Leyden Jar, 71
Magnetic Needles, to make, 7 and
118
Magneto-electric Machine, 90
for shocks, 90
unidirection, 99
Materials indispensable, 2
Microfarad Condenser, 79
Microphone, 1 80
Mounting Graphite Rods or Plates,
150
Neutralising Rods, 64
Ohm's Law, 122
Pacinotti Armature, 109
Paper, paraffined, 77
Papyroxyline, 26
Paraffin, 18
Paraffined Paper, 77
Paste, bichromated, 35
Phonograph, 170
Pith Balls, n
Pivots, brass, 9
glass, IO
Pointers, II, 129
Poles, consecutive, IIJ
Porous Cells, 147
Primary Coil, 83
Punchings for armature, 109
Relation of current to defleclon 137
Red Varnish, 13, 37
Resistance to armature coils 112
Table of wire, 112, 183
Sal-ammoniac, 6, 155
Sector, 59
Shellac Varnish, to mak, 0B
INDEX,
Solder, 4
Soldering, 3
fluids, 5
iron, to make, 3
with flame, 7
Spangle Jar, 73
Spider Wheel, 109
Stands, 37, 42, 47, 60, 66
Star Wheel, 109
Straws, for pointers, &c., II
Teaching appliances, Appendix
Telephone, 156
Thermopile, 139
Alloy, 143
Simple, 140
Clamond's, 143
Tinning, a soldering iron, 5
Tools, needful, i
Triblet Tubing, 81
Tube, glass, to work, 10
Triblet, 81
Varnish, red, 13, 37
Shellac, 58
White, hard, 13
Volt, no, 130
Volta's Electrophorus, 23
Composition, 27
Watt, no
Wheatstone's Bridge 13$
Winter's Ring, 69
Wire on armature, 113
On Field Magnets, 113
Table of carrying power, 164
Table of resistance of, 112
testing for insulation, 103
German Silver, 125
Zincs, amalgamation of, 148
to cut, 148
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