LIBRARY OF THE UNIVERSITY OF CALIFORNIA. Deceived MAR 11 1893 .... j8 9 Accessions No. $0 3 u q . class No. THE ELEMENTS OF DYNAMIC ELECTRICITY AND MAGNETISM. BY PHILIP ATKINSON, A.M., PH.D., LUTHOR OF "ELEMENTS OF STATIC ELECTRICITY" AND "THE ELEMENTS OF ELECTRIC LIGHTING." SECOND EDITION, NEW YORK: D. VAN NOSTRAND COMPANY, 23 MURRAY AND 27 WARREN STS. 1892. ..; 1 ; Engineering Library a COPYRIGHT, 1891, BV D. VAN NOSTRAND COMPANY. ROBERT DRUMMOND, Elevtrotypfr and Printer, New York. INTRODUCTION. THIS book was written for learners rather than for the learned. Previous to the last decade the demand for electric books was confined chiefly to scientific in- vestigators versed in the higher mathematics, and the authors of such books were electricians of the same class, who recognized the importance of mathematical accuracy in treating electric phenomena. Hence mathe- matical formulae became a prominent feature of such books. But the various electric industries to which the recent unprecedented electric development has given rise, have given employment to a numerous class of persons to whom mathematical books are almost un- intelligible, and yet to whom a scientific knowledge of the various kinds of electric apparatus which they are required to operate, or with which their business is con- nected, is of the highest importance. There is also a class of liberally educated persons who desire to extend their knowledge of electric principles, but have not the time or patience to follow the intricacies of mathe- matical formulae, especially in the abbreviated form usual in the books referred to. A third class are stu- dents who intend to become electrical engineers, to whom a thorough knowledge of elementary, physical, electric principles is important as a preparation for a iii VI INTRODUCTION. Company, the Electrical Supply Company, Central Electric Company, the Electrical Accumulator Com- pany, "C & C" Electric Motor Company, the Western Union Telegraph Company, the Chicago Bell Telephone Company, the American Telephone and Telegraph Company. PHILIP ATKINSON. CHICAGO, November i, 1890. CONTENTS. CHAPTER I. THE VOLTAIC BATTERY DEFINITIONS i Dynamic Electricity Defined. Discoveries of Galvani. Dis- coveries of Volta. The Couronne de Tasses. The Voltaic Pile. Value of Volta's Discoveries. Cell, Element, and Bat- tery. Battery Sign. Electrodes and Poles. Conditions of Electric Energy. Electromotive Force. Resistance. Cur- rent. Units of Electromotive Force, Resistance, and Cur- rent. Operation of the Voltaic Cell. Theory of Electric Generation in the Cell. Amalgamation of the Zinc. Insula- tion and Clamping. Polarization. CHAPTER II. ONE-FLUID CELLS 13 Smee's Cell. Zinc-Carbon Cells. Walker's Cell. Potas- sium Bichromate Cell. The Grenet Cell. The Mercuric Bisulphate Cell. The Leclanche Cell. The Law Cell. The Diamond-Carbon Cell. Dry Cells. Polarization of One- Fluid Cells. CHAPTER III. TWO-FLUID CELLS. BATTERY FORMATION 23 Construction of Two-Fluid Cells. The Daniell Cell. The Callaud Cell. The Grove Cell. The Bunsen Cell. The Silver Chloride Cell. Battery Formation. Connection be- tween Cells. CHAPTER IV. MAGNETISM 35 The Natural Magnet. Magnetic Polarity. The Mariner's Compass. The Surveyor's Compass. The Earth's Magnetic vii viii CONTENTS. Poles. Declination. Inclination or Dip. The Dipping Needle. Magnetic Maps. Terrestrial Magnetism Illustrated. Magnetic Intensity. Magnetic Force Ascertained by Oscilla- tion. Magnetic Force Ascertained by Deflection. Absolute Magnetic Intensity. Biot's Law. Origin of Terrestrial Mag- netism. Secular Variation. Secular Variation in the United States. Annual and Diurnal Variation. The Eleven Year Period. Magnetic Storms. Cosmic Variation. Exact Ob- servation. Secular Variation at Washington. Secular Varia- tion at San Francisco. Artificial Magnets. Magnetic Satu- ration. The Armature. Laminated Magnets. Magnetic Loss. Portative Force. Polar Attraction and Repulsion. Magnetic Lines of Force. Magnetic Field. Form of Mag- nets. Magnetic Penetration. Location of the Poles. Para- magnetic and Diamagnetic Bodies. Magneto-Crystallic In- duction. Magnetism as a Mode of Molecular Motion. Anal- ogy between Magnetic and Electric Phenomena. Coulomb's Torsion Balance. The Gauss-Weber Portable Magnetometer. CHAPTER V. ELECTROMAGNETISM 71 Deflection by the Electric Current. The Galvanoscope. The Schweigger Multiplier. Ampere's Rule. The Astatic Needle. Compensating Magnet. Cause of Deflection. The Electromagnet. Electromagnetic Poles. Winding. Mag- netic Strength. Core. Helix Coefficient of Magnetic In- duction. Electromagnetic Saturation. Form of Electro- magnets. Armature. Experiments in Diamagnetism. List of Diamagnetic and Paramagnetic Substances. Deflection of the Electric Current by the Magnet. Ampere's Table. The Solenoid. De La Rive's Floating Battery. Mutual In- duction of Electric Currents. Rotary Movement by Current Induction. Ampere's Theory of Magnetism. Generation of Electric Currents by Induction. Current Induced by Magnet. Current Induced by Another Current. Current Induced by Opening or Closing Primary Circuit. Current Induced by Varying the Strength of Primary Circuit. Results of Current Induction. Generation of Current Dependent on Variation of Intercepted Magnetic Force. Coefficient of Mutual In- duction. Self- Induction. Extra Current. The Spark. In- CONTENTS. IX duction of Core. Induction Coil. Condenser. Interrupter. Sliding Core. Water Rheostat. Construction of Core. Operation of Condenser. Leyden Jar as a Condenser. Special Construction. Ruhmkorff's Commutator. The Coil a Converter. Electric Perforation. Physiological Effects of Faradic Current. Discharge in Air and in Vacuo. Electric Gas Lighting. Spark Coil. CHAPTER VI. ELECTRIC MEASUREMENT. no Electric Potential. Electromotive Force. Electric Resist- ance. Insulation and Conductivity. Electric Current. Ohm's Law. Electric Units. The Volt. The Mircrovolt. The Ohm. The Megohm. The Ampere. The Milliampere. The Ampere-Hour. The Coulomb. The Farad. The Microfarad. The Watt. The Electric Horse- Power. Dif- ferent kinds of Electric Measurement. Electrometers. Gal- vanometers. Measurement of Angles. Angular Measure- ment of Deflective Force. Calibration of Galvanometer. Sine Galvanometer. Tangent Galvanometer. Astatic Gal- vanometer. Thomson's Reflecting Galvanometer. Differen- tial Galvanometer. Ballistic Galvanometer. Common Gal- vanometers. Voltmeters and Ammeters. The Weston Volt- meter. The Weston Ammeter. The Weston Milliammeter. The Wirt Voltmeter. Ayrton and Perry's Spring Voltmeters and Ammeters. Gravity Ammeters. The Cardew Volt- meter. The Edison Current-Meter. The Forbes Coulomb- Meter. Voltameters. The Water Voltameter. The Weber- Edelmann Electrodynamometer. Measurement of Electric Resistance. Resistance Coils. The Wheatstone Bridge. CHAPTER VII. THE DYNAMO AND MOTOR 165 The Magneto-Electric Generator. Commutation. The Alliance Machine. The Siemens Armature. Wilde's Ma- chine. The Dynamo. Ladd's Machine. The Pacinotti- Gramme Armature. Improved Commutator. Direction of Current. Interior Wire of the Gramme Armature. The Cylinder Armature. Closed-Circuit and Open-Circuit Arma- tures. Location of the Armature's Magnetic Poles. Mag- X CONTENTS. netic Lag. Position of the Brushes. The Field- Magnets. Series, Shunt, and Compound Winding. Constant Current Dynamo. Constant Potential Dynamo. The Edison Dyn- amo. Alternating Current Dynamos. The Gordon Dynamo. The Westinghouse Dynamo. Separate Excitation. Advan- tages of the Alternating Current Dynamo. The Converter. Development of the Electric Motor. The Dynamo as a Mo- tor. Principles of the Motor. Loss of Energy. Eddy Cur- rents. Series, Shunt, and Compound Wound Motors. Re- versible Rotation. The Alternating Current Motor. The Westinghouse Tesla Motor. The Tesla Motor as a Converter. Reversal of Rotation. Distribution of Power. Elevated- Road Distribution. Thermo-Magnetic Motors. CHAPTER VIII. ELECTROLYSIS 206 Nomenclature by Faraday. Theory of Grotthus. Elec- trolysis of Water. Conditions of Electrolysis. Secondary Reaction. Electrolysis of Mixed Compounds. Relations of Electrolysis to Heat. Lowest Required Electromotive Force. Faraday's Laws. Magnetic Effects. Chemical Equivalence. Electrochemical Equivalence. Effect of Current Reversal. Effect of Convection. Relative Conditions of Current and Electrolyte. Electroplating. Various Details. The Anodes. Plating Solutions. Auxiliary Operations. Required Elec- tric Energy. Required Time of Immersion and Thickness of Deposit. Agitation of the Solution. Electrotyping. Elec- tric Refining of Metals. Electric Reduction of Ores. The Hall Process for Aluminium. CHAPTER IX. ELECTRIC STORAGE 233 The Leyden Jar and Condenser. Grove's Gas Battery. Plante's Secondary Cell. Chemical Reaction. The Faure Cell. Chemical Reaction. Defects of the Faure Cell. Im- proved Faure Cell. Electric Preparation of the Plates. Elec- tric Energy of Improved Cell. Effects of Charge and Dis- charge on the Plates. E. M. F. of discharge. Conductivity and Buckling. Weight of Cells. Composition of Grids. CONTEXTS. XI The Julien Cell. The Pumpelly Cell. Durability of Storage Cells. Storage Capacity. Relative Time of Charging and Discharging. The Hydrogen Alloy Theory. CHAPTER X. THE RELATIONS OF ELECTRICITY TO HEAT 252 Heat Developed by Electric Transmission. Joule's Law. Joule's Equivalent. Heat Developed by Electrochemical Action. Electro-Thermal Capacity of Conductors. Electric Blasting. Electric Cautery. Electric Fuses. Ther mo-Elec- tric Generation. Thermo-Electric Diagrams. The Peltier Effect. Thermo-Electric Inversion. The Thomson Effect. The Thermopile. Electric Welding. CHAPTER XI. THE RELATIONS OF ELECTRICITY TO LIGHT 279 The Relations of Electric Heat to Electric Light. Photo- Electric Generation. Photo-Electric Reduction of Resistance in Selenium. Polarization of Light. Magneto-Optic Polari- zation Faraday's Discoveries. Verdet's Discoveries. Bec- querel's Discoveries. Kiindt and Rontgen's Discoveries. Kerr's Discoveries. Effects of Double Reflection. Summary. Maxwell's Theory. Molecular Theory. Strain in the Media. Electric Lighting. The Arc Light. The Arc. Electric Candles. The Arc Lamp. The Crater and Point. The Heat and Light. Establishment of the Current. The Carbons. Automatic Regulation. Hefner von Alteneck's Regulator. Series Distribution. Automatic Cut-Out. The Incandescent Lamp. The Filament. Filament and Lamp Attachment. Position and Current. Parallel Distribution. Multiple Series and Series Multiple. Three- Wire System. CHAPTER XII. THE ELECTRIC TELEGRAPH. . , 310 Early History. The American Morse Code. The Inter- national Morse Code. Simple Line Equipment. The Bat- tery. The Key. The Register. The Sounder. The Relay. Cut-Out, Ground-Switch, and Lightning-Arrester. Line XI 1 CONTENTS, Construction. Station Arrangement. Switch-Board. Re- peaters. The Button Repeater. The Milliken Repeater. Repeater Connections. Duplex Telegraphy. The Stearns Duplex. The Polar Duplex. The Pole-Changer. The Pola- arized Relay. Operation of the Polar Duplex. Quadruplex Telegraphy. Construction and Operation of the Quadruplex. Repeating by the Quadruplex. Substitution of the Dynamo for the Battery. The Wheatstone System of Automatic Rapid Transmission. Submarine Telegraphs. Locating Faults. The Dial Telegraph. Printing Telegraphs. CHAPTER XIII. THE TELEPHONE 359 Early History. Principles of the Telephone. The Bell Telephone. Improved Transmitters. The Edison Trans- mitter. The Blake Transmitter. Accessory Apparatus. The Signaling Apparatus. The Exchange. The Multiple Switch-Board. Hughes' Microphone. Theory of Telephonic Transmission. Multiplex Telephony. Long Distance Tele- phony. Van Rysselberghe's System. The American System. The Hunning Transmitter. Transmission on Long Distance Lines. THE ELEMENTS OF DYNAMIC ELECTRI- CITY AND MAGNETISM. CHAPTER I. THE VOLTAIC BATTERY. DEFINITIONS. Dynamic Electricity Defined. The term dynamic, from dvva}A.iS, power, is appropriately used to designate elec- tricity when employed for useful work, embracing the electric phenomena pertaining to that state of electric motion termed current, by which apparatus or machinery is operated, as distinct from that class of phenomena termed static, which pertains chiefly to electricity when stationary and not employed in this way. Hence it may be accepted as properly including all the various electric phenomena to which the terms galvanic, voltaic, current, chemical, magneto, and thermo have been applied. Discoveries of Galvani. In 1780, Galvani, a professor of anatomy at Bologna, Italy, observed certain muscular contractions in the limbs of frogs recently killed, pro- duced by electricity generated by a frictional machine. He subsequently noticed similar contractions when the frogs' limbs were hung on an iron balcony by copper hooks in contact with the lumbar nerves. Placing a pair of them on an iron plate, and touching the lumbar nerves with a copper wire the opposite e-nd of which i DYNAMIC ELECTRICITY AND MAGNETISM. was in contact with the plate, he reproduced the mus- cular movements. From this he inferred that the nerves and muscles were oppositely electrified, and that the muscular action was due to the establishment of a connection between them. Discoveries of Volta. Volta, a professor of physics at Pavia, Italy, having observed that the movements were produced by using a muscle in connection with two metals, inferred that they were due to the electricity generated by the contact of the metals when the damp muscle was placed between them, and that if the same conditions were produced in some other way, electric generation would follow. On this hypothesis he con- structed, in 1800, the apparatus known as the couronne de tasses, or crown of cups. The Couronne de Tasses. This apparatus consisted of a series of cups or glasses, arranged in a circle, each containing a zinc plate and a copper plate partly im- mersed in a solution of salt in water, the copper of each cup being joined by a copper conductor to the zinc of the next cup, the fluid intervening between the two metals. Connection being made by a conductor between the copper of the first cup and the zinc of the last, strong electric effects were obtained, and the dis- covery excited great interest in the scientific world, as friction was the only means previously known of gener- ating electricity. The Voltaic Pile. Volta subsequently invented a por- table apparatus, intended for medical, electric treat- ment in hospitals, known as the voltaic pile. This ap- paratus consisted of a series of copper and zinc disks, arranged in a pile, with disks of cloth, moistened with a solution of salt in water, between each pair ; the low- est disk being copper, the next zinc, and the next cloth; the same order being continued throughout the pile, so THE VOLTAIC BATTERY. DEFINITIONS. 3 that the topmost disk was zinc. Connection being made between the top and bottom disks, as between the ter- minal plates of the couronne de tasses, similar electric effects were obtained. This apparatus was also constructed with copper and silver coins. Water acidulated with sulphuric acid was also used instead of the solution of salt in water, both for the pile and the couronne de tasses. Value of Volta's Discoveries. These discoveries laid the foundation of the science of dynamic electricity, and Volta's apparatus is the type of all the batteries since constructed. The value and importance of his work become apparent when we consider that after nearly a century of constant experiment by eminent scientists the metals he employed are still found to be the most efficient and economical for this purpose, while his arrangement of the elements in series is still found to be the arrange- ment which produces the highest electric potential. The use of zinc in battery construction has never been superseded. It has been employed in nearly every bat- tery that has ever been invented, and enters into the construction of every one now in general use. And copper, in connecction with it, is the next metal in most general use for this purpose. Cell, Element, and Battery. A single pair of metals or their equivalent, with the fluid and containing vessel, or substance, is designated as a cell or element, and a combination of such cells is called a battery j the latter term being also applied to a single cell, when employed alone. Battery Sign. This sign, |i|i[i, is used to represent the battery; the short, heavy lines representing the zinc, and the light ones the copper or its equivalent; the number of lines varying indefinitely, according to the size of the battery, each pair representing a cell. 4 DYNAMIC ELECTRICITY AND MAGNETISM. Electrodes and Poles. Since the metals, or their equiv- alents, are the principal avenues in which the electricity travels, downward through the zinc and upward through the copper, or its equivalent, they are called the elec- trodes, from j/XeKTpov odos, electric road. The zinc, being consumed by the chemical reaction, is termed the soluble or generating electrode, and the copper the con- ducting electrode. This term is also applied to instru- ments used for conveying and applying electricity. The parts of the electrodes which project out of the fluid are known as \hzpoles; the projecting part of the zinc being designated as the negative pole, and that of the copper, or its equivalent, as \.\\e positive pole. These terms are also applied respectively to the outer ter- minals of the conducting wires connected with the poles of a battery or other electric generator. The terms positive and negative are also applied to the electrodes, the zinc being called the negative elec- trode, and the copper, or its equivalent, ihepositive. Conditions of Electric Energy. In estimating the elec- tric energy of a cell three important conditions are to be' considered, termed respectively electromotive force, resistance, and current ; any two of which being known, the third can be ascertained by calculation. Electromotive Force. Electromotive force, symbol E. M. F., has been defined as " that which moves or tends to move electricity from one point to another." It is represented by difference of electric potential; elec- tricity always moving, or tending to move, from higher to lower potential with a force, or pressure, equal to this difference. This condition, in the cell, depends on the nature of the materials employed and their mutual relations, varying in proportion to the chemical reaction between the soluble electrode and fluid, and the resist- ance to such reaction and the electric, molecular THE VOLTAIC BATTERY. DEFINITIONS. 5 ment generated by it, by the various materials com- posing the cell. Hence this is not properly a force, but a condition producing force. Resistance. Resistance, symbol R, is that which opposes the movement of electricity through a conductor; and, in the cell, it depends chiefly on the nature of the fluid, the quantity intervening between the electrodes, and a certain effect termed polarization. It varies directly as the length and inversely as the cross-section of the con- ductor; and since the distance between the electrodes may be regarded as the length of the fluid conductor, while the area of their immersed surfaces measures its cross-section, the fluid resistance of a cell varies directly as the distance between the electrodes, and inversely as the area of their immersed surfaces; hence the least resistance, dependent on these conditions, is obtained with the shortest practicable distance between the elec- trodes coupled with the greatest area of immersed sur- face. The resistance of battery fluids varies greatly; that of pure water or acid alone, for instance, is very high, but in mixtures of the two the resistance is greatly re- duced. Hence the importance of selecting the fluid with reference to its resistance as well as its chemical reaction. Current. Current, symbol C, is the electric movement produced in a conductor by electromotive force in opposition to resistance; its value being ascertained by dividing the former by the latter. Hence strength of current varies as each of these factors, increasing with increase of E. M. F. or decrease of resistance, and decreasing with decrease of E. M. F. or increase of re- sistance, but remaining constant when each varies in the same ratio as the other, DYNAMIC ELECTRICITY AND MAGNETISM. Units of Electromotive Force, Resistance and Current. THE VOLT is the unit of electromotive force, repre- sented practically by the E. M. F. of the Daniell cell, to be described hereafter, to which it is nearly equal. THE OHM is the unit of electric resistance, represented by the electric resistance of a column of mercury 106 centimeters in vertical height, and i square millimeter in cross-section, at the temperature of o C. THE AMPERE is the unit of current strength, repre- sented by an E. M. F. of i volt divided by a resistance of i ohm. As electric measurement pertains to a future chapter in which it is resumed and treated at greater length the above brief definitions of the three principal elec- tric units must suffice for our present purpose. Operation of the Voltaic Cell. If the metals are strictly pure there is no perceptible action either chemical or electric in the voltaic cell so long as there is no connec- tion between the electrodes ; but when the poles are brought into contact, or connected by a conductor, chemical reaction, accompanied by the generation of electricity, begins at once. If the metals are impure, as is usually the case, chemical reaction and electric gen- eration, in a limited degree, occur without polar connec- tion. In either case the water is decomposed, the hy- drogen collecting on the surface of the copper, and the oxygen combining with the zinc, forming oxide of zinc, which then combines with the sulphuric acid, forming sulphate of zinc. The generation of electricity may be proved by separating the poles slightly, when an electric spark will pass between them. Theory of Electric Generation in the Cell. Volta, as we have seen, attributed the electric generation to the con- tact of the metals, and this was the accepted theory among scientific observers to the time of Faraday. THE VOLTAIC BATTERY. DEFINITIONS. 7 Meantime chemistry, almost unknown as a science in Volta's time, had made rapid advancement, and Fara- day's observations having led him to the conclusion that the mere contact of the metals was not an adequate cause for the results obtained, and was not proportionate to such results, made an investigation of the relations between the chemical and electric actions of the cell, which enabled him to demonstrate that the electric generation was in exact proportion to the chemical reaction ; and his results having been fully verified by other observers, the chemical theory of electric genera- tion in the cell has since been generally accepted as cor- rect. It may be briefly stated as follows : The principal seat of chemical reaction is at the sur- face of the zinc, which is consumed by oxidation, while the copper acts as a conductor and is not consumed. Hence, since electric movement is from higher to lower potential, and the same law applies to the energy of chemical reaction, in common with other forms of physical energy, and since the electric energy of the cell is found to be strictly proportionate to its chemical reaction, it is assumed that the electric current origi- nates at the surface of the zinc and flows through the fluid to the copper. In the absence of external connection between the metals, it is evident that the difference of electric poten- tial would immediately become equalized and the cur- rent cease, but when they are brought into external contact, or connected by a conductor, the current finds an outlet through the copper, and flows back to the zinc through the external circuit ; chemical reaction is thus sustained and the current becomes continuous. The electric generation produced by Zamboni* s dry pile is adduced in proof of the contact theory. This pile was made of a large number of paper disks, some thou* 8 DYNAMIC ELECTRICITY AND MAGNETISM. sands, coated with zinc or tin foil on one surface, and with dioxide of manganese on the other, and closely compressed in a glass tube, their similarly coated sur- faces turned in the same direction, bringing those op- positely coated into contact. Such a pile, when its cir- cuit is completed, as in Volta's pile, can excite the electroscope, ring a bell, or give sparks. But this elec- tric action can be accounted for by chemical reaction, caused by dampness in the paper, rather than by the mere contact of different substances. Such experiments as the divergence of the leaves of the electroscope and the oscillations of the magnetic needle by the mere contact of different metals in their immediate vicinity are also adduced in support of the contact theory; but such electric action is doubtless due to the static charge generated by the slight friction produced in making the contacts. The law of the conservation of energy requires the expenditure of energy in one form as a condition of the production of the same amount in another form. Now in every electric generator, static or dynamic, machine or battery, this law is found to be strictly true ; there must be a complete circuit of materials differing in mo- lecular constitution, and the expenditure of energy, mechanical, chemical, or in some other form, at some point in the circuit as a condition of electric generation; and this expenditure must be equal in amount to the electric energy produced and that absorbed by friction, heat, or otherwise. Hence as chemical energy is the only energy expended in the battery, the conclusion is inevitable that it is the source of the electric energy generated. Amalgamation of the Zinc. As strictly pure zinc is too expensive for practical use in battery cells, and ordinary commercial zinc contains a certain percentage of iron THE VOLTAIC BATTERY. DEFINITIONS and other metals by which chemical and electric action is generated independent of the copper, and the energy thus, in part, expended, within the cell, without passing through the external part of the circuit, where it can be made available, a fault known as local action, the method has been adopted of amalgamating the surface of the zinc with mercury, which renders it more homogeneous and prevents any serious interference from local action, which is thus reduced to its minimum. The zinc is first cleansed with potash or otherwise, after which the mercury, mixed with acid, is applied by any convenient method, or the zinc dipped into the mixture. Sulphuric acid may be used for this purpose, but a mixture of five parts chlorhydric and one part nitric acid is preferable. The same result is also ob- tained by adding bisulphate of mercury to the solution, the mercury combining with the zinc, and the acid being set free. Amalgamation is thus more easily accomplished and better sustained. The molten zinc, before it is cast into plates, may be permanently amalgamated by the addition of about 4 per cent of mercury, and thus the frequent renewal, necessary with surface amalgamation, be dispensed with. Insulation and Clamping. When both electrodes are suspended from the support, they must be insulated from each other, either by making the support of insu- lating material, or insulating one of them from it. They must also be provided with clamps and binding-screws for making connections, and the points of contact with conductors kept clean and free from oxidation. Polarization. It has been stated that, as a result of the chemical reaction of the cell, hydrogen accumulates on the surface of the copper. As this accumulation in- creases, it weakens the electric action and finally stops 10 DYNAMIC ELECTRICITY AND MAGNETISM. it ; an effect termed, polarization. As a thorough knowl- edge of this effect and the methods used to correct it is of the highest importance in the study of the cell, it is proper first to examine its nature. If the poles of a battery of two or more cells be con- nected with platinum terminals which project into a vessel of water acidulated with sulphuric acid, hydrogen will be evolved at the terminal connected with the zinc, or negative pole, and oxygen at that connected with the copper, or positive pole, in the exact proportions which form water, two volumes of hydrogen and one of oxygen. If now the battery be disconnected, and the terminals of the wires connected with the gas tubes brought into contact, an electric current will flow through them in the reverse order to that of the original current, the gases, at the same time, recombining to form water. From which it is evident that the electric energy ex- pended in decomposing the water was stored up in the gases, and reappears when they return to their original state. Fig. i shows the apparatus by which this decomposi- tion is effected ; oxygen being evolved in the right-hand tube and hydrogen in the left. It will be noticed that the hydrogen is evolved at the pole towards which the current flows within the decomposing vessel, connected externally with the zinc of the battery, and the oxygen at the pole from which it flows, connected externally with the copper of the battery ; the external current through the wires connecting with the battery being towards the oxygen tube and from the hydrogen tube ; also that the same direction of current-flow occurs with respect to the battery, internally from zinc to copper, externally from copper to zinc, completing the circuit through the decomposing vessel. And since the current from the gas tubes, when disconnected from the battery THE VOLTAIC BATTERY. DEFINITIONS. II and brought into mutual contact, flows in the reverse order to that of the original battery current, it is evi- dent that when the gases accumulate on the electrodes within the cell, the effect must be to set up a similar re- FIG. i. verse current, which neutralizes the primary current. Hence this action is appropriately termed polarization, since it produces opposing poles. But since the oxygen, from its strong affinity for the base metals, combines with the zinc, the polarization is confined to the hydrogen, taking place on the copper. This affinity of the oxygen makes the use of a platinum terminal necessary for the oxygen at least, when it is desired to collect the gases separately, as above, since oxygen does not combine with platinum; while, if a base metal were used, it would become oxidized, and no oxygen gas could be collected. A single cell of less E. M. F. than 1.49! volts is insuf- ficient to decompose water, since the polarizing energy, in such case, exceeds the generating energy; hence two such cells at least are required. 12 DYNAMIC ELECTRICITY AND MAGNETISM, To correct polarization the accumulation of the hy- drogen must be suppressed, and to do this in the most effectual, practical, and economical way, without impair- ing the energy of the cell in other respects, is the most important problem in cell construction. It may be done either by mechanical or chemical means, the latter be- ing the most practical and effectual. Among the me- chanical means adopted are the lifting of the electrodes, or the conducting electrode alone, out of the fluid, so that the hydrogen may pass off. With a battery of two or more cells the electrodes of half the cells may thus be depolarized while the other half remain in the fluid and furnish the current. And as only a momentary ex- posure is required, any simple mechanism, operated by a weight or spring, by which this alternate exposure can be effected will answer the purpose. Another method is the injection of air into the fluid against the conduct- ing electrode. Either of these methods may be em- ployed for work which does not require a continuous, strong current ; and they are sometimes used in connec- tion with the chemical process to intensify the electric action. But all mechanical contrivances for this pur- pose are necessarily cumbersome and inconvenient, and hence undesirable. The chemical method is to introduce into the cell some substance which has a strong chemical affinity for the hydrogen, and absorbs it without interfering with the action of the cell in other respects. This is accom- plished either by the use of a single fluid holding the substance in solution, or by using two fluids separated by a porous cup or otherwise, so that the zinc shall be in contact with one fluid, and the copper, or its equiva- lent, in contact with the other. Hence arises the divis- ion of cells into two classes, vxe-ftuid and /m>-fluid cells, each of which now claims our attention. ONE- FLU ID CELLS. CHAPTER II. ONE-FLUID CELLS. Smee's Cell. This cell, represented by Fig 2, was in- vented by Smee, an English electrician, in 1840. The electrodes consisted originally of a plate of platinum suspended between two plates of zinc ; the object of this arrangement being to utilize both surfaces of the platinum, since, in any cell, only the ad- jacent surfaces of the opposite electrodes are brought into action. Depolariza- tion was effected by platinizing the surface of the platinum, electrically, so as to furnish a rough surface from which the hydrogen could escape much more freely than from a smooth surface, since a point has neither adhesion nor electric resistance, and the hydrogen atoms, being at the same electric poten- tial, are self-repellent. The platinum plate was subsequently replaced by a platinized silver plate, and this was afterwards replaced by a copper plate, covered with a rough coating of cop- per, then silver-plated and then platinized. The fluid consists of one part sulphuric acid to seven parts water. This cell is practical and efficient, where constancy of current is not required ; but, like all single-fluid cells, the current soon weakens. Zinc-Carbon Cells. The expense of constructing cells with platinum or silver stimulated the search for some FIG. 2. 14 DYNAMIC ELECTRICITY AND MAGNETISM. cheaper material, and Sir William Grove first suggested the use of carbon, but failed to reduce his suggestion to practice. In 1843 Bunsen constructed the first cell in which carbon was used. This was a two-fluid cell, and will be described under that head. Since that time carbon has been successfully employed in the construc- tion of numerous different cells which have come into general use. Carbon suitable for this purpose may be obtained from the inside of gas-retorts, and cut into plates or other convenient forms. It may also be prepared from coal, coke, graphite, or charcoal, pulverized, cemented to- gether, reduced to the proper form in moulds ; then dried, baked, and soaked in sirup of sugar repeatedly, till it acquires the requisite density and firmness. To obtain a good connection for the clamps and con- ducting wires, it is desirable that the upper part of the carbon should be soaked in melted paraffine, and then copper-plated, The paraffine fills the pores, and ex- cludes the acid, which would otherwise ascend by capil- lary attraction and destroy the copper. The advantages of carbon are: i. That it is cheap. 2. That, like platinum, it is insoluble in acid, and pos- sesses the conductivity necessary for an electrode. 3. That it has a rough surface, similar to that produced artificially with platinum in Smee's cell, by which de- polarization is assisted. 4. That, being porous, a great amount of internal surface is brought into contact with the fluid. Walker's Cell. Walker was one of the first to use car- bon as an electrode. In 1849 he constructed a cell sim- ilar to the Voltaic, substituting carbon for copper. In 1857 he platinized the carbon, copper-plated and tinned its upper end, placed the lower end of the zinc in a ves- sel of mercury, by which it was kept amalgamated; and ONE- FLU ID CELLS. 1 5 used a fluid composed of one part, by volume, of sul- phuric acid to eight parts of water. This cell has great constancy, requires but little care, and is cheaply constructed. Its electric energy is about the same as that of the Smee cell. It has been exten- sively used in England for telegraphing, with great success. Potassium Bichromate Cell. The most efficient single- fluid, carbon and zinc cell is that in which potassium bichromate is the depolarizing agent. The fluid con- sists of water, sulphuric acid, and potassium bichro- mate, and the following are recommended as the best proportions : 66 per cent by weight of water, 25 " " " " " sulphuric acid, 9 " " " " " potassium bichromate. The bichromate is decomposed by the sulphuric acid, and oxygen liberated, which enters into combination with the hydrogen while both are in the nascent state, producing water, and thus preventing the accumulation of the hydrogen. Practically, however, there is a cer- tain amount of polarization, and salts are also formed, which, if allowed to accumulate, reduce the conductivity of the carbon; so that the intensity of the electric action soon diminishes, and the fluid requires to be agitated, either by injecting air into it, or by withdrawing the electrodes, or the zinc alone. Air may be injected through a rubber tube; but this method, though very effective, is inconvenient in practice, and the withdrawal of the electrodes is preferable. Hence this cell is best adapted to work where constancy is not required; so that after a few minutes' use the electrodes may be withdrawn and the cell allowed to recuperate, while preparation is made for the next operation. Medical, surgical, and laboratory work is of this character, and for such work it is especially fitted; having the highest 1 6 DYNAMIC ELECTRICITY AND MAGNETISM. electric energy of any single-fluid cell in use ; being capable of application to a great variety of different operations; being free from noxious fumes; and easily made portable, either as a single cell, or a battery of cells. It is usually fitted with a hard-rubber cover, to which the electrodes are attached, and thus insulated. And as depolarization is in proportion to the relative amount of surface of the conducting electrode brought into action as compared with that of the soluble electrode, it is usual to have a carbon plate on each side of the zinc plate; using two carbons and one zinc, or three carbons and two zincs. As this fluid soon weakens with use, and is subject to slow chemical change when not in use, the amount should be so proportioned to the size of the electrodes as to prevent rapid exhaustion. The Grenet Cell. The bottle form of the bichromate cell, known as the Grenet, shown in Fig. 3, is conven- ient for work requiring only a sin- gle cell. The electrodes are at- tached to a close-fitting hard-rub- ber cover, and the zinc is connected with a sliding rod by which it can be drawn up into the wide neck, while the enlarged base gives the requisite capacity for a full supply of fluid. The zinc of any bichromate cell should be kept well amalgamated, and when the fluid is renewed, the deposit of chrome alum which ac- cumulates in the bottom of the vessel should be removed, and the 3- carbons soaked in warm water to remove similar deposits from their pores. OXE-FLUID CELLS. 1 7 The Mercuric Bisulphate Cell. This cell is extensively used for medical pocket-batteries, which are usually constructed with two small zinc and carbon cells, each about an inch square and half an inch deep. The car bon is placed in the bottom of a hard-rubber cup, and the zinc, resting on a ledge which insulates it, forms the cover. The fluid consists of a solution of mercuric bisul- phate in water; a few grains of the bisulphate to a tea- spoonful of water being sufficient for a cell. The acid of the bisulphate unites with the zinc, setting the mer- cury free, which keeps the zinc amalgamated. The solution can be made up quickly, and renewed when wanted; and the cell is easily cleaned and requires but little care. The Leclanche" Cell. Lec- lanche, a French electrician, was the first to use sal-ammo- niac (NH 4 C1) in theconstruc- tion of battery cells. Fig. 4 represents this cell, which consists of a glass jar, in the centre of which is placed a porous cup containing a carbon plate, which projects above it as shown, and is surrounded with crushed carbon and crystals of man- ganese binoxide, mixed in about equal - proportions. This cup is closed with Port- land cement, except two small openings left for ventilation, and its contents con- stitute the conducting electrode. The zinc is a round 1 8 DYNAMIC ELECTRICITY AND MAGNETISM. rod, about half an inch in diameter, placed in a recess provided for it in the outer vessel. The fluid is a saturated solution of sal-ammoniac in water ; about 6 oz. of the salt being required for a quart cell, which is kept about two thirds full, and its upper surface coated with paraffine, to prevent surface accumulation of the salt. This solution permeates the porous cup and materials contained in it, a little water being added through the ventilating openings. The manganese binoxide being rich in oxygen, which is evolved by the chemical action, acts as a depolarizer, the oxygen uniting with the hydrogen to form water ; the large proportion of surface in this electrode to that of the zinc greatly facilitating depolarization. But if electric action is continued too long at a time, an excess of hydrogen accumulates, oxygen not being generated with sufficient rapidity to unite with it, and polarization ensues, requiring a period of rest for the absorption of the hydrogen. Hence it is not fitted for work on a con- tinuously closed circuit. The E. M. F. of this cell is about 1.48 volts, and its resistance comparatively low, being reduced by the im- proved conductivity of the mixture constituting the conducting electrode. The current is always in full proportion to the consumption of material, there being no chemical action except with a closed external cir- cuit, and hence no waste of material by local action or otherwise. The electrodes can therefore remain per- manently immersed in the fluid without detriment, so that the cell can remain undisturbed till the fluid is ex- hausted, a little water being added occasionally to sup- ply the loss by evaporation. It contains no poisonous materials, emits no noxious fumes, and can endure a temperature of 16 C. without freezing or decrease of electric energy. ONE- FLU ID CELLS. The above style of Leclanche cell is known as the Disque. In a more recent style, known as the Prism or Gonda, the porous cup is dis- pensed with, and the conduct- ing electrode constructed with two prisms, attached by stout rubber bands to the central carbon plate as shown in Fig. 5 ; spaces for the circulation of the fluid being left between the plate and prisms. These prisms are composed of the double chloride of iron and ammonia, mixed either with manganese binoxide, graphite, or powdered retort- carbon, as preferred, and ce- mented together with any suit- able glutinous substance, as tar, rosin, or gum-lac. The greater compactness of this form of electrode gives it higher conductivity than the Disque form, while the suppression of the porous cup reduces the resistance. The electrodes are suspended from a close-fitting cover of insulating material by enlarged pole-pieces, which close the 'openings through which they pass. The Leclanche cell, in both styles, has come into ex- tensive use for open circuit work in which there are con- tinually recurring intervals of rest ; and in France it is used for telegraphing, to which it is found to be well adapted in offices where the work required is not so constant as to cause inconvenience from polarization ; cells having been used for nine years without renewal of the zincs, and only one renewal of the sal-ammoniac. FIG. 5. 2O DYNAMIC ELECTRICITY AND MAGNETISM The success of the Leclanche has given rise to a num- ber of similar cells ; carbon and manganese binoxide, variously combined, being employed as the conducting electrode ; the soluble electrode in all of them being a zinc rod, as in the Lelanche. The Law Cell. Prominent among these is the Law cell, the electrodes of which are shown in Fig. 6. The conducting electrode consists of two hollow cylinders, one inclosed within the other, with space between them. The zinc is placed in a vertical opening in the same side of both carbons, through which the fluid can cir- culate freely, and is thus brought into closer proximity to the con- ducting electrode than in the Leclanche, and the fluid resist- ance thereby reduced and de- polarization made more rapid and effective. Depolarization is also made more effective by the increased proportion of surface in the conducting electrode to that in the zinc. Both electrodes are attached to a close- fitting, insulating cover. The Diamond Carbon Cell. The Diamond Carbon Cell, shown in Fig. 7, is another of the same class, in which the conducting electrode consists of seven round rods arranged in a circle around the zinc rod, and put in electric connection with each other by attachment to a cover made of the alloy known as white metal, which is not easily oxidized ; the zinc being insulated by a porcelain bushing. This cell has the same advantages in regard to de- FIG. 6. ONE-FLUID CELLS. 21 polarization as the last. The metal cover reduces its resistance, and the separate rods are easily renovated by heating or soaking in hot water when necessary, and cheaply replaced when worn out. In other cells of this class, as the Laclede and Mi- FIG. 7. crophone, the cover is made a part of the conducting electrode and the zinc insulated from it. The ringing of electric bells is one of the most com- mon uses to which sal-ammoniac cells -are applied, and for which their constancy on open-circuit work especially fits them. Dry Cells. A cell constructed with a semi-fluid, not liable to spill, is termed dry. Cells filled with sand or sawdust, soaked with dilute acid, are instances of this construction. Portable cells, having starch or similar 22 DYNAMIC ELECTRICITY AND MAGNETISM. material to absorb the fluid, and hermetically sealed, are now becoming common, and are very convenient for many purposes ; and, when properly constructed, have a high degree of constancy and efficiency. An absolutely dry cell is an impossibility ; a certain degree of damp- ness or moisture being essential to proper chemical action. Polarization of One-Fluid Cells, All one-fluid cells, no matter how perfect their construction, are subject to polarization to a greater or less degree ; and though less complicated than two-fluid cells, and more com- venient for many uses, they are not adapted to work requiring a continuous current, or in which the intervals of rest are not sufficient for complete depolarization. TWO- FLU ID CELLS. BATTERY FORMATION. 23 CHAPTER III. TWO-FLUID CELLS. BATTERY FORMATION. Construction of Two-Fluid Cells. In the two-fluid cell polarization is either wholly prevented, or so reduced that the cell may be used for work requiring greater constancy than can be obtained from a one-fluid cell. The construction requires that the conducting electrode shall be surrounded with a fluid capable of suppressing the hydrogen, while the soluble electrode is surrounded with a fluid capable of chemical combination with the material of which the electrode is composed; and that the means of separation between the fluids shall not be such as to prevent electric or chemical action. For this purpose a porous cup, like that in the Leclanche cell, made of unglazed porcelain, is placed inside the larger vessel, and contains one of the electrodes with its fluid, while the other electrode with its fluid is placed in the outer vessel, and electric and chemical action takes place through the pores of this cup, where the fluids come into contact. Various other means of separating the fluids are used, as vessels or partitions of wood, paper, or animal mem- brane. Gravitation is also employed; a heavy fluid be- ing used in connection with a light fluid, the former settling to the bottom of the vessel, while the latter rises above it. The Daniell Cell. This is one of the oldest and best two-fluid cells in use. In was invented by Daniell, an English electrician, in 1836, and has undergone various modifications. Fig. 8 represents one of the best known styles. The outer vessel is a glass jar containing water 24 DYNAMIC ELECTRICITY AND MAGNETISM. or dilute sulphuric acid, in which is placed a hollow cylinder of zinc, having a slit in one side for the free circulation of the fluid. Inside this cylinder is placed a porous cup containing a solution of copper sulphate in water, to which some crystals of the sulphate are FIG. 8. added; in which is placed a copper cylinder, slit like the zinc. The chemical reaction is as follows: Hydrogen being liberated by the oxidation of the zinc, and the copper sulphate (CuSO 4 ) decomposed, the copper (Cu) is de- posited on the copper cylinder, and the other constitu- ent (SO 4 ) unites with the hydrogen (H 2 ), forming sul- phuric acid (H 2 SO 4 ), which in turn is decomposed by the zinc (Zn), forming zinc sulphate (ZnSO 4 ), more hydrogen being set free to unite with the liberated SO 4 , as before; the interchange taking place through the pores of the inner vessel. The hydrogen being thus TWO- FLU ID CELLS BATTERY FORMATION. 2$ entirely suppressed, depolarization is complete, and the copper cylinder, accumulating only pure copper, is al- ways in the best condition as an electrode. The E. M. F. of this cell is about 1.05 volts. It has great constancy, and is but slightly affected by changes of temperature ranging from +18 to -fioo C.; below this range the internal resistance increases, and at 5 to 7C. the solution freezes. Its chief defect is that the consumption of material is nearly as great when unemployed as when employed. Amalgamation of the zinc is not necessary. The electric resistance of the porous cup, which re- sults from the reduction of the cross-section of the fluid in passing through the pores, and from local action caused by the material of which this cup is composed, led to the invention of cells in which the fluids are sep- arated by gravity. The Callaud Cell. One of the best known gravity cells is the Callaud, represented by Fig. 9. The copper is placed in a solution of copper sulphate at the bottom of the vessel, and the zinc suspended in a solution of zinc sulphate near the top; the two fluids being kept separate by the difference in their specific gravity, the copper sulphate being the heavier. Connec- tion with the copper is made by a copper wire, insulated by gutta-percha or India-rubber to protect it from injury by local action at the junction of the fluids, and from contact with the zinc. The separation of the fluids is never quite complete; a certain percentage of the copper sulphate rising to the -upper part of the vessel, producing a copper deposit on 26 DYNAMIC ELECTRICITY AND MAGNETISM. the zinc; an effect which is increased by local action on both electrodes, evolving hydrogen and producing ascending and descending currents. As this deposit accumulates copper pendants are formed, which increase in length till they reach the copper sulphate, when this action becomes much more rapid, with increased waste of the copper sulphate. Hence they should be removed before attaining this length, by lightly tap- ping the zinc, causing them to drop off. This cell has about the same E. M. F. as the Daniell, while the reduction of resistance by the removal of the porous cup produces a corresponding increase of current in the external circuit. The Grove Cell. This cell was invented by Grove, an English electrician, in 1839. ^ ^ s constructed with an amalgamated zinc cylinder immersed in dilute sul- phuric acid, contained in a glass jar, within which is a porous cup containing a strip of platinum immersed in strong nitric acid. This acid is rich in oxygen, which unites with the hydrogen, producing complete depolar- ization; and, being a good electric conductor, greatly reduces the resistance. The chemical reaction forms water, and also nitric tetroxide (N a O 4 ), which is emitted in noxious, red fumes, and is one of the greatest objec- tions to this otherwise excellent cell. Its E. M. F. is about 1.8 volts, which is 80 per cent greater than that of the Daniell cell, while its internal resistance is about 20 per cent that of the Daniell. Hence a Grove cell of the same size as a Daniell has about nine times the current strength. It is therefore one of the most powerful cells in use. The Bunsen Cell. In 1843, Bunsen, a German elec- trician, adopting the plan originally proposed by Grove, produced a cell having carbon instead of platinum in the porous cup, but otherwise identical with the Grove TWO-FLUID CELLS. BATTERY FORMATION. 2*] cell. This substitution greatly reduced the cost without impairing the energy; the E. M. F. and internal resist- ance being about the same as in the Grove. Depolarization is complete, but the same noxious fumes occur as in the Grove cell. The Silver Chloride Cell. Silver chloride was first used in the construction of battery cells by Marie Davy about 1860; subsequently Warren De La Rue made such improvements in the construction as to bring the cell into general use. His cell, as shown in Fig. 10, consists of a small glass jar, about 5 inches in height and \\ inches in diameter, which contains a dilute solu- tion of sal-ammoniac, in the proportion of 23 grammes to i liter of distilled water, in which is placed a small rod of unamalgamated zinc of superior quality; also a strip of silver imbedded in a small cylinder of silver chloride (AgCl), which is contained in a cylinder of parchment-paper. The electrodes are shown separately, 28 D YNAMIC ELECTRICIT Y AND MA GNE TISM. Z representing the zinc, Ag.Cl. the imbedded silver strip, A the paper cylinder, and B the cylinder and inclosed strip. The cell is closed by a paraffine stop- per fitting air-tight, through which the electrodes pro- trude. This prevents evaporation and creeping salts, and insulates the electrodes from each other. Near the top of the zinc there is a hole into which the silver strip, bent over from the adjoining cell, enters as shown at C, when the cells are connected into a battery. The silver chloride in this cell acts as a depolarizer much in the same manner as the copper sulphate in the Daniell cell. By its decomposition zinc chloride is formed and silver deposited on the conducting electrode; hence there is no oxidation, no deposit of hydrogen, and consequently no polarization. The E. M. F. is 1.03 volts, and the internal resist- ance 4.3 ohms. Resistance, being chiefly due to the sil- ver chloride, is much greater when the cell is first used than subsequently when reduced by the deposit of silver throughout the mass of the chloride. The small size of this cell makes it convenient for the construction of batteries having a large number of cells, one constructed by De La Rue containing 11,000. The construction of battery cells is limited only by the number of combinations of suitable materials which may be formed; and as the principles which govern these combinations have been fully set forth and illustrated by the various cells described in the preceding pages, it is unnecessary to carry these details farther. Battery Formation. There are two principal methods of combining cells to form batteries, known by the terms series and parallel. When joined in series, the soluble electrode of each cell is connected with the conducting electrode of the adjoining cell; and when joined in par- allel, all the soluble electrodes are connected with each TWO-FLUID CELLS. BATTERY FORMATION. 2$ other, and likewise all the conducting electrodes. The latter method is also known by the terms multiple arc, side by side, and for quantity; and the series method by the term for intensity, to distinguish it from the method for quantity. But as the use of these various terms is confusing and unnecessary, it is better to confine our- selves to the terms first given above, which have received the sanction of leading electricians and are now in gen- eral use. With a given number of cells a given amount of elec- tric energy may be generated, which it is evidently im- possible, according to the law of the conservation of energy, either to increase or diminish by any method of connecting them. But it is possible to control and direct this energy in such a manner as shall best subserve the uses to which it is to be applied; and, for this purpose, either the series method or the parallel may be used alone, or the two combined to any desired extent. Fig. n illustrates the method by which six cells may be joined in series; the circles representing cells, and the lines conductors. FIG ii. Fig. 12 shows how the same cells may be joined in parallel. FIG. 12. 3O DYNAMIC ELECTRICITY AND MAGNETISM. Fig. 13 shows a combination of two series of three cells each, and these series joined in parallel; and Fig. FIG. 13. 14 shows three series of two cells each, and these three joined in parallel. FIG. 14. Since electromotive force is that which moves or tends to move electricity from one point to another, and de- pends on difference of potential, and since this difference, in a cell, depends on the nature of its materials and the method of construction, we should expect to find the E. M. F. of a small cell equal to that of a larger one of the same composition and construction; and experiment proves that such is the fact. The case is analogous to that in hydrostatics, where liquid pressure depends on difference of level and not on the size of the vessel; the liquid in a small vertical TWO-FLUID CELLS. BATTERY FORMATION. 31 tube balancing liquid of the same kind contained in a larger one connected with it, so that the level is the same in each. So when a small cell is joined to a larger one of the same kind by connecting the similar elec- trodes, so that opposing currents meet, the current from the one exactly neutralizes that from the other; proving that both currents have the same strength, and hence that the E. M. F. of each cell is the same. But this is not true of dissimilar cells, differing by construction in E. M. F., nor similarly in hydrostatics of liquids differ- ing in specific gravity. The six cells in Fig. 12, joined in parallel, are practi- cally equivalent to one cell six times the size of any one of them; for, the similar electrodes of each kind be- ing joined together, each set acts as one electrode. Hence the E. M. F. of the battery, connected in this way, is only equal to that of a single cell; just as in hydro- statics the liquid pressure in six tubes of the same size placed vertically side by side, and connected with a hor- izontal tube at bottom, is only the same as that in any one of the tubes alone. But if the six are joined end to end in a vertical series, the pressure becomes six times as great. So if the six cells are joined in series, as in Fig. n, the E. M. F. becomes six times as great. In the former instance we have liquid pressure, in the latter electric pressure. But since electric resistance va- ries directly as the length and inversely as the cross- section of a conductor, the resistance also becomes six times as great; each of the six cells with its electrodes and fluid adding a unit to the length of the conducting line, while the cross-section remains the same. And since the quantity of electricity passing through a conductor, represented by the volume of current, equals the E. M. F. divided by the resistance,the quantity obtained from the series in Fig. n is only one sixth of that obtained from the six cells in parallel, as in Fig. 12, and hence no 32 DYNAMIC ELECTRICITY AND MAGNETISM. greater than that of a single cell, though the E. M. F. is six times as great. When the six cells are joined in parallel, as in Fig. 12, the resistance is only one sixth of that developed in a single cell; for the cross-section of the united conduct- ors is six times as great, affording six times as large an avenue for the passage of electricity. Hence, though the E. M. F. is only equal to that of a single cell, the electric quantity or volume of current is six times as great, and hence also six times as great as that of the six cells in series, which has been shown to be only equal to that of a single cell. Hence in the series combination we have current in- tensity at the expense of current quantity, small current and large E. M. F. ; and in the parallel combination, quantity at the expense of intensity, large current and small E. M. F. ; one being in the inverse ratio of the other in each case. The combination proper to be used depends on the nature of the required work. If there is high resistance to be overcome, as in a long telegraph line, the intensity must be sufficient to overcome it, and leave a sufficient surplus to operate the instruments, and the series arrangement should have the preference. But if the resistance is low, and the required quantity large, as in the deposition of metal in electro-plating, the parallel arrangement is to be preferred. The practical rule is to make the internal resistance of the battery equal to the external resistance to be overcome: and our illustrations show that the variation of the rel- ative proportions of quantity and intensity by different' methods of combination is practical for this purpose to any required extent. The correctness of the rule becomes evident when we consider that if by a preponderance of the series arrange- ment the internal resistance exceeds the external, TWO-FLUID CELLS. BATTERY FORMATION. 33 greater intensity is developed than is required; and if by a preponderance of the parallel arrangement the in- ternal resistance is less than the external, the intensity is insufficient to overcome the external resistance. In the former case there is a waste of intensity at the expense of quantity, and in the latter a waste of quantity at the expense of intensity. So that the most economical arrangement is attained by following the rule given above, which is based on the intensity or quantity re- quired, to which equality of internal and external re- sistance serves merely as a convenient guide. In all the various combinations of a given number of cells which may be made, as shown, the product ob- tained by multiplication of the current strength in amperes into the E. M. F. in volts must remain the same, since each factor varies inversely as the other; hence difference of combination can produce no variation in the amount of electric energy developed, as represented by this product, the variation observed pertaining ex- clusively to the different factors. Connection between Cells. Since all unnecessary re- sistance within the battery causes a waste of energy, it is important that this resistance should be reduced to the minimum, both in the cell itself, as we have already seen, and in the connection between the cells. This can often be accomplished by clamping the electrodes of adjoining cells together without any interven- ing connection. Fig. 15 shows a form of the Grove cell spe- cially adapted to this purpose. The cell being rectangular, the porous cup thin and flat, and FlG - J 5- the electrodes flat strips, permits a compact arrange- 34 DYNAMIC ELECTRICITY AND MAGNETISM. ment of all the parts; the zinc, Z, being bent so as to inclose the porous cup, V t while its upper end is clamped in immediate contact with the platinum, P. Fig. 16 shows the silver chloride battery, in which the cells are round but small, permitting them to be placed so Fie. 1 6. close together that the top of the silver electrode can be bent over, and inserted into a hole in the top of the zinc. Where such methods as the above are not practicable, connection can be made by heavy copper wire or strip, in which the resistance is insignificant. But it is of the utmost importance, in all cases, to have perfect con- tacts, kept free from oxidation; and this requires fre- quent, careful inspection. MAGNETISM. 35 CHAPTER IV. MAGNETISM. The Natural Magnet. The natural magnet is a hard black stone, which has the property of attracting iron. It derives its name from Magnesia, a country of Asia Minor, where it is supposed to have been first dis- covered. It was also found at Heraclea, a city of ancient Lydia, and hence called also the heraclean stone. It was known at least five hundred years before the Christian era, being described by Plato and EuripU des. It is very rare, but an ore of iron, closely allied to it, known as magnetite, is more abundant, though not always magnetic. Magnetic Polarity. No practical use was made of the magnet stone till sixteen centuries after its discovery, when it was found to have the property of assuming a north and south position in the direction of its longer axis, when supported so as to have a free horizontal movement about its centre of gravity. This property was termed polarity, from its reference to the earth's poles, and the stone thereafter became known as the lodestone leading stone. It was also observed that iron, rubbed with this stone, acquired its properties of attrac- tion and polarity, and this led to the invention of the mariner's compass. The Mariner's Compass. This instrument at first con- sisted of a thin strip of magnetized iron, named from its shape the needle, attached to wood or cork and floated in & vessel of water; a light wooden pointer, attached to it, indicating the ship's course. 36 DYNAMIC ELECTRICITY AND MAGNETISM. In this rude state it became known in Europe early in tne twelfth century, but the exact date and name of the inventor are unknown. A much earlier claim for this invention is made by the Chinese, but does not seem to be well sustained. The loss of magnetic energy due to the softness of iron seriously impaired the usefulness of the compass, but the subsequent discovery of steel furnished the material for needles much more permanently magnetic. Various improvements followed till it became the per- fect instrument which we now have, as represented in Fig. 17. The needle is mounted on a pivot, and at- FIG. 17. tached to the under side of a circular card which ro- tates with it, the margin of which is graduated to thirty- MAGNETISM. 37 two divisions indicated by pointers, including the four cardinal points N., S., E., W., and also to 360 where great accuracy is required. A circular box with glass cover incloses it, so poised as to maintain a perfect level un- affected by the motion of the ship. The most accurate needles are compound, consisting of several needles connected together. The compass shown in Fig. 17 has eight needles attached to the card, four on each side of the axis of rotation. The Surveyor's Compass. This compass differs from the mariner's chiefly in having the needle exposed to view, the graduated circle stationary, and sights and a small telescope mounted above. The Earth's Magnetic Poles. The polarity of the needle was found by Gilbert to depend on the mag- netism of the earth, which produces north and south magnetic poles by which the needle is attracted; the polarity of each being opposite to that of the corresponding pole of the needle. These are not identical with the geo- graphical poles; the north magnetic pole being near the arctic circle, lat. 70 5' N., long. 96 46' W., and the south near the antarctic circle, about lat. 73 S., long. 154 E., according to Airy's maps, Figs. 18 and 19; the location of the south magnetic pole being only approximate, its position having never been accurately determined. There are indications of secondary poles also, but their existence and location are not well established; neither is it known whether the magnetic poles are stationary or slowly changing position, as no accurate observations on this point have been made since the dis- covery of the north magnetic pole by Ross in June 1831; previous to which nothing was known in regard to the location of either magnetic pole. Declination. The difference of position between these and the geographical poles produces a deflection of the 38 DYNAMIC ELECTRICITY AND MAGNETISM. needle from a true north and south position at all points on the earth's surface except those situated on what is known as the agonic line, or meridian of no dec- lination, and this deflection is termed declination; the needle's north pole being deflected toward the north magnetic pole, north of the magnetic equator, and its south pole toward the south magnetic pole, south of the magnetic equator. The exact declination at any point is the angle between the vertical plane of the true meridian and that in which the longer horizontal, axis of the needle lies at the time of observation. If the distribution of magnetic force on the earth's surface varied uniformly, it is evident that the agonic line would coincide with the meridian passing through the magnetic and geographical poles, and the declina- tion would vary as the distance east or west of this line; and the magnetic equator, being equally distant from the magnetic poles, would cut the geographical equator at opposite east and west points at an angle of about 20 ; and on this equator there could be no declination, horizontal magnetic attraction on each pole of the nee- dle being equal and opposite, while the declination on any geographical meridian not coinciding with the agonic line would vary as the distance north or south of this equator, and attain a maximum of 90 at parallel points adjacent to either magnetic pole. Hence the declination at any point could be calculated from the latitude and longitude if the position of the agonic line were known. But observation shows that this hypothesis is only approximately true, and that the distribution of mag- netic force on the earth's surface is very irregular, as shown by Figs. 18 and 19 ; and that declination, posi- tion of the agonic line, and other magnetic facts can be determined only by actual observation at each point. MAGNETISM. 39 FIG. 18. NORTHERN HEMISPHERE. 40 DYNAMIC ELECTRICITY AND MAGNETISM. Inclination or Dip. At the magnetic equator the posi- tion of the needle is parallel with the horizon, vertical magnetic attraction on each of its poles being equal and opposite, but at all points north or south of this line it is inclined at an angle known as its dip or inclination j its north pole, north of it, inclining towards the north magnetic pole ; and its south pole, south of it, towards the south magnetic pole: the inclination attaining a maximum of 90 at each, the position of the needle becoming vertical. Hence the compass needle requires a counterpoise sufficient to counteract the dip and keep it in a true horizontal position. With uniform variation of magnetic force, the inclina- tion at all points between the magnetic equator and poles would vary as the magnetic latitude ; but this is only approximately true, observation being required to determine its value at any point, and also the position of the magnetic equator, and parallels, as well as of the magnetic poles; and as the south magnetic pole has never been definitely located, the vertical position of the needle at that point can only be assumed. The Dipping Needle. The inclination is ascertained by an instrument known as the dipping nee die, constructed with a graduated circle set vertically in the plane of the magnetic meridian, around which a delicately poised needle has a free vertical movement. Magnetic Maps. By observation of the declination, dip, and otherphenomena at various points on the earth'ssur- face, maps may be prepared which are approximately cor- rect for a limited number of years. The maps of Sir George Airy, Figs. 18 and 19, and of the U. S. Coast and Geodetic Survey, Figs. 20, 21, 22, and 23, have been pre pared in this way. The magnetic poles being located with approximate accuracy, the magnetic equator is found by tracing a great circle connecting all points in MAGNETISM. 4! the equatorial region where the needle maintains a per- fect horizontal parallel. This circle, which is very irregular, cuts the geographical equator at opposite east and west points at an angle of about 13, as shown in Figs. 18 and 19. The agonic line is found by connecting the points of no declination on a great circle passing through the magnetic poles and cutting the magnetic equator at right angles approximately. Other great circles con- necting points of equal declination, and hence called isogonic lines, pass also through the magnetic poles and cut the magnetic equator, in like manner, at approxi- mately equal intervals. Parallels to the magnetic equator, connecting points of equal inclination on the isogonic lines, and hence called isoclinic lines, cut the agonic line at approximately equal intervals. All these lines, both of declination and incli- nation, show great irregularities, the irregularities of the parallels corresponding approximately to those of the magnetic equator. Terrestrial Magnetism Illustrated. If a magnetic nee- dle, free to move vertically and horizontally, be brought into the vicinity of a magnetized bar of steel, lying in a north and south position, be moved directly over it from end to end, and also parallel to it at a short distance on each side, it exhibits all the phases of declination and dip found on the various parts of the earth's surface, as already described, but in a more regular manner; which is strong proof that the earth is a great magnet, as al- ready stated, with curving lines of force radiating in all directions from its magnetic poles, like other mag- nets, as described hereafter ; thus accounting in a most satisfactory manner for the phenomena of declination and dip. These are the lines represented in part on the maps, 42 DYNAMIC ELECTRICITY AND MAGNETISM. the needle being merely the instrument by which they are traced. Magnetic Intensity. The intensity of this magnetic force constantly increases from the magnetic equator to each magnetic pole, and is represented af any point by the forces producing the declination and dip ; the former representing the horizontal component of the intensity, and the latter the vertical ; the total intensity being ascertained by dividing the horizontal force by the cosine of the angle of dip. Hence, representing the total intensity by F, the horizontal force by Jf, and the angle of dip by 6, we have the usual standard for- 1 77 H mula I 1 = 7,. cos 6 There are two methods of ascertaining the relative values of the horizontal force at different points, known respectively as the methods by oscillation and by deflec- tion. Magnetic Force Ascertained by Oscillation. The oscil- lations of the needle, when forcibly deflected from its position of rest, are accomplished, like those of the pen- dulum under the influence of gravity, in nearly equal times, though constantly decreasing in amplitude ; and the square of the number of oscillations accomplished in a given time, which in the pendulum is proportional to the force of gravity, is, in the needle, proportional to the horizontal magnetic force. Hence if a represent the number accomplished in a given time at any point on the earth's surface, and b the number accomplished in the same time by the same needle at any other point, the relative values of this force at the two points are as a* to b\ Magnetic Force Ascertained by Deflection. The hori- zontal force by which the needle is brought to rest in the plane of the magnetic meridian is the resultant of MAGNETISM. 43 two forces, one tending to rotate it into an east and west position, represented by the sine of the angle of declination, and the other into a north and south posi- tion, represented by the cosine, while the resultant force is represented by the hypothenuse of a right-angled triangle, of which the sine and cosine form the remain- ing sides (see Fig. 45, page 121); the position of the needle coinciding with that of the hypothenuse, in which the forces are in equilibrium. The relative value of the east and west force, by which the needle is deflected, is to that of the north and south force, as the ratio of the sine to the cosine, represented by the tangent. Hence the total horizontal force of the earth's magnetism, at any point, multiplied by the tangent of the angle of declination gives the deflective force at that point. Absolute Magnetic Intensity. The relative magnetic intensity being derived, as shown, from division of the horizontal force by the cosine of the angle of dip, if the absolute value of this force, in C. G. S. units, at any point is ascertained, the absolute intensity can also be ascertained. To accomplish this two observations are necessary with a needle whose magnetic moment or force by which it resists deflection is known ; a quantity ascertained by multiplying the strength of either pole by the length of the needle (or magnet). One of these observations, made by oscillation, determines the prod- uct of this moment by the horizontal force ; and the other, made by the special deflection of a small needle by the same needle (or magnet) used in the first obser- vation, determines the quotient of the moment by the horizontal force ; and dividing the product by the quo- tient and taking the square root of the result gives the absolute horizontal force. Previous to 1830 observations on magnetic intensity were made by oscillation of the dipping needle, but this 44 D YNA MIC ELE C TRICI TY A ND MA GNE TISM. method was found to be inaccurate and the observa- tions unreliable. The discovery of the method of ex- pressing this intensity in absolute measure was first made by Gauss in 1833, and the portable magnetome- ter (described in the latter part of this chapter), an im- portant aid in such measurement, was constructed by Weber in 1836. The number of oscillations made in a given time by different needles, or magnets, varies as the length, weight, form, and polar strength of each, and as the strength of the magnetic field in which it is placed. Hence, in comparing observations made by different instruments, it is necessary to correct any errors which may arise from such variation. It is also important to prevent errors due to loss of magnetism, by frequen-t testing, and remagnetizing when necessary. Parallels to the magnetic equator, connecting points of equal magnetic intensity, and hence called isodynamic lines, are traced on maps representing either the hori- zontal or the total intensity, as shown in Fig.2o. Blot's Law. The magnetic intensity at any point on the earth's surface varies with the magnetic latitude ; to which it is approximately proportional. The mag- netic force, emanating from the magnetic poles and ra- diating in curves as already stated, not only on the sur- face but into the surrounding space in all directions, varies inversely as the square of the distance from either pole, except as modified in the manner already shown. Hence the intensity is greatest at the magnetic poles and least at the magnetic equator, and may be ascer- tained approximately at any point by Biot's law, which, representing the magnetic latitude by /, makes the in- tensity proportional to Vi -\- 3 sin 2 /. Origin of Terrestrial Magnetism. The origin of terres- trial magnetism and its peculiar phenomena is to be MAGNETISM. 45 found in the reciprocal relations of magnetism and elec- tricity as explained in the next chapter, each being ca- pable of producing the other. Electric terrestrial phenomena have been described in the author's " Elements of Static Electricity," Chap- ISODYNAMIC MAP OF THE UNITED STATES FOR 1885. From U. S. Coast and Geodetic Survey. FIG. 20. ters XII and XIII, where it has been shown that differ- ence of electric potential between different parts of the earth's surface and atmosphere is apparently the result of difference of temperature, modified by the unequal distribution of land and water; hence the magnetic ter- restrial phenomena which we have been considering may be regarded as the result of the electric phe- nomena; and the peculiar phases of each, as indicated by geographical position and otherwise, leave no doubt 46 DYNAMIC ELECTRICITY AND MAGNETISM. of their intimate relationship; so that whether the mag- netic phenomena be regarded as a result of the elec- tric, or the reverse, both are undoubtedly dependent on the same physical influences. Secular Variation. Observation shows that the spe- cial phases of terrestrial magnetism are subject to great variation in respect to time as well as geographical po- sition; such variation being of three kinds, secular, an- nual, and diurnal. The first embraces long terms of years known as secular periods, whose length is deter- mined by the time in which a complete cycle of changes occurs. The discovery of this variation is due to Gelli- brand, an English electrician, and was first published in 1635- The agonic line and the isogonic lines are constantly changing position, slowly vibrating between widely separated eastern and western limits; hence the declina- tion at any point shows a corresponding variation be- tween eastern and western maxima; and the time occu- pied by the agonic line or any isogonic line in passing from its eastern or western limit, on any magnetic par- allel, until its return to the same limit again, or by the magnetic needle, at any point, in vibrating from its eastern or western maximum declination, or elongation, until its return to the same declination again, consti- tutes a secular period. When the declination has attained a maximum, it be- comes apparently stationary, change in the opposite direction being for some years imperceptible, after which the mean annual variation steadily increases for a term of years till the declination becomes zero; a cor- responding decrease of annual variation then occurs till it again becomes imperceptible, and the declination apparently stationary, at the opposite maximum; there MAGNETISM. 47 is then a return through a similar series of variations to the original maximum. This variation in rate of declination during a secular period has its exact analogy in the similar variation of rate found in a vibration of the pendulum. The length of a secular period is not definitely known, as sufficient time has not yet elapsed since observations were first made at any point for a complete cycle of changes to occur. It varies considerably in different parts of the earth; for the United States it is estimated at from 250 to 350 years, and for Paris at about 470 years ; the earliest observations having been made there, dating back to 1540. Secular vibration does not necessarily imply a change in the direction of the needle from west of north to east, or the opposite, which occurs only within the range of the agonic line; at all points east of that range the needle always points west of north, and at all points west of it east of north. Neither is it to be understood that the vibration of the needle within this range differs from that outside of it; the agonic line is simply the boundary between east and west declination, and at all points within its range the needle changes direction, once from east to west of north, and once from west to east, during the secular period, according as this line vibrates past each point in either direction. This would imply that the general vibratory move- ment in either direction is simultaneous at all points, increase of east declination and decrease of west decli- nation, or the opposite, occurring everywhere at the same time; and that when either has attained its maxi- mum elongation the other has attained its minimum, all the lines having swayed to the west or to the east simultaneously. But this is never strictly true except within a very limited area; declination may have at- 48 DYNAMIC ELECTRICITY AND MAGNETISM. tained its maximum or minimum at a remote point east or west of any isogonic line, and the opposite phase set in long before the same change occurs at intermediate points; so that it may be increasing or diminishing in opposite directions at the same time on the same par- allel. Secular Variation in the United States. It is found that this magnetic wave has thus swept across the American ISOGONIC MAP OF THE UNITED STATES FOR 1885. From U. 8. Coast and Geodetic Survey. FIG. 21. continent from east to west since observation began, and that its return eastward is now setting in. East elongation had attained its stationary phase, followed by reversal, at Halifax, N. S.,in 1713, at Eastport, Me., in 1749, at Boston in 1780, at New York in 1799, at Pitts- burg in 1808, at Cincinnati in 1815, at Chicago in 1832, MAGNETISM. 49 at Salt Lake City in 1873, and will attain it at San Francisco, as computed, in 1893. In 1890 declination, throughout the interior of the United States, was tending westward; west declination increasing and east declination diminishing, while at the extreme eastern and western points it had become stationary and the opposite phase was setting in; west declination beginning to decrease on the east coast, and east declination to increase on the west coast. ISOCLINIC MAP OF THE UNITED STATES FOR 1885. From IT. S. Coast and' Geodetic Survey. FlG. 22. The secular periods of dip and magnetic intensity are apparently the same as those of declination. The earliest reliable observations for the United States are those given in Halley's chart for the year 1700. The agonic line, mov^jjgwTflien passed 50 DYNAMIC ELECTRICITY AND MAGNETISM. a little east of Charleston, S. C.; declination at that point Jan. ist. being 36' E. In 1790 declination had attained its maximum eastern elongation, 4 54 T V E., followed by decrease till about Jan. i, 1890, when the agonic line was a little west of Charleston, declination being 4 T V W. Annual and Diurnal Variations. The annual and diurnal variations are apparently due to change of temperature. The annual maximum variation occurs in summer and the minimum in winter, corresponding respectively to the months of greatest and least change of temperature ; the diurnal maximum during the day and the minimum during the night, corresponding respectively to the hours of greatest and least change of temperature. These variations are very slight, that of diurnal declina- tion being greatest, ranging from 6' to 10^', as observed at Philadelphia and at London ; while the annual does not exceed ij'. The electric terrestrial phenomena already referred to show annual and diurnal variations corresponding in time and amount to the magnetic. The Eleven Year Period. It is found that the greatest magnetic diurnal variations take place at regularly re- curring periods of about eleven years each, correspond- ing to the periods of greatest solar disturbance, as in- dicated by the sun-spots, and of most frequent occur- rence of the electric phenomena known as the aurora ; minimum periods recurring at intervening epochs of eleven years. Magnetic Storms. Unusual perturbations in terrestrial magnetism often occur, known as magnetic or electric storms, lasting usaully only a few hours, though some- times much longer. They are indicated by sudden and unexpected deflections of the needle, and great and rapid fluctuations from its normal position, and also by MAGNETISM. 51 other magnetic and electric disturbances, which occur simultaneously over extended areas, often embracing distant parts of the globe. They usually occur in con- nection with the aurora, and are accompanied by electric currents in the earth, each phenomenon being doubtless due to the same cause, as explained in " Elements of Static Electricity," Chapter XV. Cosmic Variation. There is also slight magnetic varia- tion due to solar and lunar influence, which may prop- erly be termed cosmic. That due to solar influence depends on the rotation of the sun on its axis, and hence has a corresponding period of about 26 days. That due to lunar influence exhibits two maxima and two minima during each lunar month, the difference between which at Philadelphia is about 27", and at Toronto about 38". Exact Observation. In view of these numerous varia- tions it is evident that the magnetic needle, when light and delicately poised, so as to be sensitive to the slight- est change, is in a state of constant tremulous motion, and never absolutely at rest ; so that the record of an observation, to be of true scientific or even practical value, must specify the exact time, limited to the day and hour when made, as well as the exact location. This is especially true of declination, which is of the highest practical importance in surveying and naviga- tion, often involving important legal controversies. Secular Variation at Washington. Observation at Washington began about 1790, at which date the agonic line passed through it. In 1797 this line had attained its eastern limit, and declination at Washington its eastern maximum, being 30' E. Jan. ist. The agonic line has since been moving westward, and passed through Washington again in 1803. Fig 23 shows the position of this line in the United 52 DYNAMIC ELECTRICITY AND MAGNETISM. MAP SHOWING THE POSITION OF THE AGONIC LINE IN THE UNITED STATES AT FOUR DIFFERENT EPOCHS. from U. S. Coast and Geodetic Survey. MAGNETISM. 53 States at four different epochs, including that of its eastern limit. In 1810 the declination was 12' W.; in 1830, 39' W.; in 1850, i 58^' W.; in 1870, 2 55l V W. ; and in 1890, 4 15-^.' W. The mean annual variation in declination from 1790 to 1810 was 0.6'; from 1810 to 1830, 1.35' ; from 1830 to 1850, 3.99' ; from 1850 to 1870, 2.85' ; and from 1870 to 1890, 3.99' ; the average for the entire period from 1790 to 1890 being 2.55*'. Observations on the dip and magnetic intensity in the United States date back to the latter part of the last century; but, on account of imperfections in the instru- ments and methods of observation in general use for this purpose previous to 1838, observations made before that date are not considered very reliable. The dip at Washington was then 71 13' ; for the next 22 years there was alternate increase and decrease, the maximum being attained in 1845, when it was 71 34'. In 1860 it was 71 20' ; it then steadily declined, with slight alter- nation, at a mean annual rate of about 1.75', and in 1890 was 70 24'. The total magnetic intensity at Washington in 1840 was 0.61923 of a dyne. It decreased from that da:e to 1850, when it was 0.61370 ; increased to 0.61877 in ^65, and decreased to 0.60863 in I ^5- Secular Variation at San Francisco. The declination at San Francisco in 1790 was 13 6' E.; in 1810, 14 6' E.; in 1830, 15 E.; in 1850, 15 47^' E.; in 1870, 16 20^' E.; and in 1890, 16 34yV' E. The mean annual variation in declination from 1790 to 1810 was 3' ; from 1810 to 1830, 2.7' ; from 1830 to 1850, 2.37' ; from 1850 to 1870, 1.65' ; and from 1870 to 1890, 0.72' ; the average for the entire period from 1790 to 1890 being 2.088', 54 DYNAMIC ELECTRICITY AND MAGNETISM. There are no accurate data from which variation in dip and total magnetic intensity on the west coast of the United States, for an extended period, can be ascer- tained, reliable observation on these phenomena being very recent. The dip at San Francisco in 1885 was 62 15', was thought to have just passed its maximum and to be slowly decreasing ; and the total magnetic intensity for the same date was about 0.5456. Artificial Magnets. Various metals besides iron and steel can acquire magnetism, but only in a slight degree, especially nickel and cobalt, also chromium, cerium, and manganese ; and Faraday found that all substances apparently are susceptible of magnetic influence, as might be inferred from the magnetism of the earth itself. But steel of high temper is the only metal ca- pable of both acquiring and retaining magnetism to a sufficient degree for practical purposes, hence all perma- nent artificial magnets are made of it ; and magnetized steel, like the natural magnet, is capable of magnetizing steel or iron brought into contact with it without im- pairing its own magnetism. Steel magnets may be of any convenient size and form, but are usually made either straight or U-shaped, 2 to 12 inches in length, \ inch to an inch or more in width, and T 3 ^- to -J of an inch in thickness. They may be magnetized to a certain degree by simple contact with a natural or artificial magnet, also by electricity or, more effectually, by the electro-magnet, as explained hereafter ; or, if straight, by placing them for a con- siderable time parallel to the line of inclination in the plane of the magnetic meridian. This process may be hastened by concussion or, in the case of a wire, by torsion, both indicating that mag- netism is a molecular effect, the molecules being under a MAGNETISM. 55 strain in this position to which they yield more rapidly when assisted by the concussion or torsion. A common method is represented by Fig. 24. Two magnets having their opposite poles in contact and S[ IN FIG. 24. resting on the bar to be magnetized, at its centre, are drawn apart to its opposite ends and brought together again at the centre repeatedly an equal number of times on its opposite surfaces till it becomes magnetically saturated, the final movement on each surface terminat- ing at the centre. The polarity acquired by the bar at each end is opposite to that of the magnetic pole brought into contact with it. Bars of the U or horseshoe form are magnetized in a similar manner by taking the centre of the bend as the point for beginning and terminating. Another method is to interpose a non-magnetic body between the magnetizing poles and move both alternately from end to end over each surface an equal number of times. Magnetic Saturation. By magnetic saturation is meant the full quantity of magnetism which the bar is capable of retaining. It may be magnetized above the point of saturation, when it is said to be super-saturated, but the extra magnetism thus acquired is rapidly dissipated. The Armature. The magnetism of straight bar mag- nets and magnetic needles becomes impaired in the course of years, and the needles especially require to be remagnetized to maintain the requisite strength. The U and horseshoe magnets are each furnished with a piece of soft iron connecting the poles, and held there 56 DYNAMIC ELECTRICITY AND MAGNETISM. by magnetic attraction. It is known as the armature or keeper, from its supposed ability to prevent magnetic loss by completing the magnetic circuit. The chief ad- vantage of the U or horseshoe form is in the concen- tration of the magnetic attraction of both poles on the same armature, which is itself thus temporarily mag- netized and has poles of opposite magnetism to those by which it is attracted, which increases the force of attraction. Two bar magnets placed near each other, side by side, can also have armatures with advantages similar to those just mentioned. The term armature is also similarly applied, in the construction of electro-magnetic apparatus, to a piece of iron attracted by a single pole; also in a special, technical sense, in the construction of the dynamo, as explained hereafter. Laminated Magnets. Magnets of either form men- tioned are often made of a number of thin bars, sepa- rately magnetized and bound together with their similar poles in contact. They are known as laminated, and have greater magnetic strength than those having the same amount of steel in a single piece, an effect proba- bly due to the partial suppression of magnetic eddy currents, to which the steel massed in a single thick piece is liable. Such currents occurring within the mass, like electric " local action" within the battery cell, tend to neutralize and reduce magnetic potential differ- ence, while their external effect is lost. But the lami- nated structure tends to confine the lines of force to the laminae and give them a normal direction toward the poles. These eddy currents were first observed by Foucault, and hence are termed Foucault currents. In large electro-magnets special construction is required to guard against their deleterious effects. Magnetic Loss. In addition to the loss by gradual MAGNETISM. 57 dissipation, already referred to, steel magnets lose their magnetism by sudden or violent perturbations, such as concussion, a white heat, or such extreme cold as 100 C., which indicates, as before, that magnetic change is dependent on molecular change. Magnetized iron rapidly loses its magnetic energy, especially iron of the softer grades, but retains a small amount known as residual magnetism ; a similar amount being often acquired in the process of the manufacture of iron machinery. Portative Force. The attractive property of the mag- net is manifested in sustaining pieces of iron or steel suspended from it, both by direct contact and through the medium of other pieces, so that a number of small pieces, as nails or needles, may be suspended from each other. This property is known as its portative force. If connection with the magnet be severed, the iron quickly loses its magnetism, but the sceel retains it to an extent governed by its mass and temper. Magnets of the U or horseshoe form can sustain weights attached to their armatures to the amount, in some cases, of twenty times their own weight, and a little lodestone mounted in Sir Isaac Newton's signet- ring could sustain two hundred times its own weight. The portative force does not vary in the direct ratio of the mass; the proportion of force to mass being greater in small than in large magnets. The following rule is given by Bernoulli: Let/ represent the force, wthe weight of the magnet, and a the quality of the steel and method of magnetiz- 3 ing; then/ = a yw. The weight sustained varies also somewhat as the area of surface contact between the magnet and its armature, and the portative force is gradually increased by frequent additions to the load, but this increase is 58 DYNAMIC ELECTRICITY AND MAGNETISM. lost by sudden separation of the armature from the magnet. Polar Attraction and Repulsion. If the similar poles of two magnetic needles or magnets, free to move, are placed in mutual proximity they repel each other, but if their dissimilar poles are so placed they attract each other; from which is derived the law that like magnetic poles repel and unlike attract each other. From this it will be seen that the poles of the needle are opposite in polarity to those of the earth by which they are attracted, but for convenience the pole which points north is termed the north pole, and that which points south the south pole; though they are sometimes more appropriately termed the north-seeking and south- seeking poles. Their initial letters, TV^and S, are used to distinguish them, magnets being usually marked on the north pole only, by N or a cross-line; hence the expres- sion " marked pole" is sometimes used to distinguish it from the south or "unmarked pole." It is impossible to produce one kind of polarity with- out at the same time producing its opposite. This becomes manifest in a very striking manner when a magnet Is broken, the pieces assuming opposite polarity on opposite sides of the fracture and at opposite ends, each becoming a perfect magnet. If the parts be pressed closely together in their original position these poles disappear, leaving only the poles at the original ends; and the same thing occurs if the opposite poles of two rectangular magnets of the same cross-section be pressed together so as to make perfect joint. A magnetic pole may have sufficient strength to over- come by induction the repulsion of a weaker one of similar polarity and produce attraction when brought sufficiently close, while at a greater distance where the induction is less there is repulsion. MAGNETISM. 59 Unmagnetized iron or steel is attracted by either pole of the magnet, and apparently attracts either pole, but only as the result of reaction, being itself magnetically passive. Magnetic Lines of Force. If a sheet of paper be placed over a bar magnet and iron filings dusted over it, the sheet being lightly tapped, the filings arrange them- selves in curves corresponding to the lines of force em- anating from the poles, as shown in Fig. 25. Each filing becomes itself a magnet by induction, and their FIG. 25. dissimilar poles being, mutually attracted, attach them- selves to each other, while their similar poles are mutually repelled; hence poles of the same name all point in the same direction. The lines of force, radiating from each pole of the magnet, being mutually repelled and attracted toward the opposite pole, are under the influence of two forces, each varying inversely as the other, the one urging them directly forward and decreasing as the square of the distance from each pole increases, the other attracting 60 DYNAMIC ELECTRICITY AND MAGNETISM. them toward the opposite pole, and increasing in the same ratio as the first decreases, becoming greater as the distance to the opposite pole lessens and the dis- tance from the originating pole increases ; hence the resulting curves are formed as indicated by the filings. The space inclosed by lines of force is termed a tube of force. Magnetic Field. The filings show the lines of force only in the plane of the sheet of paper, which may be placed at any angle at which they can be sustained in position ; while close to the poles they stand on end nearly at right angles to the paper. Hence it becomes evident that the lines of force inclose the magnet in the form of a spheroid which is cut by the plane of the paper and shown in longitudinal section. The friction of the filings on the surface, and their weight, inertia, and tendency to mass together, prevent free movement, FIG. 26. so that they indicate the actual position of the lines of force very roughly. Those lines must be understood to fill the entire space inclosed by the spheroid, constitut- ing what is known as the magnetic field ; a more accurate conception of which would be such a figure as might be supposed to form itself around a magnet suspended in an atmosphere of iron vapor. In Fig. 26 the filings represent what are known as MAGNETISM. 6 1 consequent poles, which may result from imperfections in the temper of the steel or in the method of magnetiz- ing. Such poles may also be produced in a thin bar of highly tempered steel by touching it at several points with a magnet. The result in either case is similar to that of several short magnets joined together by their opposite poles. In Fig. 27, A and B, the filings "show repulsion and FIG. 27. attraction in a very instructive manner. A represents like poles in proximity with the lines curving away from each other, while B represents unlike poles in a similar position with the lines curving towards each other. In Fig. 28 is shown at A how the opposing lines of force from like poles produce mutual repulsion as al- ready described, while at B the curving lines from un- like poles interlock and produce mutual attraction. Since these lines do not radiate into vacant space, but into the air, it may safely be assumed that the air is the medium by which the magnetic field is formed, and that it becomes magnetized in a manner similar to that of the iron filings by which the field is represented in section ; which accounts rationally for the observed at- traction and repulsion. This hypothesis is strengthened by the consideration that it is impossible for energy, in any form, to exist independent of matter, so that the field could not be formed in an absolute vacuum ; mag- 62 DYNAMIC ELECTRICITY AND MAGNETISM. netism itself being doubtless an effect of energy acting on matter. Hence we must either assume that the mat- ter, in this case, is the air of which we have actual B FIG. 28. knowledge, or the hypothetical ether whose actual ex- istence has never been demonstrated. The interposition of any substance in the magnetic field except that of magnetic bodies, as iron or steel, or those termed diamagnetic, as bismuth and copper, offers no obstruction to magnetic induction ; attraction or repulsion taking place through all other bodies with the same facility as if they were not present : while steel or iron, especially soft iron, absorbs and diverts the lines of force through its own substance in propor- tion to its mass, extent, and relative position. Iron may therefore be used as a shield against magnetic influ- ence. Diamagnetic bodies also offer slight obstruction, but MAGNETISM. 63 in an opposite sense to that of iron or steel, resisting or turning aside lines of force instead of absorbing them. Form of Magnets. The forms of the magnet already described are the most convenient for practical use ; but iron or steel in any form may be magnetized, becoming polar normally in the direction of its longer axis, which would be true of magnets having the form of the spher- oid, cylinder, or ellipse, as well as of the more common forms ; but in such forms as the sphere or circle, having equal radii, the separate poles are not distinguishable, but must be supposed to exist and neutralize each other. Opposite surfaces of sheet iron or steel may be so magnetized as to acquire opposite polarity ; such mag- netic distribution being known as lamellar, in distinction from the ordinary distribution, which is termed solenoidal, and such sheet magnets are known as magnetic shells. Magnetic Penetration. The depth to which magnetism penetrates depends somewhat on the degree of magnet- ization and the size of the bar in cross-section. It is strongest in the outer layers, as may be shown by re- moving them with sulphuric acid, when the magnetism will be found to become constantly weaker as the cen- tral core is approached. The same may also be shown by magnetizing bundles of thin steel plates bound to- gether and gradually removing the outer ones ; also by means of a magnetized steel tube, whose magnetism is found to be nearly equal to that of a solid bar of the same cross-section. Location of the Poles. The poles have been thus far assumed to be at the ends of the magnet's longer axis, but this is practically true only of long, thin, narrow magnets uniformly magnetized throughout ; their true location is a little inside of the ends, the distance vary- 64 DYNAMIC ELECTRICITY AND MAGNETISM. ing inversely as the mass in cross-section, so that in thick magnets it becomes noticeable. Paramagnetic and Diamagnetic Bodies. Faraday pro- posed to call such bodies as are capable of assuming the magnetic properties of attraction and repulsion, as iron, steel, nickel, and cobalt, paramagnetic, while bodies which are repelled by either magnetic pole, as bismuth, anti- mony, copper, and phosphorus, he proposed to call dia- magnetic. But these terms have been adopted only in part, the utility of " paramagnetic" especially being questioned. Some writers accept the term diamagnetic as applied to the latter class of bodies, and designate the former as magnetic, a usage which is simpler and more con- venient. Magneto-Crystallic Induction. Bodies, whether trans- parent or opaque, having a crystalline structure are in- fluenced by magnetic action differently from bodies which do not possess such structure, and diamagnetic crystalline bodies differently from magnetic crystalline bodies. Such a body when suspended subject to mag- netic induction is most strongly influenced in a certain direction known as its magne-crystallic axis, which, in crystals having cleavage, is usually at right angles to the plane of cleavage. This axis, according to Tyndall, seems to lie in the direction of the crystal's greatest density, and magnetic crystals, free to move, take posi- tion with this axis in the direction of the lines of force, while in the position assumed by diamagnetic crystals it is at right angles to those lines. The whole subject is imperfectly understood, and opinions in regard to it are conflicting ; and in its pres- ent aspect it must be regarded as of secondary impor- tance. MAGNETISM. 65 Magnetism as a Mode of Molecular Motion. The mag- netic phenomena thus far observed indicate that mag- netism is closely related to the molecular constitution of the magnetized body. The effects of extreme heat or extreme cold, of concussion and torsion in producing or destroying magnetism, all of which affect the molecu- lar constitution, have already been noticed; it is also found that when iron filings are closely packed in a tube and magnetized the mass exhibits all the magnetic properties of a bar magnet, but if disturbed by being shaken up the magnetism disappears. The filings in this case may roughly represent the molecules of a solid bar, and the magnetic loss is analogous to that produced in such a bar by concussion. If the tube is of glass the filings can be seen to arrange themselves lengthwise, with similar poles all turned in the same direction, as already observed in the curved lines of filings, con- stituting magnetic series, from which results the polarity of the mass, as in magnets with consequent poles. It is found that magnetizing a steel bar produces a slight change in its form, the bar becoming a little longer, and its other dimensions being correspondingly reduced; an effect attributed to a change of position in its molecules. In the unmagnetized bar the molecules may be supposed to be massed together without order, but under the magnetic influence to arrange themselves symmetrically in the direction of their longer axes, with similar poles in the same direction, like the filings, the results being the change of form mentioned above and the polarity of the bar. This theory receives further support from the slowness with which steel acquires magnetism, as compared with soft iron, and its power of retention, while the iron both acquires and loses it rapidly; this quality in the steel being attributable to its rigidity, which resists change of position in the mole- 66 DYNAMIC ELECTRICITY AND MAGNETISM. cules, while those of soft iron easily yield to such change Further proof of the same character is found in the fact that when a bar is magnetized suddenly by electric action a clink may be heard in it, both at the beginning and end of the process, which can be satisfactorily ac- counted for only on the theory of molecular action. The above phenomena clearly indicate molecular change of position as a result of magnetization, and hence motion; but if the motion should cease when the molecules have assumed symmetrical position, magnetic action should also cease, for it would be absurd to sup- pose that this action could continue when the motion which gave rise to it ceased, and that it should be the result of mere symmetrical arrangement, the molecules thereafter remaining quiescent: but we find, on the con- trary, that it is then at its maximum, and, in steel, re- mains permanent for years. Hence we must infer a corresponding maximum and continuity of molecular motion. The character of this motion cannot be known. We may suppose each molecule to oscillate in the direction of either its longer or shorter axis, or to rotate around either axis, or to have a vortical motion, or to combine two or more such motions; but it is a reasonable infer- ence that this motion, whatever its character, is similar and uniform in each molecule, so that there is no inter- ference between them such as would result from a dil- igence either in their motions or position. And this condition may be supposed to constitute the difference between the magnetized and unmagnetized bar; or more explicitly, that this motion is itself that which we term magnetism. Hence if we could be endowed with some superior sense, capable of penetrating the magnet and revealing its separate molecules and their motions, we should MAGNETISM. 6/ probably see, instead of a quiescent body, a quivering mass of innumerable atoms, each moving with incon- ceivable rapidity, and all in a uniform manner and in obedience to a common impulse. This theory of magnetism has the sanction of Clerk Maxwell, Hughes, and others, and is in accordance with the similar theories in regard to heat and electricity among whose advocates Tyndall is prominent. And as we have seen that different kinds of motion may be combined in the same molecule, without interfer- ence, as in larger masses, we may attribute the heat to one kind, the electricity to another, and the magnetism to a third, all being different manifestations of that universal energy which is inherent in all matter. The theory of magnetism being a fluid, or two dis- similar fluids, once so popular, has now become obsolete, and cannot be sustained en rational grounds in the light of recent investigation. Analogy between Magnetic and Electric Phenomena. The close analogy between many of the phenomena of electricity and magnetism indicate that both are closely allied, if not identical, as will appear more fully in the next chapter. But it may here be noticed that opposite magnetic polarity has its analogy in opposite electric polarity; that in both cases the opposite kinds are coexistent and neutralize each other; that magnetic distribution is influenced by the form of bodies in a manner similar to that of electro-static distribution ; and that magnetic attraction and repulsion has its analogy in electric attraction and repulsion. But in all these analogies there is a well-defined difference observable which easily distinguishes the magnetic from the electric phenomena. Coulomb's Torsion Balance. A full description of this instrument and its application to the measurement of 68 DYNAMIC ELECTRICITY AND MAGNETISM. FIG. 29. electric force may be found in the writer's " Elements of Static Electricity." It is used in a similar manner to measure magnetic force, as shown in Fig. 29. It consists of a circular glass case, with a vertical cylinder projecting from the cover, hav- ing at its upper end a graduated circle, with a pointer to move round it attached to a milled head, from which is suspended, by a fine wire, a magnetic needle, with its poles opposite a circle on the case, graduated to correspond to the one above. The following experiment made by Coulomb shows its use: Zero of the lower scale being brought opposite one of the needle's poles, and accurately adjusted in this position by comparison with a copper needle of equal weight suspended by a thread, and the upper scale being adjusted with its zero opposite the pointer, it was ascertained, by a preliminary trial, that the milled head required to be turned 35 to produce i of deflection in the needle; hence magnetic force, in this case, was to torsion as i to 35. A magnet was then in- serted, as shown, with its pole close to the similar pole of the needle, and was found to produce 24 deflection; hence its force, as ascertained above, was 35 times 24, to which must be added the 24 of torsion, giving 24 X 35 -f- 24 = 864 as the " torsion equivalent " of mag- netic repulsion with the poles 24 apart. The milled head was then turned so as to reduce this distance one half (12), requiring 8 complete turns, which gives 8 X 360 2880. But the 12 remaining MAGNETISM. 69 at bottom represented a force equal to half the original " torsion equivalent," or 432, which must be added in, giving 2880 -f- 432 = 3312, nearly /oar times 864, as the " torsion equivalent" of magnetic repulsion at one half the distance. In like manner it can be shown that any reduction of distance, algebraically represented by a, would require a " torsion equivalent" equal to a*\ hence magnetic force is thus proved to vary inversely as the square of the dis- tance. The inaccuracy observable in the arithmetical result is accounted for by inaccuracies in the instrument, and in the angular measurement adopted, which are fully explained in " Elements of Static Electricity" referred to above. The Gauss- Weber Portable Magnetometer. This instru- ment, used for measuring the horizontal force of the earth's magnetism at any point, as shown by the mag- netic declination, is constructed as follows: A bar mag- net of convenient size is suspended horizontally, from a vertical standard, by an unspun silk fibre. To one end of this magnet is attached a lens, and to the other a glass scale adjusted to the lens's focus of parallel rays. This part of the apparatus is inclosed in a box having a small window at each end, on a line with the hori- zontal axis of the magnet, through one of which light is admitted to the rear of the scale, and through the op- posite one the parallel rays from the lens pass out and enter the field-glass of a small telescope, mounted in front, through which the scale divisions may be ob- served. This apparatus is mounted on a tripod on which it can be rotated horizontally around the axial line of sus- pension of the magnet, and the angle of rotation meas- 70 DYNAMIC ELECTRICITY AND MAGNETISM. ured on an azimuth scale with vernier attachment, mounted on the tripod underneath the apparatus. The instrument being adjusted to the proper level, the torsion is first removed from the silk fibre by sus- pending from it a small plummet of the same weight as the magnet, after which the magnet is suspended so as to come to rest in the magnetic meridian without pro- ducing torsion of the fibre, and the movable part of the apparatus rotated till some division of the magnet scale coincides with the cross-wires in the field of the tele- scope. The reading on the lower scale is then noted, and by a second rotation the instrument is adjusted to the true north, ascertained by observation of one of the heavenly bodies by means of a small transit apparatus mounted back of the magnet-box ; and the reading of the lower scale for this second position being noted and the necessary corrections made, the difference of the two readings gives the true declination for the place and time. ELECTROMAGNETISM. CHAPTER V. ELECTROMAGNETISM. Deflection by the Electric Current. Such accidental effects as the magnetizing of steel instruments by light- ning had long indicated some relation between mag- netism and electricity, but all attempts to produce similar results by artificial means had failed to give satisfactory results. In 1802 Romagnosi of Trente noticed that the magnetic needle was deflected by the voltaic current, but his discovery failed to attract at- tention. In 1819 Oersted of Copenhagen discovered that the needle was not only deflected by the voltaic current, but tended to take a position at right angles to it, and that the deflection was governed by the direc- tion of the current and relative position of the needle. The discovery, like that of Volta, marks an important epoch in electric progress; it established beyond doubt the mutual relationship of electricity and magnetism, and was the origin of the science of electromagnetism with all the great inventions to which it has given rise. Oersted's experiments are easily repeated by holding a straightened section of copper wire, connecting the poles of a battery or cell, alternately above and below a poised magnetic needle, and reversing the direction of the current in each position. The Galvanoscope. A better instrument for this pur- pose is represented by Fig. 30, consisting of a mounted, rectangular brass frame, surrounding a poised needle lengthwise, and provided with binding-screws at the terminals of the wires. The under wire has its ter- 72 DYNAMIC ELECTRICITY AND MAGNETISM. minals at A and B, and the upper wire, which is joined A B c to the under at the left and in- sulated from it at the point of support on the right, has its ter- minal at C. Hence when the battery wires are attached to A and B the current flows under the needle; when attached to A and C, over the needle; and when attached to B and C, round the needle; its direction below being the reverse of that above. When the frame is parallel to the needle in the mag- netic meridian and the current is flowing over the needle from north to south, the north pole is deflected to the east; when the current is reversed the deflection is to the west; and when the current flows under the needle from north to south or from south to north these deflections are reversed. When the flow is round the needle lengthwise in either direction, through the connections at B and C, the deflecting force is doubled, the current in the upper and under wires flowing in opposite directions, and hence both tending to deflect in the same direction, as shown above. Since this instrument may be used to indicate the presence and direction of an electric current, it is known as the galvanoscope. The Schweigger Multiplier. By multiplying the coils which pass round the needle the deflecting force may be proportionately increased within certain limits. This may be done by winding the wire, provided with an in- sulating envelope, on a frame of non-magnetic material. The effect may be increased by using a short needle which shall be included within the helix at any angle of ELECTKOMAGNETISM. 73 deflection, and which, for convenience of observation, may be connected with a light non-magnetic pointer. The first instrument of such construction was called the Schweigger multiplier , in honor of its inventor. Ampere's Rule. To determine the direction of the deflection in every case Ampere proposed the con- ception of a little human figure so placed that the cur- rent would enter at its feet and leave at its head, its face being turned constantly towards the needle and its arms extended at right angles to its sides. The left hand would then constantly indicate the direction of the north pole's deflection, and the right that of the south pole, at any point, above, below, or at either side; the deflection in the latter case tending vertically. The Astatic Needle. The force which deflects the needle as above acts at right angles to that of the earth's magnetism which tends to maintain it in the plane of the magnetic meridian, and the amount of de- flection depends on the relative strength of the electric current, the force of the earth's magnetism at any point being practically constant; but the deflection evidently can never, under these conditions, equal a right angle. But if the effect of the earth's magnetism is neutralized in the apparatus, a much more sensi- tive and effective instrument can be produced. This is done approximately by the astatic needle, shown in Fig. 31, which consists of two parallel needles of equal length attached to a short vertical support, with their poles reversed, so that each neutralizes the directive force of the other. They are usually suspended by an untwisted silk fibre so as to be uninfluenced by friction, protected from air currents by a glass case, and 74 DYNAMIC ELECTRICITY AND MAGNETISM. provided with a graduated scale to indicate the amount of deflection. The wire carrying the current passes round one of the needles lengthwise, or may pass round each, being wound alternately in opposite directions; and the poles being also reversed, it is evident that the current flow- ing in each section of the wire must produce deflection in the same direction in both needles. But it is practically impossible to construct the two needles with such mathematical precision that there shall not remain a slight deviation in mass, magnetiza- tion, and parallelism sufficient to produce a prepon- derance influenced by the earth's magnetism; so that the very best astatic needles are only approximately correct. Compensating Magnet. A similar astatic effect is pro- duced by fixing a magnet in the magnetic meridian with its poles above the similar poles of the needle, and at such a distance that its influence is just sufficient to counteract the directive force of the earth's magnetism. If too close it reverses the needle's position, but with a provision for vertical adjustment it can be maintained in the best position for directive compensation. Cause of Deflection. If a copper wire in which an elec- tric current is flowing pass at right angles through a card on which iron filings are dusted, the filings, when the card is lightly tapped, arrange themselves in con- centric circles around the wire, indicating that lines of force, due to the current, circulate in planes at right angles to it and magnetize the filings. If we suppose such a wire to pass through this page, at right angles to the paper, the current to flow from the reader, and a magnetic needle to be carried round it, the needle would constantly tend to assume position in a plane parallel to the paper, its north pole, by Am- ELECTROMA GNE TISM. 7 5 pere's rule, turning in the same direction as watch- hands move; while if the current flowed toward the reader this direction would be reversed. From such indications it is evident that there is around every current-bearing wire an electric field in which lines of force circulate in planes at right angles to its length; and since it has already been shown that lines of force in the magnetic field circulate in planes parallel to the magnet's length, it is evident that the tendency of these forces, when brought into mutual proximity, must be to cause the needle or magnet to take position at right angles to the direction of the cur- rent, in which position the planes of the magnetic and electric forces coincide. We may assume a similar physical condition for the electric field to that already assumed for the magnetic, namely, that the air, or the hypothetical ether, is the medium by which the force radiates, and is itself elec- trified throughout a space of which the current-bearing wire is the central core. The Electromagnet. It was discovered in 1820 by both Arago and Davy that iron and steel could be magnet- ized by the electric current by inclosing a bar of either metal in a helix of insulated wire through which a cur- rent is passing; the steel remaining permanently mag- netic after being withdrawn from the helix, while the iron is magnetic only while inclosed and during the pas- sage of the current ; hence the latter is technically known as the electromagnet, in distinction from the former. Electromagnetic Poles. By observing the direction in which the current from the battery or other electric generator is passing, the poles may easily be distin- guished by Ampere's rule, already given ; that being the south pole, viewed endways, around which the cur- ? DYNAMIC ELECTRICITY AND MAGNETISM. rent is passing in the same direction as watch-hands move, while that is the north pole around which it flows in the opposite direction. Winding. It is immaterial in which direction the helix is wound, whether from right to left, or from left to right, or in layers in either direction, alternately from end to end, like thread on a spool, provided the winding, in each case, is in the same direction through- out; but if reversed, in sections or in alternate layers, the result is consequent poles at the points of sectional reversal, or neutralization between layers oppositely wound. Magnetic Strength. An electromagnet is capable of acquiring magnetic strength, as represented by the force in any way exerted, far in excess of the best steel mag- net of similar size, and they have been made of suffi- cient lifting power to sustain more than a ton. The strength is dependent on the size of the iron core, the quality of the iron, the amount of wire in the helix, and the strength of the magnetizing current. Core. The core can be magnetized only to the point of saturation, beyond which increase in size of helix or strength of current can produce no increase of magnetic strength; hence its mass should be duly proportioned to that of the helix, hollow cores, of sufficient mass, having the same efficiency as solid ones. Its ends should project beyond those of the helix. The iron should be soft and homogeneous in structure to render it capable, not only of the highest degree of magneti- zation, but of rapidly acquiring or losing its magnetism at the closing or opening of the electric circuit ; a quality on which the practical value of the electromag- net largely depends. Coefficient of Magnetic Induction. This property of magnetic permeability, or conductivity for the lines of ELECTROMAGNET1SM. 77 force, is termed the coefficient of magnetic induction, and is found in various bodies in different degrees, but in none to the same degree as in soft iron, which is therefore said to have a high coefficient of magnetic induction. Helix. The helix may consist of fine or of coarse wire of any conductivity, copper being practically the best, and the total volume of current with a given mass of wire may be the same in either case, while the resist- ance may vary greatly. A helix of ten coils of wire of a given cross-section and length may equal in mass another helix of a hundred coils of one tenth the cross- section and ten times the length: and resistance varying directly as length and inversely as cross-section, and current, with a given E. M. F., varying inversely as re- sistance, the volume of current in any given section of the fine wire would be only one tenth of that in a coarse wire of equal length, while the total volume in the mass would be the same in either case, since the fine wire has ten times the number of coils. The rule is to make the resistance of the helix equal to the external resistance of the current, and thus adapt the magnets to the conditions of the work for which each is designed. The diameter of the coils, within certain limits, does not affect the strength of the magnet, since the field of magnetizing electric force surrounding the wire varies directly in area and inversely in magnetic effect as the square of its distance from the core, variation in one sense compensating variation in the other; since its strength at any point in the larger area, equally dis- tant from the wire, is the same as in the smaller area, while the distance of the wire from the core is propor- tionally greater. With such proportion between the mass of the helix and of the core as to insure saturation without excess. 78 DYNAMIC ELECTRICITY AND MAGNETISM. the diameter of the helix should not exceed one half its length. Electromagnetic Saturation. A perceptible amount of time is required to produce magnetic saturation of the core, which, in the case of very large magnets, may amount to two seconds. It has also been observed that this result is attained more rapidly with high E. M. F. coupled with high resistance than with low E. M. F. and low resistance, though the volume of current in each case is the same. Form of Electromagnets. The horseshoe or U form, shown in Fig. 32, in which both poles attract the same armature, has, as in the steel magnet, the greatest practical efficiency. The winding must be in the same di- rection in both coils, as if a straight bar were thus wound and then bent; which requires the wire to cross to the opposite side at the bend as shown. A modification of this form is seen in the rectangular form shown in Fig. 33; the wire may also be wound on separate bobbins and slipped over the cores as shown. Armature. The armature should be of the best soft iron, and of such form and mass as to embrace the greatest practicable number of lines of force, since it is itself a magnet during contact, and its force, whatever it may be, varies in the same ratio as the mag- net's force, and the portative force ' IG> 33 * equals the sum of the two. Hence if the magnet's force equals x and the armature's force y, the por- tative force is x -f y, and if the magnet's force is EL E C TROMA GNE TISM. 79 doubled, the portative force becomes 2(x -f 7); if halved, X I y ; the proportion being the same for any other variation VCtl ICtLlUll. The magnetization of the steel magnet can be accom- plished most efficiently by the electromagnet. Experiments in Diamagnetism. The electromagnet, by its superior power, affords the means of examining dia- magnetic bodies, not practicable with the steel magnet. For this nnrnnse Faradav. in i8.dc. nseH thp annaralus umguciu; uuuica, uui picn-ui^ciuic wnu LUC aicci iiicigiicu. For this purpose Faraday, in 1845, use d the apparatus .T FIG. 34. shown in Fig. 34, which consists of two powerful electro- magnets, A and B, having hollow cores, between whose opposite poles the body under examination may be suspended, as shown; the distance between the poles being adjusted as required by adjusting the movable frames FF, to which the cores are attached, to the re- quired position, as indicated by the scale RR, where they are secured by the binding-screws EE. The poles terminate in cone-shaped armatures, attached by screws, by which the magnetic force is concentrated between the rounded points. 80 DYNAMIC ELECTRICITY AND MAGNETISM. If the body under examination is repelled from the concentrated magnetic field thus formed, it is classed as diamagnetic; if attracted, as magnetic, or paramagnetic. The tests are made either with small balls of the vari- ous substances, suspended near the poles, which are brought into close proximity, or with straight bars suspended between the poles, when placed farther apart. The balls are repelled or attracted as above, according as they are diamagnetic or paramagnetic, while the bar, if diamagnelic, sets itself equatorially, as shown by the bar ab, Fig. 34, but, if paramagnetic, axially, that is, parallel to a line joining the poles. The reason of this becomes obvious when it is con- sidered that the magnetic field, as has been shown, con- sists of magnetized matter, which may be air at the ordinary density, or rarefied to any extent possible by the formation of a partial vacuum, or some other gaseous body, as oxygen; and the suspended body be- comes itself a part of this field when attracted into it. Hence if paramagnetic, like iron, it can form within itself a field which may equal or even greatly exceed in strength that of the gaseous body in which it is sus- pended, and hence is drawn into that position in which it can embrace the greatest number of lines of force, which, in a straight bar, is the axial position; but if dia- magnetic, like bismuth, it is pushed aside by the exist- ing lines of force, from the stronger to the weaker part of the field, where the lines of force are equal to its re- ceptive or magnetic inductive capacity, which in the case of a straight bar is the equatorial position, where the greater part of the bar is most remote from the cen- tral region of greatest magnetic intensity. Hence para- magnetic bodies are those in which magnetic inductive capacity is high, diamagnetic bodies those in which it is low. Such a test is not to be regarded as indicating the absolute diamagnetism or paramagnetism of the body ELECTROMAGNETISM. 8 1 under examination, but as a comparison with the mag- netic condition of the gaseous medium occupying the field, which would ordinarily be the air at its normal density; the test being analogous in this respect to that for the specific gravity of bodies by a comparison of their weight in air with their weight in water. Gases are tested by means of bubbles inflated with them showing attraction or repulsion; and liquids simi- larly, when suspended in glass vessels; but a correction is evidently required in the first case for the matter com- posing the walls of the bubble, and in the second for the glass of the vessel, since the magnetic condition of either might differ widely from that of the gas or liquid and seriously affect the result. List of Diamagnetic and Paramagnetic Substances. The principal substances found to be diamagnetic are as fol- lows : bismuth, phosphorus, antimony, zinc, mercury, lead, silver, copper, gold, water, alcohol, tellurium, se- lenium, sulphur, thallium, hydrogen, air. The princi- pal ones found to be paramagnetic are as follows: iron, nickel, cobalt, manganese, chromium, cerium, titanium, oxygen; also substances containing the above metals in combination. The proper magnetic classification of platinum is not settled; it has been assigned to each list by different observers, the weight of evidence being in favor of its paramagnetic character; but, when chemi- cally pure, Wiedemann considers it diamagnetic. Flames, smoke, and hot air tend to move, in the mag- netic field, from higher to lower potential, which would indicate that they are diamagnetic; but this is not clear, since the movement may be due to the convection of the air and its diamagnetism. It has been observed that bismuth, when pulverized, made into a paste with mucilage, and formed into a roll, sets itself equatorially in the magnetic field, like a bismuth bar; but when con> 82 DYNAMIC ELECTRICITY AND MAGNETISM. pressed into a flat plate its position becomes axial, an effect attributed to the semi-crystalline structure of the mass. Deflection of the Electric Current by the Magnet. It has been shown that a magnetic needle or bar magnet free to rotate takes position at right angles to a fixed wire bearing an electric current; conversely it may be shown that if the wire be free to rotate it will take position at right angles to a needle or bar magnet in a fixed posi- tion, a result which evidently follows from the law of action and reaction. Ampere's Table. This can be explained with the ap- paratus devised by Ampere, shown in Fig. 35, known as Ampere's table, which consists of a wire bent as shown, and suspended so as to rotate horizon- tally round its centre of gravity; its ends dip- ping into mercury cups to insure perfect con- tact; and having arms and supporting stand- ards of brass, with which the battery wires which supply the cur- rent connect at bottom, as shown, so that the FIG. 35. wire coil becomes part of the circuit. This coil will adjust itself with its plane at right angles to the length of a bar magnet thrust into it; its position being reversed when the poles or the current are reversed, and similar effects but weaker being ob- served when the magnet is held above or beneath the coil. If the magnet be entirely withdrawn the coil will ELECTROMAGNETISM. 83 assume the same position with reference to the earth's magnetism to that which it would assume if a magnet were placed beneath it with its south-seeking pole turned north, so as to represent the earth's magnetism. The plane of the coil will then be at right angles to the magnetic meridian, and that face turned north which would be indicated by the left hand of Ampere's little figure swimming with the current, face downward, at the bottom of the coil. The Solenoid. If this single coil be replaced by the solenoid represented by Fig. 36, which consists of a helix with straight portions of the wire returned to the cen- tre as shown, the mag- netic effect is greatly in- creased, each convolution assuming the same posi- tion as the single coil, so that the solenoid takes the same position as the , FIG. 36. magnetic needle and has north and south polarity. This might still be accounted for by the current's reaction setting the planes of the coils at right angles to the magnetic meridian in obedi- ence to the earth's magnetism, but it is found that the poles of the solenoid are repelled by like poles of the magnet and attracted by unlike, which indicates that it has true magnetic properties like those of the needle, which is made further apparent by the fact that two solenoids suspended in mutual proximity behave like two needles similarly placed, exhibiting polar attraction and repulsion in the same manner. Here, then, we have a current-bearing wire, which may be copper or any other metal, behaving like a steel mag- net; which furnishes strong proof, in addition to that already adduced,, of the close affinity, if not actual 84 DYNAMIC ELECTRICITY AND MAGNETISM. identity, of electricity and magnetism. It is also found that an electric current flowing in rarefied air as its medium, in a glass tube in which the highest attainable vacuum has been produced, is attracted or repelled by a magnet like the current in the wire. The polarity of the solenoid is much weaker than that of the magnetic needle, but may be greatly reen- forced by placing within it a soft-iron core, which in fact makes it an electromagnet. It now takes position in the plane of the magnetic meridian with the energy of the needle, but its polarity must be ascribed chiefly to the magnetism of the core rather than to that of the current, as shown in the solenoid without a core. De La Rive's Floating Battery. De La Rive used for the above experiments a little floating battery. It is easily constructed with little plates of zinc and carbon, or copper, attached to a large flat cork and floated on water acidulated with sulphuric acid; a light coil or solenoid attached to the plates projecting from the upper surface of the cork. It is simpler, cheaper, and more easily constructed than Ampere's table, but less effect- ive and limited to a smaller number of experiments. Mutual Induction of Electric Currents. By using the rectangular frame represented by Fig. 37 with Ampere's table, the mutual attraction and repulsion of electric -j currents may be shown. If a current is flowing round the frame, as shown, and a straightened section of another current-bearing wire be held in close proximity, parallel to either of its vertical sections, the frame wire will be attracted if the two currents flow in the FIG. 37. same direction, but repelled if they flow in opposite directions ; this will also be true when the wires are inclined to each other at an angle, attraction taking ELECTROMAGNETISM. 85 place when the currents flow either toward or from a com- mon point, but repulsion when one flows toward and the other from a common point. Ampere, who discovered this mutual action, gave to it the name of electrodynamics. Its explanation may be found in the mutual action of solenoids already de- scribed, whose like poles were shown to repel and unlike to attract, like the corresponding poles of the magnet. It is evident that when two like solenoid poles are brought into mutual proximity, the currents in adja- cent sides of their coils must flow in opposite direc- tions, one upward and the other downward; hence the inductive lines of force in the field surrounding the two poles meet in opposition and repel each other as in steel magnets, as already explained. But if unlike solen- oid poles be brought into proximity the currents in ad- jacent sides flow in the same direction, and the lines of force interlock and draw the poles together, as in steel magnets. Now if the adjacent sections of the solenoid coils in proximity be straightened, we have exactly the same conditions as in the rectangular wire frame and straightened section of wire in proximity, and the re- sults in each case are the same. The wires and surrounding air in all these cases are the media through which energy manifests itself by molecular action; and we call these manifestations elec- tric, or magnetic, or electromagnetic, according to the nature of the media, and the conditions under which the manifestation occurs. When a current flows through a conductor, it is found that the effect of induction is to produce an opposite current in any adjacent parallel conductor. The nature of this action may be represented by the following diagram: (*) + io + + + + + + -f- + +i w --* 86 DYNAMIC ELECTRICITY AND MAGNETISM. Let a represent a conductor in which a current is flowing from left to right by virtue of the difference of potential represented by -|- 10 at the left and -f- i at the right; and let b represent an adjacent parallel con- ductor. Since inductive influence radiates from a charged body equally in all directions, only a small fraction of the lines of electric force radiating from a are intercepted by b ; but for convenience we may rep- resent this fraction by -^ at the right, and 2 at the left. Now, since, as has been shown, the positive pro- duces by induction an equal corresponding negative, and vice versa, the positive potential in a, represented by -f- TO, induces, under the conditions named, a negative potential in b, represented at the adjacent point on the left by 2, while on the right -j- i in a induces in b\ and since electric movement is always from higher to lower potential, the current induced in 'b must flow from right to left opposite to that in a. Hence when currents in two or more adjacent paral- lel conductors flow in the same direction the effect of their mutual induction is to produce in each a counter- current, which reduces the volume of the primary cur- rent; the effective current by which useful work is ac- complished being then represented by the difference be- tween them. As in the illustration, if the primary current were represented by 10 and the induced oppos- ing current by 2, the effective current would be repre- sented by 8. But if the primary currents flow in opposite direc- tions, the effect of induction is reversed, and the volume of effective current in each conductor increased, as can easily be seen by the following diagram: (C)+ 10 + + + + + + + + +! (d) + I + + + + + + + + +IQ In which the current in c flows from left to right, and in which, as well as the cheeks, are insulated ELECTRO MAGNETISM. IO5 from each other by the rubber. Two brass springs con- nected with the binding-posts B and C press against the cylinder, which is mounted on brass supports connected with the binding-posts A and D. The battery wires connect also with A and Z>, and the terminals of the primary coil with B and C. When the cylinder is in the position shown in the cut, with the springs pressing against the insulating rubber, no current can pass; but when turned so as to bring the cheek V in contact with the spring attached to B, and V with that attached to C, the current flows from -f- P through A a v' V , through the coil from C to B, and thence through VvbD to N. But when the cylinder is reversed, so that V connects with C, and V with B, then the current from -\- P \.Q N flows through the coil in reverse order, by way of Aav'V, from B to C, and thence, as before, through VvbD to N. The Coil a Converter. The coil is not a generator but a converter, transforming the energy derived from the battery, or any dynamic generator, by increasing the potential difference, or E. M. F.; which must be done at the expense of a corresponding reduction in the volume of current, since otherwise there would be an increase of electric energy without a corresponding expenditure of chemical or other energy, which would be impossible. For all the energy is derived from the generator, and a certain percentage expended in operating the interrupter and overcoming the resistance of the primary coil, so that even when the interrupter is operated by external power there is still a loss from the resistance of the primary. This energy, as has been shown, first enters the pn - mary coil, which, from its low resistance, carries a large current, whiie its high coefficient of magnetic induction, derived from the core, multiplies the lines of force cut 106 DYNAMIC ELECTRICITY AND MAGNETISM. by the secondary coil. The secondary, from the ex- treme fineness of the wire, has great resistance, and hence carries a very small current, but creates a great E. M. F., or potential difference, which in a large coil may equal many thousand volts, the convolutions being very numerous and each adding its quota to the coeffi- cient of mutual induction. But since, with wire of any given cross-section, the resistance increases directly as the length, and since, as shown, the E. M. F. also increases as the length and hence in the same ratio, the current must remain con- stant. But since, with a given size of coil, any variation in cross-section of wire produces a corresponding oppo- site variation in its length, from which must result a corresponding variation in the relative proportions of E. M. F. and current, it is evident that any resulting increase of current must produce a decrease of E. M. F., and any decrease of current an increase of E. M. F. The length of the spark, or discharge, depends both on the E. M. F., or electric pressure, and on the cross- section of the perforation made through air or other in- sulating medium; for the length and cross-section of the perforation measure the resistance overcome, and any variation in either dimension must be compensated by an opposite variation in the other, otherwise there would result an increase of work without a correspond- ing increase of energy. Hence the great spark-length obtained by the coil is the result of the transformation, which, by reduction in volume of current, concentrates the electric energy on a fine line and impels it with a corresponding increase of E. M. F. The great advan- tage of the coil in this respect is shown by the fact that the longest spark obtainable without a coil from 1080 silver chloride cells, the largest battery ever con- structed, is only ^ of an inch, while Spottiswoode's ELECTROMAGNETISM. IQJ great coil gives, with 30 Grove cells, a spark of 42 \ inches in length. Electric Perforation. Perforation, as used above, re- fers to the path by which the electric energy passes through a substance, using its material as the medium of transfer; displacement of this material being often an accompaniment of the discharge, though not a necessary consequence, since energy and not matter is thus trans- ferred. Paper, for instance, when thus perforated is displaced, while glass is pulverized, on the line of dis- charge, with surrounding fracture and little or no dis- placement. A discharge through any insulating medium is termed disruptive. Physiological Effects of Faradic Current. The faradic, or alternating, current of the coil, when passed through any part of a living body, produces a tingling sensation accompanied with muscular contraction, which may be mild or painfully severe, according to the strength of the current, as regulated in the manner already de- scribed. This current is now extensively employed in medical practice, and its use constitutes an important branch of electro-medical treatment. In ordinary lecture-room experiments it is received through metal handles, connected with the coil termi- nals, which may be held in the hands or otherwise ap- plied; but, for medical use, special electrodes, such as sponges and rollers having insulating handles, are used, by which the current can be applied by the physician or attendant as required. Discharge in Air and in Vacuo. The intensely brilliant spark produced by the electric discharge in air at the ordinary density is due to the heat generated by the electric energy in this high resisting medium, which is rendered incandescent on the line of discharge by the intensely rapid vibration of its molecules. Hence the 108 DYNAMIC ELECTRICITY AND MAGNETISM. electric spark is a line, or fine cylinder, of incandescent air, often bent, contorted, or subdivided, whose molecules are in a state of intensely rapid vibration. The longest spark in air at the ordinary density is comparatively short, the energy being soon expended in overcoming the high resistance; but when the density is reduced by the production of a partial vacuum, the length of the discharge is proportionally increased. This may be done by a partial exhaustion of the air from a glass tube with the common air-pump, but is accomplished more effectively in the hermetically sealed vacuum tubes of Geissler, in which the density is re- duced by the mercury pump to T ^Q^ of an atmosphere, and platinum terminals sealed into the extremities. A discharge several feet in length may be obtained in such a tube with a small coil; the low resistance also per- mitting increase of cross-section, with change of color to the light pink seen in the aurora, which is a similarly diffused discharge. This change of color is a necessary consequence of the diffusion, since enlargement in the space occupied by the discharge produces a correspond- ing diffusion of the light and heat produced at each point and hence a proportional reduction of their in- tensity. Since the space occupied by the discharge increases in the same ratio as the reduction of the atmospheric density, a reduction to j^Vo of an atmosphere would give, with a tube of sufficient size, an enlargement of 333^ times the space occupied by the same discharge in air at the ordinary density. But when the density is reduced to T^nroTTriF f an atmosphere, as in Crooke's vacuum tubes, the medium becomes insufficient to carry the current with the same facility as in the lower vacuum, and we have resistance in the opposite sense to that found in air at the ordinary ELECTROMAGNETISM. 109 density, with many interesting phenomena described in " Elements of Static Electricity," in which this subject, and also the auroral discharge, is more fully discussed. Electric Gas Lighting. The coil and battery are ex- tensively used for lighting the gas in churches and audi- ence halls where the burners are not easily accessible. For this purpose wire, properly insulated, is connected with the chandeliers, and interrupted at each burner so as to furnish short sparks which pass in series through the escaping gas and light it. Spark Coil. The spark coil, as it is termed, is best adapted to this use; it consists of the primary coil and core, giving a short thick spark with strong current; the secondary coil and interrupter being dispensed with, reducing the resistance, risk of burning out, and expense. The term spark coil is also applied to the complete induction coil, when constructed for this or any similar purpose, where the main object is the spark rather than the current. The induction or influence machine Holtz, Tb'pler, or Wimhurst is also used for the same purpose, as de- scribed in " Elements of Static Electricity." 11O DYNAMIC ELECTRICITY AND MAGNETISM. CHAPTER VI. ELECTRIC MEASUREMENT. ELECTRIC MEASUREMENT pertains to measurement of the force exerted in any way by electric energy, or of the resistance which opposes it, or of certain effects re- sulting from the mutual relations of this energy and resistance. It is dependent on certain physical con- ditions which will now be considered in their order. Electric Potential. Potential in the physical sense is that condition of matter by virtue of which it is capable of exerting physical force. Thus we estimate the heat potential of a body by the effect it can produce on temperature; its gravity potential by the attractive force it can exert as a mass; its magnetic potential by the magnetic force it can exert; and its electric poten- tial by the electric force it can exert. Electric potential is designated relatively as positive, negative, or zero. Matter has positive electric potential, or is positively electrified, when its electric condition is higher than that of other matter to which it may be related either by contiguity of position or electric con- nection, so that it is capable of imparting electricity to it; it has negative electric potential when its electric condition is lower, so that it is capable of receiving electricity from such other matter; and zero electric potential when its electric condition is the same as that of the other matter, so that it can neither impart nor receive: and any variation in either condition must of course change these relative electric conditions. Hence ELECTRIC MEASUREMENT. Ill a body may have positive potential with reference to one of lower potential and, at the same time, nega- tive with reference to one of higher potential, or zero with reference to one of the same potential. Potential difference is conveniently represented by the symbol/, d. Electromotive Force. Electromotive force has been already briefly referred to as " that which moves or tends to move electricity from one point to another," and as being represented by difference of electric poten- tial. Hence it is the relative condition of electric force between different bodies or parts of a body; the tend- ency of electricity being always to move from higher to lower potential in the same sense as heat tends to move from higher to lower temperature. It is some- times represented as electric pressure, in the same sense as water pressure in a reservoir, or steam pressure in a boiler, and the analogy is correct so far as the pressure is concerned; but water or steam pressure tends to move matter, while electric pressure tends to move molecular force, using matter as its medium. It is independent of the quantity of electricity gener- ated, and depends solely on potential difference, just as force in each infinitesimal drop of water in Niagara Falls is derived from the height of the falls and not from association with other drops. Hence small electric quantity may be combined with large electromotive force, or the reverse. The number of battery cells joined in parallel may be multiplied indefinitely, while the electromotive force, as has been shown, is only that of a single cell, each cell being, in this respect, indepen- dent of the others; and the same is true of the generat- ing parts of any other electric generator, when joined in parallel, as the parallel pairs of plates in a Topler machine, or the parallel coils of a dynamo. But when 112 DYNAMIC ELECTRICITY AND MAGNETISM. these parts are joined in series, the electromotive force of each being added to that of the others varies as the number of such parts and as the value of this quantity in each. The symbol of electromotive force is E. M. F., as already given, but in mathematical formulae E alone is used. Electric Resistance. Electric resistance, as briefly de- fined in Chapter I, is that which opposes electric move- ment, and may consist specifically in the molecular constitution of the conductor or insulator; in counter- electromotive force, or counter-induction; in useful work; or in an artificial obstruction placed in the circuit for a useful purpose. In conductors it varies directly as the length of the conductor and inversely as its cross ssction, and also inversely as its conductivity; and as insulators must be regarded as inferior conductors, the same rule applies to them. But the relative electric resistance of the substance displaced by the insulator must also be con- sidered, since an insulator, as glass or vulcanite, required in construction, may displace dry air which has much higher electric resistance than either, and the insulation be thereby reduced. In such case resistance is in- creased by reduction in the cross-section of the insulator. But if a substance of lower resistance is displaced, in- crease in cross-section of the insulator increases the electric resistance. In its effect, electric resistance in a conductor is similar to the frictional resistance produced in a pipe conveying a fluid, by an accumulation of loose material, such as moss or cotton waste, which obstructs the flow; but in its nature it is very different, fluid matter being trans- mitted by the pipe, but electric energy by the conductor. Insulation and Conductivity. Resistance is the oppo- ELECTRIC MEASUREMENT. 11$ site of conductivity, and very high resistance, when applied to a certain class of bodies, is termed insulation. Conductivity is that quality of a body which facilitates electric transmission, while resistance or insulation ob- structs it. Each varies inversely as the other, but there is no well-defined boundary between them; every con- ductor having a certain amount of resistance, and every insulator a certain amount of conductivity. Where conductivity is found to predominate, as in the metals, the term conductor is applied, and where resistance pre- dominates, as in glass and vulcanite, the term insulator is applied. Silver and copper are metals of the highest conductivity and consequently of the lowest resistance. German-silver and bismuth have high resistance and hence low conductivity. Glass and vulcanite have high insulation and correspondingly low conductivity, so low that the term conductor is never applied to them, nor is the term insulator ever applied to silver or copper. Hence a conductor is any substance of such low resist- ance that it can be used practically for the transmission of electricity, and a non-conductor or insulator is any substance of such high resistance that it can be used practically to prevent such transmission. If electricity is a mode of molecular motion by which energy manifests itself, difference of molecular constitu- tion in different bodies would easily account for these varied results. Such difference might consist in varia- tion in the size, shape, or relative arrangement of the molecules, or in a combination of such causes. Molecu- lar arrangement in a conductor might be such as to produce harmony of movement by which undulations would be rapidly propagated, while in an insulator a different arrangement might produce conflicting move- ments, by which they would neutralize each other and 114 D YNA MIC ELEC TRICI TY A ND MA GNE TISM. thus prevent transmission, as already explained in regard to magnetism in Chapter IV. Electric Current. Current, as stated in Chapter I, is that electric condition in a conductor which results from electromotive force modified by resistance; and its mathematical quantity is ascertained by dividing the former by the latter. It pertains exclusively to what is understood as electric movement, and is used in the same sense when applied to this movement in a conductor, as the same term when applied to the flow of water, steam, gas, or any other fluid, in a pipe; and in this sense also are used the terms current intensity, quantity, volume, strength and resistance; and on this principle all the various kinds of electric apparatus pertaining to current are constructed, and current estimates and measure- ments made. In the present imperfect state of electric knowledge this conventional form of expression is convenient and admissible, provided the distinction between an electric current and a fluid current is kept strictly in mind, the former being a flow of energy, the latter a flow of mat- ter. Our actual knowledge of the nature of an electric current is very limited; the generator creates E. M. F. at one end of the conductor, and a molecular movement is supposed to take place by which electric energy is instantly transmitted. Theoretically this movement is in the form of transverse vibrations, but as a matter of fact its nature is unknown; the only well established fact concerning it being that there is no transmission of a fluid, as was formerly supposed, or of other matter, energy alone being transmitted, using matter as its medium; and it is the effect produced by the energy on this medium, which is known as the electric current, or electricity in process of transmission. Ohm's Law. The law by which the strength of an ELECTRIC MEASUREMENT. 115 electric current is determined was discovered by the German electrician, Ohm, and is briefly as follows : The strength of an electric current varies directly as the electromotive force by which the current is impelled, and inversely as the total resistance encountered. From this law are derived the following formulae by which either of the three factors represented by the symbols C, , R can be found when the other two are known : E> Formula for finding current, C = -=-. J\. Formula for finding E. M. F., E CR. p Formula for finding resistance, R = . Cx Electric Units. In order to render possible the calcu- lations required in estimating electromotive force, resist- ance, and current, certain units of measurement are required, some of which have already been briefly de- fined in connection with batteries. They are appropri- ately named after different distinguished electricians. The International Electric Congress which met at Paris in 1881, and again in 1884, revised these units, giving them a definite value referable to fixed standards, and the units thus established are distinguished as ''legal" and accepted as authoritative. The C. G. S. mechanical unit is taken as the basis of the units by which electric force may be represented in absolute measure. The initial letters, C. G. S., are the symbols of the three factors, space, mass, and time ; C. standing for centimeter, G. for gramme, and S. for second ; hence this C. G. S. unit represents the work accomplished by the movement of a mass equal to one gramme, through a space equal to one centimeter, in one second, and is known as the erg. Il6 DYNAMIC ELECTRICITY AND MAGNETISM. The Volt. The volt is the unit of electromotive force, and was formerly represented by the E. M. F. of a battery-cell nearly equal to that of the Daniell ; but as this is a variable quantity, a definite value was given this unit by the adoption of a quantity represented by 100,000,000 C. G. S. units as its equivalent, which is therefore the amount of electric energy which, if con- verted without loss, would equal this amount of mechani- cal force. But as such a large number is inconvenient to write, the equivalent expression, io 8 , has been adopted in its stead, and the same method of abbreviation fol- lowed in representing the other electric units. Hence the legal volt equals io 8 C. G. S. units of E. M. F. ; but in approximate estimates, the E. M. F. of a Daniell cell, which is about 1.05 volts, is usually sufficiently ac- curate. The microvolt equals T 7n>iT7ro"o ^ a v lt. The Ohm. The unit of electric resistance is the ohm. It was formerly represented by the resistance of a given number of feet of wire of a given gauge ; a very unre- liable standard, requiring a different length for each dif- ferent metal, and subject to great variation from differ- ence of quality or temperature, or slight difference of gauge. The standard resistance adopted by the Electric Con- gress is that of a column of pure mercury, 106 centims. in length, and i sq. millim. in cross-section, at the tem- perature of o C.; which is nearly equal to io 9 C. G. S. units ; the resistance of a similar column, 106.21 centims. in length, being the exact equivalent, but to avoid the fraction the standard was fixed as above. Hence the legal ohm equals io 9 C. G. S. units of resistance. The megohm equals a million ohms. The Ampere. The unit of current strength, or volume, is the ampere, and is derived from the two preceding units in accordance with Ohm's law, by dividing the ELECTRIC MEASUREMENT. II? unit of E. M. F. by the unit of resistance. Hence, since E io 8 i C = ^-, i amp, = ; = icr 9 = . R io 9 io Hence the legal ampere represents current strength equal to ^ of a C. G. S. unit. It does not include time as an element, but refers exclusively to the strength of current flowing in a conductor at any instant, as repre- sented in cross-section at any point. The milli-ampere is the thousandth part of an ampere. The Ampere-Hour. The ampere-hour is a unit derived from the last, in which the element of time is included. It represents a current of one ampere flowing through a conductor for one hour, or its equivalent in a greater current for a less time or a less current for a greater time, as two amperes for half an hour, or half an ampere for two hours. It is of recent origin, but is sanctioned by general use, and is often convenient in electric calcu- altions. The Coulomb. The unit of current quantity with reference to time is the coulomb. It is derived from the ampere, and represents the quantity of electricity which flows for one second with a current strength of one ampere. Hence any variation either in the time or strength of a current produces a corresponding varia- tion in the quantity represented in coulombs, while if one factor varies inversely as the other the quantity re- mains constant; a ten-ampere current flowing for one second or a one-ampere current flowing for ten seconds represents ten coulombs. And since there are 3600 seconds in an hour, 3600 coulombs equal one ampere- hour. The legal coulomb, being derived from the ampere, equals io" 1 , or -fo, of a C. G. S. unit of current quantity - Il8 DYNAMIC ELECTRICITY AND MAGNETISM. The Farad. The electric unit of capacity is the farad. It represents the storage of one coulomb of electricity in a condenser; and when such storage raises the potential to one volt, the capacity equals one farad. The legal farad equals lo" 9 , or T^oo^oVo-oWj of a C. G. S. unit of capacity. The Microfarad. The farad being inconveniently large for practical use in estimating the capacity of condensers, the microfarad, representing one millionth of a farad, has been adopted in its stead. Hence the microfarad equals io- 15 , or y-^o o-jnnroVo ouoiro o> of a C. G. S. unit of capacity. The Watt. The uni*: of electric power is the watt, named after the inventor Watt. It is derived from E. M. F. and current combined, neither of which taken alone is a correct representative of electric power; E. M. F. representing pressure, while current represents pressure modified by resistance ; hence there might be large E. M. F. with small power, or the reverse, in pro- portion to the relative resistance ; or current might remain constant while power varied. Hence, to obtain an accurate expression for electric power, or rate of work, the E. M. F. is multiplied into the current, that is, the volt into the ampere. The legal watt then equals one volt multiplied into one ampere, the product being io 7 C. G. S. units of power, io 8 X io -1 io 7 . The term volt-ampere is synonymous with watt. The Electric Horse-Power. The electric horse-power, which is the equivalent ci the mechanical horse-power, is represented by 746 watts, equal to 746 X io 7 = 7,460,000,000 C. G. S. units of power. Different Kinds of Electric Measurement. The electric measurement here considered pertains to dynamic elec- tricity ; and since much of the apparatus by which elec- tricity in this form is generated, and by which it is ELECTRIC MEASUREMENT. IIQ measured, is constructed with reference to the reciprocal relations between electricity and magnetism, the units are usually termed electromagnetic to distinguish them from electrostatic units, which represent electric force alone, and from magnetic units, which represent magnetic force alone. Instruments for electric measurement are constructed either on the principles of electric attraction and repul- sion, on the relations between electricity and magnet- ism, on the heat developed by the electric current, or on the amount of metal deposited or gas generated by electrolysis. Electrometers. The instruments by which electro- static force is measured are known as electrometers, and measure either the absolute force by which one electri- fied body attracts another by direct movement, as in the attracted-disk electrometer ; or the relative force by which one repels another by a rotary movement, as in the torsion balance; or the combined relative effects of attraction and repulsion by rotary movement, as in the quadrant electrometer. As all these electrostatic instru- ments and methods of measurement are fully described in the author's " Elements of Static Electricity," further reference to them here is unnecessary. Galvanometers. Instruments for electric measure- ment constructed on the principle that the magnetic needle tends to assume a position at right angles to that of the electric current were formerly known exclusively as galvanometers, a term still applied to the older instru- ments of this class, while certain improved instruments recently constructed on this principle are known as volt- meters and ammeters, the former used to measure elec- motive force, and the latter current strength. Instruments indicating the presence and direction of electric currents, as the galvanoscope, Schweigger mul- I2O DYNAMIC ELECTRICITY AND MAGNETISM. tiplier, and astatic needle, have already been described in Chapter IV, but none of these measure current strength, though roughly indicating its amount, while the gal- vanometer, constructed on the same principles, is a much more accurate instrument. Its general construction consists of a magnetized needle, poised so as to have a free horizontal rotary movement, and inclosed within a coil of insulated copper wire through which the electric current can flow ; the strength of the current being measured by the needle's deflection as shown on a graduated circle of 360. Galvanometers are adapted only to the measurement of direct currents, and are but slightly affected by alternating currents. In every galvanometer except the astatic, the needle and the vertical plane of the inclosing coil are set in the plane of the magnetic meridian, so that the deflecting force of the current acts at right angles to the horizontal component of the earth's magnetism, the former tending to rotate the needle into a position at right angles to the direction of the latter. Hence it is evident that the amount of the deflection never can exceed 90, since at this angle the position of the needle is normal to the direction of the current, and the force represented by the angle at a maximum. But the deflecting force does not vary in the same ratio as the angle of deflection, since the needle receives the full effect of this force only when in the vertical plane of the coil, which in this case coincides with that of the magnetic meridian, while in every other position only a portion of this force acts on it, and the strength of this effective portion varies inversely as the angle of deflection. This matter will be better understood, especially by ELECTRIC MEASUREMENT. 121 those not familiar with the measurement of angles, by reference to Fig. 45. Let the line NS represent the needle in the plane of the magnetic meridian, poised at its center C, so that it can be rotated by the deflecting force into the position WE, or any intermediate position ; the force acting on its north pole, N, tending to rotate it toward E, and that acting on its south pole, S, tending to rotate it toward W. When the needle has thus been turned from the position NS r the deflecting force acts on it obliquely, its FIG. 45. effective component on the north pole, when in the position AR, being represented by the line 1C, while the remainder acts along the needle's length, and is not represented by the angle of deflection ; a similar result being true of the deflective force on the south pole ; hence the effective part of this force in the position AR is to that in the position NS, as 1C to NC ; and so when the north pole has been deflected to /?, D, F, or H, the effective part, as compared with that represented by 122 DYNAMIC ELECTRICITY AND MAGNETISM. NC, is represented respectively by the lines JC, KC, LC, and MC. But the effective part represented by each of these lines belongs to a current of increased strength, other- wise it could not produce the increased deflection, and hence, though representing a constantly decreasing incre- ment of the total force, its actual strength is increasing directly as the angle of deflection ; so that the effec- tive part, represented by the short line MC, is as much stronger than the entire deflective force represented by JVC as the angle NCH is greater than zero. Measurement of Angles. Angles are measured by cer- tain functions known as sines, cosines, and tangents. Take any angle, as NCA, and with any part, NC, of one of the inclosing lines, as radius, and the point C, where the lines meet, as center, describe a circle ; and from the point A, where the other inclosing line meets or inter- sects the circumference, draw a line, AI, perpendicular to NC\ the ratio between the length of this line and radius is the sine of the angle. And the length of radius being taken as the unit, the sine is represented by the length of this perpendicular, which therefore is the measure of the angle. Hence each of the lines, BJ, DK, FL, HM, perpendicular to NC, is, like AI, the measure of the angle which it subtends. The length of this perpendicular may vary from radius to zero, but evidently can never exceed radius. The cosine is the ratio between the length of radius and that part of it included between the center and the point where the perpendicular representing the sine meets it. Hence, radius being unity, CI represents the cosine of the angle NCA, and may also be taken as its measure ; the value of the cosine varying inversely as that of the sine. In like manner CJ, CK, CL, and ELECTRIC MEASUREMENT. 123 represent the cosines of the other angles mentioned, and hence measure them. The tangent is a straight line which touches the cir- cumference of a circle, or arc, at any point, but which, if produced, does not cut it, as NT ; and hence it forms a right angle with radius at the point of contact. For any angle less than a right angle, it is included between the line coinciding with radius at the point of contact and a straight line drawn from the center and produced to meet it, and hence it subtends the angle formed by these lines. Thus N$ is the tangent of the angle NC* and each of the lines, ^10, N \$, Nzo, and ^25, the tangent of the angle which it subtends. The length of the tangent varies from zero to infinity; the tangent of a right angle being infinite, since it is perpendicular to one of the inclosing lines and parallel to the other, and hence can never meet the latter. Angular Measurement of Deflective Force. It has been shown in Chapter IV that the horizontal force of the earth's magnetism, by which the needle is deflected, must vary as the tangent of the angle of deflection; but in the galvanometer this force is represented by that of the current flowing in the coil, hence the same rule ap- plies ; so that if the tangent be laid off into equal spaces, as in the figure, and lines from the dividing points be drawn to the center, those spaces must represent equal increments of current strength, though the increments of the circumference included between these lines and also the cosine, which represents the effective component of the deflective force, constantly decrease as the angle of deflection increases. Hence the total deflective force, representing the current's strength, does not vary as the arc through which the needle rotates, but as the tangent of the including angle. For instance, the ratio of strength between a current 124 DYNAMIC ELECTRICITY AND MAGNETISM. producing a deflection of 10 and one producing a de- flection of 20 is not that of 10 to 20, but of tan 10 to tan 20 ; for it requires a current of much more than double the strength to double the arc, since, as already shown, only that portion of the total force represented by the cosine is effective in producing the deflection; but the tangent of 20 is much more than twice the length of the tangent of 10, and represents the total increment of force, effective and non-effective, while the cosine represents only the effective portion. Now since it has been shown that the strength of the effective in- crement varies as the angle, it is correctly represented by the angle's sine. Hence the EFFECTIVE deflective force varies as the COSINE, its STRENGTH as the SINE, and the TOTAL STRENGTH OF CURRENT as the TANGENT of the angle of deflection. Calibration of Galvanometer. A galvanometer may be calibrated by ascertaining from comparison with a simi- lar standard instrument, or otherwise, the different degrees of current strength represented by different degrees of deflection ; and these results being tabulated are a correct guide for the use of the instrument so long as the magnetic strength of the needle remains unim- paired, and the functions of other parts affecting the deflection remain constant. Melloni used the differential deflections of opposite electric currents produced by heat as a means of cali- bration ; and the term seems, perhaps for this reason, to have been derived from thermometric calibration, to which it is analogous. All instruments for electric measurement require jew- elled bearings for the rotating parts, to reduce the fric- tion to the minimum. Sine Galvanometer. The deflective force .may be meas- ured either by the sine or the tangent, according to the ELECTRIC MEASUREMENT. 125 construction of the galvanometer. Where great sensi- tiveness is required the sine galvanometer is preferred. Its essential features are a long needle and an inclosing coil of only sufficient diameter to permit the needle's free oscillation, and which can be rotated horizontally. Its construction will be understood from Fig. 46. needle is mounted at the centre of a vertical coil, FIG. 46. composed of a number of convolutions of insulated cop- per wire wound on a circular grooved brass frame, and underneath the coil is mounted a circle, graduated to correspond to a similar one in proximity to the needle above; the centre of each being in a vertical line with the centre of the needle The upper circle is attached to the frame of the coil, so that both can be moved 126 DYNAMIC ELECTRICITY AND MAGNETISM. horizontally by an index lever, shown below, through any number of degrees indicated on the lower circle by the index. The instrument being set with the plane of the coil in the magnetic meridian, parallel to the needle, which points to zero, and a deflection being produced by the passage of the current to be measured, the needle rotates out of the plane of the coil to a position where the mag- netic field is weaker ; the coil is then turned in the same direction as the needle, its approach producing further deflection, till its plane again coincides with the needle, which again points to zero. In this position the deflec- tive force of the current is evidently just equal to the opposing horizontal force of the earth's magnetism, which would bring the needle back to its original posi- tion if the deflective force were withdrawn. The num- ber of degrees through which the coil has been turned being noted on the lower scale, the sine of the corre- sponding angle indicates the current strength, to which it bears a certain definitely varying ratio, as has been already shown. This mode of measurement is approximately accurate for angles of less than 20, in which the values of the sine and tangent are nearly equal, but is not reliable for larger angles, the difference in those values being too great, as has been shown. Tangent Galvanometer. The tangent galvanometer, though less sensitive than the sine galvanometer, is sim- pler in construction and more accurate for large deflec- tions by strong currents, and hence is generally pre- ferred. Its essential features are a short needle, and a coil of relatively large diameter, varying from ten inches to one meter. The needle is usually about three quar- ters of an inch in length, diamond-shaped, with an aluminium pointer of convenient practical length attached at right angles to its polar diameter. The large diam- ELECTRIC MEASUREMENT. 127 eter and circular form of the coil are to create a field of approximately uniform strength within the small central area in which the needle rotates ; the inner lines of force from the current converging to the centre ; and the needle is made short so as to be confined as closely as possible to this small central space, where the field is most uniform. Fig. 47 shows an instrument of this class ; its coil consisting of a single turn of copper wire, having prac- FIG. 47. tically no resistance, and not requiring a supporting frame. The coil terminals are connected with two tubes shown at the base, one inside the other, insulated from each other and furnished with binding-screws. The needle-case is from four to five inches in diameter. The tangent values are sometimes laid off on the scale of the instrument in the manner shown in Fig. 45 ; 128 DYNAMIC ELECTRICITY AND MAGNETISM. but as it is difficult to do this with requisite accuracy, the scale of degrees is usually preferred, the values of the tangents corresponding to the deflections being easily ascertained from a table. The Helmholtz-Gaugain tangent galvanometer is illustrated by Fig. 48. The needle is placed at the cen- tre of a straight line connecting the centres of two separate coils of equal size, set parallel to each other at a distance apart equal to their radius. This arrange- FIG. 48. ment insures much greater uniformity of field than can be obtained from a single coil, and still greater uni- formity could be obtained by the addition of a third coil, midway between the two, and of such diameter that each of the three should be equally distant from the centre of the needle. The two-coil method was pro- posed by Helmholtz, while Gaugain proposed placing ELECTRIC MEASUREMENT. 129 the needle in the same relative position on one side of a single coil. The instrument here shown has two sets of coils, marked A and B, four in all, connected with binding- screws at the base of each circular support, as shown, one coil of each set on each support. The A set has very low resistance, only a small fraction of an ohm to each coil ; each being composed of about four turns of No. 12 copper wire. The B set has very high resistance, 10 to 12 ohms to each coil, each composed of No. 26-30 wire. The needle is suspended by an untwisted silk fibre inclosed in a vertical tube, and adjusted by the screws shown at top, so that it rotates without friction and against only slight torsion. Astatic Galvanometer. A very sensitive galvanometer, originally invented by Nobili, may be constructed with the astatic needle described and illustrated on page 73 ; which, being approximately independent of the earth's magnetism, is deflected by a very slight current. Fig. 49 shows the construction. Two short needles with poles reversed are attached to a common support, which also carries a light pointer of convenient length. The coil is flat and usually of sufficient hori- zontal diameter to inclose the lower needle entirely in any position; while the upper needle rotates over its upper surface, and the pointer over a dial-plate with scale above. The needle is suspended at the centre of the coil from a vertical sup- port by a single fibre of silk, or by two parallel fibres hung near each F other; the latter method being known as bifilar suspension, its object being to bring the needle I3O DYNAMIC ELECTRICITY AND MAGNETISM. to rest in a fixed position more perfectly than can be done by the torsion of a single fibre; the needle being raised slightly when, by its deflection, the two threads are twisted out of parallelism, and its weight tending to bring them back to the parallel position. The suspen- sion is adjusted by the thumb-screw shown above; the needle being set parallel to the vertical plane of the coil; and as it is impossible to make a needle perfectly astatic, both should also be parallel to the plane of the magnetic meridian. A glass shade affords protection from air-currents. The readings, for the reasons already given, are only approximately accurate, and for deflections greater than 20 unreliable; but the instrument can be cali- brated for larger deflections. Thomson's Reflecting Galvanometer. This instrument, invented by Sir William Thomson for telegraphing through long submarine cables, is exceedingly sensitive. Its construction is shown by Fig. 50; its principle being practically that of a tangent galvanometer with a long pointer and tangent scale. At the center of a line connecting a pair of small coils of equal size and resistance, is suspended, by a silk fibre, a diminutive concave mirror, of about i centim. diameter, with a little needle, made usually of a piece of watch- spring, attached to its back; the weight of both not ex- ceeding one or two grains. A small circular opening in the case, directly opposite the mirror, widening out- ward, admits the light from a lamp connected with the graduated scale shown in Fig. 51. This scale is placed in front of the galvanometer, at a distance of about 36 inches, and the lamp is placed in the box at the right which excludes the direct rays. The light is transmitted through a tube terminating in a small circular opening, from which a beam falls on ELECTRIC MEASUREMENT. \l\ the small mirror shown just below the centre of the scale, and is reflected to the galvanometer mirror, and thence back to the scale; the mirror adjustments being such that a small spot of light, concentrated by a lens in the galvanometer, shown in the cut, is reflected on FIG. 50. zero of the scale, when no current is passing, and moved to the right or left, according to the direction of the current, to a distance corresponding to the current strength. The shadow of a fine wire, stretched in front 132 DYNAMIC ELECTRICITY AND MAGNETISM. of the galvanometer mirror, indicates the exact centre of the spot of light, which is adjusted to zero by a curved magnet, attached above to a vertical rod, with its poles in opposition to those of the needle, and which can be moved to any required position vertical or horizon- tal. The pointer being the ray of light, 36 inches long, the slightest deflection is prominently indicated on the scale ; a current produced by dipping the points of a FIG. ST. brass pin and a sewing-needle into a drop of salt water, moving the spot of light half the length of the scale. The coils can be removed and coils of any required resistance up to 5000 ohms substituted ; and as these and similar delicate measuring instruments are liable to injury from powerful currents, which also produce de- flections too great for accurate measurement, shunts of fine wire are provided, separate from the instrument, by which fractions of the current, of measurable strength, are transmitted ; and the respective resistances of coil and shunt being known, the entire current strength can. t>e ascertained. ELECTRIC MEASUREMENT. 133 The requisite light can be furnished by an ordinary kerosene lamp, but that of an electric lamp or a lime burner, when obtainable, is far superior. Fig. 52 shows another style of the same instrument FIG. 52. with four coils in two sets, upper and lower, having any required resistance up to 8000 ohms. Differential Galvanometer. This instrument is con- structed with two coils of equal size and resistance, be- tween which the needle is mounted at the central point, and through which currents may be transmitted simul- taneously in opposite directions and their relative 134 DYNAMIC ELECTRICITY AND MAGNETISM. strength compared : if equal, there is no deflection ; but if unequal, the relative difference in strength is shown by the amount of the deflection. Ballistic Galvanometer. A ballistic galvanometer is one constructed with a needle weighted by inclosing it in lead or otherwise, so that the impulse given it by a transient current of too short duration to be measured in the ordinary way may be developed slowly by the needle's momentum, so that the amount of deflection can be more easily observed. When used to measure current quantity, as indicated by current strength in the discharge of a condenser, the sine of half the angle of deflection produced by the first swing of the needle is taken as proportional to the quantity of the transient current thus produced. Common Galvanometers. Galvanometers of various styles and sizes are constructed for ordinary practical use, usually with flat coils of various degrees of resist- ance. Such instruments are often better adapted to measurements where only approximate accuracy is required than those of finer construction, but are not suitable for strict scientific work. Voltmeters and Ammeters Galvanometers measure only current strength, usually in degrees of an arc, but it has become important in the progress of electric de- velopment to measure also electromotive force, and to express the measurements of both E. M. F. and current strength in volts and amperes, either directly or in terms easily reducible to those units : for this purpose voltmeters and ammeters are constructed. The difference between these two instruments con- sists chiefly in the respective resistance of each, and its relative position in use ; the voltmeter having high re- sistance and being placed in a derived circuit between the points whose difference of potential is to be meas- ELECTRIC MEASUREMENT. 135 ured, while the ammeter has low resistance and is placed directly in the main circuit at any point where current strength is to be measured. It will be noticed that an unmagnetized, soft-iron needle, or armature, is an important feature of many of these instruments. The Weston Voltmeter. This instrument, shown by Fig. 53, incloses within its case a powerful steel horse- FIG. 53. shoe magnet, the poles of which project into the narrow space in front and are attached to two soft iron pole- pieces, as shown in Fig. 54. These inclose a circular space, within which is mounted a soft-iron armature core, maintained in a fixed central position by attach- ment to a brass yoke which connects the pole-pieces ; part of this yoke, with its right-hand connection and a central projection for attachment of the core, being shown. A light copper frame, f of an inch wide, and wound with a coil of fine, insulated copper wire, surrounds the core, and has a limited rotary motion, on jewelled bear- ings, in the narrow space between the core and pole- 136 DYNAMIC ELECTRICITY AND MAGNETISM. pieces, which is just wide enough to allow rotation with- out contact. The terminals of the coil are connected above and below with two flat springs, oppositely coiled, and so attached to the copper frame and adjoining parts as to maintain the coil in a fixed position, when the springs a"e not under tension, and bring a light aluminium FIG. 54. pointer, attached to the frame, to zero of the scale on the left. These springs are made of a special, non-magnetic alloy, and are placed in opposition to neutralize the effects of expansion and contraction under variations of temperature. A resistance coil, mounted within the case, makes electric connection, by one of its terminals, with one of ELECTRIC MEASUREMENT. 137 .iie springs, while the other terminal is connected with the front binding-post on the left. Another connection with the rear binding-post on the same side taps this coil at a point nearer the spring, so as to include a much lower resistance. The other spring is connected with the binding-post on the right, back of which is a contact key and a calibrating coil. This part of the circuit can be closed permanently, after calibration, by depressing the key and giving it a quarter-turn. When connections with an electric source are made by the right binding-post and either of the two on the left, the current enters and leaves the copper coil through the springs, its direction and the winding being such as to produce deflection from left to right; the coil tending to rotate into a position at right angles to the lines of magnetic force, in opposition to the tension of the springs. And the instrument being calibrated in ac- cordance with the resistance of its coils, the deflection of the pointer will indicate the difference of potential in volts; since with a given resistance the E. M. F., or po- tential difference, varies directly as the current strength. The entire resistance is to that of the sectional part in the ratio of 20 to i; the divisions of the scale being in volts for the outer reading, corresponding to the high resistance, and the same in twentieths of a volt for the inner reading, corresponding to the low resistance, as shown. Hence the E. M. F. which will produce a de- flection of one division, when connection is made with the front binding-post on the left, will produce a deflec- tion of twenty divisions when connection is made with the rear binding-post. The high-resistance circuit is used for apparatus gen- erating strong currents, as dynamos, and the low-resist- ance circuit for apparatus generating weaker currents, as primary batteries, on account of its greater sensitive- 138 DYNAMIC ELECTRICITY AND MAGNETISM. ness: and as a dynamo current would be likely to injure or destroy the copper coil, if admitted through the low resistance, the rear post is protected from accidental contacts by an outer covering of hard -rubber. In some of the instruments all the posts are similarly protected; the rubber also preserving the contacts from oxidation. The scale readings also vary in different instruments. The deflection of a current-bearing coil in a magnetic field of special strength gives this instrument great superiority over instruments depending on the deflection of a steel or soft-iron needle; the magnetic action being stronger, and its relation to the current more direct. The constancy of the instrument is dependent solely on the constancy of the magnet, the springs, and the inter- nal resistance. The Weston Ammeter. The construction of the Wes- ton ammeter is similar to that of the voltmeter, but simpler ; the chief differences being that the copper coil is of coarser wire, having much lower resistance, and the resistance coil is not required: hence there are only two binding-posts and a single circuit, directly through the copper coil and springs. The scales for different instruments range from 5 amperes, with divisions of ^V f an ampere, to 100 am- peres, with divisions of i ampere, according to the rela- tive resistance of the coils. The Weston Milliammeter. This instrument has the same construction as the ammeter but lower resistance. Instruments of two different resistances, with scales of corresponding difference, are constructed; one of 300 milliamperes, with scale divisions of 2 milliamperes each; and the other of 600 milliamperes, with scale divisions of 4 milliamperes each. A milliampere being T -gVo- f an ampere, it is evident that these instruments are capable of measuring very ELECTRIC MEASUREMENT. 139 low currents, especially as the scale divisions are read- able to fifths; so that the smaller instrument can indi- cate a current of \ of 2 milliamperes, -^Vir of an ampere. The Wirt Voltmeter. This instrument, illustrated by Fig. 55, is constructed on the principle of ascertaining the E. M. F. to be measured by comparison with a known FIG. 55. E. M. F. ; each being proportional to a resistance having similar conditions through which the measurement is made. The case incloses two Clark cells, each having a con- stant E. M. F. of 1.43 volts, the connections being so arranged that either can be employed alone, or the two joined in series so as to obtain an E. M. F. of 2.86 volts. I4O DYNAMIC ELECTRIC!!^? AND MAGNETISM. Under the glass cover is shown a small galvanometer, with magnetic needle, light aluminium pointer, and terminal wires connected with the coil; also a small scale, not shown, under the pointer, having a limited range, in opposite directions, from o at the centre. Extending round the case inside is a coil of german- silver wire, having a resistance of about 2500 ohms, one terminal of which is attached to one of the binding-posts shown on the right, marked -)-, while a sliding contact, which can be moved to any required point on this coil, is connected with the other binding-post, marked ; and this contact is attached to the rim of the hard- rubber cap, shown above, which can be rotated on the interior part of the cap, on which is shown a scale graduated in volts, from ij to 120. By rotating this rim, a short index, attached to it, is moved to any re- quired point on the scale, the sliding contact being moved simultaneously, so as to include any resistance required between the terminals of the binding-posts. The galvanometer circuit also includes a certain por- tion of this coil, having a known resistance calibrated with reference to the known E. M. F. of the battery cells, which are also included in this part of the circuit. A contact key, shown on the left, closes this circuit through the galvanometer, producing deflection of the needle and attached pointer. If connection with a generator whose E. M. F. is to be measured be made through the binding-posts, so that the current shall oppose the meter's battery cur- rent, the needle will be deflected, when the contact key is closed, so long as the generator current is stronger or weaker than that of the battery. Let the instrument be so placed that the earth's mag- netism shall bring the galvanometer pointer to o on the small scale ; and let the rim be turned so as to bring the ELECTRIC MEASUREMENT. \^\ attached index near the probable E. M. F. on the large scale ; then, deflection being produced by closing the contact key, let the rim be turned so as to include suffi- cient resistance to equalize the opposing currents and bring the galvanometer pointer back to o ; the index will then show the E. M. F. of the generator in volts on the large scale. For, since with a given current, E. M. F. varies directly as resistance, if the E. M. F. of the bat- tery be represented by E and that of the generator by E'y the resistance of the battery circuit by R and that of the generator circuit by R', then R : R' : : E : E' . That is, the resistance of the battery circuit is to the re- sistance of the generator circuit as the E. M. F. of the battery is to the E. M. F. of the generator, and the calibration gives this E. M. F. in volts. A switch is shown in front by which connection can be made with either of two separate circuits, the right- hand contact, marked -fa to indicate the relative meas- urement of E. M. F., connecting with one having ten times the resistance of that connected with the left-hand contact. At the opposite corner, in the rear, three bat- tery connections are arranged, the right and left ones, marked A and B, being each through a separate cell, and the central one, marked 2, through the two cells in series ; a plug closing whichever connection is to be used. When the switch is on contact i, as shown, and the plug in A or B, the scale readings require no cor- rection, and should be the same with the plug in either hole, each cell being a check on the accuracy of the other. But when the plug is in hole 2, the cells being in series, the reading must be multiplied by 2, since the battery E. M. F. is doubled ; for R : R' : : *E : 2E'. But when a generator of low E. M. F. is to be tested, the switch is connected with the contact marked fa, which includes, in the battery circuit, a resistance of ten 142 DYNAMIC ELECTRICITY AND MAGNETISM. times that included by contact i ; hence, since the bat- tery current with this resistance is only ^ of what it was with the former resistance, y 1 ^ the E. M. F. will de- velop an opposing current of equal strength, giving the same reading, which must be divided by 10 to give the correct E. M. F. ; for \vR \ R' \ \ loE : E' . Each cell is if inches high and f of an inch in diam- eter, constructed with an inverted glass cup, inclosed in a brass case and hermetically sealed with soft rubber melted into the bottom. The electrodes are zinc and mercury, and the fluid zinc sulphate and mercuric bisulphate, formed into a paste in which the electrodes are inclosed ; connection with the mercury being made by an insulated platinum strip which represents the positive pole. This cell is selected on account of the remarkable con- stancy of its E. M. F., and the instrument is calibrated for a cell temperature of 21 C., requiring a correction in the reading of .000367 per degree of variation above or below 21 C., which must be made by subtraction for the higher temperature, and by addition for the lower. The cells are easily removed and replaced, when necessary, without disturbing the connections; and being small, hermetically sealed, and amply protected, do not interfere in the least with the handling of the instru- ment, and can be cheaply replaced when exhausted. Ayrton and Perry's Spring Voltmeters and Ammeters. The unreliability of electric measuring instruments con- structed with permanent magnets, liable to magnetic loss, or to variation of magnetism from the influence of powerful currents, and consequently requiring frequent recalibration, has led to improved methods of construc- tion, of which the spring voltmeters and ammeters of Ayrton and Perry are a result. Fig. 56 represents the ammeter, the voltmeter being of similar construction ; ELECTRIC MEASUREMENT. 143 the principle being simply the torsion of a spring by electromagnetic attraction. The current passes through a long, narrow vertical coil, of high resistance in the voltmeter and low resistance in the ammeter, within which is suspended a light soft- iron tube, which incloses a long spiral spring of phos- phor-bronze ribbon. This spring supports the tube, being attached at bottom to a brass cap in which the tube terminates, and above to a milled head which rests on the glass cover and is connected with the spring by a FIG. 56. vertical pin which passes through the glass ; a similar pin projects downward from the bottom of the brass cap and passes through a hole in a support below, in which it has a free vertical movement ; so that the two pins hold the spring and tube in a vertical position ; and the tube being shorter than the coil, its centre on a vertical 144 DYNAMIC ELECTRICITY AND MAGNETISM. line is above that of the coil. To the top of the tube it attached a light pointer which rotates over a scale graduated either in volts or amperes according to the design of the instrument. When no current is passing the pointer indicates zero on the left of the scale, but when the current passes, the tube is pulled down by magnetic attraction, in oppo- sition to the torsion of the spring, to a distance pro- portional to the current's strength ; giving it a rotary motion by which the pointer is deflected, which indicates by direct readings the E. M. F. in the voltmeter, and the current strength in the ammeter, according to the respective resistance of each instrument, and its position in the electric circuit. The tube can be turned by the milled head so as to bring the pointer to the required position in calibrating ; and a reflected image of the pointer, in a mirror placed under it, enables the observer to determine accurately its position on the scale. A little magnetic needle, shown at the front corner of the base, indicates the direction of the current; but as such a needle is liable to have its poles reversed by powerful currents, a bar magnet is preferred for this purpose. Since the deflection of the pointer depends on the magnetic attraction of the tube downwards, it must evidently be always in the same direction, and hence in- dependent of the direction of the current ; so that while this direction may be ascertained as above, it is not essential to the use of the instrument that it should be known. A light movable auxiliary coil surrounds the main coil and is connected with it in parallel ; this can be moved up or down in calibrating till a position is reached in which its inductive influence on the main coil ELECTRIC MEASUREMENT. 1 45 is best adapted to the construction, where it is made stationary. The case is ventilated, as shown, to prevent the ac- cumulation of heat generated by the current, which would expand the spring and produce inaccuracy. The usual binding-posts connected with the terminals of the coil are shown at the right and left, the left post being marked A to distinguish them in use. The voltmeters are usually constructed to measure E. M. F. ranging from 15 volts to 1000 ; the ammeters, to measure current strength ranging from ^ of an ampere to 600 amperes. Gravity Ammeters. While springs have greater con- stancy than permanent magnets in the construction of electric measuring instruments, their constancy is liable to vary, or be impaired, from well-known causes, as heat- ing, age, and use, imperfect material, or oxidation ; but the force of gravity, being always known and constant, may be utilized in such construction to produce instru- ments of grert constancy. On this principle the United States Electric Lighting Company constructed the am- meter shown in Fig. 57. Two pairs of electromagnets, wound with coils of low resistance, and having laminated soft-iron cores, are placed as shown ; each pair having its coils wound on the same core, producing consequent poles, but mag- netically insulated from the other pair. At the centre, between these magnets, is mounted a soft-iron armature, lightly poised on a horizontal axis, the end of which is seen through the circular opening, and having a vertical rotary movement parallel to the mag- nets' plane. This armature is about 2 inches long, ij inches wide at each end, f of an inch at the centre, and \ of an inch thick ; its sides concave, and its ends con- yex and slotted to correct the effects -of residual mag- 146 DYNAMIC ELECTRICITY AND MAGNETISM. netism. A pointer, attached to its axis, indicates the readings on a scale above, as shown. When no current is passing, the armature is main- tained in a fixed position by one or more little weights attached to its lower left-hand corner, its longer axis FIG. 57- being on a diagonal line between the lower left and upper right-hand corner of the instrument, and the pointer at zero on the left of the scale. But when the current passes through the coils in either direction, the armature rotates in obedience to the electromagnetic force, its longer axis tending to assume a horizontal ELECTRIC MEASUREMENT. position, and the pointer is deflected from left to right in proportion to the current strength, which is indicated by direct reading in amperes. By the removal or addition of one or more of the little weights, the sensitiveness of the instrument may be varied in calibrating, as required for different ranges of current strength. The terminals of the coils are shown at the base, and holes for ventilation at the top of the case. Instruments constructed on this principle have not been employed to any great extent as voltmeters, not being sufficiently sensitive for the light currents required. Since the weight, as it rises recedes from the vertical line which passes through its axis of rotation, the force opposing rotation increases in the direct ratio of the increase of leverage thus produced. Hence, as equal divisions of the scale would represent unequal increments of current strength, they should be made in the inverse ratio of this increase of leverage. But as it is difficult to mark off such short spaces with the requisite accuracy, a gravity ammeter has been con- structed by the Western Electric Company, with a ver- tical electromagnet having a pole-piece so curved that the rotating armature, as it rises, constantly approaches it, the magnetic attraction increasing in the same ratio as the leverage, so that equal divisions of the scale represent equal increments of current strength. The Cardew Voltmeter. The instruments thus far de- scribed are designed to be used with direct currents, and are liable to errors arising from self-induction in addition to those from the other causes mentioned. But since, according to a well-known law, the heat devel- oped in an electric conductor is in direct proportion to the square of the strength of the current passing through it, instruments can evidently be constructed on 148 DYNAMIC ELECTRICITY AND MAGNETISM. this principle which will measure either current strength or difference of potential, produced either by direct or alternating currents, and are not liable to variation from any of the causes mentioned. Among these the volt- meter, patented by Cardew in 1886, has a prominent place. Its operation depends on the expansion of metal produced by the electric development of heat. Fig. 58 gives a front view of this instrument and Fig. 59 a rear view, showing its internal construction. A fine FIG. 58. platinum wire, 8 feet long, is stretched in four lengths in a horizontal tube, by attachment to a metal frame and pulleys, as shown at a, a, t, t in Fig. 59. This tube is made of very thin metal, one third of its length being iron and two thirds brass, to maintain constancy of length between the points of attachment of the wires by such a mode of connection as to produce compensation ELECTRIC MEASUREMENT. 149 by the unequal expansion of the two metals ; and the horizontal position is given it to maintain constancy of temperature, and prevent the unequal expansion, from FIG. 59. convection of the air to which the tube and wire would be liable in a vertical position. The wire has a resistance of about 240 ohms, and at- tains a maximum temperature of about 200 C.; and its expansion varying in a certain definite ratio dependent on the difference of temperature caused by the passage of the electric current, which, as stated, varies as the square of the current's strength, produces a variation in length proportional to the E. M. F. by which the current is generated. This- produces a rotation in the pulley #', to the axis of which the pointer shown in Fig. 58 is attached, which moves in the same direction as ISO DYNAMIC ELECTRICITY AND MAGNETISM. watch-hands when the E. M. F. increases, and in the opposite direction when it decreases. This instrument should be calibrated for the average temperature of the room in which it is to be used. The Edison Current-Meter. Instruments for measuring the amount of electric current used by a consumer of light or power are constructed on various principles. Among these is the Edison current-meter, in which a small per- centage of the current is passed through two cells con- taining amalgamated zinc plates immersed in a solution of zinc sulphate. Zinc is thus deposited on the plates, which are removed and weighed at stated times, and the consumption of current being in proportion to the amount of deposition, according to the principle dis- covered by Faraday, is estimated accordingly. FIG. 60. The Forbes Coulomb-Meter. Meters like the Edison ELECTRIC ME A SUREMENT. \ 5 j cannot be used for the measurement of alternating cur- rents; but one has been invented by Forbes, operated by the heat developed by the current, which can meas- ure either direct or alternating currents. Its construc- tion is shown in Fig. 60. The current passes through a flat coil of iron wire, above which is mounted, on a paper cone having a jeweled bearing at its apex, a mica disk, with mica vanes attached. The heat developed by the current produces an ascending current of air which rotates the disk, operating a light train of clock-work which moves indexes over two dials, regis- tering the current consumption in coulombs; units being registered on one dial and tenths on the other. A glass shade protects the apparatus from external air-currents. Voltameters. Instruments like the Edison current- meter are more generally known as voltameters, a name given them by Faraday, who first proposed this method of electric measurement. They may be constructed with any substance practically susceptible of electrolysis, in accordance with Faraday's law that the amount of an element liberated by electrolysis in a given time is proportional to the strength of the current employed. Salts of copper and of silver are both employed for this purpose, also acidulated water. The Water Voltameter. This is simply a common de- composing instrument in which the liberated elements, oxygen and hydrogen, are collected in the same receiver, which is graduated in cubic centimeters or any other convenient standard. The amount of each gas pro- duced at a standard temperature and pressure, by a coulomb of electricity, being known, the entire number of coulombs consumed in a given time can easily be as- certained. This amount, at temp. o C. and press. 760 nullims., is found to be 0.0579 cubic centims. of oxygen 152 DYNAMIC ELECTRICITY AND MAGNETISM and 0.1157 of hydrogen, making 0.1736 c.c. of both, per coulomb of electricity. The use of such an instrument is confined to the labo- ratory, as the wasteful consumption of current, the re- sistance due to polarization, and the loss from recombi- nation of the gases, or escape of the hydrogen, renders it unsuitable for practical measurement. The Weber-Edelmann Electrodynamometer. This in- strument, invented by Weber and improved by Edel- mann, is constructed on the principle of the deflection of a coil, in opposition to the torsion of a wire, by the joint product of E. M. F. and current strength. Fig. 61 shows the construction. Two coarse wire coils of low resistance are mounted parallel to each other on a stand, on three transverse brass rods, sup- ported by a vertical brass ring, at the centre of which is suspended a small, fine wire coil of high resistance; its plane, when at rest, being at right angles to the planes of the larger coils. A small plane mirror is attached to the centre of the small coil, to which a ray of light from a lamp is admitted through an aperture in the lit- tle screen shown in front of it. The suspension of the small coil is by means of a wire connected with its terminals and inclosed in the vertical brass tube shown. This wire is attached to the projecting rods seen at the top of the upper section of the tube and the bottom of the lower section ; the set- screws and nuts shown being used to give proper adjust- ment to the coil and tension to the wire ; the terminal rods passing through movable disks for this purpose. The current from the generator enters by one of its circuit terminals, attached to a binding-screw at the bottom of the lower section of the tube, passes up through the inclosed wire and traverses the small coil, goes thence through the upper section of the wire and ELECTRIC MEASUREMENT. 153 returns by the upper section of the tube to the ring, passes through one of the rods to a terminal of one of the larger coils, traverses that coil and returns by FIG. 61. 154 DYNAMIC ELECTRICITY AND MAGNETISM. another rod to the other large coil, and traversing it, passes out by a binding-screw to the generator through the other terminal of the external circuit. Proper insulation and connections are provided be- tween the rods, coils, and supporting ring to insure the passage of the current as above ; and its direction may be reversed by reversing the connection with the ex- ternal circuit. The current in the three coils has practically the same E. M. F., but the difference in resistance gives the high- resistance coil small current strength and the low-re- sistance coils large current strength, so that the current of the small coil represents chiefly E. M. F., and that of the larger coils, current strength. When the current passes, its combined effect in the three coils, as represented by the product of the small current into the large, or E. M. F. into current strength, tends to bring the plane of the small coil into a position parallel to that of the other two; the amount of deflec- tion being indicated on a scale by a ray of light reflected from the little mirror, and observed through the aper- ture shown just above the ring. As this deflection represents the product of the E. M. F. into the current strength, the voltage into the amperage, it shows the electric power of the current as indicated in watts ; hence the instrument is appropriately named electro- dynamometer or electric-power-measurer. It can be used either with the direct or the alternating current, and is especially adapted to the latter, having no magnetic needle. Measurement of Electric Resistance. Since current strength depends on the mutual relations of electro- motive force and resistance, it is evident that apparatus for varying resistance by the introduction or withdrawal of a definite known quantity, and of ascertaining and ELECTRIC MEASUREMENT. 1 55 measuring it when unknown, in order to properly adjust these mutual relations, is a matter of the highest im- portance in electrical construction. Resistance may be varied, as already shown, by varying the length or diameter of the conductor, or by changing the circuit from series to parallel or the reverse ; but as this usually requires permanent construction, it becomes necessary to have also some simple means by which a resistance of known amount can be promptly introduced into any circuit or withdrawn from it without interference with the permanent construction : this is furnished by the resistance coil, or rheostat as it is also termed. Resistance Coils. Resistance coils are made of ger- man-silver wire on account of its high resistance, which is usually about seventeen times that of pure copper, and calibrated as to gauge and length for a given number of ohms resistance, the wire being properly wrapped for insulation. Fig. 62 gives an ideal view of the construc- tion. X, F, and Z are short blocks of brass, insulated from each other above, but connected below through the coils c and ^, as shown ; each coil being wound with a double strand to reduce self- induction. Two brass plugs, a and , having hard-rubber handles, fit into holes between the blocks so that when placed as shown, the three . . FIG. 62. blocks are in electric connec- tion, and having practically no resistance, a current would pass directly through them, without traversing the coils. But if a plug, as a, is removed, the current between X and Kmust then pass through the coil c. In like manner if plug b is removed, the current between I$ DYNAMIC ELECTRICITY AND MAGNETISM. Y and Z must pass through the coil d\ which, being twice the length of c y would have twice the resistance if made of wire of the same gauge, or four times the re- sistance if also the cross-section of the wire were one half that of c. In this way resistance can be varied to any practical extent required. Sets of resistance coils, calibrated for resistances vary- ing from i ohm or less to 10,000 or more, are con- veniently arranged in cases, as shown in Fig. 63. The FIG. 63. case has a hard-rubber cover by which the brass blocks are insulated above, each pair being connected through a coil below, as shown in Fig. 62. A hole in the centre of each block receives each plug when removed from between the blocks, to prevent its being mislaid, and connection with the electric circuit is made through the binding-posts shown at the right. To introduce any required resistance it is only neces- sary to remove the plug from its place between the blocks opposite which the resistance required is marked on the cover, the other plugs all remaining connected. If, for instance, i ohm resistance is to be introduced, let ELECTRIC MEASUREMENT. IS? the first plug at the front right-hand corner be re- moved, opposite which " i ohm" is marked; the current must now flow through that coil, and pass by all the other coils, through the blocks and plugs; if 50 more ohms are to be added, the last plug at the rear left-hand corner is removed, opposite which is marked " 50 ohms;" and the resistance then becomes 51 ohms. The Wheatstone Bridge. The Wheatstone bridge is an instrument for measuring an unknown resistance by comparison with a known resistance. Fig. 64 gives an ideal view of its construction. Let A, B, C, D be four wires connected at the points P, Q, M, N, and let M and N be connected with the galvanometer G, and P and Q with the battery X, by which a current can be sent from P to Q. This current will divide at P, and the portion FIG. 64. passing through each branch of the circuit will be in- versely proportional to the respective resistance of each. Now it is found that the potential between any two points in an electric circuit varies inversely as the re- sistance between them; and as the E. M. F, between any two points is represented by their potential differ- ence, the E. M. F. at M would vary as the ratio of re- sistance in C to that in Z>, and the E. M. F. at Was the 158 DYNAMIC ELECTRICITY AND MAGNETISM. ratio of resistance in A to that in B; if these ratios are equal, then the E. M. F., or ejectric pressure at M, is equal to that at N, irrespective of the amount of current in each branch, and no current can pass between these points, and hence there can be no deflection of the gal- vanometer needle. But if either ratio differs from the other, then current will pass between M and 7V in pro- portion to this difference and produce deflection. Suppose this difference to be caused by the introduc- tion of an unknown resistance into the arm D\ then by varying the resistance in B till the deflection disappears, equality between the ratios is restored, and as the resist- ances of A, B, and C are known, that of D may be com- puted; for, allowing the letters to represent the resist- ances, Since C : D : : A : B, AD = BC, and D = - A In like manner, when the respective resistances of any three of the arms are known, that of the fourth may be ascertained. The total resistance or total current in either branch, or the equality or inequality of resistance or current in the arms, are matters of indifference, equality of ratios, as above, being the principle of construction. As the potential decreases from P to Q in both branches of the circuit, it is evident that if an unknown resistance greater than that of D were substituted for jD's resistance, the effect would be to reduce the poten- tial difference, or E. M. F., between C and D, producing deflection of the needle by a flow of current from N to M, and requiring proportional increase of resistance in B to restore the equilibrium. But if this unknown resistance were less than that of D, the effect would be to increase the potential difference between ELECTRIC MEASUREMENT. 159 producing deflection by a flow of current from M to TV, and requiring proportional decrease of resistance in B. This instrument may be constructed in any convenient form in which the mutual relations of the different parts to each other are properly maintained; and sets of resistance coils may be so connected with the different arms as to vary the resistance as required. Fig. 65 shows a convenient, practical form. On an insulating strip of hard rubber are mounted five copper strips furnished with binding-screws; and between the two end strips is stretched a wire, connected with them, made of a compound metal composed of 85 parts platinum and 15 parts iridium, having high resist- ance and not easily oxidized; and parallel to it is a graduated scale on w r hich the resistances of equal divisions of the wire are marked in ohms, after proper calibration. The arms and connections for the battery and galvanometer are lettered in the cut to correspond to the lettering in Fig. 64. The arm A extends from Q round to N, including a section of the resistance wire, and the arm B from P round to N, including the re- maining section; arm C, from Q to J/, and arm D, from M to P: the battery connections being at P and Q, and the galvanometer connections at M and N. The con- nection at TV is made with a slide, mounted on the re- sistance wire, to which is attached a pointer which indicates on the scale the amount of resistance included in each of the arms A and B, The unknown resistance which is to be measured can be inserted either at C or l6o DYNAMIC ELECTRICITY AND MAGNETISM. D, as preferred, the remaining space being then filled with a known resistance. When deflection of the needle is produced by the in- sertion of an unknown resistance at either of those points, a movement of the slide, either to the right or left as required, changes the relative resistances of the arms A and JB, and restores the equilibrium by making the ratio of resistance between A and B equal to that between C and D; and the value of the former ratio being indicated on the scale, the value of the unknown resistance can be ascertained, as already explained. Keys are provided in the battery and galvanometer circuits by which each circuit can be opened or closed as required; the battery circuit being always closed first and opened last, to avoid the violent oscillation of the needle due to the extra current produced by self-induc- tion on opening or closing a circuit. Fig. 66 shows a very elaborate instrument, combining the galvanometer and a set of resistance coils, by which resistances from one hundredth of an ohm to a million ohms or more can be measured. The resistance to be measured is connected with the two binding-posts on the left, the battery with the two on the right. Resistance coils ranging from o to 10,000 ohms are arranged in four rows of ten each, marked respectively " units," "tens," " hundreds," and "thou- sands;" and in front of the galvanometer are two rows, A and -B, of three each, the corresponding ones on each side marked respectively " 10," " 100," and " 1000." In the long rows, each of the ten coils in the same row has the same resistance; each in units' row having one unit, each in tens' row one ten, and so on. But in the short rows, each coil has the resistance marked on its bolt. The coils in each long row are connected to- gether in series by the bolts, each coil being connected ELECTRIC MEASUREMENT. 161 with two bolts by its opposite ends. Parallel to each row of bolts and insulated from them is a brass bar, FIG. 66. having practically no resistance; and each of the three bars, marked " units," " tens," and " hundreds," is elec- 1 62 DYNAMIC ELECTRICITY AND MAGNETISM. trically connected underneath, at the left, to the row of bolts in front of it by the bolts marked o. When plugs are placed in each of the four holes at the left, opposite the bolts in the four long rows marked o, the current passes directly through the four bolts, plugs, and ends of the bars thus connected, without passing through any of the coils; but if a plug is removed to the right, then the current must pass through all the coils to the left of it in that row and introduce the resistance indicated by the number on the bolt and the word on the connected bar in front of it. For instance, if a plug connects units' bar with bolt 4, as shown, the current passes through coils i, 2, 3, and 4, introducing four units of resistance; in like manner the plug connecting bolt 6 with tens' bar introduces 6 tens, bolt three connected with hundreds' bar 3 hundreds, and bolt 7 connected with thousands' bar 7 thousands, making the entire resistance introduced 7364 when the plugs in the two short rows are both opposite bolts numbered alike, as shown. The two bars parallel to the two short rows are con- nected underneath by a wire, and each coil in each row has a separate connection with the electric circuit ; the three in row A being separately connected at the same point with the arm corresponding to A in Fig. 64, and the three in row B with the arm corresponding to B. The four long coils connect with the arm correspond- ing to D ; and the resistance to be measured, with the arm corresponding to C. Hence if the resistance in A equals that in B, and the plugs in the four long rows are moved to the right or left till the needle shows no de- flection, then the resistance in the four rows must equal that to be measured, since A : B : : D : C. Hence, with the plugs placed as shown, that resistance would be 7364 ohms. ELECTRIC MEASUREMENT. 163 But if a greater resistance than any represented by the four long rows is to be measured, as 100,000 ohms or more, then by changing the plug in row B to bolt 10, and that in row A to bolt 1000, the resistance of arm A is made 100 times that of arm B ; hence when the plugs in the four long rows are moved till the needle shows no deflection, the resistance to be measured must be 100 times that indicated in the four rows, which in the special case given would be 736,400. But if the plug in row A were at TOO and that in row B at 10, then, the resistance of A being only ten times that of B, the re- sistance in the above case, when the deflection was eliminated, would be 73,640. If a smaller resistance than any represented in the four rows is to be measured, as ^ of an ohm, then by placing the plug in units row opposite i, and those in the other three long rows opposite o in each, and moving the plug in row A to 10 and that in row B to 100, the resistance in A is made ^ of that in B', hence if the needle shows no deflection, the resistance to be measured is shown to be ^ of an ohm. In a similar manner, a resistance of T J of an ohm maybe measured. Hence we see that when the indicated resistance in row B is greater than in row A, the effect is to divide the indicated resistance in the four rows by the ratio of B to A ; but when the indicated resistance in B is less than that in A, the effect is to multiply the indicated resistance in the four rows by the ratio of A to B. In a similar manner any of the indicated resistances can be multiplied or divided. If, in the construction, the relative positions of arms Cand D are reversed, the effect is to reverse the rela- tive positions of arms A and B with reference to them; and hence the multiplication and division, as above. By increasing the number of coils, and range of re- 164 DYNAMIC ELECTRICITY AND MAGNETISM. sistance, in both the long and short rows, within prac- tical limits, any required resistance, great or small, can be accurately measured. The battery and galvanometer keys, marked respect- ively B and G, are shown in front. In a recent form of this instrument the battery key is placed above the galvanometer key and insulated from it, so that the same pressure closes both, the battery key first, as re- quired ; and the binding-posts for the battery are placed at the right of the galvanometer, and those for the re- sistance to be measured at the left ; a units' coil is also added to each of the short rows. In another form of this instrument, the bars are omitted and the resistance introduced by removing plugs, as shown in Figs. 62 and 63. The plugs should always be pressed in tight, to insure perfect contact. THE DYNAMO AND MOTOR. CHAPTER VII. THE DYNAMO AND MOTOR. The Magneto-Electric Generator. It has been shown in Chapter V that transient electric currents are generated in a conductor forming a closed circuit, when moved through a magnetic field in such a manner as to cut a varying number of lines of force and produce a differ- ence of potential between different parts of the circuit ; and that the E. M. F. varies as the number of lines cut per unit of time, and the strength of the current as the E. M. F. divided by the resistance. It has also been shown that when such a conductor is in the form of a coil having a soft iron core, the electric development is greatly increased by the coefficient of magnetism in- duced in the core. On these principles the little instrument known as the magneto-electric machine was invented by Pixii in 1833, in which subsequent improvements were made by Sax- ton and Clarke. It consists, as now constructed, of a short U electromagnet, mounted on an axis, with its poles close to those of a permanent magnet and at right angles to them, and made to rotate rapidly by means of a crank, band-wheel, and gearing. At each make and break thus produced, transient, alternating currents are generated in the coils ; and the coil terminals being at- tached to two brass plates fitted to opposite sides of the axis, with insulating material between them, the cur- rents are taken up and passed to an external circuit 'by two brass springs which press against these plates. Commutation. The plates being insulated from each 1 66 DYNAMIC ELECTRICITY AND MAGNETISM. other, and out of contact with the springs during the break, and brought into reversed contact with them at the instant of current reversal, which occurs at each half revolution, their position with reference to the springs is reversed as the currents are reversed, and hence the currents are all made to flow in the same direction through the external circuit. A direct current made up of these transient, alternating currents is thus produced by commutation, with perceptible intermission at each make and break, its smoothness varying with the rapidity of the rotation. Improved machines of this kind were constructed by Siemens, Wilde, and others, among which was a very FIG. 67. powerful one, made by the Compagnie 1'Alliance of Paris, of the following construction, illustrated by Fig. 67. The Alliance Machine. Six bronze wheels, mounted on a horizontal shaft, carried 16 electromagnets on each circumference, 96 in all, which rotated between 7 sets THE DYNAMO AND MOTOR. l6/ of laminated steel magnets, 8 in a set, fixed radially, poles inward, in 8 rows, on a horizontal frame, opposite poles alternating both radially and lengthwise ; so that the core of each bobbin, as it rotated between them, was alternately exposed to opposite poles at each end, 16 times at each rotation, the 96 electromagnets thus generating 16 X 96 = 1536 transient currents ; and as the shaft rotated 350 times per minute, 350X1536 537,600 currents per minute were generated. A machine with alternating current was employed for the electric light, for light- houses, and one with direct current for elec- tro-plating and similar work. The Siemens Armature. The principal im- provement made by Siemens consisted in a new style of bobbin, or armature, as it was called, illustrated by Fig. 68, invented in 1856, in which the coils were wound lengthwise, parallel to the axis of rotation, on the flat central part of a long iron core between two flanges, each convex outside and straight inside, and projecting beyond the central part at the ends as shown ; a cross-section resembling the letter H. This armature rotated between large pole- pieces attached to the poles of a powerful laminated steel magnet, the two flanges being the armature's poles, and its coils cut- ting across the lines of force ; and being more fully exposed in the magnetic field than in the old style of winding, the electric devel- opment was proportionally increased. Wilde's Machine. Wilde's improvement consisted in substituting a pair of electromagnets for the steel mag- net to produce the magnetic field, and exciting them by 1 68 DYNAMIC ELECTRICITY AND MAGNETISM. a small Siemens machine, mounted above it as shown in Fig. 69 ; the Siemens armature being used below as well as above. The current from the armature of the ex- citing machine passed in circuit through the coils of the electromagnets, while that from the lower armature FIG. 69. passed out through the external circuit, being made direct by commutation in both machines. The pole- pieces referred to are indicated in the cut by m n above and T T below, and insulated from each other by brass indicated by o and /'. The Dynamo. Iron when magnetized always retains a little residual magnetism, and when wrought into any THE DYNAMO AND MOTOR. 169 form acquires a similar quantity by the manipulation. It was proposed by Siemens and Wheatstone, in 1867, to excite the generator by the multiplication of this residual, found in the cores of the electromagnets and armature, by connecting the electromagnet coils with the armature circuit, and thus dispense with the exciting machine. The method of doing this may be illustrated as follows: In Fig. 69, the magnet coils are connected together below, and have their terminals at/ and q above; if the exciting machine be removed and one of the circuit terminals below, as that on the right, be connected with the coils at q> and the other, after passing through the external circuit, be connected at /, then a current pass- ing from the armature out through the left-hand termi- nal, and traversing the circuit, must return to the right- hand terminal by way of / and q, through the magnet coils, and thence through the armature coils to the left-hand terminal. The armature of a new machine, so constructed, being put in rotation for the first time, the incipient current generated in its coils during the first half-revolution, by the residual magnetism of the cores, passing through the magnet coils as above, increases this residual, which by its reaction increases the current in the armature coils in like ratio. At the next half-revolution these increased effects are doubled by the mutual reaction; and this doubling occurring at each subsequent hall- revolution and being repeated several thousand times per minute by the rapid rotation of the armature, the current, thus continually increasing in geometrical ratio, rises in a few moments to its full normal force, limited by the magnetic saturation of the cores and the carry- ing capacity of the coils. The machine, constructed on these principles, was I7O DYNAMIC ELECTRICITY AND MAGNETISM. designated as the dynamo-electric, in distinction from the magneto-electric, and subsequently became known briefly as the dynamo. The electromagnets producing the field were called the field-magnets, in distinction from the armature, which is also an electromagnet. The springs for taking up the current were called the brushes ; each consisting of a number of thin copper plates projecting beyond each other at the contact end and soldered together at the outer end. And the pair of insulated segments with which they made contact, and by which the commuta- tion was produced, was called the commutator. Hence the essential parts of the direct-current dyna- mo became known as the armature, the field-magnets, the commutator, and the brushes. Ladd's Machine. The current of the machine first constructed by Siemens, in 1867, alternated automati- cally between the internal and external circuits, being diverted from the latter when employed to excite the former. During the same year a machine was con- structed by Ladd, in which the current through both circuits was made continuous. It was substantially the same as the Wilde, with the steel magnet removed, the two armatures retained, one being connected with the magnet coils and the other with the external circuit, and the magnets placed in a horizontal position between armatures of equal size, and supported at each end on large vertical pole-pieces. The Pacinotti-Gramme Armature. An armature having the form of a wide ring was invented by Pacinotti in 1862, in which the coils were wound between projec- tions on an iron core. An improvement on this was made by Gramme in 1870, illustrated by Fig. 70, in which the core was com posed of annealed iron wires and entirely covered with the coils, only a few of which are THE DYNAMO AND MOTOR. 171 shown in the cut; the winding being continuous from coil to coil as shown. The covering of the core in this manner does not materially obstruct the transmission of magnetic force, copper being diamagnetic, so that such a core is prac- FIG. tically as susceptible of magnetism as that of the Siemens armature. Improved Commutator. An improved style of commu- tator was also invented, and used by Gramme in the construction of his dynamo in 1870, in connection with his improved armature. It is shown in cross-section in Fig. 70, and consisted of a number of short copper bars mounted on one end of the armature's axis, parallel to its length, and insulated from it and from each other by wood or other insulating material, filling the spaces between them and forming a cylinder under them on the axis. Each bar is attached to a coil as shown, so that the number of coils and bars is equal. As the currents reverse at each half revolution, a com- mutator having but two segments produces an inter- mittent current, as has been shown; but if it have four segments, as shown in Fig. 71, the brushes are brought into contact with two of the segments at each quarter revolution, and if each brush make contact with the approaching segment before breaking contact with the If 2 DYNAMIC ELECTRICITY AND MAGNETISM. receding segment, so as to bridge the intervening space, no intermission can occur. But as the coils, at each revolution, cut a varying number of lines of force per unit of time in different parts of the field, each alternate current must rise with the increase and fall with the decrease of magnetic force ; hence, with only four segments, the current, FIG. 71. though continuous, would be uneven, but with eight segments, as shown in Fig. 70, it becomes at a high speed of the armature practically even, being made still more even as the number of segments is increased. It is evident that the current cannot pass from one segment to another without traversing all the convolu- tions of the intervening coil; and as each convolution adds its quota to the current, and each coil is connected with the adjoining coil, all the currents thus generated THE DVNAMO AND MOTOR. I7J combine to augment the volume of current flowing through the outer circuit. Direction of the Current. If the armature, shown in Fig. 70, rotated in the direction of watch-hands and the current, transmitted from it through the field-mag- net coils, should circulate in such direction as to induce, in their cores, a north pole on the right of the armature and a south pole on the left. Then, according to the principles of electromagnetic induction explained in Chapter V, the currents generated on the outside part of the right-hand coils of the armature, between its core and the north field-magnet pole, would flow from the observer and be conducted back oppositely through the inside part, while those generated in the left-hand coils would flow in reverse order. And these currents, collected and made direct by the commutator, would enter the external circuit and field-magnet coils by the upper brush, and return to the armature by the lower brush. If the rotation of the armature were reversed, the direction of the current and polarity of the magnets would be reversed also. Interior Wire of the Gramme Armature. Iron being paramagnetic, the lines of force in the magnetic field cannot penetrate the Gramme armature core and pass through the interior of the ring, but are taken up by the core, which thus becomes magnetized. Hence the in- terior and end wire of the coils does not cut those lines, and cannot in this manner take part in the electric gen- eration, but serves as a conductor of the currents gen- erated in the exterior wire. It also increases the electric generation by the coefficient of magnetic induction received from the core. According to a theory now somewhat obsolete, the currents are generated by the lines of force threading 174 DYNAMIC ELECTRICITY AND MAGNETISM. through the coils, the interior wire thus taking part in the generation equall)'' with the exterior; but experi- ment seems to prove that this theory is fallacious, as no current is found in the interior and end wire when not continuous with the exterior. In the Sperry dynamo, interior pole-pieces, parallel to the axis of rotation, are used to render this wire active, the armature rotating between them and similar ex- terior pole-pieces projecting from the field-magnets. Another common form of construction is to wind all the wire on the exterior, passing it around projections on each end of the ring. The Cylinder Armature. The drum or cylinder arma- ture is also a common form, in which the wire is wound lengthwise on a cylinder, passing o\ r er the ends, as shown in Fig. 72. The core generally consists of a large number of thin sheet-iron disks, one of which is shown at B, mounted on a shaft and insulated from each other by tissue-paper. These are usually perforated by open- ings which, when placed opposite each other, form tubes for interior ventilation, connecting with ventilat- ing spaces between groups of disks, as in the Weston armature, shown at A, on which are also projections be- tween which the wire is wound. They are also made without openings or projections, as in the Edison arma- ture, shown at C, the wire being confined by brass bands, as shown. This construction of the core prevents the formation of the Foucault currents to which solid cores are liable, and which heat them and serve no useful purpose. And the disks, being parallel to the lines of force and at right angles to the currents, are in the best position for electromagnetic induction. Armatures of the Gramme pattern are also constructed with cores of this kind, made up of flat rings instead of disks. THE DYNAMO AXD MOTOR. The core should come as close to the pole-pieces as possible, to insure maximum magnetic induction, and A B FIG. 72. hence the wire wound on it should be evenly distributed, and of the minimum quantity and gauge requisite for proper electric induction and resistance. DYNAMIC ELECTRICITY AND MAGNETISM, Closed-Circuit and Open-Circuit Armatures. Armatures wound like the Gramme, in an endless spiral, with at- tachment to the commutator segments by radial arms, at regular intervals, are known as closed-circuit arma- tures; and the same designation is applied to those in which the coils are wound separately but connected with each other at the commutator, as in the armature of the Weston dynamo, shown in Fig. 73. FIG. 73. In another style, known as the open-circuit armature, each coil is independent of every other, its terminals being connected to two opposite segments of the com- mutator which have no connection with the other coils, THE DYNAMO AND MOTOR. as in the armature of the Brush dynamo; hence only those coils connected with the brushes through the commutator are in action simultaneously, each set coming into action as the other set passes out. Four brushes are employed in an eight-coil Brush dynamo, and the contacts are made in such a manner that six coils are in action simultaneously. Location of the Armature's Magnetic Poles. In accord- ance with the principles of magnetic induction, the polarity induced in the core of the armature by the field-magnets during rotation is opposite to that of the inducing poles, as shown in Fig. 74. But this polarity FIG. 74. is comparatively weak, the core's most effective polarity being that induced by the currents circulating through the armature's coils, the tendency of which is to induce similar poles in proximity to those of the field-magnets which, by mutual repulsion, are deflected into the posi- tion indicated by n n and $ s on a line joining the brush 1 78 DYNAMIC ELECTRICITY AND MAGNETISM, contacts; each half of the core, divided on this line, be- coming a separate magnet. The poles of the field-magnets are deflected in the opposite direction, the north pole to the lower corner of the pole-piece on the right, and the south pole to the upper corner of the pole-piece on the left; a line join- ing their centres being nearly at right angles to that joining the stronger armature poles. Hence the lines of force become contorted as shown. Magnetic Lag. The armature core does not become fully magnetized at the instant induction occurs, nor fully demagnetized at the instant it ceases; an infinites- imal moment being required for its saturation in the first instance and its demagnetization in the second, known as magnetic lag, during which its poles are car ried slightly forward in the direction of the rotation; this tends to separate the dissimilar poles induced by the field-magnets from the field-magnet poles, and thus to increase the contortion of the lines. Position of the Brushes. The brushes make contact with the commutator on or near the neutral line on which the currents reverse, as shown in Fig. 74, and where consequently no currents are generated; hence, in a closed-circuit armature, the parallel currents gen- erated on the left pass out from the armature by the upper brush, as each segment of the commutator comes into contact with it, and those generated on the right are added to the inflowing current entering the armature by the lower brush. If the brushes were shifted into the line of highest potential, which is at right angles to the neutral line, the wire in which the parallel currents are generated, on either side, would be carried round by rotation to the opposite side before the connecting commutator seg- ments reached the brush, and the currents neutralized THE DYNAMO AND MOTER. 1 79 by opposing currents generated in the wire, and the external current cease. But if the brushes made contact on a line between the neutral line and line of highest potential, a partial neutralization by opposing currents would occur, and the electric potential vary as the distance of the brushes from the neutral line; increasing as they approached it and decreasing as they receded from it. By shifting the brushes in this manner, automatically or otherwise, the potential and resulting current can be varied and regulated as required. Such regulation is common, but its range is limited, and it cannot always be used advantageously, as it tends to increase sparking at the brushes, a wasteful and in- jurious heating effect, difficult to suppress entirely. The Field-Magnets. The field-magnets of different dynamos vary greatly in construction and constitute the principal part of the framework of each machine, and hence they are so constructed as to support the various parts in the most convenient manner and give a compact, appropriate form, without interference with their special function. They have massive cores, usually of the best cast-iron, preferably annealed, malleable iron, though wrought- iron is also employed, but the advantage is not usually sufficient to compensate the extra cost. These cores should be sufficiently massive to insure the absorption of all the magnetism which can be generated in them without over-saturation. They terminate, at one end, in enlarged pole-pieces which nearly inclose the arma- ture, the opposite ends being connected by a cast-iron yoke, or bolted together by cross-bars, to complete the magnetic circuit. They are wound with heavy insulated copper wire, the winding being continuous from core to core. ISO DYNAMIC ELECTRICITY AND MAGNETISM. A single pair of such magnets may be employed, or two or more pairs, each core having a separate pole- piece, or two or more cores being joined to the same pole-piece. Series, Shunt, and Compound Winding. There are three principal methods of winding the field-magnets, known respectively as the series, the shunt, and the com- pound winding. FIG. 75. In the series method, illustrated by Fig. 75, the entire current traverses a single route of low resistance, pass- ing in series through the armature, the field-magnets. THE DYNAMO AND MOTOR. 181 and the external circuit; so that any variation of resist- ance, at any point, affects the entire series equally. In the shunt method, illustrated by Fig. 76, the cur- rent traverses two distinct routes; dividing, at the upper brush, in the inverse ratio of the resistance of each cir- cuit. The main current flows to the right through the FIG. 76. coarse wire of the external circuit, while a small cur- rent, varying from 1.5$ to 20$ of the entire volume, flows through the shunt, or fine wire with which the magnets are wound, and is employed exclusively to excite them. If the resistance of the main circuit is increased, the strength of its current is proportionally diminished. But the potential difference, or E. M. F., between the brushes, representing the electric pressure, is increased 1 82 DYNAMIC ELECTRICITY AND MAGNETISM. by the diminished flow of current in the ratio this in- creased resistance bears to itself plus the armature's resist- ance: and as the resistance of the shunt remains constant, the strength of its current is proportionally increased by this increase of E. M. F. : and the magnetism of the core being increased in the inverse ratio of its satu- ration, by this increase of current strength in the shunt, its reaction increases the current strength in both cir- FIG. 77. cuits; thus supplying electric energy to overcome the increased resistance. By this series of adjustments an equilibrium between these various factors is established, the total electric energy developed, varying as the me- chanical energy expended. Decrease of external re- sistance reverses these results. The resistance of the shunt may be varied as re- quired, by resistance coils. THE DYNAMO AND MOTOR. 183 The compound winding, illustrated by Fig. 77, is a combination of the series and shunt methods; a shunt wire of high resistance, used only to excite the magnets, being employed in connection with the low resistance wire, which is wound by the series method and excites them also. The automatic regulation is similar to that of the exclusive shunt method, except that the entire current flows through the magnet coils. Each of these methods of winding has its special adaptation to the requirements of a certain kind of work; as, for instance, in electric lighting it is found that the series-wound machine is usually the most suit- able for arc lighting, and the shunt and compound wound for incandescent lighting; arc lighting requiring high E. M. F. and comparatively small current, while the requirements for incandescent lighting are the re- verse; which leads to the classification given below. Constant Current Dynamo. To maintain a number of arc-lamps, connected in series, at a given illumination, a constant current of ten or more amperes, flowing from lamp to lamp, is required for each. If but one lamp were lighted, the required E. M. F. or potential would be comparatively small; but if two lamps were lighted, the resistance being doubled, the E. M. F. must be doubled to maintain the same current strength; and the same ratio of E. M. F. to resistance must be maintained for any number lighted or extinguished. Hence the construction and regulation of a dynamo for this work, or any work having similar requirements, must be such as to furnish E. M. F. capable of variation within the required range; and a machine so constructed is known as a constant-current dynamo, and is usually series-wound as stated above. Constant-Potential Dynamo. But if the required work were the maintenance at a given illumination of a 184 DYNAMIC ELECTRICITY AND MAGNETISM. number of incandescent lamps connected in parallel, the lamps being on branches derived from the main circuit, the variation of resistance is confined to these branches, in which it becomes adjusted to the require- ments of the current, the resistance of the main circuit FIG. 78. remaining constant; hence the E. M. F. remains nearly constant; and a machine adapted to such work, or work having similar requirements, is k lovvn as a constant-poten- tial dynamo, and is either shunt or compound wound. THE DYNAMO AND MOTOR. 185 The Edison Dynamo. Dynamos differ greatly in ap- pearance and minor details of construction, but their general construction and the relations of the different parts will be readily understood from the Edison dynamo, shown in Fig. 78, which is a direct-current, shunt-wound machine, used especially for incandescent lighting, and a fair representative of its class. The field-magnets, mounted vertically, rest on mas- sive pole-pieces inclosing the armature below, and on their left are shown the connections of the coils, the lever above by which the external circuit, represented by the projecting terminals, is connected and discon- nected, the projecting end of the armature below, with the commutator and brushes, the latter attached to a yoke, movable manually for adjustment of potential. The oil-cups, band-wheel, and screws for shifting the machine's position, to tighten or loosen the belt, are also shown below, and the lamp above, which indicates the general state of the current. Alternating Current Dynamos. The transient currents generated by the armature, when passed into the ex- ternal circuit without commutation, produce a continu- ous alternating current, and electromagnetic machines having such construction are known as alternating cur- rent dynamos. The Gordon Dynamo. The older machines of this class have a construction somewhat similar to that of the Alliance magneto-electric machine, already described; electromagnets with alternating poles taking the place of the steel field-magnets. The Gordon machine is of this construction; 64 short field-magnets being mounted transversely on the circumference of a wheel which rotates between two stationary armatures of similar construction, each having 64 coils; and the coils being oppositely wound on each alternate bobbin, both in the 1 86 DYNAMIC ELECTRICITY AND MAGNETISM. armature and field-magnets, produce alternating poles in each. The currents flow from the armature coils to the external circuit without the intervention of a collector and brushes; and the field-magnets are excited by two direct-current dynamos. The Westinghou.se Dynamo. The Westinghouse alter- nating-current dynamo represents an improved method FIG. 79. of construction, the principal features of which have been adopted by several machines of this class. Fig. 79 is a sectional view of the machine, as seen from the end of the armature shaft, representing 16 THE DYNAMO AND MOTOR. 1 87 field-magnets attached radially to a circular frame, their opposite, alternating poles inclosing a central space in which the armature rotates; their cores and winding being shown in section above. Fig. 80 is a sectional view parallel to the shaft; a side FIG. 80. view of one of the field-magnets being given below, and that of a core above. Mounted on the shaft at the left of the armature is the collector, composed of two copper rings, insulated from each other, on each of which a 1 88 DYNAMIC ELECTRICITY AND MAGNETISM. brush, connected with a separate terminal of the external circuit, makes contact. The armature core is composed of insulated sheet- iron disks, and ventilated by tubular openings in the manner already described; and the coils are wound in a single layer on its external surface and looped around projections at the ends. The manner of winding is shown in Fig. 81, a correct idea of it being obtained by FIG. 81. supposing the coils to lie at right angles to the surface of the paper, the outer ends turned from the observer and the inner ends towards him. Each alternate coil is oppositely wound as shown, and they all form a contin- uous closed circuit, the opposite terminals of which are connected with the separate rings of the collector; the current passing out from one ring and returning to the other alternately. THE DYNAMO AND MOTOR. 189 Separate Excitation. The direct current, always re- quired for exciting the field-magnets, in the Westing- house and similar dynamos, is obtained, as in the Gordon, from a separate, small machine. This separate excita- tion, which involves extra expense, complication, and inconvenience, may be avoided by the generation of a separate, direct current in the machine itself by the commutation, for this purpose, of a small portion of the alternating current. But separate excitation is found to be the most practicable for the large dynamos usu- ally employed for alternating-current work. Advantages of the Alternating Current Dynamo. The peculiar construction of the alternating-current dynamo and the elimination from it of the commutator, with its resistance and wasteful sparking, results in the genera- tion of currents of much higher potential, with less internal resistance than it is possible to obtain from the direct-current dynamo. Such currents can overcome the resistance of the external circuit more efficiently than those of low potential; and on this principle is based the practical rule that the amount of copper in the conductor should vary inversely as the square of the E. M. F.; according to which it is found possible to transmit such currents to points remote from the gen- erator by comparatively small wires, and thus distribute electric energy, for practical use, over a much larger area, at the same cost, than is possible with the direct current system; or over the same area at far less cost. This economical advantage is increased where elec- tricity can be generated more cheaply, as by water- power, at a point remote from where it is required for consumption; or where the generating station can be located on cheap property to furnish current for use on more expensive property, as often happens in cities. The Converter. Incandescent lighting is the principal DYNAMIC ELECTRICITY AND MAGNETISM. use for which the alternating current is now employed; and as this requires a large current distributed in small parallel currents among a great number of lamps, as explained in Chapter XI, 5000 being sometimes thus illuminated by a single dynamo, the conditions of high potential and comparatively small current, under which the electric energy is delivered, require to be reversed at the several points where it is to be consumed. This is done by the apparatus known as the converter or transformer, which is simply an inverted induction- coil of special construction; the primary coil consisting of fine wire which receives the high potential current from the dynamo, and the secondary coil, insulated from the primary, consisting of coarse wire in which, in consequence of the low resistance, a large current is in- duced and supplied to the lamps. Instead of an iron core inclosed by the coils, the coils are inclosed in an iron case composed of insulated sheet- iron plates, built up in the same manner as the armature cores already described; the two coils being placed side by side, so that both are equally exposed to the mag- netic induction. These converters, mounted on poles or otherwise, are distributed along the line between two parallel wires, one connected with the primary coils and dynamo, and the other with the secondary coils and lamps. At each point where light is required, a converter of the capacity requisite to furnish current for the required number of lamps is placed. Ten to eighty lamps may thus be sup- plied from the same converter. Development of the Electric Motor. Oersted's dis- covery of electromagnetic action, in 1819, and the sub- sequent development of electromagnetism to which it gave rise, led to the invention of numerous machines designed to utilize electricity as a motive power by THE DYNAMO AND MOTOR. IQI means of the electromagnet. The principle of con- struction in all these machines consisted in energizing electromagnets by a battery current, and by their at- traction and repulsion producing mechanical motion, either rotary or oscillating. In the rotary motors a number of iron armatures, with or without inclosing coils, rotated in proximity to an equal number of stationary electromagnets; the rotation being produced by the attraction of each arma- ture in the same direction by the opposite magnetism of a stationary pole, and its repulsion by the similar magnetism of the pole from which it was receding; the polarity being reversed at the instant of closest prox- imity by a commutator fixed on the rotating shaft. The Jacobi motor was among the most noted of the coiled armature class, and the Froment and Neff motors of the naked armature class; the armatures in the Neff being stationary and the magnets rotary, while in the Froment the armatures rotated and the magnets were stationary. In the oscillating machines the armatures, consisting of a pair of loose fitting pistons, were attracted alter- nately into hollow electromagnets whose polarity was reversed by a commutator at the close of each oscilla- tion, and a reciprocating motion thus produced. These pistons and magnets were placed either vertically at the opposite ends of a horizontal walking-beam, as in the Gustin motor, or horizontally, end to end, as in the Du Moncel and Page motors, in which the oscillatory motion was changed to rotary by a crank. From 1830 to 1873 various motors of the above kinds were constructed, and attempts made to operate ma- chinery and propel boats and cars with them; one of the most noted of these experiments having been made by Jacobi with his motor at St. Petersburg in 1838; with IQ2 DYNAMIC ELECTRICITY AND MAGNETISM. which he propelled a boat on the Neva, carrying 14 passengers, at the rate of three miles an hour; employ- ing first a Daniell battery of 320 cells, and subsequently a Grove battery of 138 cells. The Daniell was objec- tionable on account of its great weight, and the Grove on account of its noxious fumes, while the rate of speed was far too low to be of any practical advantage. In 1851 Page propelled a car on the Washington and Baltimore Railroad, at a maximum speed of 19 miles an hour, with a i6-horse motor of his construction and a Grove battery of 100 cells. But the limited capacity of motors constructed as above, the cost and inconvenience of batteries of the requisite size, their want of constancy for such strong currents, and the risk and difficulty of their transporta- tion when filled with fluid and employed to propel cars, were fatal defects which could not be overcome, so that all such motors were found to be impracticable. In 1861 Pacinotti invented a motor, the armature of which has already been described; the whole construc- tion being practically the same as that of the Gramme dynamo which appeared subsequently. This motor, like all its predecessors, was energized by a battery, and hence could not be made practical, but was the same in principle as the improved motors now in common use; being simply a reversed dynamo in which an electro- magnetic current produced mechanical motion, instead of mechanical motion producing an electromagnetic current. Pacinotti recognized this principle of inversion, hav- ing found that by energizing his field-magnets by the battery current, or substituting permanent magnets for them, and rotating his armature mechanically, an elec- tric current was generated; so that the machine could THE DYNAMO AND MOTOR. 193 generate motion by applying current, or current by ap- plying motion. The Dynamo as a Motor. This principle of inversion in the dynamo, as discovered by Pacinotti, received its first practical application by Fontaine at ihe Vienna exposition in 1873; when he used a Gramme magneto- electric machine, attached to a pump, as a motor, put- ting it in operation by a current from a Gramme dyna- mo. This led to the discovery that the dynamo itself could be used as a motor and operated by a current supplied by another dynamo; thus substituting the stronger, cheaper, constant current generated by me- chanical power for the weak, dear, inconstant current generated by chemical action, and the superior energy of electromagnetic action for mere magnetic attraction and repulsion. Hence the motor and the dynamo are identical in principle and in construction, and the same machine may be used either as a generator or a motor. In practical use, however, the motor usually requires to be smaller and more compact, as the power generated by a steam- engine or a water-wheel can be converted into electricity most economically by a large dynamo, and distributed for running cars or operating light machinery by nu- merous small motors; a motor of a tew pounds weight having sufficient capacity to operate a sewing-machine or a small lathe. Principles of the Motor. According to Lenz's law, re- ferred to in Chapter V, the reaction of an induced cur- rent, generated by the mechanical movement of a con- ductor, is always in opposition to the movement; hence the currents induced in the armature of a dynamo react in opposition to its rotary movement. This reaction is the result of the potential difference generated by the movement, which, as has been shown, induces opposite. IQ4 DYNAMIC ELECTRICITY AND MAGNETISM. electromagnetic poles in the adjacent parts of ,he arm- ature and field-magnets, producing attraction which tends to arrest the rotation. This attraction is electric as well as magnetic; the currents generated in the coils of opposite poles, facing each other, flowing in the same direction, and hence being mutually attracted. Now it is evident that a mechanical rotary force equal to this reaction is necessary to overcome it, and this constitutes almost the entire force required; the force requisite to overcome the friction and inertia of the armature being comparatively insignificant; and it is the rotation of the armature in opposition to this re- action which generates the current. If a dynamo, put in operation in this manner, be con- nected by conductors with another dynamo intended to act as a motor, the above conditions of potential differ- ence and reaction are produced in the second machine by the current from the first; and there being no me- chanical force in the motor to oppose this reaction, its armature rotates in the opposite direction to that of the generator, reproducing the mechanical force applied to the latter, less a certain percentage consumed in over- coming the resistance of the conductors. Hence the principle of the motor is simply that origi- nally discovered by Oersted, the rotation of a magnet by an electric current. But the motor thus operated generates a counter-cur- rent in opposition to that of the dynamo, and when the two machines are of equal capacity the opposing cur- rents vary as the relative speed of each machine; the motor current increasing and the dynamo current de- creasing till the speed is equalized, when the strength of the motor current becomes equal to that of the dyn- amo current less the amount necessary to overcome THE DYNAMO AND MOTOR. 195 the motor's friction and inertia, and no effective current flows from the dynamo. This condition is soon attained when the machines are running without "load," that is, without doing useful work. But when the motor is made to operate machin- ery its speed is reduced in proportion to the load, and the counter-current decreasing as the speed, the current from the dynamo is increased in the same ratio. Hence, as the current varies inversely as the motor's speed and directly as the dynamo's speed, and the motor's speed varies inversely as the load, it follows that the speed of the dynamo must be made to vary directly as the load of the motor in order to maintain the requisite speed in the motor for the performance of useful work. Hence variation of load at the motor requires corre- sponding variation of power at the dynamo; the com- bined machines being simply an apparatus for the con- venient application of mechanical power to useful work; mechanical energy being transformed into electric energy by the dynamo, and this electric energy trans- formed into mechanical energy by the motor. Loss of Energy. This double transformation entails a loss of about 15 % of the mechanical energy derived from the steam-engine or other source of power; this percentage being spent in overcoming the friction, in- ertia, electric resistance, and self-induction of the ma- chines, including also their incidental waste. Besides this a loss is incurred in overcoming the electric resistance of the conductors, which varies in proportion to their cross-section and required length, and may equal an additional 10 per cent or more, according to the dis- tance to which the power is to be conveyed. A con- siderable loss is also often incurred by imperfect in- sulation, unavoidable under certain conditions, as in the running of street-cars. 196 DYNAMIC ELECTRICITY AND MAGNETISM. Eddy Currents. In both the dynamo and motor, cur- rents are induced in the iron core of the armature, un- less suppressed by specific means, which in the dynamo flow in the same direction as those induced in the coils, and in the motor, in the opposite direction. These cur- rents, regarded by Foucault as magnetic, are regarded by later writers as electric; a distinction which per- tains chiefly to their direction rather than their nature, if both kinds of energy be considered identical; and since they cannot combine with the currents in the coils, they serve no useful jmrpose; circulating as eddies in the iron, wasting energy and generating heat. The laminated structure of the armature core sup- presses them almost entirely in the dynamo, as has been shown, but it has been found more difficult to suppress them in the motor. For in the dynamo the two sets of currents, being in the same direction, tend to weaken each other, while in the motor, being in opposite direc- tions, they tend to strengthen each other, in accordance with the principles of current induction. Hence, in the dynamo the useful currents tend to suppress the eddy currents, and in the motor, to increase them. So that any eddy currents induced in the core of either arm- ature, notwithstanding the lamination, become more prominent in that of the motor. As these eddy currents are regarded as the chief cause of loss of energy in motors, the importance of suppress- ing them by complete lamination, with thin disks and perfect insulation, in motor armatures of all sizes, is apparent. Series, Shunt, and Compound Wound Motors. The field- magnets of motors, like those of dynamos, are either series, shuntj or compound wound, and machines of each style are applied to the same work; practice being less definitely settled in motor work than in dynamo THE DYNAMO AND MOTOR. work, and opinion in regard to the adaptability of the different styles of winding to the different kinds of motor work varying. This arises from the complicated character of an apparatus composed of two machines having opposite functions and reversed modes of action, the adjustment of whose mutual relations, so as to adapt the apparatus to a varying external load, pre- sents a problem far more difficult of solution than the adaptation of a single machine to similar work, as in the dynamo. The shunt wound motor has the advantage of its in- ternal, automatic regulation, which adapts it to station- ary work having approximate constancy of load; while the series wound has been found better adapted to street- car work, where starting and stoppage, varying grade and speed, and varying number of passengers require manual regulation to adapt the current to this varying load on the motor, also prompt reversibility of motion and command of maximum energy at any rate of speed. In the Sprague motor, which is compound wound, differential regulation is obtained by opposing the shunt to the series current. In these different styles of winding the current enters, leaves, and circulates through the field in the reverse order to that in which it enters, leaves, and circulates through the corresponding styles of winding in the dyn- amo, except as changes are required in specific methods of regulation. Reversible Rotation. But as reversal of mechanical motion is often desirable, motors are constructed in which the rotation of the armature is made reversible. This is accomplished in a very simple manner with two sets of brushes having opposite current connections; one set being lifted out of contact with the commutator by a lever attached to the brush-yoke, as the other set 198 DYNAMIC ELECTRICITY AND MAGNETISM. is brought into contact; and the direction of the cur- rent through the armature being thus reversed, the ro- tation is reversed also. The Alternating Current Motor. The construction of the direct current motor involves the double conversion of the electric current from alternating to direct in the dynamo and from direct to alternating in the motor, the armature currents in each machine being alternat- ing and the field-magnet and line currents direct. As this double conversion requires two commutators with their wasteful resistance and sparking, various attempts have been made to eliminate it and produce a strictly alternating current motor; but previous to 1888 such motors had not been made practically success- ful. The general introduction of the alternating current system at about that date created a special demand for them, and led to the construction of a practical motor of this kind by Nikola Tesla, based on the principle of the shifting of the magnetic poles in the dynamo, which has already been described. Tesla made the important discovery that if the field- magnets of a motor were made in the form of a ring, similar in construction to that of the Gramme armature, the coils on opposite sections of the core all around being separately connected in pairs, and the terminals of alternate, opposite pairs connected with two pairs of the usual collectors of the alternating current dynamo, on which the brushes make contact, the magnetic poles, induced in the motor by the transmitted currents, would be shifted continuously round the ring from pole to pole, making a complete revolution during each revolu- tion of the dynamo's armature, the polarity of each section of the ring being reversed by each alternation of current : and opposite poles being induced in the armature of the motor, it would be put in rotation in a THE DYNAMO AND MOTOR. 1 99 corresponding manner by the resulting electromagnetic attraction and repulsion. The chief difference between this method of rotation and that of the direct current motor consists in the fact that, in the latter, the poles, having shifted to their relative positions, remain stationary, the armature ro- tating and its poles shifting continuously in the opposite direction as the armature rotates through them. Now if the field-magnets of the Tesla motor be regarded as a stationary armature, and the armature as a rotating field-magnet, we have practically the same relative con- ditions as in the direct current motor, with this differ- ence, that in the Tesla the poles rotate through a stationary armature, instead of the armature rotating through stationary poles; and the stationary poles of the rotating field-magnet maintain practically the same constancy of relative position to the rotating poles of the stationary armature as is found between the corre- sponding poles in the direct current motor. The relative conditions in the two motors are reversed but not essen- tially changed except by the elimination of the commu- tator from the Tesla. And the ability of the Tesla to start, stop, and maintain its rotation in synchronism with the dynamo is due to the reversed construction of the two machines, by which the magnetic poles are made rotary in the one where stationary in the other, and stationary in the one where rotary in the other. The Westinghouse Tesla Motor, A Tesla motor, used in connection with the Westinghouse alternating current dynamo, is shown in Fig. 82, its construction being similar to that of the dynamo with certain modifications. The core of the field-magnets, or stationary armature, is laminated, each plate being circular externally, and having an even number of poles projecting inward. These plates, arranged symmetrically and properly 20O DYNAMIC ELECTRICITY AND MAGNETISM. insulated, are bolted together between two caps which form the ends of the supporting frame; the lamination being shown on the outside in the cut, and twelve poles through the ventilating openings in the caps. The coils are in two separate series, wound oppositely on alternate poles; those of the same series being all wound in the FIG. 82. same direction, and those of the other series, which alternate with them, in the opposite direction. Each series has one of its terminals connected with the binding-post on the right, the other terminals being each connected with a separate binding-post on the left; the two left-hand posts being connected with THE DYNAMO AND MOTOR. 2OI separate collecting rings on the dynamo, and the right- hand post with a third ring; hence the current which enters by the right-hand post divides, producing oppo- site poles in the alternate, oppositely wound coils, and leaves by the two left-hand posts; while, at the next alternation, the current enters by the two left-hand posts, reverses the polarity, and leaves by the right-hand post. The armature, or rotating field-magnet, has the same construction as the armature of the Westinghouse dyn- amo, a single layer of copper wire covering the cir- cumference of a laminated iron cylinder, mounted on the shaft, the coils being looped round projections at the ends. The winding is continuous, as in the dynamo, but the two terminals are soldered together, so that the coils form a closed circuit without external connection. The insulation of the armature is not of essential im- portance, its E. M. F. being very low. In the direct current motor, with an armature rotat- ing between two pole-pieces, there are only four poles, two in the field-magnets and two in the armature on a line at right angles to them; but in this motor there are 12 alternate poles in the field-magnets and 12 in the armature; each pair of armature poles, on opposite sides, being on a line at right angles to that joining a pair of field-magnet poles on opposite sides; the polarity of each pair being similar but opposite to that of the other pair. Hence the poles rotating round the station- ary field-magnets can never be more that ^ of the cir- cumference in advance of the fixed poles which follow them; and there is a tangential pull between the dis- similar poles of each element, in opposite directions at opposite ends of each diameter, and a tangential push between the similar poles, producing the rotary force. As this attraction and repulsion varies inversely as 2O2 DYNAMIC ELECTRICITY AND MAGNETISM. the square of the distance, and as the distance between poles varies with their number, it is evident that the rotary force must vary in the same proportion. But it should be distinctly borne in mind that this force is not due to magnetism alone, but is electromagnetic, as in the direct-current motor, the poles of each element being constantly deflected by those of the other element, in accordance with the principle discovered by Oersted. The mutual relations of speed and current, and the generation of a counter current, explained in connection with direct current motors, apply also to this motor. As the speed, with a given load, varies in proportion to the rotary force, and the force varies with the number of poles, it is evident that the speed must vary as the num- ber of poles. Hence there should always be a sufficient number to insure the necessary speed for the required work, and this number may be greater or less than the number required in the connected dynamo; the relative speed of the two machines varying inversely as the relative number of poles in each. These general prin- ciples are, however, subject to various modifications dependent on special modifications of construction in each machine. The Tesla Motor as a Converter. The construction of this motor is practically the same as that of the con- verter a laminated iron case inclosing two insulated copper coils acting inductively on each other. This construction magnifies the inductive effect by bringing the coils of each element into close proximity, and especially increases the facility for rapid reversal of rotation, which is effected in the following manner: Reversal of Rotation. If, while the motor is in opera- Hon, the connections of the two left-hand binding-posts with the dynamo be reversed by the movement of a switch, the polarity will be reversed and the poles of the THE DYNAMO AND MOTOR. 203 field-magnets made to rotate in the opposite direction. This reversal of polarity in opposition to the momentum of the armature changes the motor for an instant to a dynamo, generating a strong extra current in the arma- ture coils. This is the extra current of break, described in connection with the induction-coil as having superior strength, and, its direction being the same as if there had been no reversal, it opposes for an instant the reversed current from the dynamo, arresting the rota- tion of the armature, which, at the next instant, is reversed by the reversal of polarity; the successive steps following each other so rapidly that full speed in one direction is almost instantly changed to full speed in the opposite direction. Similar effects are produced in the reversal of the direct current motor, but the construction does not admit of equally effective induction. By passing the current through a special converter, the regulation of this motor can be adapted to any practical conditions required. Distribution of Power. A number of motors may be operated on cars, or in shops, from one or more large dynamos, coupled together as generators, or employed separately, and centrally located; the parallel system of distribution, described in Chapter XI, being usually adopted, having been found more practical than the series system. This system is practically tfye same for distribution of power as for distribution of light, but in its application to cars special methods are required. One of the most common is to suspend a wire above the track and make connection between it and the motor on each car by a trolley attached to the end of a connecting-rod which projects above the car. As the direct current is usually employed, it enters by this wire, insulated in the air, 204 > V NAM 1C ELECTRICITY AND MA GNE TISM. and returns by the rails, which are electrically connected together for this purpose; the motor having electric con- nection with them through the car axle and wheels, and the dynamo similar direct connection. As the resistance between the mains varies inversely as the number of cars employing current simulta- neously, the supply of current varies in like propor- tion; self-regulation being thus obtained, as in parallel lighting; any additional regulation required being sup-i plied by resistance coils or otherwise. In cities where municipal regulations prohibit air lines, both conductors may be placed in a conduit; but as it is difficult, in such construction, to maintain proper insulation for naked wires with the open slot re- quired for connection with the motor, various ingenious methods have been devised to obtain the necessary in- sulation and connection. By placing the wires, properly insulated, in channels in the upper part of the conduit, on each side of the slot, they can be protected above and at the sides from dirt and wet; connection with the motor being made through the bottom of each channel; so that, with proper drainage, the insulation of the wires can be maintained; and by covering the motor connec- tions with insulating material they can also be protected from electric loss through contact with snow or mud. Elevated-Road Distribution. On elevated roads the l positive conductor can be connected with a central rail, both being properly insulated, and the track rails used for the return circuit, or a'ny other convenient, econom- ical method adopted; insulation and protection being comparatively easy, presenting no such difficulties as surface roads. Thermo-Magnetic Motors. The construction of these motors, which is still in the experimental stage, depends on the well-known principle that the heating- of a mag- THE DYNAMO AND MOTOR. 2O5 net reduces its magnetism; hence if a rotary, iron arma- ture be mounted between the poles of a powerful elec- tromagnet and its opposite sections, at points unequally distant from each pole, be alternately heated and cooled, a constantly varying magnetic force is developed, and rotation produced and maintained; the amount of force, speed of rotation, and permanency and economy of the apparatus being dependent on the construction. The capacity of the experimental motors of this class, constructed by Tesla, Edison, Menges, and others, is quite limited. Such machines may also, like electric motors, be em- ployed as electric generators, by rotating the armature in opposition to the magnetic force. The economic value of a practical motor of this kind, by which the direct conversion of heat into power could be accomplished, would, for some purposes, be very great; since the present system of converting heat into power by the steam-engine, power into electricity by the dynamo, and electricity into power again by the motor, is wasteful and expensive, notwithstanding its many advantages: and if the elimination of the steam-engine and the dynamo could be accomplished, and the energy developed by the furnace utilized without loss by the substitution of a magnetic or electromagnetic engine for the steam-engine, a very desirable end would be at- tained. Still it is not probable that such an apparatus could supersede the use of the dynamo and motor as a con- venient, economical means for the distribution of power from a central station for running cars and many other purposes. 206 DYNAMIC ELECTRICITY AND MAGNETISM. CHAPTER VIII. ELECTROLYSIS. WE have seen that the electric development which takes place in a battery cell is proportional to the chemical reaction, and, conversely, it is found that the chemical reaction developed by an electric current de- rived from the cell, or otherwise, is proportional to the electric development. In explaining polarization it was shown how water may be decomposed by the electric current. This decomposition was discovered by Carlisle and Nicholson in 1800, and it was subsequently ascer- tained that many other chemical compounds could be decomposed in a similar manner. Nomenclature by Faraday. Faraday, who made a very thorough investigation of this subject, gave to this pro- cess the name of electrolysis, a term derived from Xvoo, to loosen, or separate, combined with 7/Ae/cr/ooF, and he called substances capable of such decomposition elec- trolytes; hence the term electrolytic is applied to the cell in which the process is conducted, and sometimes also to the products of the decomposition, to distinguish them from the same substance obtained by other means; as an " electrolytic " metal. The term electrode, which has already been defined, is also due to Faraday, and was first given to each of the wire terminals of the electric circuit connected with the electrolytic cell, though its use in other connections has since been found convenient, as already shown. He called the terminal by which the current enters the ELECTROL YSIS. 2O/ cell the anode, and that by which it leaves, the cathode; the former term being derived from ava odoS", ascend- ing way, and the latter from Kara 060$, descending way. He gave the name ions to the products of the decom- position, designating those which appear at the anode, or positive pole, as anions and those which appear at the cathode, or negative pole, as cations. Hence the anions are regarded as electronegative, and the cations as electropositive; each being attracted by the pole whose electric potential is supposed to be different from its own, and repelled by the one whose electric potential is supposed to be the same; electric energy overcoming chemical affinity. In the electrolysis of water, oxygen, appearing at the anode, is regarded as electronegative, and hydrogen, appearing at the cathode, is regarded as electropositive. Theory of Grotthuss. The transfer of these atoms, or " migration of the ions," in opposite directions, which is a salient fact, is supposed to occur in accordance with a theory proposed by Grotthuss in 1805, and subsequently modified by Clausius. In every liquid a mutual inter- change of relationship is supposed to be constantly oc- curring among the molecules and atoms which compose them, producing motion in every conceivable direction, old groups being continually dissolved and similar new ones formed, and constancy of constitution thus main- tained under continual change. In Fig. 83 we have, in line i, an ideal view of this heterogeneous movement; each little oval representing a molecule of water, the two hydrogen atoms, which compose the hydrogen molecule, being shown by the shaded part and the oxygen atom by the unshaded part, an infinite number of such chains making up the mass of the liquid. In line 2, these molecules, under 208 DYNAMIC ELECTRICITY AND MAGNETISM. the influence of an electric current from A to B, are sup- posed to be reduced to a symmetrical phase, in which the oxygen part of each is turned towards the anode and the hydrogen part towards the cathode, Now a new grouping is supposed to take place, the oxygen of each molecule moving to the left to recombine with the hydrogen of the adjoining left-hand molecule, and the hydrogen moving to the right to recombine with the \ FIG. 83. oxygen of the adjoining right-hand molecule; the new formation being represented by line 3, in which the oxygen part of the molecule at the left end of the chain and the hydrogen part of the molecule at the right end is each left without a mate, and being prevented by electric action from combining with each other, each is given off as gas Thus while an interchange of atoms and molecules is taking place all along the infinite num- ber of lines of electric action throughout the entire mass, the opposite ions make their appearance only at the electrodes. The decomposition of every other elec trolyte is supposed to take place in a similar manner. ELECTROLYSIS. 2CX) Electrolysis of Water. Different compounds vary greatly in their relations to electrolysis, and the elec- trolysis of the same compound often shows great varia- tion under different conditions. The feeblest current produces electrolysis in some cases, while in others the most powerful fails to produce it. Pure water, for in- stance, resists the strongest electrolytic action, while water slightly acidulated with sulphuric or chlorhydric acid is easily decomposed; the acid remaining appar- ently unchanged, while its presence reduces the electro- lytic resistance of the water. It has been suggested, in explanation of this, that there is a decomposition and recomposition of the acid, in this connection, in such a manner as to leave it un- changed; the decomposition of the water being indirect, through the agency of the acid; one or both gases be- ing derived from the acid, which in turn receives from the water the sarr^ amount of one or both which it has surrendered. In case sulphuric acid (SO 4 H 2 ) is used, it could furnish both ; but in case chlorhydric acid (HC1) is used, it could furnish only hydrogen, while the hydrogen, taken from the water to replace this, would set free the proper combining proportion of oxygen. The theory given above shows how this may occur. It is evident that whether the decomposition of the water is direct or indirect, the final result would be just the same; the two gases being evolved in the exact pro- portions in which they recombine to form water. The high resistance of pure water to electrolysis does not absolutely prevent its decomposition. Gladstone and Tribe have effected it with zinc coated electrolyti- cally with spongy copper or spongy platinum, also with iron or lead similarly coated with copper: but, in this case, the electrodes being intimately connected, the re- sistance is reduced to a minimum, while decomposition 210 DYNAMIC ELECTRICITY AND MAGNETISM. ind the evolution of both gases with two platinum electrodes, separated, has not been found possible. But where the anode is an oxidizable metal, as copper, with which the oxygen, in the nascent state, can unite chemi- cally, the decomposition may be effected. This also occurs when sodium or potassium is brought into con- tact with water, the oxygen uniting with the metal and hydrogen being given off. Authorities differ in regard to the electrolysis of water under variation of pressure. It has been main- tained that electrolysis is influenced by pressure much in the same manner as evaporation is thus influenced; that under a pressure of 300 atmospheres about 4500 pounds to the square^inch even acidulated water can- not be decomposed, while in vacuo its decomposition may be effected by currents too weak to effect it under ordinary atmospheric pressure. But Bouvet claims to have effected it under a pressure of several hundred atmospheres, and to have found that the amount de- composed was independent of the pressure. As water is the usual solvent in solutions, its electroly- sis is usually inseparable from that of the substances held in solution, and becomes an important factor in the work required. Conditions of Electrolysis. The required conditions of electrolysis are that the substance must be a liquid, either naturally or by liquefaction, a conductor of electricity, and a compound, one of whose constituents is usually a metal. Ice, though of the same chemical constitution as water, and a conductor of electricity, cannot be electrolyzed, because it is a solid. All the oils, and nearly all melted fats and resins, being non- conductors, are not subject to electrolysis : carbon bisulphide, the liquid chlorides of carbon, and many other substances belong to the same class. Solutions ELECTROLYSIS. 21 1 of the salts of copper, silver, gold, potassium, and sodium are among the substances most easily electro- lyzed. The metallic elements usually appear at the cathode and are regarded as electropositive, and the nonmetal- lic at the anode and are therefore regarded as electro- negative. Hydrogen, which is considered a metal, appears, as we have seen, at the cathode. But in the liberation of the same element from differ- ent compounds, it may be either electropositive or electronegative according to the positive or negative character of its associate elements; positive and nega- tive expressing merely relative differences of potential under different conditions, and not absolute differences of physical constitution. Temperature has a very important influence; rise of temperature increasing bol'.i the electric conductivity and the electrolytic action. The time in which the action takes place is also of great importance; the results of rapid action generally differing considerably from those of slower action. Thus a simple metal, deposited at the cathode, may vary con- siderably in structure, or an alloy may differ in its com- position, according as the process of deposition is slow or rapid. Secondary Reaction. Secondary reaction often occurs in electrolysis by which the liberated ions form new com- binations with each other or with the electrodes them- selves. We have had an instance of the former kind in the supposed decomposition and recomposition of acid in the electrolysis of acidulated water, and of the latter, in the union of the oxygen of water with potassium, sodium, or copper, used as the anode. Such secondary reaction may occur at either elec- trode or at both, with marked characteristics peculiar 212 DYNAMIC ELECTRICITY AND MAGNETISM. to each. At the anode, the most common phenomena are corrosion of the anode, evolution of gas, and the adhesion of the ions to the anode, either as simples or new compounds; while, at the cathode, the ions, liber- ated either in a solid, liquid, or gaseous form, may either adhere to the cathode, be absorbed, dissolve, or escape. Alloys of the metals may also be formed at the cathode by the deposition of one metal upon another, also amalgams with mercury. Hence the permanent products of electrolysis may differ greatly from the elementary substances liberated, owing to the formation of new combinations during the process. The specific gravity of the liquid at each electrode often changes also, usually becoming heavier at the anode and lighter at the cr*hode. Electrolysis of Mixed Coir Bounds. In the electrolysis of mixed compounds, the different elements are usually liberated in the order of their electropositive affinities; the least electropositive cation first, since it has the weakest chemical affinity, and the stronger ones subse- quently, in proportion to increase of current strength, or as reduction in the size of the electrodes increases the potential difference or E. M. F. But, by making the quantity of each substance in the solution proportionate to its electropositive strength, several elements may be liberated simultaneously; and, by increasing the proportion, the stronger may, in some instances, be liberated in larger amount than the weaker. By varying the proportions and other conditions in this manner, Favre was enabled to obtain from a mix- ture of the sulphates of cadmium, copper, and zinc, each metal separately, and also two, or all three, simultane- ously; and found that the various results depended on ELECTROLYSIS. the energy of the battery, the electrolytic resistance of the salts, and the relative time of electrolytic action; and hence he concludes that, by thus varying the con- ditions, the different metals may be separated succes- sively from any mixture of metallic salts capable of electrolysis. Relations of Electrolysis to Heat. The evolution of heat is a necessary result of all electrochemical work, and is due both to chemical action and to electric re- sistance. When elements combine chemically, as in the battery cell, heat is generated, and when they are sepa- rated electrically, as in the electrolytic cell, heat is ab- sorbed; and the amount thus generated or absorbed bears a certain definite proportion to the work accom- plished and may be taken as its measure. This heat is distinct from that generated by the elec- tric resistance of the circuit, which varies in proportion to the amount of that resistance, and hence may be modified or controlled, while that due to chemical action is beyond control. In the battery cell there is always, in connection with the chemical reunion by which heat and current are generated, a certain amount of electrolysis by which heat and current are absorbed; and in the electrolytic cell there is always, in connection with the chemical separation by which heat and current are absorbed, a certain amount of chemical reunion by which heat and current are generated; and in both cells there is also the generation of heat by electric resistance. Hence when heat is absorbed, in either cell, there must be cor- responding electrolytic action, and, conversely, when such electrolytic action is developed there must be cor- responding absorption of heat. The electrochemical work required for the electroly- sis of any compound must be equal to that required to 214 DYNAMIC ELECTRICITY AND MAGNETISM. develop the amount of heat which would be generated by its chemical recombination, plus that required to overcome the electric resistance of the circuit. Hence the heat developed by electrochemical action in the battery is the measure of the electric work accomplished by the current, minus that expended in overcoming the electric resistance of the circuit; otherwise the results would not be in accordance with the law of the conser- vation of energy. Lowest Required Electromotive Force. It has been shown that, in polarization, electrolytic action opposes electric generation; in like manner the action of the. electrolytic cell opposes that of the battery, and when the opposing forces are equally balanced action in both must cease. Hence it is impossible to produce electroly- sis with a battery whose E. M. F. is only just equal to that of the electrolytic cell. If, for instance, the cell contain acidulated water, whose electrolytic reaction is 1.49^ volts, its electrolysis, with platinum electrodes, would be impossible with a current from a single Daniell cell, whose E. M. F. is only about i volt; hence two such cells would be the least number by which it could be effected, or a single cell having a higher E. M. F. than i 49 J volts, as a Grove or a Bunsen. The minimum E. M. F. required in each case varies with the nature of the compound to be electrolyzed, but it must always be in excess of that of the electrolytic cell, unless re-enforced by secondary action in that cell. When the anode is soluble and forms a new chemi- cal combination with the liberated anion the minimum E. M. F. required for the battery is greatly reduced. This is the case when a copper anode is used in the elec- trolysis of acidulated water; the chemical reaction, pro- ducing combination of the oxygen and copper generates a current which re-enforces that of the battery, making ELECTROL YSIS. 2 1 5 the electrolysis of even pure water possible, as already shown. In this case the water may be decomposed by a single Daniell cell, or even one of less E. M. F. It is claimed that polarization does not occur with an anode of the same metal as that deposited on the cathode, and hence that a current of the lowest E. M. F. will produce electrolysis under these conditions. The opposing current set up in the electrolytic cell does not rise at once to its full E. M. F. Hence elec- trolysis may begin with a current of less E. M. F. than the required minimum, but cannot continue. This incipi- ent electrolysis has been attributed by Helmholtz to the presence, in the solution, of such uncombined atoms as, according to Clausius, have become separated from their former associates, but have not yet formed new combi- nations; hence their segregation can be effected by a current of less E. M. F. than that required to separate atoms already combined. Faraday's Laws. The following laws were established experimentally by Faraday: 1. The quantity of an ion liberated in a given time varies directly as the strength of the current. 2. The weights of the different ions liberated from a series of different solutions by the same cur rent in the same time vary directly as their chemical equivalents. 3. Electrolysis is independent of the relative position of the electrolytic cell in the circuit. 4. The number and amount of chemical equivalents which enter into combination in the battery are equal to the number and amount liberated by electrolysis in the circuit. It is immaterial from what electric source the current is derived. Faraday produced electrolysis even with the slight current from an electrostatic machine; and Sir Humphrey Davy, in 1807, separated the metals potassium and sodium from their bases, for the first 2l6 DYNAMIC ELECTRICITY AND MAGNETISM. time, by the powerful current of a voltaic battery of 274 cells. It is still an unsettled question whether an electric current can pass through a liquid without producing electrolysis. Observation seems to show that in some instances this may occur, and that in others the electro- lytic effect is small in proportion to the conductivity. Magnetic Effects. Neither is it known to what extent magnetism influences electrolysis, as observation on this point has been very limited; but experiments by Rem- sen show certain marked peculiarities of manner in the deposition of copper, from its sulphate, under magnetic influence, which vary in proportion to the magnetic force, though the amount deposited remains unchanged. Peculiar magnetic effects have also been observed by S. P. Thompson in the deposition of lead. In 1826 Nobili observed that the deposition of lead, from a solu- tion of its acetate, upon a platinum anode, occurred in the form of rings which gave rise to very beautiful chromatic effects, and are known as Nobili s rings. Thompson has found that when the deposition is made in a magnetic field, it ceases to have the circular form, and assumes a form peculiar to the magnetic influence. Chemical Equivalence. Faraday's second law may be illustrated as follows : There is in every molecule of water 2 atoms of hydrogen and i atom of oxygen, but each oxygen atom weighs 16 times as much as each hydrogen atom, hence the chemical equivalent of oxygen is 8, that of hydrogen being i ; its volume being only half that of hydrogen, though its atomic weight is 16 times as great : that is, a given volume of oxygen, as a cubic foot, weighs 16 times as much as the same volume of hydrogen, but there are 2 cubic feet of hydrogen in a given volume of water for every cubic foot of oxygen ; ELECTROL YSIS. 2 1 7 and the liberation of these elements by electrolysis is therefore in this ratio. Hydrogen being the lightest of all known substances, its chemical equivalent is taken as the standard of com- parison for the chemical equivalents of all other sub- stances. The chemical equivalent of copper, for in- stance, is 3iyV, that being the weight of its atom as compared with that of 2 atoms of hydrogen ; and the chemical equivalent of silver is 108, that being the weight of its atom as compared with i atom of hydro- gen. Now let the same electric current be passed, for the same time, through three vessels, one containing acidu- lated water, another some salt of copper, as its sulphate, and the third some salt of silver, as its nitrate ; and, at the end of the time, let the products be weighed, and it will be found that for every gramme of hydrogen liber- ated there have been 31^ grammes of copper liberated, and 108 grammes of silver. Electrochemical Equivalence. The weight of any sub- stance liberated by a current of i ampere in i second is known as its electrochemical equivalent, and this is found to correspond practically with its chemical equivalent, in accordance with Faraday's law. Hence, if the chemical equivalent of any substance be multiplied by the electro- chemical equivalent of hydrogen, the product is the electrochemical equivalent of that substance. The electrochemical equivalent of hydrogen is found to be 0.000010352 of a gramme ; multiplying the chemical equivalent of copper by this, we get 31.7 X 0.000010352 = 0.0003281584 of a gramme as the electrochemical equivalent of copper. In like manner the electro- chemical equivalent of silver is found to be 0.001118016. Effect of Current Reversal. Faraday's third law must not be understood as applying to the relative positions 2l8 DYNAMIC ELECTRICITY AND MAGNETISM. of anode and cathode with reference to the direction of the current. If both are of the same substance and merely serve as conductors, as the platinum electrodes used in the electrolysis of water, their relative position is, of course, immaterial ; but if they are of different materials, one or both of which is soluble, reversal of relations by change of current or otherwise changes the results. Such reversal, during the process, removes the ions already deposited on the electrodes. Hence an alternating current is not adapted to electrolysis. Effect of Convection. In a perfectly homogeneous solu- tion the strength of the current is the same in every part, and hence the liberation of the ions is uniform ; but the different parts of a solution are liable to a change of density during the process of electrolysis, producing differences in the liberation of the ions at different points on each electrode. This is especially the case when vertical electrodes are employed, with a metallic salt as the electrolyte. The specific gravity of the upper and lower parts of the solution, in proximity to each electrode, changes in consequence of difference of saturation ; increase of saturation occurring at the anode with descent of the more highly saturated portion of the electrolyte, and decrease of saturation at the cath- ode with ascent of the less saturated portion, which has been deprived in part of its metal. This convection produces difference of resistance, causing the main direction of the current to be from the upper part of the anode to the lower part of the cathode ; in consequence of which there is increased deposition of metal on the lower part of the cathode and a more rapid consumption of the upper part of the anode. This is more especially the case when a strong current is employed ; action being more uniform with a weak current. It is also more uniform with a horizontal posi- ELECTRO L YSIS. 2 1 9 tion of the electrodes,, also with solutions of a viscous character, in which this convection occurs more slowly. Relative Conditions of Current and Electrolyte. Ac- cording to Quincke, electrolysis is proportional to the strength of the current per unit of sectional area of the electrolyte, varies with the E. M. F., is inversely pro- portional to the distance between the electrodes, and is independent of the cross-section and conductivity of the electrolyte, when the resistance of the rest of the circuit is small in comparison. The conditions of electrolysis thus far considered are those in which a current from an external source passes through an electrolytic cell, but it may also be effected, to a limited extent,, by currents generated by contact between the electrodes and electrolyte, as follows : i. By dipping a metal into a solution consisting of a single liquid, as iron into a solution of copper sulphate or nitrate, which results in the deposition of a thin film of copper upon the iron ; the coppering of iron wire being done in this way. 2. By employing two solutions of different specific gravities, the lighter one resting on the surface of the heavier, and using, as in the former case, a single electrode in contact with both solutions. The current generated between the two liquids produces deposition on that part of that metal which, under the conditionSj becomes the cathode ; the same result being produced by separating the solutions by a porous cell or partition, and bending the metal so that its opposite ends dip into each solution, 3. By immersing, in a single solution, two metals, in contact externally, or con- nected by a conductor ; this produces a current from one metal to the other within the cell, causing elec- trolysis, the circuit being completed through the exter- nal connection. 4. By immersing two metals, connected 22O DYNAMIC ELECTRICITY AND MAGNETISM. externally* in two solutions separated by a porous cell or partition. These various methods will be recognized as instances of the partial electrolysis, already referred to, which oc- curs in every battery ; the cells being in fact various styles of battery cells. Electroplating. The electrolytic deposition of a metal upon another metal is termed electroplating^ and is the principal means by which plating is now accomplished; its first introduction as an art being by Richard Elking- ton of England, in 1840, though it had been performed experimentally by Wollaston in 1801, and by Brugna- telli in 1805. Various Details. The principal metals used for this purpose are gold, silver, nickel, and copper, though other metals also are deposited in this way, as platinum and tin; also alloys, as brass, bronze, and german-silver. It is estimated that 125 tons of silver are used annually for electroplating in different parts of the world, 25 tons being thus used in Paris alone. A vat of suitable size is provided, usually made of pine, and lined with lead or gutta percha; enamelled cast-iron is also used for this purpose; and, for small establishments, vessels of glass, china, or stoneware are used. The current is furnished by a battery or other genera- tor, usually a dynamo in large establishments, and must be always maintained in the same relative direction. The solution consists of some salt of the metal to be deposited, which yields it pure in sufficient quantity, and in the most economical and efficient manner; and the solvent is strictly pure water, usually distilled rain- water. The anode consists of one or more plates, usually of the same metal as that which is to be deposited, while ELECTROl YS1S. 221 the articles to be plated constitute the cathode; and both electrodes are suspended in the solution from cop- per bars which rest on copper strips, insulated from each other and connected respectively with the termi- nals of the generator, as shown in Fig. 84. FIG. 84. The Anodes. The surface area of the nickel anode plates, used for nickel-plating, should equal or exceed that of the cathode surface, each cathode surface being exposed to an anode surface, and the depth of submer- sion of the anodes should be about two thirds the depth of the solution. For plating with gold and silver, less anode surface is required, and the exposure of both sur- faces of the cathode is less important, as the solutions part with their metal more easily than the nickel solution, and the anodes are more soluble. For nickel-plating plane, even surfaces, the distance 222 DYNAMIC ELECTRICITY AND MAGNETISM. between the surface of anode and cathode should be aboi i 3 inches, but for surfaces having prominences or cavities the distance should be increased to 5, 6, or even 10 inches, according to the amount of unevenness. For silver-plating, this distance should never be less than 4 inches. It is evident that while the distance, in either case, should not be such as to produce too great resistance, increase of distance tends to greater evenness of deposit on uneven surfaces, the ratio of difference in distance produced by such surfaces, as compared with the entire distance, being proportionally reduced by in- crease of distance, producing greater evenness of elec- trolytic action. Hooks of nickel wire are used for the suspension of nickel anodes, but copper wire may be used if it does not come in contact with the solution; and these hooks should be sufficiently numerous to insure full conduc- tivity. Silver anodes are suspended by iron wires or lead ribbons, and completely submerged, to prevent cor- rosion of the anode at the surface of the solution. As the metal of the anode, when the same as that de- posited, replaces that taken from the solution, its purity is a matter of great importance, affecting the color, brilliancy, and general quality of the work. But insolu- ble anodes of a different metal, or of some other sub- stance, are sometimes used with advantage. Platinum anodes are especially recommended for nickel-plating, being indestructible. But their exclusive use is not de- sirable, as the electrolytic resistance is greater with insoluble anodes, requiring increased expenditure of electric energy to overcome it; besides^ the metallic con- stancy of the bath is continually changing, being weak- ened by the abstraction of the metal, requiring repeated additions to maintain the requisite degree of saturation. Hence it is important that a certain proportion of the ELECTROLYSIS. 223 anodes, usually about one third, should be of nickel, both to maintain constancy and reduce resistance. Carbon anodes may be used wtrere platinum is con- sidered too expensive. But even the best carbon anodes are liable to disintegration, and require to be renewed from time to time, which is a serious objection to their use. The beautiful deposits of green, red, and pink gold, seen on watchcases and jewelry, are obtained by the use of silver anodes for the green, and copper anodes for the red and pink, the operation being finished with a gold anode of the same color as the deposit. Plating Solutions. The solution most generally used for nickel-plating consists of a double salt of nickel and ammonium, obtained by mixing in proper proportions, in distilled water, either nickel chloride with ammonium chloride, or nickel sulphate with ammonium sulphate. For certain purposes the character of the solution is modified by the addition of a little citric or chlorhydric acid. Solutions for silver-plating are composed variously as follows: silver nitrate; silver potassic cyanide; silver chloride and potassic cyanide; chlorides of silver and of soda; cyanides of silver and of potassium; silver nitrate, potassium carbonate, and ferricyanide of calcined po- tassium. The solution generally used for gold-plating consists of gold chloride combined with potassium cyanide; the chloride being obtained by dissolving the pure metal in a mixture of 2 parts chlorhydric and i part nitric acid, known as aqua regia. Auxiliary Operations. The preparation of articles for plating and their subsequent finishing are among the most important parts of the process, requiring a series of operations which cannot be described here in full. 224 DYNAMIC ELECTRICITY AND MAGNETISM, The principal preparatory steps are termed buffing, cleansing, pickling, and scouring. The buffing consists in polishing the surfaces by means of revolving disks and brushes, and finely powdered substances, as fine sand, pumice, emery, lime, and crocus, before plating, and in polishing nickeled surfaces in a similar manner after plating The cleansing, which is one of the first operations, is done by immersion of the article in hot potash or caustic soda; the pickling by its immersion in water acidulated with sulphuric acid; and the scouring by its immersion in a bath of nitric and sulphuric acids, to remove any re- maining traces of oxide. Special baths are also used with different metals for scouring and other purposes. Zinc is given a light coating of copper before nickel- plating, by immersion in a solution of copper acetate, combined with salts of soda and potassium; this being necessary to procure adhesion of the nickel; and iron is sometimes similarly coated by immersion in a solution of copper sulphate and sulphuric acid. Zinc may also be amalgamated as a preparation for nickel-plating; and for silver-plating, amalgamation is a prerequisite for all metals, the bath for this purpose consisting of a solu- tion of mercuric binoxide in water acidulated with sul- phuric acid. Articles prepared in the above manner are suspended on hooks of suitable metal before immersion in the last preparatory bath, and must not be touched with the hand again before immersion in the plating bath, as the slightest contact of the bare hand, in nickel-plating especially, greases the surface sufficiently to produce an imperfect spot. The time of immersion in each preparatory bath varies from a few seconds to 15 minutes, the longest time being required for the potash bath, and the opera- ELECTROLYSIS. 22$ tions being accompanied with frequent rinsings; and, after plating, the articles are rinsed in water and dried in hot sawdust before polishing or burnishing. Required Electric Energy. The electric energy of E. M. F. and current strength required varies with the metal to be deposited. For nickel-plating especially it should be vigorous at the beginning and weaker towards the close, the E. M. F. varying from 5 volts to i volt; and when furnished by a battery, 3 Bunsen cells in series, or their equivalent, may be used at the beginning, and i Smee, or its equivalent, at the close. For silver-plating, an E. M. F. of not more than two or three volts is employed at the beginning, and a current strength of 50 amperes per square meter of cathode surface. For gold-plating the E. M. F. must not exceed one volt, as the solution has very low resistance, and the current strength must not exceed 10 amperes per square meter of cathode surface. Required Time of Immersion, and Thickness of Deposit. The time of immersion in the plating solution varies with the metal to be deposited, with the metal to be plated, with the thickness of deposit required, and with the source of current employed. For nickel-plating, the time with a dynamo current varies from 15 minutes to an hour, and with a battery current from 2 to 5 hours. The average deposit is about 2 grammes per square decimeter, which gives a thickness of about -fa of a millimeter. Heavier plating is liable to peel, unless special precautions are used; but the hardness of nickel renders heavy plating unneces- sary. The time for silver-plating varies from 3 to 4 hours with the dynamo current, and from 8 to 12 hours with the battery current. The average deposit does not ex- 226 DYNAMIC ELECTRICITY AND MAGNETISM. ceed 3 grammes per square decimeter ; the average deposit on forks and similar-sized table-ware being from 80 to 100 grammes per dozen. Gold is deposited with great rapidity; the practical difficulty of the process being greatly increased by the necessity of rendering it immediately successful. A few minutes' immersion is sufficient to insure a good surface, which is usually very thin; gold-plating cover- ing more perfectly, and producing a better finish in proportion to thickness, than plating with other metals. Agitation of the Solution. In all kinds of electro- plating frequent or constant agitation of the plating solution is important to insure the homogeneousness necessary for evenness of deposit; this agitation being sometimes maintained, in large establishments, by some special mechanical device. Special precautions are also necessary to insure good work on articles having deep cavities and sharp angles. Electrotyping. The deposition of copper by elec- trolysis for the production of copies of woodcuts and similar engraved surfaces, and also of type, is known as electrotyping) and is the process by which the metal plates called electrotypes, used for all the finer grades of book and map printing, are prepared for the press. The details, which are comparatively simple, are briefly as follows: Impressions of the type or cuts are taken with a press on plates composed of beeswax and graphite, each plate having sufficient surface for sev- eral such impressions. The surface is then shaved smooth and even, and " built up" to a thickness of about jig- of an inch by additions of the melted composition to all the blank spaces; after which it is brushed with finely powdered graphite. It is then covered with a thin coating of copper, precipitated upon it from a solution of copper sulphate by iron filings; and the plates, thus ELECTROL YSIS. 22*] prepared, are suspended by copper hooks in a solution of equal parts by weight of copper sulphate and sul- phuric acid in distilled water, at a distance of about 2 inches from anodes of pure copper, of equal surface. With a dynamo-current, two hours' immersion gives a coating of the requisite thickness, but with a battery- current 12 to 14 hours is required. A single very large Smee cell furnishes a strong current of low E. M. F., about T %- of a volt, which does not produce electrolytic resistance by decomposition of the water. The copper coating is then stripped from the plates, being loosened by expansion with hot water, its reverse surface coated with solder, and melted type-metal poured over it so as to produce a plate about \ of an inch thick. The different impressions are then cut from the large plates, straightened, planed smooth on the under sur- face, trimmed to symmetrical shape, and mounted, at the regular type height, on wood or metal bases. Electric Refining of Metals. The refining of various metals by electrolysis has become an important art. It consists in obtaining them in certain required states of purity from the crude smelted products, and extracting, by the same process, such percentage of the precious metals as they may contain. Copper is one of the principal metals thus refined; the art having originated with Elkington, to whom pat- ents for the electric refining of this metal were issued, in England, in 1866. In a single copper refinery at Hamburg the average daily product, at a recent date, was 8 tons of refined copper, 2\ tons of which were chemically pure; and the gold extracted in a single year equalled ii tons. The electricity in this refinery is generated with specially constructed dynamos, from which currents can 228 D YNA MIC EL EC TKICI TY A ND MA GNE TISM. be obtained in parallel or in series, the parallel current from the largest dynamo having an E. M. F. of about 4 volts and a strength of about 3000 amperes, and the series current an E. M. F. of about 8 volts and a strength of about 1500 amperes; the electric energy represented by the joint product of E. M. F. and current strength being the same in either case 12,000 watts. Two series of 20 baths each are employed in connec- tion with this dynamo; each bath having an anode sur- face of 30 square meters, making a total of 1200 square meters ; the cathode surface being of equal amount, and the distance apart of the two surfaces from 2 to 4 inches. The anodes are thick plates of the crude metal, and the cathodes, plates of the chemically pure metal, about i millimeter thick. The solution consists of cop- per sulphate, and the deposit occurs in thick layers, which are easily removed from the cathodes. The cop- per thus deposited liberates its combining equivalent of sulphuric acid, which unites with the copper of the anodes, furnishing a supply of sulphate by which the constancy of the bath is maintained. The precious metals are precipitated into the sedi- ment; from which they are separated, and subsequently refined by a separate process. Electric Reduction of Ores. The separation of metals from their ores is another important application of electrolysis. As such separation cannot be made suc- cessfully from crude ores, they must first be reduced chemically to salts capable of being electrolyzed, and the success of the process and its economy consists largely in the proper preparation of the. ore in this man- ner; different salts of the same metal, treated by differ- ent methods, yielding to electrolysis with different degrees of facility, and producing the metai in varying degrees of purity and in variable quantity with the ELECTROL YS1S. 22Q same current. Hence the nature of the preliminary process is often the sole condition of success or failure. Among the various ores reduced in this manner, the principal ones are those of zinc, lead, copper, silver, gold, aluminium, sodium, and magnesium. Sir Humphrey Davy, as we have seen, obtained po- tassium and sodium from potash and soda, experiment- ally, in 1807; but the first practical application of elec- trolysis to the reduction of ores was made by Bunsen in 1854. He obtained aluminium, sodium, magnesium, barium, and other rare metals by this process in quan- tities comparatively large, operating on the chlorides of most of these metals. His process consisted chiefly in submitting the fused chlorides to electrolysis in a glazed porcelain crucible, maintained at a red heat, and divided into two com- partments by a porous earthenware partition reaching nearly to the bottom. By this method he electrolyzed aluminium and magnesium with a current of 15 to 20 volts, derived from a Bunsen battery, using electrodes of coke carbon, the metals going to the cathode and the chlorine to the anode. H. E. S. C. Deville subsequently improved Bunsen's process for the reduction of aluminium. He mixed 2 parts by weight of aluminium chloride with i part common salt (sodium chloride) pulverized, fused the mixture at a temperature of 218 C, and electrolyzed it in a glazed porcelain crucible, maintained at a temper- ature of 183 C. He used a cylinder of charcoal for the anode, immersed in a portion of the fused mixture con- tained in a porous cell placed in the crucible ; salt being added in this cell to fix the chloride and prevent its volatilization. The cathode was a platinum plate, and two battery cells furnished sufficient electric 230 DYNAMIC ELECTRICITY AND MAGNETISM. being very low. The deposit contained a percentage of common salt, which was subsequently removed by dis- solving it in water ; and the metal was further purified by successive fusions, and treatment with the double chloride of sodium and aluminium as a flux. About 1885 C. E. Becquerel separated silver, copper, and lead from their ores by electrolysis ; the silver ore being first reduced to a chloride and the copper and lead ores to sulphates. Instead of a current derived from an external battery, he used the electrolytic cell as his battery, the liquid being a solution of the ore, while the electrodes were composed of zinc, iron, or lead for the anodes, and cop- per, tin, or carbon for the cathodes ; grouping the cells as required for E. M. F. or current strength. For copper he arranged them as gravity cells, a light solution of iron sulphate being superposed on a denser solution of copper sulphate, a cast-iron anode being placed in the iron solution, and a copper cathode in the copper solution, the deposit being made on the cathode. The production of electric energy at economical rates by the dynamo has revolutionized these earlier methods of the electrolytic reduction of ores by battery currents which were too expensive to be practical ; and their chief value now consists in indicating the nature of the re- quired preliminary preparation. But even at the pres- ent comparatively low cost of electric energy, the electric process is not always the most practical or economical, and its application in any particular case must be de- termined by the attendant circumstances. Where water- power is cheap and abundant, and fuel expensive and scarce, its application is likely to be more practicable than where these conditions are reversed ; power being easily convertible into electric energy, while the fuel ELECTROLYSIS. 23! required for the smelting process might make it more expensive than the electric process. The chlorides are still the salts most generally em- ployed, though the sulphates, nitrates, and acetates are preferable for some metals and for some processes. These salts are prepared from the ores by roasting, fusing, pulverizing, mixing with various substances, treating with acids, and other operations, according to the nature of the ore or the process ; and are then re- duced to the liquid condition for electrolysis, either by fusion or by solution in water, the nature of the ore or process, as before, determining the method required. The Hall Process for Aluminium. The process of C. M. Hall of Oberlin for the electric reduction of alumin- ium from its ores, patented in 1889, has been put into successful operation by the Pittsburg Reduction Com- pany, resulting in the production of the metal, nearly pure, in large quantities and at a greatly reduced price. It is substantially as follows : A steel crucible lined with carbon contains a bath, lighter and more electropositive than aluminium, com- posed, by weight, of 234 parts calcium fluoride, or fluor- spar ; 421 parts of the double fluoride of aluminium and sodium, or cryolite ; 845 parts aluminium fluoride, ob- tained by saturating hydrated alumina, A1 2 HO 8 , with hydrofluoric acid. The bath's chemical composition is represented approximately by the formula Na 2 Al 2 F 8 + CaAUFs ; to which is added 3 or 4 per cent of calcium chloride, CaCla, to prevent the abnormal increase of elec- tric resistance, otherwise liable to occur from the for- mation of certain impurities. The crucible being set in a furnace, the bath is fused at a red heat, and alumina in the form known as bauxite, an anhydrous oxide of aluminium, or the pure anhydrous oxide, AlaOs, artificially prepared, is dissolved in this 232 DYNAMIC ELECTRICITY AND MAGNETISM. fused bath and subjected to electrolysis with a dynamo current of 4 to 8 volts E. M. F.> which is sufficient to electrolyze the alumina, but not the bath. Carbon electrodes are employed, the anode being im- mersed in the bath and the carbon lining of the crucible forming the cathode. The aluminium goes to the ca- thode, sinking to the bottom on account of its greater specific gravity, where it can easily be drawn off; and the oxygen goes to the anode, where it unites with the car- bon, forming carbonic acid, CO 2 , which escapes as gas; the anode being thus consumed at the rate of about i pound of carbon to i pound of aluminium produced, and requiring frequent renewal. The presence of the calcium chloride prevents more rapid carbon consump- tion by suppressing the formation of carbonic oxide, CO, otherwise liable to occur, and which consumes double the amount of carbon, the oxygen uniting with the car- bon in the proportion of i to i instead of 2 to i, as shown. The electrolysis proceeds continuously, aluminium being drawn off and alumina added in sufficient quan- tity to keep the bath saturated with it, though an ex- cess is not injurious, as it merely sinks temporarily and is subsequently taken up. The bath also requires occa- sional additions of material to renew the loss due to volatilization and other causes; the calcium chloride volatilizing most rapidly, and its abnormal reduction being indicated by a fall of current due to increase of electric resistance. ELECTRIC STORAGE. 233 CHAPTER IX. ELECTRIC STORAGE. The Leyden Jar and Condenser. The storage of elec- tric energy in the Leyden jar was one of the earliest discoveries in electric science. A full description of this instrument and its principles is given in the author's " Elements of Static Electricity," so that it is only neces- sary here to say, that it is simply a glass vessel, coated with metal on both surfaces to within a few inches of the top, which is left bare for insulation. An electrostatic charge, positive or negative, given to either coating, usually the inner, by a static machine or spark coil, produces by induction a charge of oppo- site potential on the other coating, when connected with the earth or opposite pole of the machine or coil, and the two coatings remain in this electric condition till gradually restored to the same potential by the slow convection of the air; but an instantaneous discharge may be produced, attended with spark and snap, by making a connection between them by a conductor. And it is characteristic of this instrument, that while the charge is received gradually, occupying usually some seconds or minutes, the discharge, produced as above, is always instantaneous, and nearly complete. The condenser, described in connection with the in- duction coil, is an instrument of the same character, re- ceiving and surrendering its charge in a somewhat sim- ilar manner. Grove's Gas Battery. Polarization is another instance of electric storage, and observation of this phenomenon, 234 DYNAMIC ELECTRICITY AND MAGNETISM. and of the fact that the oxidation of the copper anode in an electrolytic cell produces similar storage, led to some experimental investigation of the subject by Gau- therot and Ritter early in the present century. In 1842 FIG. 85. Grove constructed a gas battery on this principle, which is illustrated by Fig. 85. A three-necked flask, V, contains acidulated water, into which are inserted two inverted tubes, containing respectively oxygen and hydrogen, designated by O and ELECTRIC STORAGE. 235 ff. Platinum wires, sealed into the upper ends of these tubes, are connected with platinum electrodes in contact with the gases above and water below, and the external circuit is completed through copper conductors whose terminals dip into mercury cups. When the circuit is closed the gases recombine to form water, generating an electric current, which in the cell is from hydrogen to oxygen, and externally from oxygen to hydrogen, and whose E. M. F. is equal to that required to electrolyze water, 1.49^ volts. It is evident that the gases could either be evolved by a separate chemical process and admitted to the tubes previous to the latter being connected with the cell, or be evolved directly from the acidulated water of the cell by a battery or dynamo current. In the lat- ter case the total amount of electric energy generated by the recomposition would equal that expended in the decomposition, and in the former the amount of electric energy obtained would be in the same proportion for the same amount of gas recomposed. In either case there is storage of electric energy by chemical decomposition, which is recovered by chemical recomposition; and this is the principle of chemical electric storage as developed in the various styles of the apparatus, known as the storage battery, accumulator, or secondary cell. The gener- ation of electric energy must always follow the recom- bination, whether the elements are evolved in the gas- eous form by insoluble electrodes like platinum, or in the solid form by combination with soluble electrodes, of which the oxide formed on a copper anode in the electrolysis of water is an instance. Grove constructed similar batteries with other gases, and also with plates covered with metallic peroxides. Wheatstone, Siemens, and Kirchoff made similar ex- periments, but Gaston Plante's discovery, in 1859, of 236 D YNA MIC ELEC TR1CI TY AND MA GNE TISM. the special adaptation of lead plates for this purpose, opened the way for the practical success of electric storage. Planters Secondary Cell. Plante constructed a cell, using as electrodes two large sheets of lead rolled to- gether and electrically insulated from each other with strips of gutta-percha, as shown in Fig. 86; the method FIG. 86. of rolling being shown at A, and the sheets, rolled and clamped, at B, projecting strips of lead being left at- tached to each for terminals. They were then im- mersed in water acidulated with ten percent sulphuric acid, in a tall glass jar, and subjected to the action of a battery current supplied by two or more cells. A por- tion of the water being decomposed, the oxygen evolved at the anode combined with the lead, forming a dioxide, and the hydrogen was occluded on the cathode. When the anode ceased to absorb oxygen, as indicated by the escape of the gas, the cell was disconnected from the battery, and discharged by making an external con- nection between the terminals of the electrodes, and then recharged with a reversed current. This process was repeated during a period of several ELECTRIC STORAGE. 237 months, the time of charging being continually increased from a few minutes at first to several hours subse- quently, with long and increasing intervals of repose previous to each discharge and reversal; its object being to cover one of the plates with a thick coating of dioxide of lead, and the other with a coating of spongy lead of equal thickness. Chemical Reaction. The chemical reactions, as de- scribed by Gladstone and Tribe, are substantially as follows: The first charging produces only a thin film of the dioxide on the anode and of the hydrogen on the cathode; but the discharging changes the dioxide, PbO 2 , which is insoluble in sulphuric acid, to monoxide, PbO, which is at once reduced to sulphate, PbSO 4 , by the acid of the solution; the liberated oxygen atom uniting with the lead of the cathode and forming mon- oxide, which is also reduced to sulphate, as on the anode; the result of the discharge being a thin film of lead sulphate on each plate. During the second charging the sulphate of the plate, now made the anode by reversal of current, is decom- posed, the sulphuric acid, absorbed to form the sul- phate during the repose and subsequent discharge, is restored to the solution, and the lead, thus liberated, combines with the oxygen liberated simultaneously from the water and forms the dioxide. The hydrogen, also liberated from the water, goes to the plate now made the cathode and decomposes its sulphate; restoring the sulphuric acid to the solution, and liberating the lead, which adheres to the plate as a spongy coating. The respective results of each subsequent charging and discharging are the same as those just described; and as the spongy lead affords increased interior sur- face, the chemical reactions and formation of dioxide are proportionally increased. 238 DYNAMIC ELECTRICITY AND MAGNETISM. But increased thickness of the dioxide produces in- creased resistance to chemical reaction; hence arises the necessity for the period of repose before discharging, during which the chemical reaction of the anode plate, by the strong affinity of the lead for oxygen, changes some of the dioxide to monoxide, which the acid im- mediately changes to sulphate, and thus the resistance is lessened. There is also a resistance arising from an interior coating of sulphate, not reduced to dioxide by the charging, which remains in immediate contact with the plates and impedes the local action of repose, making longer periods of repose necessary as the coatings in- crease in thickness. Hence the electric formation of the plates consists of three distinct processes, the charging for the formation of dioxide on the one and spongy lead on the other, the repose for local action, and the discharging for the pro- duction of sulphate on both; the plates when completed consisting respectively of lead dioxide and spongy lead adhering to interior supports of sheet lead; and sub- sequent charging, for practical use, is always in the same direction, alternation being discontinued. The charge may be given either by a battery or a dynamo, usually the latter; the chemical reactions, when the cell is in practical use, being just the same as during the preparatory process; the electric effect being the absorption of electric energy by the conversion of sulphate into dioxide during the charging, which is re- covered by the conversion of dioxide into sulphate dur- ing the discharge. It has been observed that often when a partially dis- charged cell is given a short period of repose, the sub- sequent discharge shows increased electric energy. This is accounted for on the hypothesis that when the ELECTRIC STORAGE. 2$$ discharge is rapid some of the sulphate, formed on the anode from the dioxide, is reconverted into dioxide by the excess of oxygen developed, producing a propor- tional reduction of potential difference between the plates; but that during the short repose this dioxide is again reduced to sulphate and the potential difference restored. The maximum E. M. F. of the Plante cell is about 2.54 volts. By means of a commutator of special con- struction, Plante could instantly join a battery of 20 such cells either in series or in parallel. He used the parallel connection for charging, which he accomplished with 2 Bunsen cells, the resistance, with this connec- tion, being very low, and the series connection for dis- charging, by which he obtained a maximum current equal to that of 30 large Bunsen cells. The duration of the discharge depends on the resist- ance of the external circuit, varying from a few minutes to several hours according to the amount of current re- quired; and it ceases when the dioxide is all changed to sulphate, but should be terminated sooner to prevent injury to the plates from the excessive formation of sul- phate. The Faure Cell. The tedious, expensive process re- quisite for the electric formation of the Plante plates led to the construction by Camille A. Faure, about 1880, of plates prepared by covering sheet lead with a paste made of red lead and sulphuric acid; the coating being confined to the surface by a covering of paper and by felt placed between the plates, which also served the purpose of insulation. Thus prepared and rolled together, they were placed in a glass jar, in water acidulated with sulphuric acid, and the coating subjected to electrolysis with alternation 240 DYNAMIC ELECTRICITY AND MAGNETISM. of current, by which the red lead, known also as minium, Pb 3 O 4 , was changed in a few days to lead dioxide and spongy lead, on each plate respectively, and the cell was ready for practical use. Chemical Reaction. The chemical reaction, according to Gladstone and Tribe, is substantially as follows: On the immersion of the plates in the acidulated water, there is, at first, a purely chemical reaction, by which the minium on both plates is gradually changed, from the surface inwards, into a mixture of the dioxide and sulphate of lead, with evolution of water, thus, Pb 3 O 4 -+- 2H 2 SO 4 = PbO 2 + 2 PbSO 4 +2H 2 O. But oxygen and hydrogen being liberated by the electric current, the oxygen goes to the anode and changes the sulphate into dioxide and sulphuric acid, thus, 2PbSO 4 -j- 2H 2 O -\- O 2 = 2PbO 2 -f- 2H 2 SO 4 . The hydrogen goes to the ca- thode, changing the dioxide to spongy lead, with evolu- tion of water, thus, PbO 2 -|- H 4 = Pb -|- 2H 2 O; and chang- ing the sulphate to spongy lead and sulphuric acid thus, 2PbSO 4 + H 4 = 2Pb -f 2H 2 SO 4 ; reversal of current being necessary to electrolyze the heavy coatings com- pletely. The chemical reaction of the discharge is the for- mation of lead sulphate on both plates, and that of sub- sequent charging the reconversion of this substance to dioxide and spongy lead, as before. Defects of the Faure Cell. While the Faure cell could be produced much more economically than the Plante, and was equal to it in electric energy, it had many serious defects which proved fatal to its practical suc- cess. The felt, preventing the free circulation of the fluid, seriously impeded electrolysis; it soon became corroded by the acid and partly removed in patches, and ceased to insulate, The coating failed to adhere ELECTRIC STORAGE. 2 4 I properly, sloughing off and falling to the bottom. Hence in a short time the cell became worthless. But its invention demonstrated the possibility of practical suc- cess by some similar method of construction, to ascer- tain which the investigations of Brush, Swan, Sellon, Volckmar, and others were immediately directed. Improved Faure Cell. The result of these investiga- tions was the production, about 1886, of an improved cell, the principal feature of which is the improved style of plate illustrated by Fig. 87, which consists of a lead grid, shown at A y having its openings wider at the sur- u n A FIG. 87. faces than in the interior, as shown by the enlarged sec- tion at B. These openings are filled with a paste made of minium and sulphuric acid for the positive plates, and of litharge and sulphuric acid for the negative; litharge being a red lead monoxide, PbO, more easily reduced than minium, for which reason it is preferred for the 242 DYNAMIC ELECTRICITY AND MAGNETISM. negatives, to facilitate the reduction of the paste to spongy lead, which is more difficult than its reduction to lead dioxide on the positive plates. The advantages of this style of plate are that it gives a firm support to the paste, the plugs being held in the grids by the form given them by the openings, which obviates the necessity for the intervening felt and paper, allowing free circulation of the fluid and more perfect electrolysis. The cells are made of different sizes, stationary and portable; the stationary cells having glass vessels, and the portable, hard-rubber vessels. The 23-plate stationary cell, shown in Fig. 88, has n positive plates and 12 negative; each set attached to a lead cross-bar above and at the center, by which the plates are held at a fixed distance apart; the two sets interlocking, so that positive and negative plates alternate and are insulated from each other by two rows of hard-rubber forks. Each plate is \ of an inch thick, and the space between adjacent, positive and negative plates, -f% of an inch wide; and the two outside, negative surfaces being in- active, each set has 22 interior, active surfaces. A thick plate of glass, under the central cross-bar and plate connections on each side, supports the plates, so as to leave a space underneath for the free circulation of the fluid; each set being supported, on its opposite side, by plate projections which rest on an insulating hard- rubber strip above each cross-bar as shown; two stout rubber bands holding these supporting plates and the lower parts of the lead plates in position. A lead bar, projecting from the cross-bar of each set, can be bent into any convenient position for making connection with adjoining cells. These plates are immersed in water acidulated with ELECTRIC STORAGE. 243 36$ sulphuric acid, contained in a glass jar roj inches long, 8 inches wide, and 9j inches high ; the entire weight of jar and contents being 50 Ibs. FIG. 88. The glass jar has the advantage of allowing inspec- tion of the interior without disturbing the contents, by which the condition of the plates may be observed, and 244 DYNAMIC ELECTRICITY AND MAGNETISM. short-circuiting from the buckling of plates or the lodg- ing of loose paste plugs between them remedied; but its comparative frailty and weight are objections to its use for the portable cells required on cars and else- where. Hence a portable cell of the same capacity and number of plates is constructed with a covered, hard- rubber jar, made shorter below than above so as to furnish supporting ledges for the plates at the opposite ends, which take the place of the glass supporting plates employed in the stationary cell. The weight of this cell is 40 Ibs., its height about the same as that of the stationary cell, and its other dimensions about one fourth less. The i5-plate stationary cell has 7 positive plates, each -$ of an inch thick, and 8 negatives, each -fa of an inch thick, contained in a glass jar lof inches long, 12^ inches wide, and i3f inches high; the entire weight being 130 Ibs., and the storage capacity 300 ampere-hours, double that of the 23-plate cell. The supporting plates are of hard rubber, with openings for inspection, and are each held in position by two metal rods which pass through loops in the positives at one end and in the negatives at the other, binding the plates of each set together below and furnishing electric connection between them. Electric Preparation of the Plates. Each set of plates, positive and negative, is electrolyzed separately before they are combined in the cell intended for practical use; special sets of temporary plates, or dummies, of each kind being used for this purpose, which makes the old process by reversal of current unnecessary. The nega- tives, although thinner than the positives, require six days for the reduction of the litharge to spongy lead, while the minium of the positives is reduced to diox- ide in 24 hours. Electric Energy of Improved Cell. The E. M. F. of this ELECTRIC STORAGE. 24$ cell is about 2 volts, and its internal resistance .001 to .005 of an ohm. Its current, as in the Plante, depends on the external resistance; 30 amperes for 10 hours being considered an economical working rate for the large i5-plate cell. If less current is required the time of discharge becomes proportionally longer; and a cur- rent of 300 amperes may be obtained for an hour, but such rapid discharge is injurious to the cell. Effects of Charge and Discharge on the Plates. As both the charge and discharge result in different forms of chemical reaction, it is obvious that ample time should be allowed for this reaction to produce the required chemical changes; the normal rate under varying con- ditions being ascertained better by practical experience than by arbitrary rule. Charging is alw r ays accompanied by the evolution of gas, which, as has been shown, is chiefly absorbed, while a certain percentage escapes; hence if the rate of charging is excessive there is an abnormal escape of gas and useless consumption of current: there is also an abnormal development of heat, which may result in de- struction of the plates. As there can be no further absorption of gas when the chemical reaction of charging is completed, its ab- normal escape with a normal current indicates an over- charge, resulting as before in waste of current, but not in injury to the plates. But as the chemical reaction of 'the discharge results in the absorption of sulphuric acid and the formation of lead sulphate, a hard unyielding substance, in con- siderable quantity on both plates, and in excess of the material which it replaces, due to the absorption of acid, it is evident that if the action is too rapid, the plugs on the surfaces most exposed to chemical and electric action will become sulphated to a greater degree 246 DYNAMIC ELECTRICITY AND MAGNETISM. than on the opposite surfaces, producing unequal ex- pansion, with warping, or buckling of the plates, as a result; the same result also occurring from an excessive formation of sulphate if the discharge is continued too long. E. M. F. of Discharge. The E. M. F., during the first half-hour of discharge, is about 2.25 volts, being slightly increased by the supplementary reaction of a small amount of gas which adheres to the plates after charg- ing; it then drops to about 2.4 volts, remaining nearly constant, with a slight decline to 2 volts or less, till the dioxide is mostly reduced to sulphate, when it begins to decline rapidly; which indicates that the discharge should cease to prevent injury to the plates. Conductivity and Buckling. It is important that there should be a sufficient supply of sulphuric acid present to maintain the requisite conductivity during the dis- charge, when it is rapidly absorbed to form the sulphate; otherwise the fluid will soon be deprived of its normal quantity and the resistance abnormally increased. An excess of the acid is also injurious, causing the sulphate to form too rapidly, with buckling of the plates as a result. Such excess is liable to occur in the lower part of the cell, where the acid, from its greater specific gravity, accumulates, causing a corresponding reduction in the upper part. This increases the conductivity and chemical and electric action below, with corresponding decrease above. Hence buckling usually increases downward. Buckling, if not excessive, and if in the sarne direc- tion on all the plates, does not interfere with the action of the cell. But it always tends to loosen the plugs, so that they are liable to drop out and fall into the space between the plates, producing a short circuit. There is also liability to short-circuiting by contact between ELECTRIC STORAGE. 247 positive and negative plates, if the buckling is in oppo- site directions. Weight of Cells. When a storage battery is used on a car to furnish light or motive power, reduction of weight becomes highly important, as a large number of cells are usually required, and their aggregate weight will often equal 3500 pounds. Composition of Grids. Various metals and alloys have been tried for grids, but lead still has the preference, and is in general use. The addition of a small per- centage of antimony, as a flux, aids in producing more perfect castings; lead alone failing to flow into the nar- row spaces in the molds with the requisite facility. The further addition of a very small percentage of mercury to increase the durability, has also been tried, but its use has proved detrimental; and the antimony, though advantageous, as a flux, is not so durable as the lead. The Julien Cell. Various improvements of the Faure cell have been attempted, the chief objects of which have been to obtain greater durability, reduced weight, and to prevent the buckling of the plates. Prominent among these is the cell of Edward Julien of Belgium, whose general construction is similar to that of the im- proved Faure cell. Its chief claim is a special composi- tion of superior durability for the grids, which, so far as can be ascertained, is 94.5 per cent lead, 4.2 per cent antimony, and 1.3 per cent mercury. The Pumpelly Cell. A cell has recently been invented by J. K. Pumpelly of Chicago, the principal features of which are a horizontal position of the plates, supporting material between them, copper electrodes centrally located on opposite sides of the cell and reaching to its bottom, and a containing vessel of light durable mate- rial. The construction, in other respects, is similar to that of the Faure cell, and the same materials are em- 248 D YNAMIC ELECTRICITY AND MA GNE TISM. ployed for the grids, paste, and fluid. A slight burr at the narrow part of the grid openings holds the paste more securely, 12 per cent of antimony enters into the composition of the grids, and 20 per cent of sulphuric acid into that of the fluid. Fig. 89 shows the construction. The positive and negative plates alternate in position, the top and bottom plates being negative, and they are supported and in- sulated by cellulose made from wood, said to be a good insulator, an excellent absorbent, and in- destructible in sulphuric acid. Each plate has, at the centre of one of its edges, an opening about an inch square, and, at the same point on the opposite edge, a round, vertical, tubular projection about an inch high and half an inch in diameter, on the under FIG. 89. . , . , . , . side of which is a small socket fitted to the upper end of a similar projection frotn the alternate plate underneath. When the plates are built up in the cell, with the cellulose between them, each set has the projections al, on the same side and the openings on the opposite side; the projections of each alternating with the openings of the other on the same side; so that each projection from a negative passes up through an opening in a posi- tive, with ample space for insulation, and helps to sup- port the next negative above; and each projection from a positive passes similarly to the next positive through an opening in the intervening negative. These projec- tions form a continuous tube on each side, from top to bottom, in which are placed the copper electrodes, and melted lead is poured in around them, giving perfect ELECTRIC STORAGE. 249 metallic contact, and holding each set of plates firmly in position. The plates, thus built up, are immersed in the fluid in a hard-rubber vessel, rest on wooden blocks, and are charged, without reversal of current, in the cell designed for use. The E. M. F. is about 2 volts. Durability of Storage Cells. Manufacturers usually guarantee for the positive plates a durability of one year in constant practical use, with a normal current. The negatives are far more durable, not being subject to oxidation; and, unless injured by buckling, last for an indefinitely long period. Storage Capacity. The storage capacity of the I5-L. Faure cell, or the 3oo-ampere Pumpelly cell, is about 1,080,000 coulombs, equal to 30 ampere-hours. Hence such a cell may be discharged in i hour with a 3oo-am- pere current, or in 10 hours with a normal, 3o-ampere current; the time in seconds or hours, for a normal dis- charge, being estimated at -^ of the storage capacity in coulombs or ampere-hours. Relative Time of Charging and Discharging. The time required for charging a cell is estimated at 18 to 20 per cent more than that required for discharging it with the same current strength; that being the usual per- centage of loss of energy between the charge and dis- charge. Hence if a 300-ampere cell is discharged in 10 hours with a 3o-ampere current, 12 hours would be required to charge it with a 3o-ampere current, or 36 hours with a lo-ampere current. The current strength required for charging is estimated at 5 amperes per square foot of positive plate surface. The preparatory charging of the Pumpelly cell at the factory occupies only the same time as each subse- quent charging in actual use. Hence only about 60^ 25O DYNAMIC ELECTRICITY AND MAGNETISM. of the litharge on the negatives is reduced, at first, to spongy lead; the remainder being gradually reduced by use; which probably accounts in part for the fact ob- served, that the cell increases in energy during the first six months of use. The Hydrogen Alloy Theory. A new theory of the electrochemical action of accumulators has been pro- posed by Dr, Paul Schoop, based on the following facts: It has long been known that certain metals, as plat- inum, palladium, mercury, and iron, combine, under certain conditions, with hydrogen; and on the theory that hydrogen is a metal, these combinations are regarded as alloys. It is also well known that when platinum sponge, charged with hydrogen, is exposed to the air it be- comes rapidly heated to redness by the absorption of oxygen ; also that the charged cathode plate of an accumulator, when similarly exposed, has its tempera- ture raised, from the same cause, often to the melting point. Hence Dr. Schoop assumes that the spongy lead of the cathode, like the platinum sponge, absorbs the hydrogen liberated from the solution by the electric current during the charging; the hydrogen combining with the lead and forming an alloy, and the liberated oxygen combining with the material in the anode and forming the lead dioxide: and that during the discharge, oxygen liberated by the current from the dioxide com- bines with the hydrogen of the cathode, reducing the alloy to spongy lead and restoring water to the solu- tion; leaving the material in the anode with the same proportion of oxygen as before charging. This theory is simple, but defective in failing to ac- count for the formation of the lead sulphate, and its varying proportions during the charging and discharg- ing. It is not easy to see how hydrogen can unite with ELECTRIC STORAGE. 251 spongy lead incrusted with sulphate; so that unless the formation of sulphate, under normal conditions of the cell, be ignored, or a cell produced from which its for- mation shall be eliminated, the correctness of this theory must be considered questionable. DYNAMIC ELECTRICITY AND MAGNETISM. CHAPTER X. THE RELATIONS OF ELECTRICITY TO HEAT. THE mutual relations of heat and electricity are among the most important in electric science, whether considered with reference to the generation of elec- tricity, its transmission, its measurement, or its numer- ous forms of practical application. There can be no expenditure of electric energy without the simultaneous development of heat ; and it may also be assumed, though not so manifestly proved, that there can be no expenditure of heat energy without the simultaneous development of electricity. Heat Developed by Electric Transmission. According to the best evidence we have, electricity and heat are different kinds of molecular motion, and the transmis- sion of either is simply the extension of this motion through a material substance connected with the gen- erator, known as the conductor. When electricity is thus transmitted, its transmission is always attended by the evolution of heat, which must be considered a legit- imate part of the work done, whether useful or other- wise, and not a mere adjunct. This heat is found to be always in direct proportion to the electric resistance encountered; hence if the use- ful work to be done is the production of heat, or its concomitant, light, .the resistance is increased at the point where the heat or light is required: but if other THE RELATIONS OF ELECTRICITY TO HEAT. 253 useful work is to be accomplished, the heat is suppressed by lessening the electric resistance, as required. Thus the ratio of heat work to other work can be made to vary by varying the resistance. The analogy to this is found in the friction attendant on mechanical action, which may produce heat for a useful purpose, or be suppressed by the use of a lubri- cant when the mechanical energy is to be otherwise expended. Joule's Law. To determine accurately the relations between the electric current and the heat developed by it, Joule, who made this branch of electric science a specialty, passed a battery current through a fine wire coil inclosed in a vessel of alcohol, in which was also in- serted a thermometer. The resistance and current strength being known, were compared with the temperature to which the liquid was raised in a given time, and by this means were es- tablished the facts embodied in the following law: The heat developed in a conductor by an electric current passing through it varies as the CONDUCTOR'S RESISTANCE, the SQUARE OF THE CURRENT'S STRENGTH, and the TIME THE CURRENT LASTS. Representing the heat by H, the current by C, the | resistance by R, and the time by /, we get H '= C*Rt as the algebraic expression of this law, by which the heat developed in any electric circuit can be ascertained. Joule's Equivalent. Joule found that the amount of heat required to raise the temperature of i gramme of water i C. is equivalent, in work, to 42,000,000 ergs in C. G. S. measure; and this is known as Joule' 's equivalent. When the heat is produced by an electric current, the formula given above must be multiplied by 0.238 to re- duce the electric C. G. S. units to heat units; that being the ratio, expressed decimally, of 10,000,000, the C. G. S. 254 D YNAMIC ELECTRICITY AND MA GNE TISM. value of the electric units represented by C 2 -/?/, to 42,000,- ooo (10,000,000 -4- 42,000,000 0.238), and the formula is then^T^ C*Rt X 0.238. Heat Developed by Electrochemical Action. The ex- periments of Favre on the electrochemical development of heat fully establish the correctness of the principle, that the evolution of heat by electric action is in the inverse ratio of other work accomplished by the same action; and that the heat developed in the battery cir- cuit is the exact equivalent of the chemical energy expended in the cells, as first verified approximately by Joule. In these experiments he used a mercurial calorime- ter, so constructed that the mercury should surround the apparatus in which the heat was to be generated, and by its expansion register the amount of heat de- veloped. Placing in this instrument a vessel containing zinc and sulphuric acid, he found that the simple chemical consumption of 33 grammes of zinc produced 18,682 units of heat. He then replaced this vessel by a Smee battery cell, and noted the electrochemical con- sumption of the same amount of zinc ; varying the experiments by using connecting wires of different resistance, and also by comparing the evolution of heat when the cell was placed in the instrument and the connecting coil was outside, with its evolution when the coil was placed in the instrument and the cell was out- side. The results varied but slightly from that of the first experiment, the consumption of 33 grammes of zinc producing 18,674 units of heat. The first experiment showed the amount of heat developed by a given amount of chemical action, measured by the consumption .of the zinc; and the second proved that practically the same amount of heat was developed in the battery circuit by this amount of chemical action in the cell. THE RELATIONS OF ELECTRICITY TO HEAT. To show the mutual relations between electric heat and other electric work, a battery of 5 Smee cells, joined in series, was placed in the calorimeter, and connected with a small electromagnetic engine; and the evolution of heat during the consumption of 33 grammes of zinc noted in three different experiments, as follows: i. With the engine at rest the heat evolved was 18,667 units. 2. With the engine running, but doing no work except to overcome its own friction and inertia, the heat evolved was 18,657 units. 3. When the engine by raising a weight did 12,874,000,000 ergs of work, the heat evolved was 18,374 units. Dividing the number which repre- sents the work in the last experiment by Joule's equiva- lent (42 X io 6 ) gives 306 heat units, and 18,374 -j- 306 = 18,680. Hence, with proper allowance for unavoidable discrepancies, we find that in the last three experiments, as in the first two, the heat evolved was equivalent to the chemical energy expended; while the last experi- ment proved that the evolution of heat is in the inverse ratio of other work; the heat which disappeared being reproduced as work; a result conformable to the doc- trine of the conservation of energy. Electro-Thermal Capacity of Conductors. Since the heat developed in a conductor by an electric current varies as the resistance, and the resistance varies with the nature of the material, and also directly as the length and inversely as the cross-section of the conductor, it follows that material, mass, and ratio between length and cross-section must each be considered in estimating the conductor's electro-thermal capacity. In conductors of equal length and cross-section but different conductivity, the heat developed in each by the transmission of currents of equal strength varies in- versely as the conductivity, or, which is the same, directly as the resistance due to difference of material. 256 DYNAMIC ELECTRICITY AND MAGNETISM. Thus german-silver having about 13 times the electric resistance of copper, the heat developed in a german silver wire would be about 13 times that developed in a copper wire of the same dimensions, carrying a current of equal strength. But in conductors of the same material and mass, the resistance, and hence the heat development, varies directly as the ratio of length to cross-section, and in- versely as the ratio of cross-section to length. Suppose 100 units of heat to be developed by the passage of a current through a wire 10 feet long, then only 10 units would be developed by the same current in a section of the same wire i foot long; hence if the wire be re- garded as made up of 10 sections arranged in series, 10 units is the amount developed in each section suc- cessively. Now suppose a current of the same strength passed through a wire of the same material and mass, i foot long; the cross-section of this wire would evi- dently be 10 times as large as that of the other wire, consequently the resistance and heat development would be only ^, that is 10 units; the effect being the same as if the current passed through the 10 sections in parallel. But as the 10 units are equally distributed through the mass, only i unit of heat is developed in each section; that is, T V the amount developed in each section, or same mass, of the long wire, or series con- nection. Suppose a current of the same strength passed through a wire of the same material and mass, 100 feet long; the length being 10 times that of the original wire, the cross- section would evidently be only T ^; hence the resistance and heat development would be 10 times as great, equal to 1000 units, or 10 units to each foot. But these 10 units being developed in -fa of the original mass per foot would raise the temperature to 10 times the original THE RELATIONS OF ELECTRICITY TO HEAT. temperature per foot, or 100 units. Now since the cross- sections of wires vary as the squares of their diameters, and the heat development varies inversely as the cross- section, -fa the cross-section producing 10 times the heat, it is evident that the rise of temperature in a conductor, or heat development per unit of mass, varies inversely as the fourth power of the conductor s diameter. The heat development per unit of mass, as illustrated by the last example, deserves special notice. The number of heat units developed in a foot of the ten-foot wire was found to be just the same as in a foot of the hun- dred-foot wire, 10 units in each, though the rise of tem- perature in the last was 10 times as great, being in- versely proportional to the reduction of mass. Hence each wire, if immersed in an equal mass of the same liquid, to which its 10 heat units should be impacted, would produce the same rise of temperature, as indi- cated by a thermometer; for though the section of small wire becomes 10 times as hot as that of the large wire, it has only ^ of the mass, and hence only the same heating power. These principles have a highly important useful ap- plication, especially in electric lighting, which will be separately considered in a future chapter; but there are several minor applications, some of the more important of which may be noticed here. Electric Blasting. The explosion of a blast can be safely and expeditiously effected at any required dis- tance, by inclosing a fine wire of high resistance, usually platinum, in the fuse, and connecting it with a battery circuit of low resistance, conveying a current of the re- quired strength. When the circuit is closed, the cur- rent, which produces scarcely a perceptible change of temperature in the main conductor, instantly raises the platinum wire to a white heat, producing the explosion. 258 DYNAMIC ELECTRICITY AND MAGNETISM. In this way blasts under water are fired, and mines and torpedoes exploded. The explosion of the great blast under the ledge of rock in Hellgate, New York harbor, by the touch of a child's finger closing the circuit, is a noted instance of this. Electric Cautery. In surgery a fine platinum wire, heated to incandescence by an electric current, is pre- ferred to the knife for certain purposes; the operation, which is known as electric cautery, being more safely and expeditiously performed in this way; as in amputation of the tongue for cancer, the removal of an excrescence, or of superfluous hair from a lady's face. Electric Fuses. As conductors carrying strong cur- rents are liable, from accidental causes, to become over- heated and ignite inflammable matter in close prox- imity, a short section of a soft compound metal of high resistance, technically known as a fuse, is introduced into the circuit at any convenient point. The cross- section of this fuse is so adjusted to the normal strength of the current carried, that if, from any abnormal in- crease, the temperature approaches an unsafe degree, the fuse melts and opens the circuit. The metals forming the compound are usually lead, tin, bismuth, and antimony, combined in different pro- portions according to the melting temperature, and other properties required. Fuses are usually from J to | of an inch long, and from ^ to J of an inch in diame- ter, and adjusted to carry currents of from ^ an ampere to 200 amperes, without fusion; the melting temperature being made, by adjustment of cross-section, about 20 per cent above the carrying temperature in the large fuses, and about 50$ above in the small ones, when inclosed. The reason of this is found in the nature of the composition required for each; bismuth and lead, which melt at a comparatively low temperature, THE RELATIONS OF ELECTRICITY TO HEAT. 259 entering largely into the composition of the small fuse to give it the requisite tenacity, while tin and antimony, which have a higher melting temperature, but are more brittle and less expensive than bismuth, predominate in the large fuse, in which there is less risk of fracture, and in which economy of material is less of an object. In the open air the melting temperature of the large fuses is about 5$ higher than when inclosed, and that of the small ones about 8$ higher. As the conductivity of metallic conductors decreases with rise of temperature, and as the radiation of heat increases with increase of cross-section, both these points must also be considered; so that the proper con- struction of fuses, including material, cross-section, carrying capacity, and melting temperature, adapted to varying conditions, is a difficult scientific problem, and one of great practical importance. If a fuse melts too easily it becomes a source of annoyance from frequent, unnecessary interruption of current, while if its tem- perature of fusion is too high it fails to afford protec- tion against fire. Fuses are connected by binding-screws to insulating blocks, to which the terminals of the conductor are also similarly attached, and hence are easily replaced at a nominal expense when melted; several fuses, connected with different circuits, or different branches of a circuit, being often arranged in the same block. Thermo-Electric Generation. Before entering fully upon the consideration of thermo-electric generation, it is important to present certain general principles of electric generation which have a special bearing on this branch of our subject. An examination of the various kinds of apparatus by which electricity is generated shows that the con- struction, in every case, involves the following con- 260 DYNAMIC ELECTRICITY AND MAGNETISM. ditions: i. A complete insulated electric circuit composed of heterogeneous materials. 2. Molecular excitation at one or more points in this circuit. And it may be safely assumed that in all cases where these conditions are fulfilled, either by natural or artificial means, electricity is gen- erated, even though such generation may not be appar- ent. These conditions are a legitimate consequence of the law of the conservation of energy as applied to elec- tricity considered as a mode of molecular motion. For if the circuit were not complete, molecular motion, ex- cited at any point, must soon cease; for the continuous storing of energy in one place implies its removal from another place, and this cannot continue indefinitely, nor for any considerable time, without a connection between the two places by which the transferred energy can re- turn to the place of its origin. The same would be true if the circuit were complete but perfectly homogeneous throughout, both as to material and resistance, for mo- lecular motion would then be transmitted equally in opposite directions, and the meeting of these equal, opposing currents would stop the flow, producing a re- sult similar to that in the former case. But if, from difference in the nature of the materials, or in their resistance, or both, molecular motion is more free to extend itself in one of two opposite directions than in the other, and by a transfer of energy, incident to such extension, there occurs a corresponding reduc- tion of such motion in the opposite direction, that is, in electric language, if the current becomes positive in one direction and correspondingly negative in the oppo- site, it is evident that this motion must extend itself round the circuit continuously, or the current continue to flow from higher to lower potential, so long as the exciting cause continues; the transferred energy, which THE RELATIONS OF ELECTRICITY TO HEAT. 261 produces the molecular motion, being again restored to the place of its origin. Just as water in a circular trough, receiving a continued impetus in the same direction at any point, would flow round continuously. This is precisely what occurs in a battery circuit or in the circuit of an electrostatic machine; materials differing in molecular constitution and resistance, as brass, glass, hard-rubber, pointed conductors and spherical conduc- tors, in the machine, and zinc, fluid, carbon or its equiv- alent, and copper, in the battery, forming the circuit, which is so arranged in each case that electric action, beginning at a certain point of junction of different materials, is continuously transmitted more easily in one direction than in the opposite; mechanical action being the exciting cause in the machine and chemical action in the battery; and the energy, whether mechanical or chemical, thus absorbed, reappearing as electricity. We may now consider the application of these prin- ciples to thermo-electric generation. In 1821 Seebeck made the discovery that an electric current could be generated by heating or cooling the junction of two dis- similar metals connected in an electric circuit. See- beck's experiment may be repeated by soldering or fusing together the ends of short pieces of any two metals, differing materially in molecular constitution, as bismuth and antimony, or german-silver and iron, and connecting their free ends, electrically, with a deli- cate galvanometer. On heating the junction to a tem- perature above the rest of the circuit, by a spirit-lamp or otherwise, the needle will be deflected, showing the generation of an electric current, and the same effect, with reversed current, will be produced if the junction be correspondingly cooled, as may be done by the appli- cation of ice; the direction of the current when the junction is heated being from bismuth to antimony, and, 262 DYNAMIC ELECTRICITY AND MAGNETISM. when cooled, from antimony to bismuth; the E. M. F., or potential difference, being in proportion to the differ- ence of temperature between the junction and the other parts of the circuit. Hence, in such a combination, composed of one or more couples, if each alternate junction be heated and the intervening junction simul- taneously cooled, the E. M. F. is proportionally in- creased, the current being from bismuth to antimony across each heated junction, and from antimony to bismuth across each cooled junction, and hence in the same direction round the circuit; and the same would be true of a circuit composed of any other metals hav- ing similar molecular differences. As the capacity of bismuth for heat is much lower than that of antimony, its rise of temperature with the same increment of heat is proportionally greater, and also its fall of temperature with the same abstrac- tion of heat; and as we find that the electric current flows from bismuth to antimony across the heated junc- tion, and oppositely across the cooled junction, it is evident that its flow in each case is from the hotter to the colder metal. But we know that the flow of an electric current is always from higher to lower potential, and in the direction of least resistance, and also that rise of temperature in a metal increases its electric re- sistance; hence we must infer that, in this case, increase of potential and resistance accompany rise of temper- ature, and decrease of potential and resistance accom- pany fall of temperature in each metal respectively,, creating a preponderance of both in the hotter metal. On the molecular theory, it is the propagation of molecular motion, with heat as the exciting cause, which constitutes the electric current; heat undulations being,, in some occult manner, transformed into electric un- dulations. Only a part of the heat supplied is thus; THE RELATIONS OF ELECTRICITY TO HEAT. 263 transformed, the remainder being either radiated, or appearing as heat in elevation of temperature in the circuit; and likewise when heat is abstracted, the re- maining heat, set in motion toward the junction by the cooling, is in part transformed into electricity, while the remainder is either radiated, or appears as heat in the reduced temperature of the circuit. In the above experiment the complete circuit is com- posed of three metals, copper forming the galvanometer coil and connections, though the generating part, or thermal battery, as it might be termed, is composed of only two metals. But the experiment may be performed with a circuit composed strictly of but two metals. For this purpose let a frame be constructed with a strip of copper or iron, of any convenient length, having its ends bent and soldered to a parallel bar of bismuth; or let it be bent so as to have parallel sides, and its free ends be connected by a cube of bismuth soldered to each. If this frame be mounted on an insulating stand, and a magnetic needle poised on a pivot at its center, the needle will be deflected by heating or cooling one of the junctions, or by heating one junction and cooling the other, as in the former experiment. No thermo-electric current can be generated in a cir- cuit composed of a single metal of perfectly homoge- neous molecular structure; but even with a slight dif- ference, such as may be produced by a coil or twist in a wire, a perceptible current may be obtained, which be- comes more marked with increased difference of struc- ture, as between differently manufactured kinds of the same metal, having different degrees of hardness, brit- tleness, or tenacity: and with continued increase of molecular difference, as between different metals, thermo- electric development increases in like proportion. Which proves that this development is dependent on molecular 264 DYNAMIC ELECTRICITY AND MAGNETISM. structure, indicating an intimate relation, if not actual identity, between electricity and molecular motion. It is found that lead shows no perceptible difference of thermo-electric potential at different temperatures, like other metals; hence it has been chosen as the stand- ard by which the relative thermo-electric potentials of other metals may be compared. In making such com- parison the same mean temperature must be chosen for the various metals, since the relative thermo-electric potentials of different metals varies greatly at different temperatures. Taking the microvolt (y-^ooooF f a volt) as the unit of potential, and i C. as the heat unit, the following metals, at a mean temperature of 19 to 20 C., show, according to Matthiesen, the relative thermo- electric potentials indicated, in microvolts, when the temperature of the junction between any two of them is raised i C. above the rest of the circuit: Bismuth, Commercial, Pressed Wire 97 Bismuth, Pure, Pressed Wire 89 Bismuth, Crystal, Axial 65 Bismuth, Crystal, Equatorial 45 Cobalt 22 Mercury .418 Lead o Tin .1 Copper, Commercial .1 Platinum .9 Gold - 1.2 Antimony, Pressed Wire 2.8 Silver, Pure Hard - 3 Zinc, Pure Pressed - 3-7 Copper, Electrolytic 3.8 Antimony, Commercial, Pressed Wire 6 Arsenic 13. 56 Iron, Soft 17.5 Antimony, Axial 22.6 Antimony, Equatorial 26.4 Tellurium 502 Selenium 807 THE RELATIONS OF ELECTRICITY TO HEAT. 26$ Thus any two or more of these metals, arranged in this order in a series, would acquire this relative poten- tial difference with a heat increment at the junction, or junctions, of i C., and a mean temperature of 19 to 20 C.; each being electropositive to all that follow, and electronegative to all that precede it. The potential difference between bismuth and cobalt, for instance, is 97 22 = 75, and between copper and antimony, 26.4 3.8=22.6, while between bismuth and antimony it is 97 -|- 26.4 = 123.4; difference between any two above or below zero being obtained by subtraction, while differ- ence between one above and one below is obtained by addition. In a series composed of any of these metals, arranged as above, the entire potential difference, or thermal E. M. F., is found, as in a battery series, to be equal to the sum of all the differences between each separate couple. Each couple may thus be regarded as a ther- mal cell, or element, the two metals corresponding to the two electrodes in a battery cell, heat energy, instead of chemical energy, being the exciting cause. Hence in any similar series of metals, ABCD, the sum of the potential differences between each couple, as A and B, B and C, C and Z>, is the same as the potential differ- ence, or E. M. F., between the extremes A and Z>; so that if a direct junction were made between A and Z>, and the intervening couples omitted, the E. M. F. would be the same as in the full series, as may be verified numerically in any series chosen from the table. In a circuit composed of several couples of any two metals, alternate junctions require to be either heated or cooled, or each alternate junction heated and the in- tervening one simultaneously cooled, as the heating or cooling of two adjacent junctions to the same tempera- ture would produce opposite, neutralizing currents. 266 DYNAMIC ELECTRICITY AND MAGNETISM. The table is not intended to embrace all the sub- stances which manifest electro-thermal properties, but only a few of the metals in which those properties are prominent; such properties being common to a large class of bodies, both metallic and non-metallic. The potential differences given must be understood to apply only in a general sense, as differences of molecular structure produce, as shown, great variation in this re- spect; so that in different experiments the results ob- tained from the same metals procured from different sources would be only approximately the same. Thermo-Electric Diagrams. Sir William Thomson pro- posed a graphic representation of the relative thermo- electric potentials of different substances at different degrees of temperature, consisting of a diagram having vertical lines representing the differences of tempera- ture, and lines approximately horizontal representing the thermo-electric differences of potential, and in 1856 used such a diagram for the first time to illustrate a lecture. Fig. 90 shows a diagram by Prof. Tait con- structed in this way. Lead, being thertno-electrically constant at different temperatures, is represented by a perfectly horizontal line, marked o, while the other metals are represented by lines tilted at the various angles required to show their relative thermo-electric differences, at different temperatures, with respect to lead, and hence with respect to each other. Lines rep- resenting metals whose potential difference with respect to lead increases negatively or decreases positively with increase of temperature descend from left to right; while those representing metals whose potential differ- ence increases positively or decreases negatively with increase of temperature ascend from left to right. Thus zinc, at 19 to 20 C., is shown to be about 3.7 below THE RELA TIONS OF ELECTRICITY TO HEA T. 267 268 DYNAMIC ELECTRICITY AND MAGNETISM. lead, as given in the table, while at 480 C. it is 15' below. Thermo-electric differences may be represented also by the areas formed by the lines. Thus in a zinc-iron couple, with one junction at 100 C. and the other at o C., the thermo-electric difference is represented in the diagram by the area a, b, c, d. For small tempera- ture differences, of one or two degrees, the superficial contents of the areas are practically the same as be- tween rectangles, and hence are obtained by simply multiplying the temperature differences by the poten- tial differences; but for large temperature differences the irregular shape of the areas requires special calcula- tion in each case. The Peltier Effect. Peltier's observations on the ther- mo-electric circuit led him to the natural conclusion that if difference of temperature at the junctions could generate an electric current, then, conversely, the pas- sage of an electric current through such a circuit must generate a corresponding difference of temperature at the junctions, and experiments made by him in 1834 verified this conclusion. Hence this phenomenon, which is now a well-established thermo-electric law, has been called the Peltier effect, in distinction from the genera- tion of heat by the resistance of the circuit, as observed by Joule, which is known as the Joule effect. These two effects are entirely consistent with each other and may occur simultaneously in the same cir- cuit. For, in any circuit, whether composed of one metal or several, the temperature varies in proportion to the square of the current's strength, in accordance with the Joule effect; but if the circuit is composed of different metals, or different kinds of the same metal, there occurs also a transfer of heat from one junction to another in proportion simply to the cur- THE RELATIONS OF ELECTRICITY TO HEAT. 269 rent's strength, so that one junction is heated while the other is correspondingly cooled, in accordance with the Peltier effect. In the Joule effect the amount of heat generated in the circuit, as a whole, is not varied by the direction of the current, while in the Peltier effect the transfer of heat is reversed by reversal of current; so that junctions heated by a current flowing in a given direction, as from antimony to bismuth, are correspondingly cooled by the same current flowing in the opposite direction, as from bismuth to antimony, while the alternate junctions cooled in the first instance are correspondingly heated in the second. The reduc- tion of temperature in a bismuth-antimony combination may thus become so great as to freeze water in a cavity at the cooled junction. Thermo-Electric Inversion. Prof. J. Gumming found that, in a copper-iron couple, iron ceases to be electro- negative to copper when the temperature of the junc- tion is raised to 280 C.; the current from copper to iron ceasing, and the Peltier effect also disappearing, when a current is transmitted in either direction. But when the temperature of the junction is raised above 280 C., iron becomes electropositive to copper and the Peltier effect is also renewed. This is illustrated by the iron and copper lines in the diagram which cross each other at the neutral point, iron being represented as electronegative to copper on the left of this point and electropositive on the right: similar thermo-electric inversion being also shown in other metals. The potential differences given in Tait's diagram vary somewhat from those given in Matthiesen's table, with which Cumming's experiments seem to accord more closely; only approximate accuracy being attainable in different experiments, for the reason already given. The Thomson Effect, This thermo-electric inversion 2/0 DYNAMIC ELECTRICITY AND MAGNETISM. led Sir William Thomson to conclude that since there is no heat development, or Peltier effect, at the junction of a copper-iron couple, at 280 C., by the passage of an electric current through it, therefore, conversely, there can be no accumulation of heat at this point, at like temperature, when the current is generated by heat supplied to it, and therefore the heat supplied must be absorbed by other parts of the circuit than the junctions, and hence must pass between differently heated parts of the same metal. Experiments with different metals verified this con- clusion, showing that when a thermo-electric current passes through a conductor, from a hotter to a colder part, there is a transfer of heat, which in some metals, as copper, is from the hotter to the colder part, while in others, as iron, it is from the colder to the hotter part: but when the direction of the current is from a colder to a hotter part this transfer is reversed. This electric convection of heat in the same metal is known as the Thomson effect, in distinction from the Peltier and Joule effects. It follows from the above that in a copper-iron cir- cuit, when a current is generated by heating a junction to any temperature below 280 C., the current being from copper to iron is from cold to hot in the copper and from hot to cold in the iron, so that in both metals heat is transferred to the junction; but when the tem- perature of the junction is raised above 280 C., the current being reversed, the heat transfer is also reversed, and is from the junction in both metals: while with the junction at 280 C., there being no current, there is no transfer of heat in either direction. And the same principles apply to any circuit having similar thermo- electric inversion. The Thermopile. It has not yet been found possible THE RELATIONS OF ELECTRICITY TO HEAT. to constiuct electric generators of general practical efficiency on the principle of the direct conversion of heat into electricity. Generators of this kind, con- structed by Clamond and others, have not fulfilled the hopes raised by their first apparent success; the genera- tion of strong currents, combined with the heat neces- sary to produce them, seems to effect, in a short time, such permanent change of molecular structure as to reduce the production and maintenance of potential difference between the different metals below the point of practical efficiency. The difficulties in such construction become further manifest when we consider that comparatively few of the metals given in the table are practically available for this purpose, either in consequence of small poten- tial difference, extreme rarity, as in the case of tellurium and selenium, volatility when heated, or other cause. Of the available metals, bismuth and antimony have the highest potential" difference, and can be used at moder- ate temperatures; bismuth melting at 264 C. and antimony at 450 C. The thermopile, represented by Fig. 91, is constructed with a number of small, short metal bars, usually of bismuth and antimony, arranged side by side in couples, junctions being formed between each pair of ends in alternate order, by soldering or fusing; the arrangement being such that the current must pass from one metal to the other through the entire series. By having as many layers as there are bars in a layer, a compact, cubical form is obtained. The bars thus arranged and properly insulated are inclosed in a brass case, open at the ends, and mounted on a stand provided with apparatus for elevating and adjusting them to any required height or angle. Coni- cal caps are fitted to the ends to admit the heat radiated 2/2 DYNAMIC ELECTRICITY AND MAGNETISM. from any object whose temperature is to be tested, and to exclude radiation from other sources; and binding- screws are provided for the galvanometer connections. The alternate junctions being at opposite ends, one set may be exposed to heat while the alternate set are cooled; and the entire potential difference, or E. M. F., being equal to the sum of the potential differences in FIG. 91. the series, a very sensitive apparatus, for investigating slight differences of temperature, may be obtained, when the instrument is used in connection with a sensitive galvanometer; the needle responding instantly with a prominent movement, easily read from the scale, to temperature differences hardly perceptible in the ther- mometer: the heat generated by the bending of a copper wire being sufficient to produce a deflection of several degrees. The highly important investigations of Melloni and of Tyndal on heat were conducted with the aid of such an apparatus. THE RELATIONS OF ELECTKICITY TO HEAT. 273 The relative E. M. F. of the thermopile as compared with other generators is very small. Taking 200 micro- volts as the average E. M. F. attainable by the simul- taneous heating and cooling of the opposite junctions in a single bismuth-antimony couple, the total E. M. F. in a thermopile, or multiplier, of 50 such couples would be 50 X 200 = 10,000 microvolts, or T iHHHHr = TOO of a volt; so that the combined E. M. F. of 100 such genera- tors would be only equal to that of a single Daniell cell. But as the comparative resistance of the thermopile is also small, the current is comparatively large: suppos- ing the resistance in the above case to be -j 1 ^ of an ohm, then 3^ - ^C, or ^ of an ampere. In the largest Clamond thermo-electric batteries, consisting of 150 iron-galena elements, the estimated E. M. F. is only 5^ volts, and the internal resistance 2 ohms, which would give a current of 2 f ^ amperes, in an external cir- cuit of no resistance. A Daniell battery of the same number of elements similarly joined, in series, would have an E. M. F. of about 150 volts and an internal resistance of about 300 ohms, which would give a current of an ampere, in a similar external circuit, less than that of the thermo-electric battery, though its E. M. F. is 28 times as great. Electric Welding. This highly important application of electricity has been largely developed by Prof. Elihu Thomson since 1886, and has now attained a wide range of practical work. It consists in uniting pieces of metal by pressing them together, end to end, and heating the juncture by an electric current till the metal becomes sufficiently plastic to form a perfect joint; only so much of it being included in the circuit as may be necessary for this purpose. The alternating current is employed, and applied by 2/4 DYNAMIC ELECTRICITY AND MAGNETISM. the welder shown in Fig. 92, which consists of a con verter and clamping apparatus combined. The con- verter, shown in the rear, is constructed with a laminat- ed iron core inclosing a massive copper tube, equivalent FIG. 92. to a single coil, which forms the secondary circuit. The primary circuit consists of an insulated copper coil wound in two sections through the interior of this tube, as shown, and inclosing its upper and lower parts together with the adjacent parts of the core. This circuit is connected with the dynamo by the terminal THE RELATIONS OF ELECTRICITY TO HEAT. 2?$ wires shown in the rear, and the secondary circuit is connected, on the right, with a massive grooved copper bar, to which is fitted the copper sliding-block A. Two massive copper clamps, C and C, grasp the two bars to be welded together, the right one movable in connection with the sliding-block A, and the left fixed; and to this fixed clamp the secondary circuit is con- nected on the left. Pressure being applied to the block A by the crank B and connected gearing, the right bar to be welded is forced against the left; the circuit being opened and closed by a switch connected with a treadle, and the current regulated by a reactive coil connected with the primary circuit. A dynamo, specially adapted to this work, furnishes a current which, in the 2o,ooo-watt welder, has an E. M. F. of 300 volts at the terminals of the primary circuit, which is reduced, in the secondary circuit, to about i volt ; and the efficiency being about 80 per cent, the maximum current is about 16,000 am- peres. The welding capacity of a welder of this size, for bar-iron, ranges from bars f of an inch in diameter to bars of i inch diameter; the range for brass being three fourths of this, and for copper one half. To adapt the welder to different kinds of work, its primary circuit is connected in series to an auxiliary converter of special construction, by which the E. M. F. can be more fully controlled. The primary circuit of this converter is wound on a section of a laminated iron core, composed of a split ring, the slit being on the opposite side from the coils, which are so arranged that they can be joined either in series or parallel by a switch. The core incloses an iron armature, upon which is mounted the secondary circuit, consisting of a massive brass casting, which also includes a section of 2/6 DYNAMIC ELECTRICITY AND MAGNETISM. the core and may be rotated so as to include either the primary circuit or the slit, as required. When rotated so as to include the primary, the E. M. F. is reduced to one half that which it is when the secondary is opposite the slit, across which no lines of force can pass, and where the magnetism is therefore at its minimum. The E. M. F. can also be either increased or diminished by joining the coils of the primary, either in series or in parallel; hence its variation in the welder, by these vari- ous means, includes a wide range. The above, known as the indirect method, is em- ployed for the heavier and more complicated kinds of welding, and where several welders are operated by current from a single dynamo ; while for the lighter, simpler kinds the direct method is employed, in which the welder is connected directly to the dynamo, the armature of which has a high potential circuit of fine wire, in series with the field-magnet coils, which acts inductively on a low potential circuit, composed of a massive, U-shaped, copper bar, connected directly to the welding apparatus, the construction of which is the same as already described. The direct current may be employed, but the alter- nating is preferred on account of its higher efficiency and freedom from electrolyzing effects, a point of spe- cial importance in the welding of alloys. The ends to be welded together are rounded so that contact shall be first made at the center, and the weld being from the center outwards, oxidized particles and other impurities are forced out as the ends are pressed together, making a perfect joint, superior to any which pan be made by forging; and the entire process being thus open to the inspection of the operator, flaws cannot escape observation. Manual pressure can be employed in ordinary cases ? but for more difficult welding, where THE RELATIONS OF ELECTRICITY TO HEAT. 277 great accuracy is required, pressure by hydraulic or other mechanical power is preferred. The greatest heat is developed at the center of the weld, extending only a short distance on either side, and varying directly as the resistance, which increases with the rise of temperature. Bars of inch iron become red hot for a distance of i^ inches on either side of the weld, but are comparatively cool at a distance of 2\ inches ; and the operation being completed in 40 seconds, the time is too short for diffusion of the heat by conduction ; hence waste of energy from this source is reduced to the minimum. The time varies for metals of different kinds and sizes, from i or 2 seconds for fine wires to 2 or 3 minutes for heavy bars; wrought-iron bars, 2 inches in diameter, requiring an average time of about 97 seconds ; 2j-inch iron pipes, \ inch thick, 61 seconds ; the average time for copper bars being about -| that required for wrought-iron bars. The E. M. F. is so low that the enormous current re- quired for heavy work is perfectly safe; and conductors carrying currents of many thousand amperes, but having an E. M. F. of only a fraction of a volt, may be handled with impunity and without sensible effect. The range of application is almost unlimited, em- bracing not only all welding hitherto considered practicable, but a large amount considered either wholly impracticable or extremely difficult, ranging from the most refractory metals to alloys fusible at 90 C. Not only can such metals as cast-iron, copper, lead, tin, zinc, brass, german-silver, and bronze be welded, each to its own kind, but any of these dissimilar kinds can be welded together. Steel cables composed of a large number of fine wires, tubing, and various kinds of metal work usually united by screws, rivets, soldering, or brazing, can be welded by this process; 278 DYNAMIC ELECTRICITY AND MAGNETISM. also articles which, from their peculiar shape, are diffi- cult or impossible to weld in the ordinary way. It has also a highly important application in the expeditious repairing of broken machinery on ships, in factories, and elsewhere. Welds made by this process have been subjected to the severest practical tests by the United States naval authorities and various, eminent, civil and electrical engineers, and have received their unqualified approval for superior strength and tenacity. THE RELATIONS OF ELECTRICITY TO LIGHT. 2/9 CHAPTER XL THE RELATIONS OF ELECTRICITY TO LIGHT. The Relations of Electric Heat to Electric Light. Tt has been shown that heat is always a result of electric resistance, and is in proportion to such resistance; and as a certain degree of resistance is found in every con- ductor, it follows that heat always accompanies electric transmission. When the heat increases to a sufficient degree of intensity, light is produced, either by incan- descence or combustion according to the nature of the medium of transmission. Hence the electric genera- tion of light follows that of heat and is dependent on heat intensity; heat being produced without light, but light never being produced without heat. Heat and light, according to well-established theories, being considered different modes of molecular motion, if electricity also be so considered, the difference between the three would seem to consist in the nature of the motion in each case, and may be attributed to differences in the length, amplitude, rapidity or phase of undulation peculiar to each, as pertaining both to the molecules of the conductor, and to the medium of transmission through space. Neither phenomenon is developed at the expense of the other, except as the nature of certain conductors produces variation between the development of heat and electric current; hence if these are different kinds of molecular motion, they must occur in such a manner as not to neutralize each other. It has been shown how such different kinds of motion may coexist without interfer- 280 DYNAMIC ELECTRICITY AND MAGNETISM. ence in the magnet, and similarly here, motion whose general direction is in lines, straight, curved, or spiral, would not interfere with transverse undulations, nor would either interfere with rotary motion of the mole- cules. It is not impossible that two or more of these phe- nomena may be identical; that the heat undulations, or the electric undulations, or both, are, at a certain degree of intensity, recognized as light; though, in the present state of our knowledge, it is more in accordance with observed facts to assign to each a distinct mode of motion; that of heat being comparatively slow, with considerable length and amplitude of undulation, while those of light and electricity are inconceivably rapid, with undulations of a corresponding character. We have seen that heat and electricity reproduce each other directly, but that in the electric production of light, heat intervenes, and that the light is appar- ently a result of the heat rather than of the electricity; for when the heat is produced by any other method, light usually follows increase of heat intensity in the same manner, though we have no direct evidence of the presence of electric action: and yet it is not im- possible that electricity, though occult, may be present as an active agent, or that the light and the electricity may be identical. Photo-Electric Generation. While the direct generation of light by electricity is not clearly apparent, the direct generation of electricity by light has been effected ex- perimentally, though it has not yet been found possible to construct practical generators on this principle. The first experiment of this kind was made by Bec- querel about 1850, who found that when one of two silver plates, freshly coated with silver chloride and im- mersed in water, is exposed to light, an electric cur- THE RELATIONS OF ELECTRICITY TO LIGHT. 281 rent, indicated by a connected galvanometer, flowed to the exposed plate from the opposite pole. In 1875-6 Adams and Day, English electricians, made a very extensive series of experiments to ascertain the electric relations of selenium to light; one result of which was the discovery of electric generation by this metal under the influence of light. A small piece of selenium, whose electric resistance had been reduced by annealing, had platinum terminals fused into its oppo- site ends; the platinum wire being formed into little rings on the inserted ends, to giver fuller contact. On exposure of the selenium to candle-light the passage of an electric current was indicated by a prominent deflec- tion in a connected galvanometer; the direction of the current being from the part least exposed to the part most exposed, a result similar to that in Becquerel's experiment. That this was not a thermo-electric current was proved in various ways: i. The current began promptly with the exposure, and ceased promptly with the ex- clusion of the light, instead of showing the more grad- ual increase and decrease of current due to heating and cooling. 2. The current, in most of the experiments, was the result of exposure of the body of the metal, while the thermo-electric current results from exposure of the junctions. 3. When the light was focused on a junction the direction of the current was from selenium to platinum, while that of a thermo-electric current would have been from platinum to selenium; this direc- tion being also, as will be perceived, from the least to the most exposed part of the selenium, as before. In 1887, Prof. Edlund constructed a generator by melting a very thin layer of selenium on a disk made of a metal with which it could unite chemically, and covering this layer with gold-leaf made so thin that the 282 DYNAMIC ELECTRICITY AND MAGNETISM. sunlight could penetrate to the selenium. Connection with a galvanometer being made between the gold-leaf and lower disk, an electric current was developed on exposure to the sun's rays, which responded promptly to the influence of the light, and ceased promptly with its exclusion, thus proving its photo-electric character, as in the former example. Photo-Electric Reduction of Resistance in Selenium. The electric resistance of ordinary, vitreous selenium is 3.8 X io 10 38,000,000,000 times that of copper, but when annealed by being kept for several hours just be- low the point of fusion, 220 C.,and then cooled slowly, it becomes crystalline and its resistance is materially re- duced. The difference of crystalline structure produced by the more rapid cooling of the exterior than the in- terior has been assigned by Gordon as a probable reason for its property of photo-electric generation. It was found by Adams and Day that the resistance of this annealed selenium, when a battery current is passed through it, is much less in the light than in the dark; the resistance varying directly as the square root of the quan- tity representing the illumination. Bell and Tainter utilized this property of selenium in the construction of their phot l ophone. A narrow strip of selenium connected at the edges with broad plates of brass furnishes a photo-receiver of large surface ex- posure and of low resistance as respects form; the se- lenium furnishing a resistance highly sensitive to light and varying under its influence from 300 ohms to 150. This photo-receiver being placed in a battery circuit connected with a telephone receiver, the varying light reflected from a distant point by a thin mirror, con- stituting the disk of a telephone transmitter, which re- sponds to the undulations of the voice, produces corre- sponding variations in the battery current, by which the THE RELATIONS OF ELECTRICITY TO LIGHT. 283 voice is reproduced in the telephone receiver, as ex- plained in connection with the telephone. Tellurium has the same photo-electric properties as selenium in less degree, and carbon also shows similar properties. Polorization of Light. The ether undulations, supposed to constitute light, are believed to be transverse to the direction of the rays. This transverse undulation is supposed to be equal in all directions within a circular space, so that the theoretical conception of a ray viewed endways in cross-section would be that of a circle com- posed of an infinite number of planes of undulation in which the undulations, by mutual adaptation, occur without interference. As if numerous fine wires, each bent into short curves, in the same plane, at right angles to the wire's length, were fitted together so as to form a long slender cylinder, with these curves crossing each other at all possible angles along its central axis. But under certain conditions of transmission and re- flection, the ray becomes flattened, as if compressed be- tween opposite lateral forces, so that these undulations all occur in one plane, and the ray is then said to be polarized. This happens when light is transmitted through cer- tain crystals, especially tourmaline. If two thin plates of tourmaline be placed with their surfaces parallel to each other, and a ray of light be transmitted through them at right angles to their surfaces, and to a certain direction in each, known as its optical axis, the light will pass freely through both to a screen beyond, so long as these axes* are parallel. But if either crystal be turned so that the optical axes are at an angle, the sur- faces being still parallel, the light which passes through one is obstructed in the other, gradually disappearing from the screen as the angle increases, till at 90 it is en- 284 DYNAMIC ELECTRICITY AND MAGNETISM. tirely extinguished. If the rotation be continued in the same direction, the light gradually reappears on the screen, and regains its original brightness when the axes again become parallel. The crystal on which the light is first received is known as the polarizer and the other as the analyzer. The theory of this phenomenon is that the undula- tions in passing through the polarizer are changed from the phase of a circle to that of a plane, in which form they readily pass through the analyzer so long as the optical axes of both crystals lie in the same plane; but when the planes of the axes cross, it is as impossible for the polarized light to pass through the analyzer as it would be for a metal rod, compressed into a sheet be- tween rollers, to pass crossways through the wires of a bird-cage. Light when reflected at certain angles from certain substances becomes polarized as well as when trans- mitted, the polarizing angle varying according to the nature of the reflecting substance. The analyzer in this case may be either a reflector or a transmitter, and the polarized ray is reflected, transmitted, or extinguished according to the angle at which it meets the analyzer. Magneto-optic Polarization. Faraday's Discoveries. In a series of experiments, made in 1845, Faraday found that polarized light is influenced by the electro- magnetic current. A polished piece of " heavy glass"- silicated borate of lead about 2 inches square and ^ an inch thick, was interposed edgeways in the path of a ray of lamp-light, polarized by reflection from a plane glass surface; the analyzer being turned so as to ex- tinguish the ray. A U electromagnet was placed close to the glass, in such position that a line through its poles, which were about 2 inches apart, was parallel to the direction of the ray. On the passage through its THE RELATIONS OF ELECTRICITY TO LIGHT. 285 coils of an electric current from a battery of five Grove cells, the extinguished ray again passed through the analyzer, proving that its plane of polarization had been rotated into a new position by the electromagnetic ac- tion; which was confirmed by the fact that, by a further rotation of the analyzer, an angle was found in which the magnetized ray was extinguished, but in which the ray was transmitted when no current was passing a reversal of the conditions of transmission and extinc- tion found in the first position. Faraday found that, to produce these results, a solid or a liquid medium of transmission was necessary for the reception of the magnetic action, but failed to ob- tain 'them by such action on air or other gaseous medium, or in vacuo. He also found that the direction in which the plane of polarization was thus rotated coincides with that in which the magnetizing current passes round the magnet ; reversal of current conse- quently producing reversal of this rotation. But it was subsequently ascertained by Verdet that this coincidence of direction is true only of diagmagnetic bodies, while, in certain paramagnetic bodies, this rotation is opposite to the direction of the magnetizing current. It should be especially noticed that the direction of the magnetic lines of force, from pole to pole, was, by the position given to the magnet, made parallel to the ray. Faraday varied his experiments by using different kinds and different forms of magnets, and placing the glass, or rather medium, in different relative positions; but to obtain the effect described, the parallel position of the ray to a line through the poles was requisite. He also used a pair of bar electromagnets with tubular cores, so placed that a ray could be transmitted through both and received on any medium placed between dis- 286 DYNAMIC ELECTRICITY AND MAGNETISM. similar poles, which, as in the U magnet, were about 2 inches apart. Passing the ray horizontally across a single pole, with the magnet in a horizontal position, he found the ray's rotation, when the glass was on the side next the analyzer, to be the reverse of what it was when the glass was on the opposite side; change of pole or reversal of current producing reversal of rotation. But, with the glass above, below, or in front of the pole, no rotary effect was produced. The cause of these various effects becomes obvious when we consider that the lines of magnetic force radiate in all directions from a single pole: hence, when the glass was in the horizontal plane of the magnet, these curved lines, in that plane, were nearly parallel to the short portion of the ray trans- mitted through the glass, but radiated in opposite directions on opposite sides of the pole; so that on one side they coincided with the direction of the ray's trans- mission, and on the opposite side were opposed to it; but above or below the pole they were at right angles to the ray, while in front of it radiation was equal in opposite directions. Another rule given by Faraday for finding the direc- tion of the ray's rotation, with diamagnetic bodies, which has a special application to the case of a single pole, is substantially as follows: A ray of light, coming to the observer, is rotated in the same direction as watch-hands move, when the magnetic lines of force parallel to it are radiated from a north pole in the same direction as the ray, or from a south pole in the opposite direction; reversal of the ray's direction producing re- versal of rotation. Faraday obtained the same effect, in a limited degree, from steel magnets as from electromagnets; also from coils without iron cores; proving that the effect is chiefly THE RELATIONS OF ELECTRICITY TO LIGHT. 28? magnetic, though also electric. He also found that this effect is independent of any specific polarizing property normally pertaining to the diamagnetic body through which the ray is passed; the electromagnetic polarizing effect being either increased or diminished by such specific property, according as it produced rotation in the same or in the opposite direction. He could not produce any change in this effect by any degree of mo- tion given to the dielectric while under the joint in- fluence of magnetism and light. He noticed that the rotation increased slowly, requiring about two seconds after the closing of the circuit for the attainment of the full effect, but that it ceased promptly on opening the circuit. The first result he attributes to a lag in the magnetic saturation of the core, while the second showed the intimate relation of this effect to electromagnetic action. His conclusion in regard to magnetic lag was confirmed by the fact that there was no lag when the coil alone, without a core, was used; the rotation respond- ing promptly both to the opening and closing of the circuit. He also found that any addition made to the dielectric on either side, and not in the line of the ray, produced no difference in the rotary effect. His final conclusion is, that since this effect is essen- tially the same in character under all these varying conditions, and is independent, in this respect, of the nature of the dielectric, or its own specific rotative force, therefore the magnetic force and the light have a direct, mutual relation, but require the intervention of matter as the medium of action. Verdet's Discoveries. Experiments made by Verdet in 1852 confirmed the results obtained by Faraday, except in regard to the direction of the rotation produced by certain paramagnetic bodies, as already explained. His apparatus consisted of two powerful electromagnets 288 DYNAMIC ELECTRICITY AND MAGNETISM. with hollow cores, similar to those used also by Fara- day, through which light could be transmitted to the medium interposed between dissimilar poles, parallel to the lines of force. He also used a U electromagnet with massive, slotted pole-pieces, through which the light could be transmitted at any desired angle; the magnet having also a rotary movement by which, the angle between the lines of force, from pole to pole, and the ray could be adjusted and measured with a gradu- ated scale and vernier. The principal substances used as media were the " heavy glass," used by Faraday, common flint glass, and carbon bisulphide. Verdet endeavored to ascertain not only the facts in regard to electromagnetic polarization, but also the laws which govern it; and to determine the specific electromagnetic rotative force of different substances. His principal deductions are embodied in the following law: The rotation of the specific electromagnetic plane of polarization for any substance is directly proportional to the strength of the magnetic action, to the thickness of the medium traversed jointly by the magnetism and light, and to the cosine of the angle between the ray and the lines of magnetic force. Verdet chose water as his standard of comparison for specific rotative differences; but Gordon, who subse- quently made a special investigation of this subject, found carbon bisulphide a more reliable standard. Hence taking the specific magneto-rotative force of this substance ac unity, that of water is found to be 0.308 and that of " heavy glass " 1.422. Becquerel's Discoveries. These are the specific differ- ences for white light; but this force has been found to vary for different colored rays, and since difference of color is believed to be due to difference of wave-length, H. Becquerel, who, in 1880, made a special investigation of this branch of the subject, claims to have found that THE RELATIONS OF ELECTRICITY TO LIGHT. 289 the rotations of different colored rays vary (very nearly) in the inverse ratio of the squares of their wave-lengths. Thus taking the rotation produced by carbon bisulphide in green light as unity, that produced in red light is 0.6 and in blue light 1.65. The wave-lengths assigned to each, in ten-millionths of an inch, being 211 for green light, 256 for red, and 196 for blue, if each number be divided by 211 and the quotients squared, the recip- rocals of the squares, expressed decimally, correspond approximately to the respective rotations given above, in accordance with Becquerel's law. Kiindt and Rontgen's Discoveries. In 1879, Kiindt and Rontgen, with a 65 cell Bunsen battery, and electro- magnets wound with 2400 turns of wire, discovered the magnetic rotative force of air and other gaseous bodies, which Faraday with a 5-cell Grove battery failed to discover. They found that air, oxygen, nitrogen, car- bonic acid, coal-gas, ethyl, and marsh-gas, all rotate the ray in the direction of the magnetizing current, like water and carbon bisulphide; that the degree of rota- tion, which is very slight, varies greatly in differen* gases, and is proportional in each to the density of the gas; and that light, traversing the atmosphere in the plane of the magnetic meridian, is rotated, by the earth's magnetism, i for every 316 miles of air traversed. Becquerel, whose experiments were made a year later, found that the rotation of oxygen is opposite in direction to that of the other gases mentioned; such difference in observation being easily accounted for by the small de- gree of the observed rotation. Kiindt discovered, in 1884, that light transmitted through a film of iron, of such tenuity as to be trans- parent, is rotated in the direction of the magnetizing current, as in diamagnetic bodies. Kerr's Discoveries. In 1875 Dr, Kerr discovered that 2QO DYNAMIC ELECTRICITY AND, MAGNETISM. light, polarized in a plane, when transmitted through a dielectric, at certain angles, under intense electric strain, suffers double refraction and is changed into that mode of polarization known as elliptical, in which the undulations occur in two planes crossing each other at right angles. For this purpose he used a rectangular prism of plate glass, in which holes were drilled at each end to within J of an inch of each other at the center, into which were inserted the wire terminals of a powerful induction coil. A receptacle of similar shape, and of special con- struction, was also provided for experiments on various liquid dielectrics, as carbon bisulphide, benzol, paraffine oil, kerosene, oil of turpentine, and olive oil. The light, after passing through a polarizing crystal, was transmitted through the dielectric at right angles to the direction of the wires; the polarizer being turned as required to cause the plane of the polarized ray to form with this direction any angle desired; and the ray, thus transmitted, was received by the analyzer. This will be better understood if the dielectric be conceived as lying across this page, the direction of the wires being the same as that of the printed lines, and the ray, polarized in a plane, transmitted at right angles to the surface of the paper; the plane of the ray being turned so as to form an angle with the printed lines; as if a thin knife-blade, turned at an angle to the lines, were thrust through the paper. The ray being thus transmitted, and the analyzer turned so as to extinguish it, reappeared, on the passage of the current, when the electric strain reached a high degree of intensity; being brightest when the plane of the ray was at an angle of 45 to the direction of the wires or electric strain but becoming dimmer as the angle either increased or diminished; and being extinguished THE RELATIONS OF ELECTRICITY TO LIGHT. 29 1 when the plane of the ray was either parallel to the direction of the electric strain or at right angles to it. Dr. Kerr's conclusion from these experiments is, that, in any given dielectric, the quantity of tHis optical effect or intensity of electro-optic action per unit of thickness of the dielectric, varies directly as the square of the resultant electric force produced in the dielectric. In 1877 Dr. Kerr discovered that light reflected from the end of an electromagnetic pole having a polished surface, is rotated in a direction opposite to that of the magnetizing current, and hence in opposite directions by dissimilar poles. In order to concentrate the magnetic force on the polarized ray, he used a block of soft-iron which he called a " submagnet," having a rounded angle which was placed within ^V f an i cn f one P^ e of a U elec- tromagnet. The ray, polarized in a plane either parallel or perpendicular to the plane of the angle of incidence, met the pole's surface in this narrow space, and was thence reflected to the analyzer, through which it passed when magnetized, being rotated as above, but by which it was extinguished when not magnetized. When the ray was polarized in a plane forming an oblique angle with the plane of the angle of incidence, the magnetism produced elliptic polarization, as in transmission through a dielectric under electric strain, and the ray could not be extinguished as before. The angle of incidence is that included between the incident ray and a perpendicular to the reflecting surface; its plane being known as the plane of incidence. Dr. Kerr also found that when polarized light is re- flected from the side of an electromagnet, the resulting rotation, except under certain conditions, is in the same direction as that of the magnetizing current. In this investigation he dispensed with the submagnet, DYNAMIC ELECTRICITY AND MAGNETISM. and he used, for a reflector, the side of a soft-iron arma- ture, laid across the ends of the poles of a U electro- magnet. The ray, received through a slit in a screen, passed through the polarizer, and was reflected to the analyzer from a side of the armature perpendicular to that across the poles, in a plane at right angles to the magnet's plane. When the ray was polarized in a plane parallel to that of the angle of incidence, the rotation was in the same direction as that of the magnetizing current, for any angle of incidence; but when polarized in a plane per- pendicular to that of the angle of incidence, the rota- tion was in this direction only for angles of incidence between 75 and 80, and in the opposite direction for angles between 75 and 30. Effects of Double Reflection. It has been observed that when light is polarized by reflection from a plane sur- face, a second reflection, in the opposite direction, from a parallel plane surface, at the same angle and in the same plane, annuls ordinary polarization but doubles magnetic polarization. Hence, with ordinary polariza- tion, an even number of such reflections annuls, while an odd number gives the same amount as a single re- flection: but, with magnetic polarization, the effect, under these conditions, is multiplied by the number of reflections. Summary. The results of all these various observa- tions, in which are comprehended about all that is known of the relations of electricity to light, may be briefly summarized as follows: i. Light can be generated by electricity and electrici- ty can be generated by light. ?. Polarized light, transmitted through a dielectric, has its plane of polarization rotated either by electro- magnetic fo.rce, by magnetic force alone, or by the forcq THE RE LA TIOXS OF ELECTRICITY TO LIGHT. of an electric current alone, in the same direction as the current which produces, or would produce, the result- ing magnetism. 3. Polarized light, reflected from the end of an elec- tromagnetic pole, has its plane of polarization rotated in a direction opposite to that of the magnetizing current, when polarized either parallel or perpendicular to the plane of incidence. But when reflected from the side of an electromagnetic armature, the rotation is, for nearly all positions of polarization, in the same direction as that of the magnetizing current. 4. Light transmitted through a dielectric under elec- tric strain undergoes double refraction when polarized at an angle of 45 to the direction of the strain. 5. Reflection which annuls ordinary polarization mul- tiplies magnetic polarization. Maxwell's Theory. It has been already suggested that magnetism may be a mode of molecular or other motion having the phase of a vertical whorl around a central axis of propagation. This is the theory of Clerk Maxwell, in which he attributes magnetism to an un- dulatory motion of this kind in the ether. Applying this theory to the magnetic polarization of light, he conceives that the polarized ray, passing through the magnetic field, has its plane of polarization rotated into a new angle, in this magnetic whorl, in which it can pass through the analyzer, where it was before extin- guished. This theory certainly accounts in a very satisfactory manner for the opposite phases of rotation produced by opposite poles, and otherwise, under the various condi- tions of transmission and reflection which we have been considering. For if such a vertical whorl exists in the magnetic field, it is evident that the rotation of the polarized ray, in passing through it, would depend on 294 DYNAMIC ELECTRICITY AND MAGNETISM. the angle between the plane of the ray and that of the whorl; so that the different phases observed to exist are just those which should result from such conditions. Molecular Theory. It is not improbable that these phenomena may be due to modes of molecular motion, magnetic or electric, in the substance of the media, rather than to undulations of the hypothetical ether ; such a theory being as consistent with the various ef- fects observed as that of the undulating ether. The rotation produced by reflection of the ray from a mag- net is no exception to this; the molecular motion of the reflecting surface producing the rotation, which is intensified by the passage of the ray through the mag- netic field having the air for its medium, to which the molecular motion of the magnet is communicated. Strain in the Media. It is evident that the rotation of the ray, and the other effects observed, seem to result from magnetic or electric strain in the media rather than in the light itself, and that the effect on the light is secondary: still it is none the less evident that these effects are as truly modes of polarization as the polar- ization which occurs in the ordinary way in the crystal; the latter being, as we have seen, due to the peculiar crystalline structure, by which the undulations are all forced into the same plane, while, in the former, the structure of the media, solid, liquid, or gaseous, changed by magnetic or electric action, forces this plane into a new angle. The motion, given the media by Faraday, would not disprove this, since it is probable that the magnetic action would produce change of structure in the me- dium in each new position much more rapidly than the mechanical action could produce change of position; so that the direction of the strain would be the same as if the medium were stationary. THE RELATIONS OF ELECTRICITY TO LIGHT. 2g$ Quincke attributes the double refraction obtained by Dr. Kerr to an electrostatic strain producing either expansion or contraction in the media according to the substance employed. Fontana noticed that the Leyden jar becomes slightly expanded when charged; an effect attributed by Volta, Priestley, and Duter to electric compression of the glass. Electric Lighting. It has been shown that when the heat developed in a conductor by its resistance attains a sufficient degree of intensity light is produced; and on this principle, by the use of conductors of high re- sistance, we obtain the electric light, either as the result of incandescence or combustion. The Arc Light. The electric light was discovered in 1813 by Sir Humphry Davy, who obtained it by the passage of a current from 2000 voltaic cells through two rods of wood carbon, placed end to end, and, after the establishment of the current, slightly separated, producing a light of the most intense brilliancy having the form of an arc; hence the origin of the term voltaic arc or arc-light by which light, similarly produced, is designated, since it always assumes this form. It was subsequently produced with 40 Grove or Bunsen cells and rods made of carbon obtained from | gas retorts, but remained as a laboratory experiment till brought into practical use, 60 years after its dis- covery, by the economical generation of electricity by the dynamo. Electric Candles. One of the earliest and simplest methods of producing this light for practical use was by the electric candle; that of Jablochkoff, invented in 1872, being the first. It consisted of two carbon rods, each about 8 inches in length and \ of an inch in diameter, imbedded in a cylinder composed chiefly of porcelain 296 DYNAMIC ELECTRICITY AND MAGNETISM. clay, known as kaolin, at a distance apart of about ^ of an inch, and mounted vertically on a base. A dynamo current, passed up one rod and down the other, produced the arc ligh't between them above. The kaolin being an insulator, the current was established between the rods by a carbon primer, connecting their upper ends, which was immediately consumed, and the current subsequently maintained by the incandescent carbon vapor. The rods burned slowly downward, con- suming the kaolin also, which increased the light by its incandescence. If a candle was accidentally extin- guished, a new primer was required to renew the current. The average duration of a candle was about i hours, but by using a group of 6, with automatic transfer of current, 9 hours continuous light could be obtained. The upward radiation with downward shadow, and the liability to accidental extinction, led to improve- ments, among which was the Jamin candle, constructed with 2 carbon rods, inclined toward each other at an angle, and fed downward by clock-work, making con- tact at the lower extremities for the establishment of the current, and having subsequent automatic separa- tion to form the arc. The sun lamp of Clerc and Bureau was another simi- lar device, in which the rods were fed downward by gravity, and maintained at the requisite angle and dis- tance apart by a block of marble or magnesia through which they passed. As they did not come into contact, a primer was necessary to establish the current, which was subsequently maintained by the conductivity which the block acquired by the heat, and which served also to prevent accidental extinguishing; the incandescence of the lime in the marble or magnesia increasing the light and modifying its color. The arc was from | an inch to 2- inches or more in length, while in the other can- THE RELATIONS OF ELECTRICITY TO LIGHT. 297 dies its length was only ^ to \ of an inch; and the duration of this candle, with one pair of carbons, was about TO hours. The Arc Lamp. But all these devices were compara- tively short-lived, and were superseded by the arc lamp, now in general use, which, with various modifications, consists essentially of two carbon rods, as shown in Fig. 93, maintained in a vertical position by automatic feed- ing devices controlled by the current which produces the light; being at first in contact, for the establishment of the current, but subsequently separated by the su- perior current strength thus acquired, to the normal distance required to form the arc; further permanent separation being prevented by the increased resistance which the arc acquires by increase of length, which weakens the current, causing the mutual approach of the carbons when the arc becomes abnormally long, or their contact for instantaneous relighting when acci- dentally extinguished. The Arc. The arc thus formed consists of carbon vapor in union with oxygen. Its usual length varies from -Jg- to \ of an inch, but for exceptionally strong lights it may be increased to J of an inch. Its electric resistance varies from \ an ohm to 100 ohms, and its illumination from 1000 to 2000 candle-power; its heat intensity being sufficient to volatilize the most refractory substances, not excepting the diamond. Its charac- teristic form is due to the difference of electric potential between it and the external air, by which it is attracted outward at the center while retaining its attachment to the carbons above and below; the potential difference on its opposite sides being unequal on account of its posi- tion being at the side of the central line of the carbons as shown below. When a direct current is employed, as shown by the 298 DYNAMIC ELECTRICITY AND MAGNETISM. -j- and signs in Fig. 93, a crater is formed in the upper carbon and a point on the lower, and the current pro- ducing the arc, following the path of least resistance, passes to the point of the lower carbon from the lowest projection on the irregular rim of this crater. As this FIG. 93. projection burns away the arc shifts to the next lowest point and thus travels continuously round the crater above, as if pivoted on the point of the lower carbon. The Crater and Point. The formation of the crater is due, in part, to the checking of the current and conse- quent accumulation of energy above by the high resist- ance of the arc, causing increased consumption of car- THE RELATIONS OF ELECTRICITY TO LIGHT. 299 bon. The exterior of both carbons is consumed more rapidly than the interior, consumption increasing to- ward the tips, producing a cone on each, the lower pointed and the upper truncated. There is also, prob- ably, a certain degree of electrolysis, producing excess of oxidation at the anode, or upper carbon, and correspond- ing diminution at the cathode; carbon vapor forming the electrolytic bath; the intensity of this action at the center, where the vapor is densest, producing the crater and point. In short arcs particles of carbon and fused im- purities are deposited on the cathode, forming the mush- room tip, shown in Fig. 93, which is burnt off at the base and again renewed as the consumption proceeds. With the direct current, the positive carbon is con- sumed about twice as fast as the negative, but with the alternating current the consumption of both is equal, and both become pointed. The Heat and Light. The heat is greatest in the car- bon vapor, and the light greatest in the incandescent carbon, 65$ of it being from the crater, the downward radiation from which is of special importance in the arc light, whose elevation for safety and convenience be- comes necessary in consequence of its intense brilliancy and the powerful currents required to produce it. Establishment of the Current. The contact of the car- bons for the establishment of the current becomes necessary from the fact that a current sufficient to maintain the longest arc cannot pass through an air space of JOTW f an i ncn > w h^e the momentary condensation of electric energy, and consequent high potential dif- ference produced between the carbons previous to their separation, is sufficient to overcome the high resistance of the air film and cold carbon, and establish the arc, which is then maintained, through the reduced resist- ance, by the normal current. 300 DYNAMIC ELECTRICITY AND MAGNETISM. The Carbons. Carbon, originally used by Sir Hum- phry Davy in the discovery of the electric light, is still found to be the only substance suitable for its success- ful production; and it is of the highest importance that it should be pure and of homogeneous composition. Various carbonaceous substances have been employed for the production of the arc-light carbons, as coke, coal, charcoal, lampblack, graphite, and sugar; but pe- troleum coke, a residuum of the distillation of crude petroleum, has given the most satisfactory results. It is ground and then mixed with some hydrocarbon, as gas-house pitch, and after being thoroughly ground again, is molded in steel molds, heated and condensed by heavy pressure and the infiltration of hydrocarbon, ind hardened and purified by repeated baking at vari- us temperatures. The process involves numerous manipulations and equires great circumspection; the result being the production of carbons of remarkable purity and homo- geneousness. They are usually about 12 inches long, and vary in diameter from T ^ to j\ of an inch, or more, in proportion to the current and candle-power required. They are beveled for concentration of the current, at the end intended for lighting, and usually copper-plated to within an inch of the point, for increase of conduc- tivity. Automatic Regulation. The automatic regulation of the light is accomplished either by a train of clock-work or by a solenoid; both methods being in general use. The first is the oldest and was invented by Foucault, receiving various improvements in its earlier stages by Duboscq, Serrin, and Lontin; further improvements being subsequently added. In both methods the carbons are attached by sockets and binding-screws to brass rods supported vertically, THE RELATIONS OF ELECTRICITY TO LIGHT. 301 which, in the first method, are operated by the clock- work by means of electromagnets, through the coils of which the current passes. When the carbons are in contact or too close, the strong current through the magnet coil attracts the armature operating the clock- work and separating them, in opposition to the force of a spring, a weight, or an opposing current, which tends to bring them together; and as the current producing the separation becomes weakened by the increased re- sistance of the arc a balance between the opposing forces is obtained, by which the arc is maintained at its normal length. In the solenoid method, used by Siemens, Brush, and others, the upper carbon holder is lifted against the force of gravity by an armature to which it is attached, which moves ver- tically in the interior of a solenoid coil through which the current passes. As the armature is attracted upward, a clutch attached to it grips the edge of a loose washer, which being tilted grips and lifts the carbon holder which passes through it. Fig. 94 illustrates this and shows its application to the double carbon lamp, shown in Fig. 95. The clutch on FIG. 94. FIG. 95. 302 DYNAMIC ELECTRICITY AND MAGNETISM. the left being narrower than the one on the right, the left pair of carbons are kept apart by this simple device till the pair on the right are consumed, when the change of resistance instantly brings the left pair into contact, and the light is renewed. Hefner von Alteneck's Regulator. The regulator of Hefner von Alteneck, of which Fig. 96 is an ideal illus- FIG. 96. tration, has an important application to the solenoid lamp and to arc lighting in general. The current from L to Zi divides at /, the main branch going through the low resistance coil R\ and the lamp, as shown, while a shunt current of about \% of the entire strength goes through the high resistance coil R and round the lamp. The armature ss is drawn down by the greater magnetism induced by the lower current, separating the carbons and establishing the arc. As the resistance of the arc increases with its length, the potential difference, or E. M. F., between L and L\ in- creases, and the strength of the lower current decreases in like proportion. But as the resistance in R remains constant, the strength of its current is increased by the increased E. M. F. in the same ratio as that in R\ is diminished by the increased resistance, tending to draw THE DELATIONS OF ELECTRICITY TO LIGHT. 303 the armature ss upward by the increased magnetism in- duced and shorten the arc, which thus becomes adjusted to its normal length and a balance is maintained. These coils may be arranged in any convenient man- ner, as by winding in opposite directions, one outside the other; the shunt current thus opposing and par- tially neutralizing the magnetic effect of the main . current, as in the Brush arc lamp. Series Distribution. As currents of 10 to 15 amperes are usually required for arc lamps, the series method of distribution is found to be the most economical, and the only practical method; the entire current passing from lamp to lamp through a series often embracing 50 or more, distributed over a large building, or area of a town. Automatic Cut-Out. As any variation of resistance in a lamp affects every lamp in the series, regulators, con- structed on the principle of Hefner von Alteneck's, are required in the series system; also automatic short- circuiting apparatus for the exclusion of extinguished lamps, without which the extinction of a single lamp would interrupt the current, causing the extinction of every lamp in the series. Such apparatus, in the Brush lamp, consists of an electromagnet wound with two coils, a fine wire coil on a closed circuit connected with the shunt, and a coarse wire coil on an open circuit connected with the magnet's armature. The ordinary shunt current does not induce sufficient magnetism to attract the armature, but the increased current, caused by the extinction of the lamp, is sufficient for this pur- pose; the attracted armature closing the coarse wire circuit, by which the full current is carried past the extinguished lamp. The Incandescent Lamp. In the first attempts to' pro- duce the electric light by incandescence exclusively, 304 DYNAMIC ELECTRICITY AND MAGNETISM. platinum wire was employed and also iridium, but the superior advantages of carbon were soon demonstrated; consisting in its high electric resistance, 250 times as great as that of platinum, its infusibility at the highest temperature, and its greater illuminating power. But as it is volatilized at high tempera- tures in the presence of oxy- gen, its exclusion from the air became necessary, and this was accomplished by inclosure in a glass bulb in which a high vacuum was subsequently pro- duced by a mercury pump. Such are the general principles of construction of the incan- descent lamp as we now have it, as illustrated by Fig. 97. The Filament. The carbon, prepared from a variety of dif- ferent substances, as bamboo, bass broom, cotton, linen, and silk, consists of filaments bent into any convenient form which will fit in the glass bulb. They are subjected to numerous ma- FIG. 97. nipulations to give them the requisite hardness, tenacity, elasticity,homogeneousness, and durability. The principal steps are the forming; carbonizing by baking at a high temperature with ex- clusion of air; and " flashing^" which consists in heating the carbonized filaments to incandescence by the elec- tric current or otherwise, in a bath of carbon vapor, the carbon from which is thus deposited on them, forming an even, dense, hard, homogeneous coating. The car- THE RELATIONS OF ELECTRICITY TO LIGHT. 305 bon of some filaments is entirely built up in this way on a base of fine platinum wire. There are also filaments made of hollow tubes for increase of surface. The average durability of a filament, in the 16 can- dle-power lamp, is from 600 to 1000 hours; the heating and cooling, molecular action, and general wastage, finally terminating in its rupture, requiring renewal of both filament and containing bulb. Its electric resist- ance, when heated to incandescence, is about half its cold resistance, ranging from 50 to 200 ohms, accord- ing to its length, cross-section, and composition. Filament and Lamp Attachment. Each filament, when completed, is attached at both ends, as shown, to plati- num terminals sealed into the glass, after which the air is exhausted and the bulb hermetically sealed. Each bulb is then attached to a socket from which it can be easily removed for replacement; in which is a device, operating with springs, for closing or opening the circuit by turning the insulating handle shown, by which the current is passed through the filament or ex- cluded from it for lighting or extinguishing the lamp. Position and Current. The position of this lamp when in use is entirely a matter of convenience, as its illumi- nation seldom exceeds 16 candle-power, and its current J to f of an ampere. The current may be either direct or alternating according to the system of lighting, each system having numerous distinctive features. Parallel Distribution. The large number of lamps re- quired on an incandescent lighting circuit and the small current required for each makes the parallel sys- tem of distribution the most economical and practical. This system is illustrated by Fig. 98, in which are rep- resented two heavy copper mains issuing from the dynamo, between which the lamps are mounted on fine wire connections, 306 DYNAMIC ELECTRICITY AND MAGNETISM. These mains may extend to any required distance through a building, or through streets, with branch mains extendinginto the build- ings; but when the direct current is em- ployed, they must be of sufficient size to reduce the resistance to a required mini- mum. A copper conductor capable of carrying a current sufficient to feed 5000 16 candle-power lamps at a mean distance of 4000 feet from the dynamo would require a cross-section of 12.57 square inches, the size being proportionally reduced as the line branches into parallel circuits, while wire of No. 14 to 16 gauge is large enough for the lamp connections. If a circuit, like that shown in Fig. 98, have a resistance, including that of the dynamo, of i ohm, and each filament a hot resistance of 199 ohms, and the dynamo an effective E. M. F. of 100 volts, then, if a 100 volts i single lamp be lighted, it has - = an am- 200 ohms 2 pere current. But if two lamps be lighted, the current has two paths instead of one between the mains, which is the same, in effect, as doubling the cross-section of the filament and thus halving its resistance ; which gives 199^? , 2R 20l7? , .. , .. -ZZ__ j _ - = iooi/c ; then, if the fraction be 222 nesrlected, = iC for the 2 lamps, i an ampere to lOO/t each, as before. For any small number of lamps the resistance varies inversely and the entire current directly as the number lighted, and the current per lamp remains practically constant, as shown, being equally divided among the FIG. 98. THE RELATIONS OF ELECTRICITY 7^0 LIGHT. 307 entire number lighted. But as the resistance of the dynamo and circuit remains constant while that of the lamp filaments varies, it is evident that in the lighting of any considerable number of lamps the fraction, neg- lected above, would make a sensible difference in the ratio of resistance to E. M. F. Suppose that 100 were lighted, then the entire filament resistance would be - = 1.99^, and, adding in the i ohm constant re- sistance, we have 2.99 ohms as the entire resistance; hence - = 33i%^ which, divided among the 100 lamps, gives about of an ampere per lamp, instead of J an ampere, with only one or two lighted. There is also a certain amount of current wastage, making an entire current variation of 15$ to 20$, which, must be provided for in order to maintain constancy of current and illumination. This, in the direct current system, is done by the introduction of resistance coils into the circuit, by which the current can be varied by variation of the resistance, and in the alternating cur- rent system by a direct variation of current in the con- verter. Hence when the indicator at the station shows a variation of current below or above the normal, by the lighting or extinguishing of any considerable number of lamps, the attendant makes the necessary correction by moving a switch either in the resistance box or con- verter according to the system of lighting employed. Multiple Series and Series Multiple. A number of short series of lamps may take the place of single lamps on a parallel circuit, producing what is termed a "mul- tiple series" installation; or a number of groups with lamps in parallel in each may be placed in series, pro- ducing what is termed the " series multiple" installation. 3O8 DYNAMIC ELEC7RICITY AND MAGNETISM. Three-Wire System. In the Edison three-wire system, illustrated by Fig. 99, two parallel circuits with two dynamos are combined, the dynamos be- ing connected together in series as shown; and a single central main, attached to the short connector which joins them, takes the place of the two interior mains, and equalizes the current through the lamps, in the following manner. When an equal number of lamps is lighted on each cir- cuit, the resistance between the circuits being equally balanced, the entire current flows across through the several pairs of lamps in series between the two external mains. But the lighting of a greater number on one circuit than on the other reduces the resistance and increases the current in that circuit; and this surplus current flows through the central main; in a negative sense if the increase is in the left-hand circuit, but in a positive sense if the increase is in the right-hand circuit. Three mains are thus enabled to do the work for which four are usually required. But a further reduc- tion in the required amount of conducting metal results from the fact that this amount is found to vary in- versely as the square of the required E. M. F., which being doubled by joining the two dynamos in series, the cross-section of each main should be reduced to J the usual amount, its length remaining the same if there were no change of filament resistance. But the joining of each pair of lamps in series increases the filament re- sistance to four times the amount of that of each pair joined in parallel, the current traversing twice the fila- ment length with half the cross-section. Hence the FIG. 99. THE RELATIONS OF ELECTRICITY TO LIGHT. 309 ratio of resistance to E. M. F. would be doubled if mains of only i the usual size were employed; therefore, to maintain constancy of current, mains of J the cross- section and same length would be required; that is, 3 mains, each \ the usual size, or f the usual amount of copper, if each main were required to carry a full cur- rent. But, as the central main carries only the required surplus of current, its cross-section can be reduced about 1 below that of the other two. 310 DYNAMIC ELECTRICITY AND MAGNETISM. CHAPTER XII. THE ELECTRIC TELEGRAPH. Early History. The experimental stage of the elec- tric telegraph extends back to the middle of the last century; static electricity having been first employed for the transmission of signals; a plan for alphabetic signaling by this means being described in Scot's Mag- azine for 1753. Lesage constructed the first electric telegraph, in 1774, at Geneva; in which he employed 24 wires, each connected with a separate pith-ball electro- scope, representing a letter of the alphabet. Similar methods were employed later, by Lomond in 1787, and Ronalds in 1816. Reusser, in 1774, suggested the illu- mination of letters made with metal spangles on glass plates, as an improvement on the pith-ball method of Lesage. Sommering, in 1808, first employed voltaic electricity for telegraphing, using 35 water voltameters, each con- nected with separate wires and giving separate signals: and similar methods were subsequently tried by Coxe, Smith, Bain, and others. Ampere, in 1820, proposed to employ 24 galvanometer needles, each connected with a separate wire. Schilling, in 1832, and Weber and Gauss, in 1833, employed a single needle, indicating alphabetic signals by right and left deflections. Steinhill subsequently developed this system, employing two needles, one for the positive and the other for the negative current, both deflected in the same direction; alphabetic signals being given by bells THE ELECTRIC TELEGRAPH. 311 struck by the needles, and also by dots made with ink on a moving strip of paper, as well as by observation of the movements by the eye. Steinhill, while constructing a telegraph line at Mu- nich, in 1838, made the very important discovery that the current could be carried by a single wire, and the earth employed for the return circuit by making con- nection with it at the terminals of the line; from which he inferred that the earth took the place of the return wire as a conductor; but subsequent experiments seem to prove that the earth, in this case, is to be regarded as an electric reservoir, giving and receiving electric energy, rather than as a conductor. Cook and Wheatstone, in 1837, introduced the needle telegraph, as it was designated, into England, and con- structed, on the London and Birmingham Railway, the first line ever employed for commercial use. It con- sisted of five underground wires connected with five separate needles; a system which they subsequently modified, employing two wires connected with two needles in one method, and a single wire and needle in another method. The signal for the transmission of a message was given by a bell rung by an electromagnet. In 1831 Henry transmitted signals by sounds pro- duced by the movements of the armature of an electro- magnet; and Morse, in 1835, invented a telegraph oper- ated in a similar manner, in which alphabetical signals, consisting of lines and dots, were made on a moving strip of paper, first by a pencil, but subsequently by a steel point which embossed them on a grooved roller over which the paper was moved by clock-work oper- ated by a weight. Morse constructed the first commercial telegraph line in the United States, between Washington and Balti- more, and sent the first message over it May 27, 1844. 312 DYNAMIC ELECTRICITY AND MAGNETISM. This line was mounted on wooden poles and consisted of two iron wires, the practicability of employing the earth for the return circuit being then imperfectly understood. The American Morse Code. The original Morse code for letters, numerals, and punctuation, now employed in the United States and Canada, is as follows: A B C D E F G HI J K L M N O P Q R S T U V W X Y Z & Period Semicolon Comma Exclamation Interrogation Paragraph Parenthesis Italics 1234 56 It will be noticed that this code consists of long dashes, short dashes, dots, and spaces; L for instance being indicated by a long dash, T by a short dash, R by three dots with space between the first and second, C by three dots with space between the second and third. Hence the number and relative positions of these four elements constitute the distinction between the different characters; the spaces having equal sig- nificance with the dashes and dots. The International Morse Code. The Morse code has been found so well adapted to telegraphing, that it has superseded all others for this purpose, and come into general use throughout the world. But on its in- troduction into Europe some changes were necessary to THE ELECTRIC TELEGRAPH. 31 3 adapt it to .the various languages, and also to remedy defects which had been developed by its practical use in America. This led to the adoption, by a telegraphic convention assembled at Vienna in 1851, of the inter- national Morse code, now employed in all countries ex- cept the United States and Canada. In this code long spaces between the elements of a letter are eliminated, as they are liable to be misunderstood for the spaces between letters; each numeral is represented by five elements, and each punctuation mark by six. The dif- ferences between this code and the American are as fol- lows: C F J L O P Q R X Y Z Ch A O U E N Period Comma Exclamation Interrogation Apostrophe Hyphen Parenthesis I 23 5678 9 As it was soon found that messages could be read more easily and rapidly by the click of the instrument than by the record on the paper, the dots, dashes, and spaces came to indicate sounds and pauses, and the registering instruments were replaced by sounders in all the principal offices. 3H DYNAMIC ELECTRICITY AND MAGNETISM. Simple Line Equipment. The principal apparatus re- quired for the equipment of a simple Morse telegraph line are a battery, signal key, sounder or register, and relay; all of which must be duplicated at each end of the line; the duplication of the battery, on such a line, being de- sirable though not always strictly necessary. A light- ning arrester, ground switch, and cut-out are also required. The Battery. The principal requirements of the bat- tery are strength and constancy, and any good battery fulfilling these conditions can be employed. The gravity battery is one of the best and cheapest, and hence is extensively used for this purpose. It requires com- paratively little care, is free from noxious fumes, and not liable to polarize. The Key. The key, one form of which is shown in ". 100, is a lever of steel or brass, so mounted as to FIG. 100 have a vertical movement, limited by two set-screws which can be adjusted to any required range of motion; the upward movement being produced by a spring con- nected with one of the screws, and the downward by pressure on the hard-rubber knob at the left, which THE ELECTRIC TELEGRAPH. 315 closes the circuit by bringing a little projection under- neath into contact with an anvil attached to the left hand bolt; the points of contact being faced with plati- iium to prevent fusion by the extra current at break. This bolt and anvil are insulated from the support- ing frame, while the bolt at the fight is connected with it, and both are connected with the terminals of the electric circuit. When the key is not in use the circuit is closed by a lever pushed under a metal projection at- tached to the anvil. The Register. A simple form of the embossing register is shown in Fig. 101. The armature of an electromag- net Mis attached to the bent lever Z, pivoted at d so FIG. IOT. as to have a vertical movement limited by the adjust- able screw m and stop underneath. A steel point/, attached by an adjustable screw to the bent end of the lever, makes contact in a little groove with the roller r, and embosses the message on a strip of paper carried between the rollers, which are operated by clock-work impelled by a weight attached to a cord wound on the drum W, and controlled '^y the brake a. The electromagnet is connected with the line by binding-posts, one of which is shown at s, and when the 3l6 DYNAMIC ELECTRICITY -AND MAGNETISM. current, transmitted from the distant station, attracts the armature, the lever L is drawn down against the force of the spiral spring #, bringing the point into con- tact with the paper, and registering the message as de- scribed. Double embossing registers, operated on the same principle, are now in common use, by which two sepa- rate messages can be registered in parallel lines on the same strip of paper, or one message only, on a single line, as required. Inking registers, both double and single, are also in common use, and are generally pre- ferred to the embossing instruments. The clock-work, in all the new registers, is operated by a spring. The Sounder. One of the best known forms of the sounder is shown in Fig. 102. A bent lever, having a FIG. 102. vertical and a horizontal arm, is pivoted on an arched support as shown, the vertical arm being concealed by the support. The horizontal arm has a vertical move- ment between the poles of an electromagnet, limited by the adjustable set-screws shown above on the left; and is held in contact with the upper screw, when the cir- THE ELECTRIC TELEGRAPH. 3 1/ cuit is open, by a retractile spring connecting the lower end of the vertical arm with an adjustable screw which passes through the supporting post on the left. The instrument is connected with the line by the binding-posts on the right, and when a current is sent through the coils of the magnet the attraction of the armature brings down the lever, the point of the screw striking with a sharp click on the curved brass sounding piece. When the current ceases the spring brings the lever up with a light click against the screw above; and by means of these two clicks, signals indicating the dots, dashes, and spaces are distinguished. The sharp click indicates the beginning of a dot or dash, and the light click its termination; a pause following a sharp click indicates a dash, and a pause following a light click in- dicates a space. The screws can be adjusted to any required range of motion, both in the sounder and register; and the arma- ture, in both instruments, is kept out of contact with the magnet poles, to prevent magnetic adhesion. The Relay. On short, well-insulated lines, not ex- ceeding 20 or 30 miles in length, the sounder, or register, if its resistance is not too high, can be operated by the line current; but, on longer lines, resistance and im- perfect insulation usually weaken this current too much for direct action; but by the aid of a relay it can per- form this work indirectly through the agency of a local battery current. A common form of the relay is shown in Fig. 103. An electromagnet supported in a horizontal position by an adjustable screw, on the right, and a curved stand- ard, on the left, has its armature attached to a vertical lever, pivoted below, but having a horizontal movement above limited by two screws by which its range of mo- tion can be adjusted. A retractile spring holds it 3l8 DYNAMIC ELECTRICITY AND MAGNETISM. against the point of the left hand screw when the cir- cuit is open, while a weaker spring below tends to force it in the opposite direction; the tension of the upper spring being capable of adjustment as shown. The two binding-posts on the left are connected re- spectively with the curved standard and lever support, and, exteriorly, with a local battery which embraces in FIG. 103. its circuit the sounder or register; and the electromag- net is connected with the line by the two binding-posts on the right. Hence, when the line current passes through the magnet coils, the armature is attracted and the local circuit closed by contact between the platinum points attached to the lever and right hand screw, and the sounder or register operated by the local current. The distance between the magnet and its armature can be adjusted by the supporting screw on the right, which moves the magnet through the openings in the curved support on the left. Hence the adjustment by this means and that of the retractile spring and upper screws can be adapted to any current which may be sent over the line. THE ELECTRIC TELEGRAPH. 319 Cut-Out, Ground Switch, and Lightning Arrester. These three instruments, of which there are several different forms, are employed separately, or may all be combined in one. A common form of the latter kind is shown in Fig. 104. Three brass plates, with binding-posts attached, are FIG. 104. mounted on an insulating block, the central plate hav- ing a row of points on each side. This block is attached to the wall in any convenient position, the end plates connected with the terminals of the line, and the office instruments placed in circuit between them, and the central plate connected with the earth. When a brass plug is inserted between the end plates, as shown, the office instruments are cut out of the circuit, but when the plug is removed the current must evidently pass through the instruments. If, under the latter arrangement, lightning should strike anywhere on the line, its high potential would cause its current to pass to the earth by way of the points and central plate, instead of taking the longer route through the instruments. When the line connections, at a way station, are inter- rupted, the direction in which the interruption has occurred may be ascertained, and current from the 320 DYNAMIC ELECTRICITY AND MAGNETISM. opposite direction obtained by making connection with the earth. This is done by inserting the plug between the central plate and one of the end plates. If the inter- ruption were on the right and the plug should happen to be first inserted on the left, no current would be ob- tained ; if then the plug were inserted on the right, the instruments would be placed in circuit between the earth and the uninterrupted connection on the left and current obtained; and in like manner connection could be established on the right, if the interruption were found to be on the left. The cut-out should always be closed during a thunder- storm or the absence of the operator, to prevent acci- dents to the instruments. Line Construction. The ordinary telegraph line is con- structed with No. 6-7 iron wire, B. and S. gauge, but for short lines No. 8-9 wire can be used. It is coated with zinc to prevent oxidation, and mounted on wooden poles, provided with supporting cross-arms to which it is attached by glass insulators; a large number of parallel wires, arranged in tiers, being often mounted on the same poles. In cities where air lines are prohibited, the wires, coated with insulating material and combined in cables, are laid underground; the cables being inclosed in lead pipes, or otherwise protected against moisture and abrasion, and often placed in conduits so as to be acces- sible without disturbance of the pavement. Joints are made, where required, by twisting the wires firmly together, and then soldering them to insure perfect electric connection and prevent its interruption by oxidation ; but electric welding, now coming into use, is preferable for this purpose. Where wires pass through walls, for office connections, they require to be well insulated by hard-rubber tubes. THE ELECTRIC TELEGRAPH. 321 Station Arrangement. The arrangement of the instru- ments and connections of a terminal station are shown in Fig. 105. The line is connected with the earth at G, the wire being soldered to a mass of buried metal for good connection, or to water or gas pipes, where they are available for this purpose. It is connected with the main battery shown at , which has its positive pole connected to the earth and its negative to the line; this arrangement being reversed at the opposite end of the line; hence the line current, at this station, always flows towards the battery, and at the opposite terminal sta- tion, from the battery, while the earth current, at each station, flows in the opposite direction. The instruments and battery may be cut out, when necessary, by a switch or plug connection, at X, with the ground wire shown at the left. When a message is being received, the circuit is closed through the key K, and the line current, entering through the lightning arrester at X, traverses the relay 322 DYNAMIC ELECTRICITY AND MAGNETISM. M by the binding-posts i and 2, and goes through K and battery E to the earth at G. The circuit of the local battery ' ', being closed by the relay, its current passes from the positive pole, by the binding-posts 3 and 4 and platinum points, through the sounder S and thence to the negative pole of E' . When a message is being sent, the circuit is opened at K) and the instruments at the distant station, traversed by the outflowing current, respond to the manipulations of the key at the home station, where the instruments are traversed by the inflowing current as before. The same conditions, with reversal in the direction of the current, occur at the opposite end of the line. It is important that the instruments should always be in circuit during business hours, as a station is liable to be called at any moment; and their constant click is a notice to the operator that his connections are right and the line in working order. All the messages and station calls are therefore heard at every station on the line, but responded to only at the station called. The arrangement of a way station is the same as that of a terminal station except that only the local battery is required; the current from the main batteries enter- ing by one branch of the line, traversing the relay and key and operating the sounder by the local battery, and leaving by the other branch of the line. Messages can therefore be received or sent in either direction, the current being positive to all the stations on one side and negative to all those on the other. The current is derived from both terminal batteries, which may be regarded as a single battery, with the instruments interposed between its poles in one branch, and the earth in the other. Hence if either battery is THE ELECTRIC TELEGRAPH. 323 324 DYNAMIC ELECTRICITY AND MAGNETISM. cut out by a ground switch, either at a terminal or way station, the current is proportionally reduced. Switch Board. When a number of different lines have connections through the same office a switch-board, like that represented by Fig. 106, is required. The vertical brass bars represent line connections and the rows of brass disks, battery connections. The disks in each horizontal row are electrically connected together at the back, but insulated from the other rows; and each row, except the lowest, connected with a separate bat- tery, the lowest being connected with the earth. By the apparatus known as a spring-jack the instru- ments may be connected with the line as shown at SJ'"', a wedge W having brass plates on its opposite sides, insulated from each other, but connected with the terminals of the instrument circuit, being inserted be- tween a pair of springs, which close the circuit again when the wedge is withdrawn. The bars B and B* being thus connected with the line, the insertion of a plug at H puts battery MB 1 in connection with the line L by the vertical bar B; and, in like manner, battery MB is connected with L' by the plug F and bar B 9 . Any line may be connected with the earth for testing by inserting a plug between its bar and the lowest row of disks. Thus the insertion of a plug at M gives the line connected with j9 3 an earth connection. Repeaters. As the distance to which messages can be transmitted is limited, even with the aid of the relay, it becomes necessary to have them automatically repeated by a special apparatus which employs local batteries. By this means stations four or five thousand miles apart can hold communication with almost the same facility as those on short lines. Press despatches can also, in THE ELECTRIC TELEGRAPH. 325 this way, be received simultaneously by all the princi- pal stations on a line. Before the introduction of this method the message had to be repeated by the operator, involving great de- lay and liability to errors. Two sets of instruments are required for repeating, each connected with a separate branch of the line, and including a relay, sounder, and also two or more local batteries to each set. The Button Repeater. The button repeater, invented by Wood in 1846, and still employed to a limited extent, was one of the first in use. It consists, as improved, of a button switch placed between the two sets of instru- ments, having double contacts on each side, one pair of which may be closed when the other pair is opened, or both pairs opened as required. Each set of instruments is in circuit between each pair of contacts, and when either pair is closed the two branches of the line are connected through both sets of instruments, and each connected also with a separate main local battery. When a message is to be repeated, the operator at the repeating station, on being notified, closes the switch contacts through the branch of the line wishing to transmit, giving it a closed connection to the earth through the main local battery of the repeater, with which it is connected; which enables the operator at the sending station to repeat into the opposite branch through the sounder connected with his branch, which also acts as a transmitter. When a message from the opposite direction is to be repeated, the connections are reversed by reversal of the switch. In this repeater both sets of instruments respond to the manipulations of the sending operator's key, but the closed switch contacts prevent any break in the through connections. When, however, the switch i 326 DYNAMIC ELECTRICITY AND MAGNETISM. disconnected from the contacts on both sides, the through connection being opened, each branch of the line becomes independent of the other, and terminates in its main local battery at the repeating station. Mes- sages can then be sent or received by each set of in- struments, separately, by connecting them with local keys, or through connections, independent of the re- peating apparatus, made with a single set. The Milliken Repeater. The annoyance and delay occasioned by the absence of operators at repeating FIG. 107. stations led to the invention of repeaters which can be reversed from distant stations by the current, automatic- ally. The Milliken repeater, shown in Fig. 107, is one of the best known of these. It consists of the ordinary elay, shown at the right, by which the sounder is THE ELECTRIC TELEGRAPH. 327 operated in the usual manner, and an extra magnet, mounted at the left, whose armature is arranged to close the circuit automatically, and keep it closed, through one branch of the line, by the aid of connected apparatus, during the repeating of a message by a sim- ilar companion instrument connected with the other branch. The upper retractile spring has greater ten- sion than the lower, so that when the attraction of the extra magnet ceases, its armature is pulled back, bring- ing an insulated stop against the armature of the relay, as shown, and closing the local circuit through the con- nected sounder. Repeater Connections. The arrangement and connec- tions of the Milliken repeater, at a station, are shown in Fig. 108. T and T* are sounders, used also as trans- mitters, and connected respectively, at Y and V, with the extra magnets ExM and ExM' of the opposite re- peaters through the circuits of the extra local batteries XL and XL'. The levers of T and T are furnished with continuity-preserving springs s and j', insulated from them, which make contact with stops above, when the levers are attracted, and close the circuits of the main local batteries MB and MB' respectively; the lever projections, atr and x' t limiting the upward move- ment of the springs, when the attraction ceases, and breaking the contact. In this way each line circuit is closed by a spring through its main local battery before the closing of the circuit at the opposite end of the lever through the extra local battery, and remains closed till after the latter circuit is opened. Thus when the lever of T is attracted, the circuit of MB is closed at s before that of XL is closed at Y, and remains closed till after the contact at Y is again opened. The dark space at the mounting of these springs shows hard-rubber in- 328 DYNAMIC ELECTRICITY AND MAGNETISM. THE ELECTRIC TELEGRAPH. sulation, which is similarly indicated at various other points. The main local battery MB is connected with the eastern line through relay R ', and MB' with the western line through R\ each battery being connected with the earth as shown. When a message from the east is to be repeated to the west, the eastern operator opens the circuit through his key, and the operator at the western terminal station, finding this to be the case, closes the circuit through his key; hence there is current in the western line through relay R, but none in the eastern line through R'\ arma- ture B is therefore attracted, closing the local circuit EE at C '; and transmitter T, being attracted, first closes the eastern line at j, through battery MB, and then the circuit of XL at Y. The magnet ExM* therefore at- tracts its armature, allowing the spring S'" to open the local circuit E' E' atC': this releases 7 1 ', opens the cir- cuit of XL' at K', and then the western line at j', break- ing connection with battery MB' . This break stops the current on the western line, demagnetizing relay R\ but fi's armature is still held closed by the superior force of the spring 5 over that of S"', hence the connections on the left remain closed while the instruments on the right respond to the manipulations of the eastern oper- ator's key, enabling him to repeat into the western line. In a similar manner the western operator, by opening the circuit through his key while that through the key at the eastern terminal station is closed, can produce automatic reversal of the connections at the repeating station, and repeat into the eastern line. Duplex Telegraphy. The simultaneous transmission of messages in opposite directions on the same wire occupied the attention of various inventors from 1852 to 1872. The first suggestion of the practicability of 33 DYNAMIC ELECTRICITY AND MAGNETISM. this method was made by Moses Farmer, an American, in 1852, and the first invention of the kind by Gintl, an Austrian, in 1853. Gintl's invention not proving suf- ficiently practical, improvements on it were made, in 1854, by Frischen, Siemens, and Halske, and as the re- sult of their labors the duplex system was first put in successful, practical operation, in 1855, between Munich and Vienna, and subsequently, in the same year, between Vienna and Trieste. Preece, Nystrom, Maron, and other European invent- ors made various valuable contributions to the duplex system between 1855 and 1863, but it was almost un- known in America till 1868, when Stearns began a series of experiments based on the European methods which resulted, in 1872, in the practical adoption of his system in the United States. The Stearns Duplex. The construction and operation of this system is shown in Fig. 109, in which the con- nections at terminal stations on the right and left are represented. R and R' are differential relays connected with sounders not shown, in each of which the two bob- bins are each oppositely wound, as shown, so that cur- rents of equal strength in each would neutralize each other's magnetic effect on the cores, while a current in either branch of the circuit alone, or of greater strength in one branch than in the other, would magnetize the cores. Rh and Rh' are rheostats whose resistance is so adjusted that the resistance from the central point a or a' of either relay through the rheostat to the earth is just equal to the resistance in the opposite direction through the line. T and T' are transmitters (not sounders) operated by the keys and small local batteries shown, and whose levers A and A' have the continuity preserv- ing springs, z and z', already described. MB and MB' are the main batteries, connected with the line through THE ELECTRIC TELEGRAPH. fr 331 B 33-2 DYNAMIC ELECTRICITY AND MAGNETISM. z and s', and also connected with the earth. SC and SC' are resistance-coils, also connected with the line and earth, whose resistance is made equal to that of the batteries MB and MB' respectively, so that the line resistance to the earth shall be the same whether the connection is through the coil or through the battery. Cand C"' are condensers adjusted to absorb a charge equal to the static charge absorbed by the line, and return an opposing current equal to the return current produced by that charge, and thus neutralize it and prevent a false signal. When a message is to be sent from the station on the left, the depression of key K closes the local circuit through the magnet of transmitter 2\ and the conse- quent attraction of lever A closes the circuit of battery MB, through spring z, and opens the ground circuit through SC at x by the depression of the spring as shown. A current from MB therefore flows through both branches of relay R, and the resistance being the same in each, divides equally at a, one half going to the line through , and has a free vertical movement be- tween two curved, metal springs s and /, connected with opposite, poles of the main battery; s being, for THE ELECTRIC TELEGRAPH. 335 convenience, supposed to be connected with the positive pole, and / with the negative. These springs are attached to the disk by insulated connections, and in proximity with them are two metal blocks b and b' attached to the disk by uninsulated connections, and hav- ing adjustable stops with which each spring can make contact alternately. The disk is connected with the line wire, giving these blocks a line connection, and the lever, which is insulated from it, is connected with the earth wire. When the key is depressed and the lever attracted, it makes contact above with spring s', lifting it and breaking its contact with block b' and also its own contact with s, which therefore springs up into contact with block b. Hence the positive pole of the battery being now connected with the line through s, current flows from it to the line through s and b, and thence to the earth through the apparatus at the distant sta- tion; and the negative pole being connected with the earth through / and j', current flows to it from the earth through / and s', and through the battery to the positive pole, completing the circuit. But when the key is opened, the attraction ceases, and the lever being pulled down by the retractile spring, makes contact below with s, depressing it and breaking its contact with b and also its own contact with j', which therefore springs down into contact with b' Hence, the negative pole of the battery being now con- nected with the line, the direction of the current is reversed, and it now flows from the positive pole through s and / to the earth, and from the earth at the distant station through the apparatus to the line, and from the line through b' and s' to the negative pole, and through the battery to the positive pole, complet- ing the circuit. 33^ DYNAMIC ELECTRICITY AND MAGNETISM. Hence, the polar connections being reversed with each opposite manipulation of the key, the direction of the line and earth currents is correspondingly reversed, FIG. in. while the direction of the current through the battery remains, of course, unaltered. The Polarized Relay. The polarized relay, shown in Fig. in, is constructed with a curved steel magnet, THE ELECTRIC TELEGRAPH. 337 whose north pole, N, we may, for convenience, suppose to be at the upper end, and its south pole, S, at the lower end, as marked. On the south pole is mounted an electromagnet, attached by its soft-iron yoke Y, Y, having oppositely wound coils, M and J/, between whose cores the soft-iron armature a, hinged to the north pole, has a free horizontal movement, limited by the stops attached to c and c, with which its projecting brass tongue makes contact alternately. This armature derives north polarity from its attach- ment to the north pole of the permanent magnet, while the cores of the electromagnet derive south polarity from their connection through the yoke with the south pole. Hence, when there is no current in the electromag- net, its poles exert equal and opposite attraction on the armature; and the same is true when currents of equal strength flow through both coils, neutralizing each each other's magnetic effect. But when current flows through only one coil, or is stronger in one than in the other, the permanent magnetism of the cores is over- come, and each acquires polarity in accordance with the direction of the current. Hence the armature, having north polarity, is attracted by the south pole and repelled by the north, and therefore vibrates between the poles in response to the changes in the direction of the current, produced by the pole-changer through the manipulations of the key; alternately closing and open- ing the local circuit of the connected sounder. The various connections are made through three pairs of binding-posts on the left; and the positions of the electromagnet, armature, and contacts, and the range of the armature's movement, are adjustable by screws, as shown. It is practically impossible to prevent the armature from being attracted against one of the stops when 33^ DYNAMIC ELECTRICITY AND MAGNETISM. there is no current in the magnet coils, notwithstanding the equal and opposite attraction; but such contact is then of no consequence, and, when the instrument is in operation, the armature is always so attracted, normally, by the electromagnetism. Operation of the Polar Duplex. The connections and operation of the polar duplex, at opposite stations, are shown in Fig. 112. The line circuit, and the earth, or " artificial," circuit have equal resistances, as in the Steams duplex. Let battery B have its positive pole to the line and its negative to the earth, and the corresponding polar connections in battery B f be reversed. The positive current frorn battery B then divides at into currents of equal strength, traversing the oppositely wound coils a and b of relay R without magnetic effect; but the cur- rent through a is doubled by the negative current of battery B' ', which flows in the same direction, producing a south pole in the core of a and a north pole in that of b, and hence armature M, having north polarity, is attracted to the pole of a and repelled from the pole of b. The negative current of B' flows in through coils a' and b' of relay R' equally, producing no magnetic effect, but the current through coil a' is doubled by the inflow- ing positive current from B, producing a south pole in the core of a' and a north pole in that of b' \ hence armature L' is attracted to pole of a' and repelled from pole of b' . Supposing the above conditions to exist when mes- sages are about to be sent in opposite directions simul- taneously; let the connections through pole-changer A' be reversed by depression of the key, putting the positive pole of B' to the line; the two batteries being now in opposition positively, there is no current in the line, but there is still a current to the negative pole of each bat- THE ELECTRIC TELEGRAPH. 339 in' NAM 1C lU.HVTNU'lTY AND MAtiNIWIUM, frry from Mir rMil.li. This <;niTrnl floww lo Mir nrgMllvr polr of // from rlj,jlif to Irff Mil'oilgh //, roll />, Mild polr HiiingiM' //; Mini limrr rrvrrarft Mir polMiify of frUiy /', MtllMf liny ^ liom polr of // lo polr of //; whilr llml to I ln< nr^Mllvr polr of //' flow, n<* hrl'MiT, fioin Irfl In , Mini hi'iiri' prod if COM no cluuitffi of pobnify In A", MiiiiMlmr M* llirirfoir I'dttUtlllW Mlll/iclrd to polr ot //' MS hufnrii, Now Irl Mir pobll rniinri lloiih of //' lr /i^iiin irvrrfiril, '"I Mir foi inn < onnr< I lout) hrhig ir^loirH, llir rmirnt fliron^h // of irhiy /' U rrvrmrd, Mini M Jillrnrfrd from polr of /' lo polr id ,i, wlillr /)/' i'. '.till MttiMf Ird to polr of it' M3 hrforr, llrmr rrhiy /' rr^pomU to thr < lutnj/rn of pobirlly produced !>y polr ( Imn^rr //', whilr rrhiy A" Is llllMffrrlrd hy Ilirm, II, iindrr rlllirr of llir Mhovn rondlll(nw, Mir pclr connrrlloiiH of // lr rrvrrwrd, rni'i'rMpmidiug r(Trrl. Mrr piodncrd In rrhiy /", wlillr rrlwy /i' irimiln* nn/iffrclrd, Sonndri ft, ronnri Ird wild r/irh rrl/iy, Mir oprnilrd l>y I'M d lMllriirt In Mir ii'-nnl iiuilinrr, Tlirrr in Mho it <) 1 1 ui 1 1 i liru'ilnl ron i MM |rd wild rn< li < ondrimrr, Mtt Kliown, In irjjiilolr lit] MI lion hy irlMidin^ llw dim liMi^r, ollirr- whr lUihlr in lir prrinMliirr on lon^ llnr^, Uiiiulriipltix TnlnKi'iiphy. hi |, li, ShiiU id Virnini, In 1855, Invriilrd llir finl rxprrlmrnlHl inrlliod of ////// i , or dniiMr, liMliwinU^lon, in llir ;>iinr dlrrclioii, (Ml n hln^lr wlrr, Ih, IJuNMCllU of Lrydrn M(HM Invrnlrd M diplrx mrlliod, nl llir SMinr llnir; Mini Kunnri, diiim-- llir wnmr yrMi' f intnlr- MII linprovrniriil in SlMik'w nirl hod, mid MMIOII ol Hrilln, in iHO^, Improvrd MHHNCIIM'N mrlliod, lint none nl llirr mrlliodw cMitir into pim'llntl nHr, lliougll lrnr llrUil In oprninj/ llir wjiy for fulnir Invrnloi'M, Mini n<> Inilliri ioirrtn in dilrx U'ttMHmUlduil WMN liunlr till ////*>tft* 4b* w> >,>,'*> ,-. ' ' . '> \',>, '.. . -,:.! ',/,.'! \\,. ,,.,:. , Mtity/fitmttiMHtw*iv*Afi' 1 ' rwvf^ ww9 MWv^P^^KR/ '*l 0tt**r tfantoM*, * ' : ' v "' "^ ' '^'"^ ' " f " ^'X' * " '' ..... "' .. . ., . .. . . ( . . . . ., . ....,...; ,,,.,.... i,/J , , , , ' i * r ^F"F VV P WlF ^WW/ J^P"lPFWr^r3P^ / j - - / - < > / j ^ ' , v PWWPKiWW^P ^ ^^Pw Pw W^Kf ^F0&& &&m^WWE0&&wUfwW wRGr^JWF P^r wJ9 . . : , ..... . . , .... ..... , , . . , ; -. - -. "-,: ' ' ' " S^'WWW^pvfir^F ^VFW^P JF F 4RPv0^vv<^ F^FVwHFwV^F H ..,..,....'.., -,/,'. .. , ,.,. < , /f .,; x< , . . , ,,... . " ' ' ' - '" * /' '--" '^" "" '.< ' ' V,' ..... - ,'.* .*.,.*., 4 . . t . . i t . t t rW R^^F^pF^rVfl ^ " : / .-.... .............. ? ! ..... " - - '' > ' ' ' " " " >'- -'> ' - ' ; ...... ''-/-'.".<}' '/ X" ' ;- > : 44. ^^F^^^^^^W^^W^F ^^^^^ ^^^^RP^FV^KV 1 WlF ^R*^ ^^ V-W / V Jrf* ' ..... : ' - .....-- v, : ,, ., ,.,. .,, . ;/. . .. !?*&* T, through both coils of neutral relay N and polar relay R with currents ot equal strength; the right-hand current going through the artificial line to the earth at , and the left hand current through the line \.o JB\ through coils a' of relays R' and N\ the upper tongue of pole-changer P\ the tongue and center post of transmitter T l to the negative pole z* of the smaller battery at X' , through this bat- tery to the positive pole, thence through the lower tongue and lever of P l to the earth at G'\ from the earth at G, through the lever and upper tongue of pole- changer P, to the negative pole of battery X, and through the battery to the positive pole, completing the circuit. There is now a 150 cell positive current, from battery X, traversing both coils of relays JV and R equally, and hence producing no magnetic effect on them. But this THE ELECTRIC TELEGRAPH. 345 current is increased in the left hand coils, a and a y by the 50 cell negative current of the small battery at X'\ and this increase produces sufficient magnetic effect to attract the armature of relay J? to the left hand stop, as shown, but not sufficient to attract that of JV against the force of its spring. But this 200 cell current trav- erses the right hand coils, a' and a' y of both relays at B, magnetizing their cores, so that the armature of relay JV 1 is attracted to its left hand stop, as well as that of relay R l to its right hand stop, as shown, producing a down click in the connected sounder of each relay. Now let the key of pole-changer P l be closed, revers- ing the polarity of the smaller battery at X', so that its positive pole is to the line, the current coming from the earth at G'. Its 50 cell positive current now flows through both coils of relays N l and It 1 equally, produc- ing no magnetic effect; the current through the artifi- cial branch going to the earth at E' , while that through the line branch changes the inflowing 50 cell negative current to an outflowing 50 cell positive current; so that now the inflowing current through a' and a' of both relays is only that of 100 cells; but, on account of the opposite winding in each branch, this 100 cell in- flowing current, on the right, and the 50 cell outflowing current, on the left, both magnetize each core in the same sense, so that both armatures are held in their former positions by the magnetism produced by a 150 cell current. But the current through a, of relay R, being also thus reduced from that of 200 cells to that of 100, while the current through b still remains that of 150 cells, as before, armature W \s attracted by the magnetism of a 50 cell current from the left to the right stop, producing a down click in the connected sounder, while the same 34-6 DYNAMIC ELECTRICITY AND MAGNETISM. preponderance of current does not affect the armature of relay N. Now let the key of transmitter T l be closed, and the connection to Z l through the transmitter tongue, lever, and center post being opened at r, and that through its tongue and left hand post, to the negative pole of the full battery at X' closed; the 150 cell positive current of X* being now to the line, neutralizes the 100 cell positive line current from battery X, but adds a positive current of 100 cells to the 50 cell current through the artificial line at B\ the armatures of both relays at B are therefore still held attracted as before. But the 100 cell line current through coils a and a at station A be- ing neutralized, while the 150 cell current through b and b and the artificial line still remains, the magnetism in relay N becomes strong enough to attract the arma- ture, producing a down click in the connected sounder, while the armature of relay R is still held on the right hand stop as before. Now let the connections of battery X' be reversed by opening the key of pole-changer P\ putting the nega- tive pole to the line. The negative current of X', being now added to the positive current of X, produces a line current of 300 cells, which flows in through coils a' and a', holding the armatures of both relays at B attracted as before, being in the opposite direction to the former outgoing current through b' and b' y which has ceased, and hence magnetizing the cores in the same sense. It also flows out through coils a and a, at station A, giving them a preponderance of 150 cells of current, over the current through b and b. The armature of relay N is therefore held attracted as before, while that of relay R is attracted from the right to the left stop, producing an up click in the connected sounder. Now let the key of transmitter T l be opened, and the THE ELECTRIC TELEGRAPH. 347 result is a change of the negative connection of battery X' to that of the smaller battery at Z 1 , restoring the conditions first considered, with both keys open at B and both closed at A; releasing the armature of relay N, which is pulled back by the spring, producing an up click in the connected sounder, all the other armatures remaining undisturbed. All the conditions which can occur with both keys closed at one station, and one or both keys either opened or closed at the other station, have now been considered; the results being, evidently, practically the same with the operations at the respective stations reversed: and it has been shown that, in each case, both relays, at each station, can be operated without mutual inter- ference; each responding to the manipulations of the key connected with the corresponding relay at the distant station, while the relays at the home station are unaffected by the manipulations of the home keys. There are minor points of practical importance, in the adjustment, which require attention, to prevent false signals, liable to occur during the momentary change of battery connections, and which, even with the best regulation, cannot be wholly prevented, but are not of sufficient importance to prevent the practical operation of the system. Repeating by the Quadruplex. The quadruplex system is also employed for repeating in a similar manner to that of the repeaters already described; messages being repeated either from one relay to another of the same kind, or from one side of the system into the other, as from neutral relay to polar, or the reverse; the latter being the usual method. Substitution of the Dynamo for the Battery. The dyna- mo was first substituted for the battery in telegraphing by Stephen D. Field, and put in operation by the 34** DYNAMIC ELECTRICITY AND MAGNETISM. Western Union Telegraph Company at New York in 1880, the Stemens-Halske dynamo being employed. But the result not being entirely satisfactory, the bat- tery was reinstated in 1887. Meantime great improve- ment had been made in dynamo construction, and in 1890 the dynamo was again substituted for the battery, not only at New York, but at all the principal stations of the Western Union. The improved Edison dynamo was adopted; the no volt machines, grouped in series of five each, and operated at a working potential of 70 volts to each machine, being employed for the line cir- cuits, and the 5 to 7 volt machines for the local circuits; a single dynamo being sufficient for the operation of several of the latter circuits, arranged in parallel. Some changes are required to adapt telegraphic transmission to the dynamo current, especially in the use of the duplex and quaduplex systems. The re- versals of polarity are made between two series of dynamos, one series furnishing the positive current and the other the negative. And the increase and decrease of current is produced by causing the currents to traverse routes of different resistance. By the use of the dynamo a full supply of current can be obtained at far more economical rates than by the battery, and also greater constancy of current; the loss due to battery exhaustion being obviated. The Wheatstone System of Automatic Rapid Transmis- sion. As telegraphic transmission by manual manipula- tion of the key does not exceed 25 to 50 words per minute, it becomes important in telegraph offices doing a large business to have some more rapid method. This is furnished by the system of automatic rapid transmission invented by Wheatstone, and employed in Great Britain for the Postal Telegraph, and in the United States by the Western Union at all its principal THE ELECTRIC TELEGRAPH. $49 offices. Its relations to manual transmission are similar to those of printing to writing. It consists in the prep- aration of the message for transmission by recording it with perforations made in a strip of tough paper, and the subsequent automatic operation of a transmitter by this perforated strip, by which the message is transcribed in Morse characters on a similar strip by an inking register at the distant station. Its instruments are a perforator and a transmitter, both of special construction, and an inking register of the ordinary type. The perforation of the paper is a purely mechanical operation. Three parallel rows of holes are perforated by punches which cut oi>t the bits of paper; the central holes, which are smaller and less perfect than the others, being made in a continuous series, equally spaced, by the teeth of a star-wheel by which the strip is simultaneously drawn through, while the message is punched in the two side rows. The punches are operated by three keys pressed down by two little mallets in the hands of the operator against the force of spiral springs which bring them up again. The depression of the left-hand key punches a hole on each side of the central hole, preparing the paper for the transmission of a dot. The depression of the cen- tral key carries the paper forward one wheel-tooth space, making only the central hole, preparing the paper for the transmission of a space, the length of which can be doubled by two consecutive depressions of this key. The depression of the right-hand key carries the paper forward two wheel-tooth spaces, making a hole on the left when the first wheel-tooth space is passed, and on the right when the second is passed, each opposite a central hole, preparing the paper for the transmission of a dash. The appearance of a strip prepared in this manner for the transmission of the word "operator" is 35 DYNAMIC ELECTRICITY AND MAGNETISM. shown in Fig. 114, a care ul examination of which will verify the above description. The paper, when -thus prepared, is placed on the transmitter, and carried forward at a regular rate of speed by a star wheel whose teeth fit into the central holes, and which is operated by a spring or weight. Each side row of holes passes over the points of two vertical rods, connected with light apparatus contained in a box underneath, by which a pole-changer is oper- ated. These points are pressed against the paper by spiral springs connected with the rods by bent levers, which operate the pole-changer; their upward move- ments being limited by two stops attached to an insu- O p & T a t 3T FIG. 114. lating walking beam, to which a regular, alternate motion is imparted by the force which operates the star wheel. As each point meets a hole in the moving paper it passes up till its connected lever meets a stop on the walking beam below; and each alternate move- ment of this beam, made simultaneously with the ad- vance of the paper one tooth space, pushes down the point which is up, and permits the other point to ascend till it is either stopped by the paper or passes up through a hole. The ascent of the left-hand point through the per- forated paper connects the positive pole of the battery with the line, bringing the pen of the register, at the distant station, into contact with the receiving paper', THE ELECTRIC TELEGRAPH. 351 and the ascent of the right-hand point through the per- forated paper reverses the polarity, withdrawing the pen from the receiving paper. And as the left point is adjusted one tooth space in advance of the right, if the two holes intended to transmit a dot pass over the points, the left point will ascend through its hole first, and be immediately depressed and the right point as- cend through its hole; and hence the registering pen will touch the paper and be at once withdrawn, making the dot. If now the paper move forward one tooth space, and the points meet no hole on either side, the negative pole being still to the line, and hence the registering pen still withdrawn, a space occurs on the receiving strip, the length of which depends on the distance the perforated strip moves before the left-hand point meets a hole. When this point meets a hole, the polarity being again reversed and a positive pole put to the line, the registering pen again touches the paper and is kept in contact till the right hand point meets a hole. If two tooth spaces are passed before this occurs, a dash is registered on the receiving strip, as shown at a or /, Fig. 114; but if only one is passed, a dot is registered, as before, as shown at e. Hence it appears that the office of this perforated paper is simply to reverse the polarity, and that when the positive pole is put to the line, either a dot or a dash is registered according to the time elapsing before re versal; and when the negative pole is put to the line, either a short or a long space is registered according to the time elapsing before reversal. In this way dots, dashes, and long and short spaces can be registered automatically as rapidly as the instruments can be made to operate; the usual range of speed being from 35 2 DYNAMIC ELECTRICITY AND MAGNETISM. 125 to 250 words per minute, which is about five times that of manual transmission. Hence one wire, by this method, can do the work of five wires by the ordinary method, and thus a large volume of telegraphic business be rapidly disposed of. This is especially advantageous in case of an accidental break in the connections, such as often occurs, since messages can still be received and prepared for trans- mission, and the accumulation rapidly despatched when the break is repaired. The simultaneous transmission of duplicate press despatches to numerous points from a central station can also, in this way, be greatly facilitated. Submarine Telegraphs. Submarine telegraph lines are constructed with cables like that shown in Fig. 115. FTG. 115. Seven or more No. 16 copper wires, thoroughly insu- lated, are inclosed in jute and protected by an exterior armor of iron wires wrapped with hemp; the interior being made water-proof. Deep sea cables, such as are used for the Atlantic lines, are made much lighter than those designed for shallow water, or shore ends, where the cables are more exposed to injury. There are two important points of difference between the operation of long ocean lines, like those across the Atlantic, and ordinary land lines. One is, that the use of powerful electric currents, such as are required to oper- THE ELECTRIC TELEGRAPH. 353 ate the ordinary Morse instruments on long land lines, are liable to damage the insulating material of long sub- marine lines, and produce faults which soon render such lines worthless. The other is, that the peculiar condi- tions of great length, submergence in water, and an incasing armor of conducting material, insulated from the interior conductors, produces an excessive static charge, similar to that of the Leyden jar, which seri- ously impedes electric transmission. Hence the first requisite is very sensitive apparatus, capable of responding to a low accompanying current, just sufficient to operate it, but not to injure the insula- tion ; such a current also reducing the static charge to its minimum. The second requisite is a condenser by which an induced, operating current is sent to the line and also a subsequent, reactive, opposing current, which neutralizes the return current of the static charge in the manner already described. Thomson's reflecting galvanometer and accompany- ing scale, described in Chapter VI, page 130, furnishes the sensitive apparatus required, and is employed as the receiving instrument; the movements of the spot* of light, on the scale, to the right or left of the zero mark, being made to indicate dots in one direction and dashes in the opposite direction, the pauses at zero indi- cating spaces. Thomson's siphon recorder, also a very sensitive instru- ment, is employed for the same purpose. It consists of a capillary glass siphon, lightly poised, to which an oscillating, horizontal movement is given by apparatus operated by the line current through a local circuit, by which its point is made to vibrate, without contact, across a moving strip of paper, making a continuou: , irregular line of dots with ink ejected in fine drawn from a vessel in which the opposite end of 354 DYNAMIC ELECTRICITY AND MAGNETISM. siphon is immersed; the ink and paper being oppositely electrified, and the various contortions of the line made to indicate the Morse characters. Fig. 116 represents a cable with the transmitting G CABLE FIG. 116. apparatus in connection at the terminal station on the right, and the receiving apparatus at the terminal sta- tion on the left, each kind being, of course, duplicated at each station, and the connections for transmitting or receiving changed by switches as required. The trans- mitting apparatus consists of the key K, permanently connected, by its rear contact, with the battery B, and arranged for connection by its front contact, when de- pressed, with the earth at , with which the battery is also connected by its opposite pole. The receiving ap- paratus consists of the galvanometer G, connected with the earth at ', through the condenser C, on one side, and through the rheostat ^, on the other. The con- denser contains 40,000 or more square feet of tinfoil, and hence is capable of accumulating a very large charge, and the rheostat has a very high resistance. The connection of the negative pole of the battery with the line through K, and of the positive pole with the earth, sends a positive current to the earth at , and, as a result of the negative potential thereby produced in THE ELECTRIC TELEGRAPH. 355 the line, a positive current flows from the earth at E' into the conductor C, repelling an equal amount of electricity from the opposite side of the condenser to the line and negative pole of the battery, completing the circuit. The line and line side of the condenser thus become negatively charged, but being at the same potential on both sides of the galvanometer G, this charge does not affect the needle after the first deflec- tion produced by the charging ; the needle therefore remains at zero. Now let the key at K be depressed, closing the front contact, and a positive current flows to the line, and into the line side of the condenser, from the earth at E, producing a deflection of the needle ; but the condenser and line becoming charged positively, to the same potential, on both sides of the galvanometer, the needle returns immediately to zero. Now let the front contact at K be opened, and the former condition being restored, and the current reversed, a deflection in the opposite direction occurs, after which the needle returns to zero as before. The advantage of a constant charge of the line in this manner becomes apparent when we consider that the complete charging or discharging of a submarine cable, two or three thousand miles in length, occupies several minutes, the time varying in proportion to the square of the length. The currents transmitted by the manipu- lations of the key rise and fall in waves ; about T 2 ^ of a second elapsing before any perceptible effect is produced at the opposite station by the closing or opening of a battery connection, and about 3 seconds being required for the wave to attain its full strength, and 3 more for it to decline. Hence, if the full phase of a wave inter- vened between signals, transmission would be exceed- ingly slow. But, by keeping the line charged, and em- ploying the condenser, the current can be quickly re- 356 DYNAMIC ELECTRICITY AND MAGNETISM. versed after each signal, without waiting for the wave to attain its full strength. And as the deflections indi- cating dots and dashes respectively are in opposite directions, their amplitude is of no consequence. Hence if, for instance, a succession of dots is required, as for the letter H, the first deflection has large amplitude, but is checked by a momentary reversal of current, and then further increased by another reversal; and thus, by four impulses in rapid succession, each less prominent than the preceding one, but all in the same direction, the four dots required are indicated. In like manner dashes are indicated by similar opposite deflec- tions. A speed of 15 to 20 words per minute can thus be attained. The constant charge of the cable and condenser in this manner reduces the interference of earth currents, often serious on lines of such length, to its minimum, so as to be of no practical importance ; the charge being sufficient to counteract the effect of such currents, under ordinary conditions, and leave a working surplus. The earth connection at O allows a small percentage of current to pass through the rheostat, by which the potential on opposite sides of the galvanometer is equal- ized when the needle is at zero, without interference from the cable charge ; this potential being nearly the same as that of the line, on account of the high resist- ance of R. Locating Faults. When a fault or break occurs in a submarine line, it can be located approximately by the Wheatstone bridge test, invented by De Sauty. One arm of the bridge is connected with the cable, and the ppposite arm with a condenser of known resistance ; and a current being sent through the instrument, flows into the condenser, through one side, and into the cable through the other, and to the earth at the fault ; and. THE ELECTRIC TELEGRAPH. 357 by this means the resistance from the shore to the fault can be ascertained. And the cable resistance per mile, in ohms, being known, the distance to the fault is easily ascertained. As the resistance of the fault itself often varies con- siderably, it is important to test from each end and com- pare the results. And by subtracting the added results from the known resistance of the cable, the loss of re- sistance due to the fault can be accurately ascertained ; and the proportionate amount of the remaining resist- ance from each end should give the resistance to the fault, from which the distance in miles can be ascer- tained as before. When a cable contains separately insulated conduc- tors, and a fault occurs in one of them, the loop method, which is considered one of the most accurate, can be adopted. Let the faulty wire be connected with a per- fect wire at one end of the cable, and the two opposite terminals connected with the bridge at the other end ; a current being transmitted, goes to the earth at the fault, passing round the loop from one side of the bridge, and direct to the fault from the other side. The known resistance of the cable being subtracted from the ascertained resistance of the loop, the re- mainder is the resistance to the fault from the end where the two wires are joined. The resistance from each end to the fault being thus ascertained, the dis- tance can be calculated. The Dial Telegraph. In the dial telegraph, different forms of which have been invented by Wheatstone, Breguet, and Siemens, messages are indicated by ordi- nary letters displayed on a dial. In Breguet's receiving instrument the letters of the alphabet are arranged on the dial in a circle, around which a pointer rotates, stopping at the required letter. This pointer is oper- DYNAMIC ELECTRICITY AND MAGNETISM. ated by an electromagnet by means of clock-work in response to the closing or opening of the circuit by the transmitter; each make or break moving it one letter, always in the same direction ; hence by a number of such changes in rapid succession it is brought to the required letter. The transmitter has a similar dial around which a lever is moved, by which a toothed wheel is rotated, which either closes or opens the circuit alternately at each letter, by a connected lever, till the required letter is reached. Such telegraphs are well adapted to the requirements of private lines, where the services of skilled operators are not available, and have been so employed in Europe ; but, in the United States, printing telegraphs have been preferred for such lines. Printing Telegraphs. Telegraphs by which the mes- sage is printed in ordinary type have been invented by House, Hughes, Phelps, and Pope and Edison. A de- tailed description of these various instruments can be found in " Prescott's Electricity and the Telegraph" and similar books, to which the reader is referred. Their general principles of construction are similar to those of the dial telegraphs, and may be illustrated by supposing the lettered keys of a type-writer con- nected with telegraphic apparatus in such a manner that the depression of a key transmits a current by which a letter, corresponding to the one marked on the key, is printed on a strip of paper in the receiving in- strument at the distant station, and the paper moved as required by the same apparatus. In fact, the print- ing telegraph is such a telegraphic type-writer. THE TELEPHONE. 359 CHAPTER XIII. THE TELEPHONE. Early History. The reproduction of music and speech by electric transmission was first accomplished by Phil- ipp Reis of Friedrichsdorf in 1861, though the transmis- sion of sound and speech by the ordinary vibrations of a tightly stretched wire, as in the mechanical telephone, had been known for 200 years previous. The produc- tion of sound by electromagnetic vibrations was first observed by Page in 1837, in connection with the alter- nations of magnetism induced in an iron bar by an in- termitting electric current in proximity ; which, when occurring rapidly and rhythmically, gave rise to a musi- cal tone, In 1854 Bourseul described a method which he had tried, by which speech could be electrically transmitted, which is practically the same as that now employed ; and predicted its ultimate success when sufficiently developed. During the next twenty years the only progress made consisted in improvements of the musical telephone by various inventors, About 1874 Elisha Gray began a series of experiments on the musical telephone, in Chicago, which led to the invention of a method by which it could be employed for the transmission of speech, for which, in 1876, he filed a caveat in the United States Patent Office. Meantime Alexander G. Bell had been making similar experiments, in Boston, expressly for the transmission of speech, and he also invented a method for which he applied for a patent on the sane day that Gray filed his caveat. 360 DYNAMIC ELECTRICITY AND MAGNETISM. A patent was granted to Bell March 7, 1876, his tele- phone was exhibited at the Centennial exposition at Philadelphia the same year, and its practical application to commercial use soon followed. Claims for priority of invention were made by Gray, also by Daniel Drawbaugh of Pennsylvania, who claimed to have invented a similar telephone in 1872, and by Dr. Cushman, whose claims extended back to 1851, The litigation between Bell and Gray ended in a compromise, and that with the other parties was decided in Bell's fa- vor. The transmission of speech by Reis's telephone was so imperfect as not to be considered entitled to priority as against the more perfect method invented by Bell. Principles of the Telephone. Sound, in the telegraph, is the arbitrary symbol of speech, while that in the tele- phone is its reproduction, the result of undulations of the air produced by the speaker at one end of the line, and reproduced by the transmitted currents at the other end. Tones, whether of music or articulate speech, are caused by the occurrence of such undulations in rhythmical order, their pitch depending on the number of undula- tions per unit of time, and their volume on the ampli- tude of those undulations. The property known as timbre, which distinguishes tones of the same pitch and volume from each other, depends on the manner in which the undulations are produced by different voices; a graphic representation showing undulations of differ- ent form. In the telephone these three properties, pitch, volume, and timbre, are accurately reproduced by the undula- tions, so that the characteristic quality of the voice, and the manner of utterance, can be distinctly recognized, as well as the words spoken. And these undulations are produced by variations of current strength, and not by intermissions of current as in the telegraph. It is the property of reproducing sonorous tones THE TELEPHONE. 3 6i which was first recognized, and caused the invention of the musical telephone to precede that of the speaking telephone, the properties of articulation and timbre being subsequently developed; and it is the development of these latter properties which distinguishes the Bell telephone from the musical telephones of Reis and Gray. As the Bell telephone embodies the leading principles of the musical telephones of Reis and Gray, a detailed description of the latter is unnecessary. The Bell Telephone. The Bell telephone is strictly a magneto-electric apparatus, generating its electric cur- rents by the movements of a magnetized armature in proximity to a conductor forming a closed circuit. The construction of its principal instrument, employed now as a receiver only, but formerly both as a transmitter and a receiver, is shown in Fig. 117. A round, hard-rubber case, 6J inches long and i-J inches in diameter, enlarged at one end as shown, incloses a round bar magnet, of an inch in diameter, to whose north pole, N, is attached a wooden bobbin, bb, wound with fine, silk-covered, cop- per wire, whose terminals, gg, are at- tached to larger wires, c c, connected with the binding-posts h h. A very thin sheet-iron disk, PP, 2\ inches in diameter, varnished to protect it from oxidation, covers the circular space within which the bobbin and magnet pole are placed, and is held in position, at its edges, by the ear piece W, which is screwed over it as shown. The center of this disk comes close to the magnet, the distance being adjusted by the screw d\ sufficient space being allowed for a slight vibration of the disk, without con- DYNAMIC ELECTRICITY AND MAGNETISM. tact. There is also a similar amount of space between the disk and the ear piece: and the vibrations produced by the variations of magnetic energy are limited by the elasticity of the disk; its center vibrating very slightly, while its edges are held fast. This disk is known as the diaphragm. In the bottom of the cavity of the ear piece, opposite the center of the diaphragm, is an opening, -^ of an inch in diameter, through which the undulations produced in the air by the vibrations are transmitted to the ear. If one terminal of the coil, as Z, be connected with an ordinary telegraph line, and the other, E, with the earth, and corresponding connections be made with a similar instrument at the opposite end of the line, one instru- ment can be used as a transmitter and the other as a receiver. When a person speaks into the transmitter, the undulations of the air cause the diaphragm to vibrate ; each vibration varying the distance between the disk and the magnet, producing corresponding varia- tions of magnetism in the disk as an armature, which induce an alternating current of varying strength in the coil. This current, transmitted to the coil of the re- ceiver, at the opposite end of the line, reproduces like variations of magnetism in its diaphragm, and hence corresponding vibrations and undulations, which repro- duce the words spoken into the transmitter. The improved telephone, as above described, was patented by Bell Jan. 30, 1877; the chief claims of the patent being the substitution of the iron disk for the stretched animal membrane previously employed, and the magneto-electric current for the battery current. Improved Transmitters. It is evident that much of the energy of the transmitted voice is spent in overcoming the various interposed resistances, so that when heard in the receiver it is comparatively weak, and the feebler THE TELEPHONE. 363 tones are liable to be indistinct. Hence various means have been devised to counteract the effects of this con- sumption of energy and render the delivery more dis- tinct. The Edison Transmitter. It was observed by Du Mon- cell that increase of pressure reduces the contact resist- ance between conductors ; and that this effect is increased by reduction of hardness and increase of electric resistance in the conductors themselves: and hence that variations of current strength may be pro- duced in this way. It was also observed by Edison that carbon is peculiarly adapted to fulfill these conditions; and in accordance with these observed facts he con- structed, in 1878, the first transmitter in which carbon was employed. He also employed platinum, as pro- posed by Gray, using a disk of each material in contact, and producing a slight contact between the platinum disk and the vibrating diaphragm by an ivory button; the high resistance of each substance, the difference of their hardness, and the varying pressure produced at the numerous contacts by the vibrations of the dia- phragm, being intended" to improve the conditions of transmission. Edison also employed an induction coil, as had pre- viously been done by Gray to increase the E. M. F. of the line current; connecting its primary coil with the circuit of a local battery, whose current traversed the transmitter, and its secondary with the line, by one terminal, and with the earth by the other, completing the circuit by corresponding connections of a Bell receiver with the line and earth at the opposite station. By this means the relative conditions of E. M. F., resistance and current strength, in the two circuits, could be varied, so that a large current of low E. M. F. and resistance, in the primary, could be converted into 364 DYNAMIC ELECTRICITY AND MAGNETISM. a small current of high E. M. F., in the secondary, capa- ble of overcoming the line resistance, and producing sufficient amplitude of vibration in the diaphragm of the receiver to render the tones audible and distinct. The Blake Transmitter. The transmit- ter invented by Blake, and now employed by the Bell Telephone Company through- out the United States, is an improved form of the Edison transmitter. A ver- tical section of its principal parts is shown in Fig. 118, attached to the door of a little cabinet in which it is con- tained; Fig. 119 giving a rear view, with the door open, showing also the induc- tion coil and connections. . 118. Opposite the mouth-piece a, formed V FIG. 119. by a funnel-shaped opening in the door, is fixed the sheet-iron diaphragm , connected with the same cord as the plug inserted in the hole bearing the calling subscriber's number in compartment B, and inserts it in the hole, in compartment C, bearing the number of the subscriber called for, which puts the two in communication; and having rung a call to the sub- scriber wanted, by pressure on the signal button, she opens the cam connection and closes the annunciator. All the spring-jacks through which any subscriber's line passes are in connection through a separate wire, and it is on a connection with this wire that the test above described is made. As each clearing-out annunciator is in circuit only when two subscribers are connected and can be operated by either, a comparatively small number is sufficient; but each subscriber being constantly in circuit with one THE TELEPHONE. 3/1 of the other annunciators, there must be as many of these as there are subscribers. Hughes' Microphone. In 1878 Hughes invented the instrument known as the microphone, by which feeble sounds can be transmitted, and reproduced, greatly amplified, in a telephone receiver. In his first experi- ments a wire nail, making loose contacts with two other nails, was employed, but when the superior quality of carbon for telephonic transmission was demonstrated by Edison, Hughes substituted a small carbon rod, pointed at both ends, and loosely mounted vertically between two carbon supports attached to a thin sound- ing board. The terminals of a battery circuit, in which the receiver is included, are attached to these supports, and the slightest sound made near this simple instru- ment, as the ticking of a watch or the walking of a fly, can be distinctly heard in the receiver, at a distance of several feet. The instrument is too sensitive to be used for ordinary transmission, but the advantage of loose contacts, as thus demonstrated, has been utilized to increase the sensitiveness of carbon transmitters, by the substitution of granulated carbon for carbon plates. Theory of Telephonic Transmission. Opinion is divided in regard to the fundamental principles of telephonic transmission, and especially in regard to the functions of carbon as a transmitter. It is well known that heat- ing reduces the electric resistance of carbon, and hence it is maintained that the variation of heat generation in the carbon, produced by the variation of pressure due to the loose contacts, produces a corresponding variation of resistance and hence of current strength. While the heat thus generated must be almost infinites- imal in quantity, nevertheless its ratio to the molar and molecular vibrations, in an apparatus of such delicate 372 DYNAMIC ELECTRICITY AND MAGNETISM. sensitiveness as the telephone, is believed to be sufficient to account for the improved transmission; and observa- tion shows that continuous use produces a perceptible rise of temperature in a transmitter. This theory ap- plies more particularly to the microphone, but its appli- cation to carbon transmitters in general is obvious. The generally accepted theory of molar vibrations of the diaphragms, as already explained, is disputed by some, who maintain that telephonic transmission is chiefly, if not wholly, due to molecular vibrations pro- duced by the variations of magnetism. In proof of this it is shown that such transmission is possible with in- struments constructed with thick disks, incapable of the molar vibrations ascribed to the thin ones. This theory also receives confirmation from the slight in- crease of length shown to be produced in a steel bar by magnetizing it, indicating that variations of magnetic strength in a magnetized bar must produce correspond- ing variations of length. The click due to magnetiza- tion, as observed by Page, also shows that magnetic, molecular vibrations may become sonorous. It is probable that all these theories are more or less applicable; that molar vibrations, molecular vibrations, and variations of resistance due to variations of tem- perature, all contribute to the observed results. The circular shape of the diaphragms and plates is also im- portant in contributing to evenness and regularity of vibration, and hence producing corresponding evenness and regularity in the atmospheric undulations, and should not be overlooked. Multiplex Telephony. The ordinary conditions of telephonic transmission require that each subscriber should have a separate wire, since each must operate his own line, on which strict privacy is required, and THE TELEPHONE. 373 be in constant connection with a central exchange through which he can be put in communication with others; while, in telegraphic transmission, the messages of numerous persons are sent, in rotation, from a central station, over the same wire, by experts, to whom alone their contents are known. Hence a telegraph line can be kept constantly occupied by thousands of persons, and rapid, automatic transmission employed, while a telephone line is occupied, usually, only a small propor- tion of the time by a single person. As this difference makes the expense of telephonic transmission enormous, as compared with telegraphic, various methods have been devised for simultaneous duplex telephonic transmission, similar to that of telegraphic, and also for the occupancy of the same line by several persons in rotation. While experiments of this kind have been, to some extent, successful, their success has not been such as to warrant their general, practical adoption. The occupancy of the same line by different sub- scribers in rotation is, however, in practical use for intercommunication between different central stations; the trunk lines, already referred to, being employed in this way; the proportionate number of such lines to the subscribers' lines being dependent on the amount of intercommunication. A description of the various experimental methods, referred to above, may be found in Preece and Maier's book on " The Telephone." Long Distance Telephony. The multiplicity of lines required for the operation ofca practical telephone sys- tem, as shown above, and the difficulty of overcoming resistance and induction so as to reproduce speech dis- tinctly at the terminals of long lines, has till recently 374 DYNAMIC ELECTRICITY AND MAGNETISM. confined the use of the telephone chiefly to the limited areas of towns and cities. Experiments in long distance telephony were formerly made on telegraph lines; and as these were composed of single, grounded, iron wires, arranged in numerous parallel lines, in close proximity on the same poles, and hence subject to high mutual induction, the results obtained were not sufficiently encouraging to induce the investment of capital in in- dependent telephone lines, and the true causes of failure were not at first clearly perceived. The effects of induction beween parallel telephone lines in proximity, which causes a person with a receiver to his ear, waiting to be put in communication, to hear, indistinctly, conversation between others, is well known. But the fundamental difference between telephonic and telegraphic transmission, as already shown, aggravates this inductive effect, when lines used for both purposes are in proximity ; the strong, intermittent current on the telegraph line overcoming the light, undulatory current on the telephone line to such an extent as to interrupt the undulations and render transmitted speech indistinct, especially on long lines ; producing a con- tinuous, accompanying crackle. Van Rysselberghe's System. This effect can be reduced by giving the telegraphic current an undulatory motion similar to that of the telephonic. This has been done by Van Rysselberghe, a Belgian electrician. He intro- duced two electromagnetic primary coils, having iron cores, into the telegraph circuit, one between the battery and the key and the other between the key and the line, and passed the ground wire through a condenser. The rise and fall of the current at each intermission, caused by the charge and discharge of the condenser, in con- nection with the retardation due to self-induction and THE TELEPHONE. 375 magnetic lag in the coils, produces an undulatory effect, similar to that in the ocean cable lines. By connecting the telephone apparatus with the line through a sepa- rate condenser, the same line can be used for simulta- neous telegraphic and telephonic transmission. As this system requires that all parallel lines, mounted on the same poles, shall be constructed in this manner, and shall also have special apparatus to prevent the tele- phonic induction referred to above, all of which entails considerable extra expense ; and as it also retards tele- graphic transmission, it has not come into general prac- tical use, though employed on some of the Belgian lines. The American System. In the long distance telephone system now employed by the American Telephone and Telegraph Company the lines are composed of No. 12 copper wire, and are complete metallic circuits, without ground connections ; each line having two wires, on each of which the current flows in opposite directions. The superior conductivity of copper, as compared with iron in one branch of the circuit, and the earth in the other, is apparent ; besides which the mutual induction of opposite currents in the two parallel lines proportion- ately increases the current strength in each direction ; while the freedom from grounded connections at the terminals prevents interruption from the inductive effects of contiguous, grounded, telegraph and tele- phone lines, usually numerous at such terminals. Where several such lines are mounted on the same poles, the mutual induction between currents, which would produce " cross talk " between the lines, is neu- tralized by introducing lines constructed with numer- ous transpositions between the straight lines. These transpositions are made at regular intervals of a few 376 DYNAMIC ELECTRICITY AND MAGNETISM. poles apart, by crossing the two branches of the line, without contact, so that each takes the place of the other on the cross-arms. By this means the adjacent lines, on either side, are alternately brought into prox- imity, through short sectional distances, with wires bear- ing reversed currents in each section, and thus the effects of induction are neutralized. The instruments employed are the Running trans- mitter and the Bell receiver, with the signaling appa- ratus already described. The Running Transmitter. The diaphragm of this transmitter is a disk of platinum foil, supported by a metal ring, and protected by wire guards in front. A thicker disk of brass, gold-plated, is placed back of this one, and parallel with it, at a distance of about -f% of an inch, and the space between filled with finely granulated carbon, sifted free from dust, whose superior quality as a transmitter has been already referred to. The whole is inclosed in a wooden box, to which is attached a metallic, funnel-shaped mouth-piece, in front, and, at the back, are attached the binding-posts for the battery terminals, one connected with the supporting ring of the diaphragm, and the other with the rear plate ; so that the current must pass through the carbon. Transmission on Long Distance Lines. Lines 600 miles long are now in practical working order, speech being reproduced with distinctness, and lines 1000 miles in length are projected. On the line between Chicago and Milwaukee, 90 miles in length, whispered conversation and the ticking of a clock can be reproduced distinctly, also a hiss, the most difficult sound to transmit by the telephone. For the accommodation of the Bell Telephone Com- pany's subscribers, connections are provided at the cen- THE TELEPHONE. 377 tral stations between their lines and the long distance lines, and the extra price for such service charged to the subscriber's account whenever such connection is made. But the reproduction of speech through such connec- tions, is not so perfect as by direct connection. INDEX. A. PAGE Absolute Magnetic Intensity 43 Accessory Apparatus, for the telephone 365 Accumulator 235 Action, heat developed by electrochemical 254 ' ' , local, in the battery cell 9 Advantages of the Alternating Current Dynamo 189 Agonic Line 38 " Map of the United States 52 Agitation of the Solution, in electroplating 226 Alliance Machine, the 166 Alternating Current Dynamo, advantages of the 189 " , the Gordon 185 " " " , " Westinghouse 186 " " Dynamos 185 " , separate excitation 189 " " Motor, the 198 Aluminium, Bunsen's process for 229 " , St. Clair Deville's process for 229 " , the Hall process for 231 Amalgamation of the Zinc, in battery cells 8 American Morse Code ... 312 ' ' System of long distance telephony 375 Ammeter, the Weston 135 Ammeters, gravity 145 ' ' , voltmeters and 134 Ampere, the 6, 1 16 Ampere- Hour, the 117 379 380 INDEX. Ampere's Table 82 " Theory of Magnetism 89 " Rule 73 Analogy between Magnetic and Electric Phenomena 67 Analyzer, the, in the polarization of light 284 Angles, measurement of 122 Angular " " Deflective Force 123 Anions 207 Annual and Diurnal Variation 50 Anode, defined 207 Anodes, the, in electroplating 221 Apparatus, accessory, for the telephone 365 " , signaling " " " 366 Arc, the 297 " Lamp, the 297 " Light, " 295 ' ' , multiple, defined 29 Armature, of the magnet 55 " , " " electromagnet 78 " , " " dynamo 170 " , " " " , the cylinder 174 " , " " " , Gramme, interior wire of the 173 41 , " " " , the Pacinnotti-Gramme 170 " , " " " , " Siemens 167 Armatures, of the dynamo, closed circuit and open circuit 176 Armature's Magnetic Poles, location of the, in the dynamo 177 Arrangement, station, in the telegraph 321 Artificial Magnets 54 Attraction and Repulsion, polar 58 Astatic Galvanometer 129 " Needle, the 73 Automatic cut-out, for arc lamps 303 " Rapid Transmission in telegraphy, the Wheatstone sys- tem 348 Regulation, in the arc lamp. . . 300 Auxiliary Operations, in electroplating. . . 223 Ayrton and Perry's Spring Voltmeters and Ammeters 142 B. Balance, Coulomb's torsion. . 67 Ballistic Galvanometer 134 INDEX. 38l PAGE Battery, the, for the telegraph 314 " cell, element and 3 " , De La Rive's floating 84 " Formation 28 " " , Two- fluid cells 23-34 " , Grove's gas 233 " , Sign 3 " , substitution of the dynamo for the, in telegraphing 347 " , the voltaic. Definitions I-I2 Becquerel's Discoveries in the electric relations of light 288 Bell Telephone, the 361 Bichromate Cell, potassium 15 Bifilar Suspension 129 Biot's Law . 44 Bodies, paramagnetic and diamagnetic 64 Blake Transmitter, the, for the telephone 364 Blasting, electric 257 Break, and make, in the electric circuit 93 Bridge, the Wheatstone 157 Brushes, dynamo 1 70 " " , position of the 178 Buckling, conductivity and, in storage cells 246 Bunsen Cell, the 26 Button Repeater, the, in the telegraph 325 C. Calibration of Galvanometer 124 Callaud Cell, the 25 Candles, electric 295 Capacity of conductors, electro-thermal 255 " , storage, of storage cells 249 Carbons, arc lamp 300 Cardew Voltmeter, the 147 Cations 207 Cathode " 207 Cause of Deflection, of the needle 74 Cautery, electric 258 Cell, the Bunsen 26 ", " Callaud 25 ", " Clark 139,142 ", ' Daniell 23 382 INDEX. PAGE Cell, diamond-carbon 20 " , Element, and Battery 3 " , the Faure Storage 236 ", " " " , defects of 240 " , " " " , improved 241 " , " " " , electric energy of 244 ", " Grenet 16 " , "Grove 26 " , " Julien storage 247 " , " Law 30 " , " Leclanche. 17 " , " mercuric bisulphate 17 " , Plante's secondary 236 " , " " , electric energy of 239 " , potassium bichromate 15 ' ' , the Pumpelly storage 247 " , " silver chloride 27 " , Smee's 13 " , theory of electric generation in the 6 " , the voltaic, operation of 6 " , Walker's 14 Cells, connection between 33 " ,dry... 21 " , durability of storage 249 " , gravity 25 " .one-fluid 13-22 ' ' , polarization of one-fluid 22 " , two-fluid. Battery formation 23-34 " , " *' , construction of 23 4 ' , weight of storage 247 " , zinc-carbon 13 Charge and Discharge, effects of, on storage cell plates 245 Charging and Discharging storage cells, relative time of 249 Chemical Equivalence 216 " Reaction in the Faure cell 240 " " Plante " 237 Circuit, open, defined 19 Clamping, insulation and, in the battery 9 Closed Circuit and Open Circuit Armatures 176 Code, the American Morse 312 " , " International Morse 312 r INDEX. 383 PAGB Coefficient of Magnetic Induction 76 " " Magnetism 98 " " Mutual Induction 95 Coil, induction 98 " a Converter, the 105 " , spark 109 Coils, resistance 155 Common Galvanometers 134 Commutation 165 Commutator, dynamo 170 " , '* , improved 171 " , Ruhmkorff's 104 Compass, the mariner's 35 " , " surveyor's 37 Compensating Magnet 74 Composition of Grids, for storage cell plates 247 Compound Winding, in the dynamo 183 Compounds, electrolysis of mixed 212 Condenser 99 *' , Leyden Jar and 233 " , " " as a 102 " , operation of 102 Conditions of Electric Energy in the battery cell 4 " " Electrolysis 210 Conductivity and Buckling, in storage cells 246 , insulation and 112 Conductor 113 Conductors, electro-thermal capacity of 25 5 Connection Between Cells 33 Connections, repeater, telegraph 327 Consequent Poles in the magnet 61 Constant Current Dynamo 183 Potential " 183 Construction of Core in induction Coil 101 " " " dynamo armature 174 " " " " field-magnets 179 ' " " "electromagnets 76 , line, in the telegraph 320 " and Operation of the Quadruplex 341 special, in induction coils 102 of Two-Fluid Cells 23 Convection, effect of in electrolysis 218 384 INDEX. PAGF Converter, the 189 " , the coil a , 105 " , " Tesla Motor as a 202 Core of dynamo armature 1 74 " " " field-magnets 179 " " electromagnet 76 " " induction coil 97, 98 " " " ", construction of 101 " " " , induction of 97 " " " ", sliding TGI Cosine 122 Cosmic Variation 51 Coulomb, the 117 Coulomb-Meter, the Forbes 1 50 Coulomb's Torsion Balance 67 Couronne de Tasses, the , 2 Crater and Point, the, in arc-light carbons 298 Current, deflection by the electric 71 " , direction of, in the dynamo ' . 1 73 " , electric 5, 114 " , " , deflection of by the magnet . 82 " and Electrylote, relative conditions of 219 " , establishment of, in the arc lamp 299 ' ' , extra 96 " , faradic, physiological effects of 107 " , generation of, dependent on variation of intercepted mag- netic force 95 " Induced by Another Current 91 " " Opening or Closing Primary Circuit 93 "Magnet 90 " " " Varying the Strength of Primary Current 94 " Induction, results of 94 " , rotary movement by 87 " Meter, the Edison. .. 150 " , position and, in the incandescent lamp 305 4 ' Reversal, effect of, in electrolysis. . 217 Currents, eddy 56, 196 " , electric, generation of by induction 90 " , Foucault . . .56, 196 ' , mutual induction of electric 84 Cut-out, automatic, for arc lamps 303 INDEX. 385 PAGE Cut-out, Ground-Switch, and Lightning Arrester, for telegraph. . . 319 Cylinder, armature, in the dynamo 1 74 D. Daniell Cell, the 23 Declination 37 Defects of the Faure Cell, 240 Definitions. The voltaic battery 1-12 Deflective Force, angular measurement of 123 Deflection, cause of 74 " , by the Electric Current 71 , of " " " by the Magnet 82 " , magnetic force ascertained by 42 De La Rive's Floating Battery , 84 Deposit, thickness of, in electroplating 225 De Tasses, the Couronne 2 Details of electroplating, various 220 Development of the Electric Motor 190 Diagrams, thermo-electric 266 Dial Telegraph, the 357 Diamagnetism, experiments in 79 Diamagnetic Bodies, paramagnetic and 64 ' ' and paramagnetic substances, list of 81 Diamond-Carbon Cell, the 20 Differential Galvanometer 133 Relay 333,336 Different Kinds of Electric Measurement 118 Dip, inclination or 40 Diplex Transmission, in telegraphy 340 Dipping Needle, the 40 Direction of the Current, in the dynamo 173 Discharge in Air and in Vacuo 107 " , charge and, effects on storage cell plates. ,. . . . 245 " , E. M. F. of, in storage cells 246 Discharging Storage Cells, charging and, relative time of 249 Discoveries, Becquerel's, in the electric relations of light 288 , Faraday's " " " " " " 284 " of Gal van i " " battery i , Kerr's, " " electric relations of light 289 , Kiindt & Rontgen's, " " " ' 289 " , Verdet's, in the " " " " 287 386 INDEX. PACK Discoveries of Voita, in the battery 2 Disque Leclanche Cell 19 Distribution, elevated road electric 204 " , magnetic, lamellar 63 " , multiple series and series multiple 307 " of electric Power 203 " , parallel, in electric lighting 305 " , series, " " " 303 " , three wire system of, in electric lighting 308 Double Reflection, effects of, on magneto-polarized light 292 Dry Cells 21 " Pile, Zamboni's 7 Duplex Telegraphy 329 " , the polar 333 " , " " , operation of 338 " , " Steam's 330 Durability of Storage Cells 249 Dynamic Electricity Denned i Dynamo, the 168 " , advantages of the alternating current 189 " Brushes 170 " Commutator 170 " , constant current 183 " , " potential 183 , the Edison 185 " , " Gordon 185 ' ' and Motor, the 165-205 " , the, as a Motor 193 " , " , substitution of, for the battery in telegraphing 347 " , " Westinghouse 186 Dynamos, alternating current 185 E. Early History of the telegraph , 310 " " " " telephone 359 Earth's Magnetic Poles, the 37 Eddy Currents 56, 196 Edison Current-Meter, the 1 50 " Dynamo, the 185 " Transmitter, the, for the telephone 363 Effect of Convection in electrolysis , , 218 INDEX. 387 PAGB Effect of Current Reversal in electrolysis 217 *' , the Peltier, in the electric relations of heat 268 " , "Thomson, " " " " " 269 Effects of Charge and Discharge on storage cell plates 245 " " Double Reflection on magneto-polarized light 292 "" , Magnetic, in electrolysis 216 Electric Blasting 257 Candles 295 " Cautery 258 " Current 5, "4 " " , deflection by the 71 " " , deflection of, by the magnet 82 " Currents, generation of, by induction 90 " " , mutual induction of 84 " Energy, conditions of, in the battery cell 4 " " of improved Faure cell 244 " " , loss of, in the dynamo 195 " " required for electroplating 225 " Fuses 258 Gas-Lighting 109 " Generation in the Cell, theory of 6 " Heat to Electric Light, the relations of 279 Horse-Power, the 118 " Intensity 29,32 " Lighting 295 " Measurement 110-164 ' " , different kinds of 118 " Motor, development of the 190 " Perforation ... 107 " Potential no " Preparation of Plates for Faure Cell 244 " " " Plante " 236 " Pressure in " Reduction of Ores 228 " Refining of Metals 227 " Resistance 5, 112 " " , measurement of 154 " Storage 233-251 " Telegraph, the 310-358 " Transmission, heat developed by 252 ' Units.. 121 388 INDEX. PAGE Electric Welding 273 Electricity, dynamic defined I 4 * , relations of, to heat 252-278 "."light 279-309 Electrochemical Action, heat developed by 254 '* Equivalence..... 217 Electrodes and Poles .... 4 Electrodynamometer, the Weber- Edelmann 152 Electrolysis 206-232 " , conditions of 210 " of Mixed Compounds 212 " f relations of, to heat 213 , secondary reaction in 211 ' ' of water 10, 209 Electrolyte, relative conditions of current and 219 Electrolytes 206 Electromagnet, the 75 Electromagnets, form of 78 Electromagnetic Poles . 75 " Saturation 78 Electromagnetism , 71-109 Electrometers 119 Eletcromotive Force 4, 1 1 1 " of discharge in storage cells 246 " , lowest required in electrolysis 214 " , Resistance, and Current, units of 6 Electroplating 220 " , agitation of the solution 226 " , the anodes 221 , auxiliary operations in 223 " , plating solutions 223 " , required electric energy for 225 , " time of immersion and thickness of deposit. 225 ' ' , various details 220 Electro-Thermal Capacity of Conductors 255 Electrotyping 226 Element, and Battery, cell. 3 Elevated Road Distribution 204 Eleven Year Period, the 50 Energy, electric, conditions of, in the battery cell 4 *' f " , of improved Faure cell. .... 244 INDEX. 389 Energy, electric, of Plante cell 239 " t " , loss of in the dynamo 195 " " , required for electroplating.. 225 Equipment, simple line telegraph 314 Equivalence, chemical 216 " , electrochemical 217 Equivalent, Joule's , 253 Establishment of the Current, in the arc lamp 299 Exact Observation, of the earth's magnetism. 51 Exchange, the telephone 367 Excitation, separate of the dynamo 189 Experiments in Diamagnetism 79 Extra Current 96 F. Farad, the 1 1 8 Faraday's Discoveries, in the electric relations of light 284 " Laws, for electrolysis 215 Faraday, nomenclature by 206 Faradic Current, physiological effects of 107 Faults, location of, in submarine telegraph lines 356 Faure Cell, the 239 " " , ", chemical reaction in 240 " " , ", defects of 240 " " , " improved 241 " " , " '* , electric energy of 444 Field, magnetic , . . . , 60 " Magnets, the, in the dynamo 170,179 Filament, the, in the incandescent lamp 304 ! ' and Lamp Attachment 305 Forbes' Coulomb-Meter, the 150 Force, deflective, angular measurement of .... 123 " , electromotive 4, in " , magnetic, ascertained by oscillation. 42 " , " " " deflection 42 " , " , lines of. , 59 " , " , portative 57 " , " , tube of ,., , , 60 Formation, battery 28 Form of Electromagnets .-.-., 78 " "Magnets ,....., 63 390 INDEX. PAGE Foucault Currents , 56, 196 Fuses, electric 258 G. Galvani, discoveries of I Galvanometer, astatic 1 29 " , ballistic 134 " , calibration of 124 " , differential 123 " , sine 124 ' .tangent 126 " " , the Helmholtz-Gaugain 128 " , Thomson's reflecting 130 Galvanometers 119 , common 134 Galvanoscope, the 71 Gas Battery, Grove's 233 ' ' Lighting, electric 109 Gauss- Weber Portable Magnetometer, the. . 69 Generation of Electric Currents by Induction 90 " " Current Dependent on Variation of Intercepted Magnetic Force 95 " , photo-electric 285 , thermo-electric 259 Generator, the magneto-electric 165 Gonda Leclanche cell 19 Gordon Dynamo, the 185 Gramme Armature, interior wire of the 173 Gravity Ammeters 145 Cells 25 Grenet Cell, the 16 Grids for storage cell plates, composition of 247 Grotthus' Theory of electrolysis 207 Ground-Switch and Lightning Arrester, cut-out, in the telegraph.. 319 Grove Cell, the , 26 Grove's Gas Battery 2-33 H. Hall Process for Aluminium, the 231 Heat Developed by Electric Transmission 252 " " " Electrochemical Action 254 INDEX. 391 PAGE Heat, electric, relations of, to electric light 279 ' and Light, in the arc lamp 299 4 ' , relations of electricity to 252-278 " , " " electrolysis to 213 Hefner von Alteneck's Regulator 302 Helix of electromagnet 77 Helmholtz-Gaugain Tangent Galvanometer, the 128 History , early, of the telegraph 310 " " , " " telephone 359 Horse-Power, the electric 118 Hughes' Microphone 371 Hunning Transmitter, the, for the telephone 376 Hydrogen Alloy Theory, the, in electric storage 250 I. Immersion, required time of, in electroplating 225 Improved Commutator ... 171 Faure Cell , 241 " " , electric energy of 244 Transmitters, telephone 362 Incandescent Lamp, the , 303 Inclination or Dip 40 Induction, coefficient of magnetic 76 " , " "mutual 95 Coil 98 of Core, in coil 97 " " Electric Currents, mutual 84 " , generation of electric currents by 90 " , magneto-crystallic 64 " , results of current 94 " , rotary movement by current 87 " .self 96 Insulation and Clamping, in batteries 9 " Conductivity 112 Insulator 113 Intensity, absolute magnetic, the earth's 43 Intensity, electric 29,32 " , magnetic, the earth's 42 Interior Wire of the Gramme Armature. . . 173 International Morse Code, the 312 Interrupter, in the induction coil 99 Inversion, thermo-electric 269 39 2 INDEX. PAGE Ions 207 Isoclinic Lines. 41 Isodynamic Lines 44 Isogonic Lines 41 Isoclinic Map of the United States 49 Isodynamic Map of the United States 45 Isogonic " " " " " 48 J. Joule's Equivalent 253 " Law 253 Julien Cell, the, storage 247 K. Kerr's Discoveries, in the electric relations of light 289 Kiindt and Rontgen's Discoveries, in the electric relations of light 289 Key, the telegraph - 314 L. Ladd's Machine. 170 Lag, magnetic, in the dynamo 178 Laminated Magnets 56 Lamp, the arc 297 " Attachment, filament and, in the incandescent lamp 305 " , the incandescent 303 Law, Biot's 44 " Cell, the 20 ", Joule's 253 " , Lenz's 93 " , Ohm's 1 14 Laws, Faraday's, for electrolysis 215 Leclanche Cell, the 17 Lenz's Law 93 Leyden Jar as a Condenser. 102 "and " , the 233 Light, the arc 295 " , Becquerel's discoveries in the electric relations of 288 " , electric, the relations of electric heat to 279 " , Faraday's discoveries in the electric relations of 284 " , the heat and, in the arc lamp 299 " , Kerr's discoveries in the electric relations of 289 INDEX. 395 PACK Light, Kiindt and Rontgen's discoveries in the electric relations of 289 " , Verdet's discoveries in the electric relations of 287 " , magneto-optic polarization 284 " , Maxwell's theory of the electric relations of . . . 293 44 .molecular " " " " " " 294 " , photo-electric generation 280 " , polarization of 283 " , the relations of electricity to 279-309 " , strain in the media 294 ' ' , summary of the electric relations of 292 Lighting, electric 295 " , the arc 297 " , " "lamp 297 '* , " " " , automatic cut-out 303 " " , " " " , " regulation 300 " , " " " ,thecarbons 300 " , " " " , " crater and point 298 " , " " ** , establishment of the current 299 " , " " " , the heat and light 299 " , " " " , Hefner von Alteneck's regulator. 302 " , " " " , series distribution 303 " , " "light 295 " , " incandescent lamp 303 " . " " , the filament 304 " " , " " " , filament and lamp at- tachment 305 " , " " " , multiple series and se- ries multiple distribu- tion 307 " " , " " " , parallel distribution 305 *' , " " " , three-wire system 308 Lightning-Arrester, cut-out, ground-switch and, for the telegraph 319 Line, agonic 38 " construction, telegraph 320 Equipment, simple, in the telegraph 314 Lines of Force, magnetic 59 " " " , isoclinic 41 " " " , isodynamic 44 " " " , isogonic 41 " , long distance telephone, transmission on 376 List of Diamagnetic and Paramagnetic Substances 81 394 INDEX. PAG Local Action, in the battery 9 Location of the Armature's Magnetic Poles, in the dynamo 177 " *' " Poles, in the magnet 63 Locating Faults, in submarine telegraph lines 356 Lodestone, the 35 Long Distance Telephone Lines, transmission on 376 " " Telephony 373 Loss of Energy, in the dynamo 195 " , magnetic, in magnets 56 Lowest Required Electromotive Force, in electrolysis 214 M. Machine, the Alliance 166 " , Ladd's 170 " , Wilde's 167 Magnet, compensating. 74 ' ' , current induced by 90 ' ' , the natural 35 Magnets, artificial 54 " , the field, in the dynamo 170, 179 " , form of 63 " , laminated 56 Magnetic Distribution, lamellar 63 " Effects, in electrolysis 216 ' ' and electric phenomena, analogy between 67 Field 60 ' ' Force Ascertained by Deflection 42 " " " " Oscillation 42 " " , generation of current dependent on variation of intercepted 95 " Induction, coefficient of 76 *' Intensity, the earth's 42 " " , absolute, the earth's 43 " Lag, in the dynamo 178 *' Lines of Force 59 " Loss, in magnets . , 56 " Maps 40 " " , of the hemispheres 39 " Penetration 63 " Polarity 35 " Poles, the earth's 37 INDEX. 395 PACK Magnetic Poles, location of the armature's, in the dynamo 177 Saturation 55 Shells 63 " Storms 50 " Strength, in the electromagnet 76 Magnetism 35-?o " , Ampere's theory of 89 " . coefficient of 98 4 ' , origin of terrestrial 44 " as a Mode of Molecular Motion 65 " .residual 57 " , terrestrial, illustrated 41 Magneto-Crystallic Induction 64 Magneto-Electric Generator 165 Magneto-Optic Polarization 284 Magnetometer, the Gauss- Weber, portable 69 Make and Break, in the electric circuit 93 Map, agonic, of the United States 52 " , isoclinic," " " " 49 " , isodynamic, of the United States 45 " , isogonic, " " " 48 Maps, magnetic 40 " , " , of the hemispheres 39 Mariner's Compass, the I 35 Maxwell's Theory of the electric relations of light 293 Measurement of Angles .... 122 " " Deflective Force, angular 123 " , electric 110-164 " " , different kinds of 118 " of " Resistance 154 Media, strain in the, in the electric relations of light 294 Megohm, the 116 Mercuric Bisulphate Cell, the 17 Metals, electric refining of 227 Microfarad, the 118 Microvolt, " 116 Microphone, Hughes' 371 Milliammeter, the Weston 138 Milliampere, the 117 Milliken Repeater, the, in the telegraph 326 Mixed Compounds, electrolysis of 212 Molecular Motion, magnetism as a mode of 65 INDEX. PAGE Molecular Theory of the electric relations of light 294 Morse Code, the American 312 " " , " International 312 Motor, the alternating current .' . . 198 " , development of the electric 190 " , the dynamo as a 193 " , " " and 165-205 " , principles of the 193 " as a converter, the Tesla 202 " , the Westinghouse Tesla 199 Motors, series, shunt, and compound wound 196 " , thermo-magnetic 204 Multiple Arc defined 29 " Series and Series Multiple distribution , 308 " Switch-Board, the telephone 367 Multiplier, the Schweigger 72 Multiplex Telephony 372 Mutual Induction of Electric Currents 84 " " , coefficient of 95 N. Natural Magnet, the 35 Needle, the astatic 73 " , " dipping 40 " Telegraph, the 311 Neutral Relay, the telegraph 342 Nobili's Rings . , 216 Nomenclature by Faraday 206 O. Observation, exact, of the earth's magnetism 51 Ohm, the 6, 116 Ohm's Law 114 One-Fluid Cells 13-22 " " " , polarization of 22 Open Circuit defined 19 Operation of Condenser 102 " " the Polar Duplex 338 " " " Quadruplex, construction and 341 " " Voltaic Cell 6 Operations, auxiliary, in electroplating 223 INDEX. 397 PAGE Ores, electric reduction of 228 Origin of Terrestrial Magnetism 44 Oscillation, magnetic force ascertained by 42 P. Pacinotti-Gramme Armature, the 170 Parallel Distribution, in electric lighting 305 Paramagnetic and Diamagnetic Bodies 64 Peltier Effect, the, in the electric relations of light 268 Penetration, magnetic 63 Perforation, electric 107 Period, the eleven year 50 Periods, secular 46 Photo-Electric Generation 280 " " Reduction of Resistance in Selenium 282 Photophone, the 282 Physiological Effects of Faradic Current 107 Pile, dry, Zamboni's 7 " , the Voltaic 2 Plante's Secondary Cell 236 Plates for Faure cell, electric preparation of the 244 " " Plante " , " " " " 236 " " storage cells, effects of charge and discharge on 245 Plating Solutions 223 Point, the crater and, in arc light carbons 298 Polar Attraction and Repulsion 58 " Duplex, the 333 " , ", operation of 338 Polarity, magnetic 35 Polarization 9 " of One-Fluid Cells 22 " of Light 283 , magneto-optic 284 Polarized Relay, the 336 Polarizer, the, in the polarization of light 284 Pole-Changer, the 334 Poles, consequent, in the magnet 61 " , the earth's magnetic 37 " , electrodes and 4 " , electromagnetic 75 " , location of the, in magnets 63 INDEX. Position of the Brushes, in the dynamo 178 " and current, of the incandescent lamp 305 Portable Magnetometer, the Gauss-Weber 69 Portative Force, magnetic 57 Potassium Bichromate Cell 15 Potential, electric no Power, distribution of. 203 Pressure, electric in Primary Circuit, current induced by opening or closing 93 " Current, " " " varying the strength of 94 Principles of the Motor 193 " "Telephone , 360 Printing Telegraphs 358 Prism or Gonda Leclanche Cell 19 Pumpelly Cell, the 247 Q. Quadruplex, construction and operation of the 341 " , repeating by the 347 Telegraphy 340 Quantity electric 29, 31, 32 R. Rapid Transmission, the Wheatstone system of automatic, in telegraphy 348 Reaction, chemical, in the Faure cell 340 " , " , " " Plante " 337 " , secondary, in electrolysis 211 Recorder, siphon, Thomson's 353 Reduction of Ores, electric 228 Refining of Metals, " 227 Reflection, effects of double, on magneto-polarized light 292 Reflecting Galvanometer, Thomson's 130 Register, the telegraph 315 Regulation, automatic, in the arc lamp 300 Regulator, Hefner von Alteneck's 302 Relations of Electricity to Heat, the 252-278 Relations of Electricity to Light, the 279-309 " " Electric Heat to Electric Light, the 279 " " Electrolysis to Heat 213 INDEX. 399 PAGE Relay, the telegraph 317 " " differential 333,336 " " " neutral ....342 " , polarized 336 Rheostat, water 101 Relative Conditions of Current and Electrolyte, in electrolysis.. 219 " Time of Charging and Discharging storage cells 249 Repeater Connections, telegraph 327 Repeater, the button 325 " , " Milliken 326 Repeaters, telegraph 324 Repeating by the Quadruplex 347 Repulsion, polar attraction and 58 Required Electric Energy, in electroplating 225 " Time of Immersion and Thickness of Deposit in elec- troplating 225 Residual Magnetism . . 57 Resistance Coils 155 " , electric 5, 112 " , " , measurement of 154 " in Selenium, photo-electric reduction of 282 Results of Current Induction , 94 Reversal, effect of current, in electrolysis 217 " of Rotation, in the motor 202 Reversible " " " " 197 Rings, Nobili's 216 Rotary Movement by Current Induction 87 Rotation, reversal of, in the motor 202 " , reversible , " " " 197 Rules, Ampere's 73 Ruhmkorff's Commutator 104 S. San Francisco, secular variation at 53 Saturation, electromagnetic 78 " , magnetic 55 Schewigger Multiplier, the 72 Secondary Cell, Plante's 236 Reaction, in electrolysis 211 Secular Periods , . . . 46 " Variation . .. 46. 400 INDEX. Secular Variation at San Francisco 53 " " in the United States 48 " " at Washington 51 Selenium, photo-electric reduction of resistance in 282 Self-induction 96 Separate Excitation of the dynamo 189 Series Distribution in electric lighting 303 " multiple, multiple series and, distribution 307 " , Shunt, and Compound Winding, in the dynamo 180 " , " , " Wound Motors 196 Shells, magnetic 63 Shunt and Compound Winding, Series 180 Siemens Armature, the 167 Sign, battery 3 Signaling Apparatus, for the telephone 366 Silver Chloride Cell, the 27 Simple Line Equipment, telegraph 314 Sine denned 122 " Galvanometer 124 Siphon Recorder, Thomson's 353 Sliding Core, in the induction coil 101 Smee's Cell 13 Solenoid, the 83 Solution, agitation of, in plating . . 226 Solutions, plating 223 Sounder, the telegraph 316 Spark, " 97 " Coil 109 Special Construction, in induction coils 102 Station Arrangement, telegraph. 321 Stearns Duplex, the 330 Storage Cell, composition of grids for plates 247 " " , conductivity and buckling in 246 " , effects of charge and discharge on the plates 245 " " , preparation of the plates 244 " " , E. M. F. of discharge 246 " " , the Faure 239 " " , " " , chemical reaction in 240 " ", " " , defects of 240 " , hydrogen alloy theory 250 " , Improved Faure , 241 INDEX. 4 O1 PAGE Storage Cell, Improved Faure, electric energy of 244 " " , " , " preparation of the plates. .. 244 ' " , the Julien 247 " " , Plante's, electric energy of 239 " " , " , chemical reaction in 237 " " , " , electric preparation of the plates 236 " " , the Pumpelly 247 " Cells, capacity of 249 " " , durability of . . . . i 249 " " , relative time of charging and discharging 249 " " , weight of 247 Storage, electric 233-25 1 Storms, magnetic 50 Strain in the Media, in the electric relations of light 294 Strength, magnetic 76 Submarine Telegraphs 352 Substitution of the Dynamo for the Battery, in the telegraph. . . . 347 Summary of the relations of electricity to light 292 Surveyor's Compass, the 37 Suspension, bifilar 129 Switch-Board, the telegraph 321 " , " telephone multiple 367 System of automatic rapid transmission, the Wheatstone 348 " " long distance telephony, the American 375 " , Van Rysselberghe's 374 " , three-wire, in electric lighting 308 T. Table, Ampere's 82 " of Thermo-Electric Potential of Metals 264 Tangent defined 125 " Galvanometer 126 , the Helmholtz-Gaugain 128 Telegraph, the electric 310-358 " , the American Morse code 312 " , " battery 314 " " " , " button repeater 325 " " " , cut-out, ground-switch, and lightning arrester 319 , construction and operation of the quad- ruplex 34I 402 INDEX. PAGE Telegraph, the electric, the dial 357 '* , early history of 310 " " , the international Morse code 312 " " " key 314 " " , line construction 320 " , locating faults in submarine lines 356 " " " , the Milliken repeater 326 " , needle ... 311 " " " , operation of the polar duplex 338 " " " , the polar duplex 333 " " " " polarized relay 336 " " , " pole-changer 334 " " , printing telegraphs 358 " " , the register 315 " , " relay 317 " " , repeater connections 327 '* 4< , repeaters 323 " " , repeating by the quadruplex 347 " " " , simple line equipment , 314 " " , the sounder 316 " " , station arrangement 321 " " , the Stearns duplex 330 " " , substitution of the dynamo for the bat- tery 347 " " , switch-board. 323 " " , the Wheatstone system of automatic rapid transmission 348 Telegraphs, printing 358 " , submarine 352 Telegraphy, diplex transmission in 340 " , duplex 329 " , quadruplex 340 Telephone, the 359~377 " " , accessory apparatus 365 " the Bell 361 " , early history of 350, " , the exchange 367 " " , Hughes' Microphone 371 " " , multiple switch-board 367 " , principles of 360 " , signaling apparatus 366 INDEX. 403 Telephone, the, theory of telephonic transmission 371 " " , transmission on long distance lines 376 " transmitter, the Blake 364 , " Edison 363 , " Hunning 376 , the, improved transmitters 362 Telephonic Transmission, theory of! 371 Telephony, long distance 373 " , " '* , the American system 375 " , " " , Van Rysselberghe's system 374 Multiplex 372 Terrestrial Magnetism Illustrated 41 " , origin of 44 Tesla Motor as a Converter, the 202 " , the Westinghouse ........ 199 Theory of Electric Generation in the Cell. 6 " " Grotthus, in electrolysis 207 " , the hydrogen alloy, in electric storage 250 " of Magnetism, Ampere's 89 " , Maxwell's, of the electric relations of light .... 293 " , the molecular of the electric relations of light 294 " of Telephonic Transmission 371 Thermo-Electric Diagrams 266 " " Generation 259 " " Inversion 269 " " Potential of Metals, table of 264 Thermo-Magnetic Motors 204 Thermopile, the 270 Thickness of Deposit, in electroplating 225 Thomson Effect, the, in the electric relations of heat 269 Thomson's Reflecting Galvanometer 130 Three-Wire System of distribution, in electric lighting 308 Time of Immersion and Thickness of Deposit, in electroplating. 225 Torsion Balance, Coulomb's 67 Transformer, or converter, in the alternating current system. . . . 190 Transmission, diplex, in telegraphy 340 " , heat developed by electric 252 " on Long Distance Telephone Lines. . . 376 , theory of telephonic 371 , the Wheatstone system of automatic rapid 348 Transmitter, the Blake telephone 364 404 INDEX. PAGE Transmitter, the Edison telephone 363 " , " Hunning " 376 Transmitters, improved " 362 Tube of Force 60 Two-Fluid Cells. Battery Formation 23-34 " " " , construction of 23 U. Units, electric 121 " of Electromotive Force, Resistance and Current 6 V. Vacuo, discharge in air and in 107 Van Rysselberghe's System of long distance telephony 374 Value of Volta's Discoveries 3 Variations, annual and diurnal, in terrestrial magnetism 50 " , cosmic " " " 51 " , secular " " " 46 " , " , at San Francisco 53 " , " , in the United States 48 " , " , at Washington 51 Various Details of electroplating 220 Yerdet's Discoveries, in the electric relations of light 287 Vibrator, in induction coil 99 Volt, the 6, 1 16 Volt-ampere 1 18 Volta, discoveries of 2 Volta's " , value of 3 Voltaic Battery, the. Definitions 1-12 " Cell, operation of the 6 Pile, the 2 Voltameter, the water 151 Voltameters 151 Voltmeter, the Cardew 147 " , " Weston 135 , " Wirt 139 Voltmeters and Ammeters 134 " " , Ayrton & Perry's spring 142 W. Walker's Cell 14 Washington, secular variation at o . . . , 51 INDEX. 45 PAGE Water, electrolysis of 10, 209 Rheostat 101 " Voltameter, the 151 Watt, the 118 Weber-Edelmann Electrodynamometer, the 152 Wheatstone Bridge, the 157 " System of Automatic Rapid Transmission, the 348 Weight of Storage Cells 247 Welding, electric 273 Westinghouse Dynamo, the 186 Tesla Motor, the 199 Weston Ammeter, the 138 " Milliammeter, the 138 " Voltmeter, " 135 Wilde's Machine 167 Winding electromagnets 76 " , series, shunt, and compound 180 Wirt Voltmeter, the 139 Z. Zamboni's Dry Pile 7, 8 Zinc, amalgamation of the, in battery cells 8 Zinc-Carbon Cells 13 TJHIVBRSITT Southern Electrical Supply Co,, Gate City Electric Co., 823 LOCUST ST., 522 DELAWARE ST., Sx. Louis, Mo. KANSAS CITY, Mo. Central Electric 116 & 118 FRANKLIN STREET, CHICAGO. MANUFACTURERS, DEALERS, AND IMPORTERS OF ALL KINDS OF Electrical Supplies. TRADE MARK. GENERAL WESTERN AGENTS OKONITE WIRE AND PRODUCTS. IMPROVED CANDEE WIRE. THE BUTLER HARD RUBBER GOODS. THE CELEBRATED PACKARD LAMP. THE BRADY MAST ARM, ETC., ETC. We carry at all times a complete assortment of the latest improved electrical specialties, and solicit correspondence. CONNECTED BY PRIVATE WIRE WITH POSTAL TELEGRAPH-CABLE COMPANY. Electric Power Transmission Co., Western Electrical Supply Co,, 1722 LAWRENCE ST., 418 S. FIFTEENTH ST., t, COLO. OXIAHA NEB FACTORIES: ANSONIA, CONN. TfiE. * f IJtf^DOLf A, gf. MANUFACTURERS AND DEALERS IN QENERAL ELECTRICAL SUPPLIES WESTERN AGENTS FOR A HIGH GRADE OF RUBBER INSULATION. SOLE MANUFACTURERS OF \VIREX SUNBEAM LAMPS AT SUNBEAM PRICES. EDISON LAMPS AT EDISON PRICES. LIST OF WORKS ELECTRICAL SCIENCE PUBLISHED AND FOR SALE BY D. VAN NOSTRAND COMPANY, 23 Murray and 27 Warren Streets, New York. ATKINSON, PHILIP. The Elements of Electric Lighting. Including Electric Generation, Measurement, Storage and Distribution. Sixth edition. 104 illustra- tions, 260 pages. 12mo, cloth $150 Elements of Static Electricity, with full description of the Holts and Topler Machines and their mode of operat- ing. 65 illustrations. 12mo, cloth 1 50 BADT, F. B. The Dynamo Tender's Hand-Book. With 70 illustrations. 16mo, cloth 1 00 Incandescent Wiring Hand-Book. With 41 illustrations and five tables. Second edition. 12mo, cloth 1 00 Bell-Hanger's Hand-Book. 97 illustrations. 12mo, cloth 1 00 BOTTONJK, S. JR. Electrical Instrument-Making for Ama- teurs. A Practical Hand-book. With 48 illustrations. Fourth edition. Enlarged by a chapter on The Telephone. 12mo, cloth. Reduced to 50 The Dynamo, How Made and How Used. A Book for Amateurs. Sixth edition, with additional matter and illustrations. 39 illustrations 1 00 Electric Bells and all about them. A Practical Book for Practical Men. With more than 100 illustrations. 12mo, cloth. Reduced to... 50 Electro-Motors. How Made and How Used. A Hand- book for Amateurs and Practical Men. Many illustra- tions. 12mo, cloth 120 CUMMING 9 L. Electricity Treated Experimentally. For the use of Schools and Students. New edition. 12mo, cloth 150 DVMONCEL, COUNT. Electro- Magnets. The Deter- mination of the Elements of their Construction. 16mo, fancy boards. (Van Nostrand's Science Series, No. 64.). $0 50 FISKE, Lt. BRADLEY, A. 9 U. S. N. Electricity in Theory and Practice; or, The Elements of Electrical En- gineering. Eighteenth edition. 8vo, cloth. 180 illustra- tions 250 HASKINS, C. H. The Gahanometer and its Uses. A Manual for Electricians and Students. Fourth edition, revised. 12mo, morocco, illustrated 1 50 HEAP, Major D. P. Electrical Appliances of the Present Day. Being a report on the Paris Electrical Exhibition of 1881. 8vo, cloth, fully illustrated 2 00 HOBBES, W. R. P. The Arithmetic of Electrical Measure- ments. With numerous examples fully worked. 12mo, cloth 50 HOPKINSON, DR. JOHN. Dynamic Electricity: Its Modern Use and Measurement, chiefly in its Application to Electric Lighting and Telegraphy; including I. Some Points on Electric Lighting^ II. On the Measurement of Electricity for Commercial Purposes. By T. N. Shoolbred. III. Electric Light Arithmetic. By R. E. Day. 16mo, boards. (Van Nostrand's Science Series, No. 71.) 50 INCANDESCENT ELECTRIC LIGHTING. A practical description of the Edison system by L. H. Lati- mer, to which is added a description of the Edison Electro- lytic Meter and a paper on the maximum efficiency of In- candescent Lamps. By John W. Howell. (Van Nos- trand's Science Series, No. 57.) 16mo, boards 50 INDUCTION COILS. How Made and How Used. (Van Nostrand's Science Series, No. 53.) 16mo, boards. . 50 KAPP, GISBERT. Electric Transmission of Energy and its Transformation, Subdivision, and Distribution. A practical Hand-book. 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