The Standard Electrical Dictionary - A Popular Dictionary of Words and Terms Used in the Practice - of Electrical Engineering
by T. O'Conor Slone
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Synonyms—India Rubber—Rubber.


Capacity, Dielectric. The capacity of a dielectric in retaining an electrostatic charge; the same as Specific Inductive Capacity. 'The number expressing it is sometimes called the dielectric constant. (See Capacity, Specific Inductive.)

Capacity, Electric, or Electrostatic. The relative capacity of a conductor or system to retain a charge of electricity with the production of a given difference of potential. The greater the charge for a given change of potential, or the less the change of potential for a given charge the greater the capacity. The measure of its capacity is the amount of electricity required to raise the potential to a stated amount. The unit of capacity is the farad, q. v. Electric capacity is comparable to the capacity of a bottle for air. A given amount of air will raise the pressure more or less, and the amount required to raise its pressure a stated amount might be taken as the measure of capacity, and would be strictly comparable to electrostatic charge and potential change. The capacity, K, is obviously proportional to the quantity, Q, of the charge at a given potential, E, and inversely proportional to the potential, E, for a given quantity, Q, or, (1) K == Q/E and (2) Q = K * E, or, the quantity required to raise a conductor by a given potential is equal to the capacity of the conductor or system multiplied by the rise of potential. The capacity of a conductor depends upon its environments, such as the nature of the dielectric surrounding it, the proximity of oppositely charged bodies and other similar factors. (See Dielectric-Condenser-Leyden jar.)

The dimensions of capacity are found by dividing a quantity of electricity by the potential produced in the conductor by such quantity.

Quantity ( ((M^.5)*(L^1.5)) / T ) / potential ( ((M^.5)*(L^.5)) / T ) = L.

Capacity, Instantaneous. The capacity of a condenser when connected only for an instant to a source of electricity. This is in contrast to electric absorption (see Absorption, Electric), and is capacity without such absorption taking part in the action.


Capacity of a Telegraph Conductor. The electric capacity of a telegraphic conductor is identical in quality with that of any other conductor. It varies in quantity, not only for different wires, but for the same wire under different environments, as the wire reacting through the surrounding air or other dielectric upon the earth, represents one element of a condenser, the earth, in general, representing the other. Hence, a wire placed near the earth has greater capacity than one strung upon high poles, although the wires may be identical in length, material and diameter. The effect of high capacity is to retard the transmission of intermitting signals. Thus, when—as in the Morse system—a key is depressed, closing a long telegraph current and sending a signal into a line, it is at least very probable that a portion of the electricity travels to the end of the wire with the velocity of light. But as the wire has to be charged, enough current to move the relay may not reach the end for some seconds.

Capacity of Polarization of a Voltaic Cell. The relative resistance to polarization of a voltaic cell, measured by the quantity of electricity it can supply before polarization. A counter-electromotive force may be developed, or the acid or other solution may become exhausted. The quantity of electricity delivered before this happens depends on the size and type of cell and other factors.

Capacity, Residual. When two insulated conductors are separated by a dielectric, and are discharged disruptively by being connected or nearly connected electrically, on removing the discharger it is found that a slight charge is present after a short interval. This is the residual charge. (See Charge, Residual.) Shaking or jarring the dielectric facilitates the complete discharge. This retaining of a charge is a phenomenon of the dielectric, and as such, is termed residual capacity. It varies greatly in different substances. In quartz it is one-ninth what it is in air. Iceland spar (crystalline calcite) seems to have no residual capacity. The action of shaking and jarring in facilitating a discharge indicates a mechanical stress into which the electrostatic polarization of the conductor has thrown the intervening dielectric.

Capacity, Specific Inductive. The ratio of the capacity of a condenser when its plates are separated by any substance to the capacity of the same condenser when its plates are separated by air.

A static accumulator consists of two conducting surfaces separated by an insulator. It is found that the capacity of an accumulator for an electric charge, which varies with or may be rated by the potential difference to which its conductors will be brought by the given charge, varies with the nature of the interposed dielectric, and is proportional to a constant special to each substance. This constant is the specific inductive capacity of the dielectric.

The same condenser will have a higher capacity as the dielectric is thinner, other things being equal. But different dielectrics having different specific inductive capacities, the constant may be determined by ascertaining the relative thicknesses of layers having the same total inductive capacity. The thicker the layer, the higher is its specific inductive capacity.

Thus it is found that 3.2 units thickness of sulphur have the same total inductive capacity as 1 unit thickness of air. In other words, if sulphur is interposed between two conducting plates, they may be separated to over three times the distance that would be requisite to retain the same capacity in air. Hence, sulphur is the better dielectric, and air being taken as unity, the specific inductive capacity of sulphur is 3.2.


The specific inductive capacity of a dielectric varies with the time and temperature. That of glass rises 2.5 per cent. between 12 C. (53.6 F.) and 83 C. (181.4 F.). If a condenser is discharged disruptively, it retains a small residual charge which it can part with later. If a metallic connection is made between the plates, the discharge is not instantaneous. Vibration shaking and jarring facilitate the complete discharge. All this shows that the charge is a phase of the dielectric itself, and indicates a strained state into which it is brought.

The following table gives the specific inductive capacity of various substances:

Specific Inductive Capacity. Substance Specific Inductive Authority Capacity. Vacuum, air at about 0.001 millimeters pressure 0.94 about Ayrton Vacuum, air at about 5 millimeters 0.9985 Ayrton 0.99941 Boltzmann Hydrogen at about 760 millimeters pressure 0.9997 Boltzmann 0.9998 Ayrton Air at about 760 millimeters pressure 1.0 Taken as the standard Carbon Dioxide at about 760 millimeters pressure 1.000356 Boltzmann 1.0008 Ayrton Olefiant Gas at about 760 millimeters pressure 1.000722 Boltzmann Sulphur Dioxide at about 760 millimeters pressure 1.0037 Ayrton Paraffin Wax, Clear 1.92 Schiller 1.96 Wllner 1.977 Gibson and Barclay 2.32 Boltzmann Paraffin Wax, Milky 2.47 Schiller India Rubber, Pure 2.34 Schiller India Rubber, Vulcanized 2.94 Schiller Resin 2.55 Boltzmann Ebonite 2.56 Wllner 2.76 Schiller 3.15 Boltzmann Sulphur 2.88 to 3.21 Wllner 3.84 Boltzmann Shellac 2.95 to 3.73 Wllner Gutta percha 4.2 Mica 5 Flint Glass, Very light 6.57 J. Hopkinson Flint Glass, Light 6.85 J. Hopkinson Flint Glass, Dense 7.4 J. Hopkinson Flint Glass, Double extra dense 10.1 J. Hopkinson


Capacity, Unit of. The unit of capacity is the capacity of a surface which a unit quantity will raise to a unit potential. The practical unit is the surface which a coulomb will raise to one volt, and is called the farad, q. v.

Capacity, Storage. In secondary batteries the quantity of electrical current which they can supply when charged, without undue exhaustion. It is expressed in ampere-hours. The potential varies so little during the discharge that it is assumed to be constant.

Capillarity. The reaction between liquid surfaces of different kinds or between liquid and solid surfaces due to surface tension. Its phenomena are greatly modified by electric charging, which alters the surface tension. Capillarity is the cause of solutions "creeping," as it is termed. Thus in gravity batteries a crust of zinc sulphate often formed over the edge of the jar due to the solution creeping and evaporating. As a liquid withdraws from a surface which it does not wet, creeping as above is prevented by coating the edge with paraffin wax, something which water does not moisten. It also causes the liquids of a battery cell to reach the connections and injure them by oxidation. The solutions creep up in the pores of the carbons of a battery and oxidize the clamps. To give good connections a disc of platinum or of lead is used for the contact as not being attacked. Another way is to dip the upper ends of the dry and warm carbons into melted paraffin wax, or to apply the wax to the hot carbons at the top, and melt it in with a hot iron.


Carbon. (a) One of the elements; atomic weight, 12. It exists in three allotropic modifications, charcoal, graphite and diamond. In the graphitic form it is used as an electric current conductor, as in batteries and for arc lamp, electrodes and incandescent lamp filaments. It is the only substance which conducts electricity and which cannot be melted with comparative ease by increase of current. (See Resistance.)

(b) The carbon plate of a battery or rod of an arc lamp. To secure greater conductivity in lamp carbons, they are sometimes plated with nickel or with copper.

(c) v. To place carbons in arc lamps. This has generally to be done once in twenty-four hours, unless the period of burning is very short.

Carbon, Artificial. For lamps, carbons and battery plates carbons are made by igniting, while protected from the action of the air, a mixture of carbon dust and a cementing and carbonizable substance. Lamp black may be added also. Powdered coke or gas carbon is mixed with molasses, coal tar, syrup, or some similar carbonaceous liquid. It is moulded into shape. For lamp carbons the mixture is forced from a vessel through a round aperture or die, by heavy pressure, and is cut into suitable lengths. For battery plates it may be simply pressed into moulds. The carbons are ignited in covered vessels and also covered with charcoal dust, lamp black or its equivalent. They are heated to full redness for some hours. After removal and cooling they are sometimes dipped again into the liquid used for cementing and reignited. Great care in securing pure carbon is sometimes necessary, especially for lamps. Fine bituminous coal is sometimes used, originally by Robert Bunsen, in 1838 or 1840; purification by different processes has since been applied; carbon from destructive distillation of coal tar has been used. The famous Carr carbons are made, it is said, from 15 parts very pure coke dust, five parts calcined lamp-black, and seven or eight parts sugar—syrup mixed with a little gum. Five hours heating, with subsequent treatment with boiling caramel and reignition are applied. The latter treatment is termed "nourishing." Napoli used three parts of coke to one of tar. Sometimes a core of different carbon than the surrounding tube is employed.


The following are the resistances of Carr's carbons per meter (39.37 inches):

Diameter in Diameter in Resistance in Ohms. Millimeters. Inches. @ 20 C. (98 F.) 1 .039 50.000 2 .078 12.5 3 .117 5.55 4 .156 3.125 5 .195 2.000 6 .234 1.390 8 .312 .781 10 .390 .5 12 .468 .348 15 .585 .222 18 .702 .154 20 .780 .125

At high temperatures the resistance is about one-third these amounts. A layer of copper may increase the conductivity one hundred times and prolong the duration 14 per cent. Thus a layer of copper 1/695 millimeter (1/17300 inch) thick increases the conductivity 4.5 times; a coating 1/60 millimeter (1/1500 inch) thick increases the conductivity one hundred and eleven times.

Carbon, Cored. A carbon for arc lamps with a central core of softer carbon than the exterior zone. It fixes the position of the arc, and is supposed to give a steadier light.

Synonym—Concentric Carbon.

Carbon Holders. In arc lamps, the fixed clamps for holding the ends of the carbons.

Carbonization. The igniting in a closed vessel, protected from air, of an organic substance so as to expel from it all the constituents except part of the carbon; destructive distillation. (See Carbonized Cloth.)

Carbonized Cloth. Cloth cut in discs and heated in vessels protected from the air, until reduced to carbon. The heating is sometimes conducted in vacuo. They are placed in a pile in a glass or other insulating tube, and offer a resistance which can be varied by pressure. The greater the pressure the less will be the resistance, and vice versa.

Carbon Dioxide. A compound gas, CO2. It is composed of Carbon, 12 parts by weight. Oxygen. 32 " Specific gravity, 1.524 (Dulong and Berzelins). Molecular weight, 44.

It is a dielectric of about the resistance of air. Its specific inductive capacity at atmospheric pressures is 1.000356 (Boltzmann). 1.0008 (Ayrton).

Synonyms—Carbonic Acid—Carbonic Acid Gas.


Carbon, Volatilization of. In arc lamps the heat is so intense that it is believed that part of the carbon is volatilized as vapor before being burned or oxidized by the oxygen of the air. The same volatilization may take place in incandescent lamps which are overheated.

Carcel. The standard of artificial illumination used in France. It is the light yielded by a standard lamp burning 42 grams (648 grains) of colza oil per hour, with a flame 40 millimeters (1.57 inch) in height. One carcel is equal to 9.5 to 9.6 candles.

Carcel Lamp. The lamp giving the standard of illuminating power. The wick is cylindrical, giving an Argand or central draft flame. It is woven with 75 strands, and weighs 3.6 grams (55.5 grains) per decimeter (3.9 inches) of length. The chimney is 29 centimeters (11.3 inches) high, 47 millimeters (1.88 inch) in diameter at the bottom, contracting just above the wick to 34 millimeters (1.36 inch).

Carcel Gas Jet. A standard Argand gas burner, made with proper rating to give the light of a definite number of carcels illuminating power. Cognizance must be taken of the quality of the gas as well as of the burner used.

Carrying Capacity. In a current conductor, its capacity for carrying a current without becoming unduly heated. It is expressed in amperes. (See Wire Gauge, American.)

Cascade. The arrangement of Leyden jars in series on insulating supports, as described below.

Cascade, Charging and Discharging Leyden Jars In. An arrangement of Leyden jars in series for the purpose of charging and discharging. They are placed on insulating supports, the inner coating of one connected with the outer coating of the next one all through the series. The actual charge received by such a series, the outer coating of one end jar being grounded, and the inner coating of the other being connected to a source of high potential, or else the same being connected to electrodes of opposite potentials is no greater than that of a single jar, but a much higher potential difference can be developed without risk of perforating the glass of a jar. The difference of potential in each jar of the series is equal to the total potential difference divided by the number of jars. The energy of discharge is equal to the same fraction of the energy of a single jar charged with the same quantity.

[Transcriber's note: The equal distribution of potential assumes all the jars have the same capacity. The charge on all jars is the same since they are in series.]


Case-hardening, Electric. The conversion of the surface of iron into steel by applying a proper carbonaceous material to it while it is heated by an electric current. It is a superficial cementation process.

Cataphoresis. Electric osmore; the transfer of substances in solution through porous membranes under the influence probably of electrolysis, but without themselves being decomposed.

Cautery, Electric. An electro-surgical appliance for removing diseased parts, or arresting hemorrhages, taking the place of the knife or other cutting instrument. The cautery is a platinum wire heated to whiteness by an electric current, and when in that condition used to cut off tumors, stop the flow of blood and parallel operations. The application is painful, but by the use of anaesthetics pain is avoided, and the healing after the operation is greatly accelerated.

The heated wire of the cautery can be used for cutting operations in many cases where excision by a knife would be almost impracticable.

Synonyms—Galvano-cautery—Galvano-caustry—Galvano-electric, do.—Galvano-thermal, do.

C. C. A contraction of cubic centimeter. It is often written in small letters, as 100 c.c., meaning 100 cubic centimeters.

Cell, Constant. A cell which yields a constant and uniform current under unvarying conditions. This implies that neither the electro-motive force or the resistance of the cell shall vary, or else that as the electro-motive forces run down the resistance shall diminish in proper proportion to maintain a constant current. There is really no constant cell. The constancy is greatest when the external resistance is high in proportion to the internal resistance.

Cell, Electrolytic. A vessel containing the electrolyte, a liquid decomposable by the current, and electrodes, arranged for the passage of a decomposing current. The voltameter, q. v., is an example.

Cell, Standard Voltaic. A cell designed to be a standard of electro-motive force; one in which the same elements shall always be present under the same conditions, so as to develop the same electro-motive force. In use the circuit is closed only for a very short time, so that it shall not become altered by polarization or exhaustion.

Cell, Standard Voltaic, Daniell's. A zinc-copper-copper sulphate couple. Many forms are used. Sometimes a number of pieces of blotting paper are interposed between two plates, one of copper—the other of zinc. The paper next the copper is soaked in copper sulphate solution, and those next the zinc in zinc sulphate solution, of course before being put together. Sometimes the ordinary porous cup combination is employed. The cut shows a modification due to Dr. Fleming (Phil. Mag. S. 5, vol. xx, p. 126), which explains itself. The U tube is 3/4-inch diameter, and 8 inches long. Starting with it empty the tap A is opened, and the whole U tube filled with zinc sulphate solution, and the tap A is closed. The zinc rod usually kept in the tube L is put in place, tightly corking up its end of the U tube. The cock C is opened, which lowers the level of the solution in the right-hand limb of the U tube only. The tap B is opened and the copper sulphate solution is run in, preserving the line of separation of the two solutions. The copper rod is taken out of its tube M, and is put in place. India rubber corks are used for both rods. As the liquids begin to mix the mixture can be drawn off at C and the sharp line of demarcation re-established. In Dr. Sloane's standard cell two test tubes are employed for the solutions and a syphon is used to connect them.

Oxidation of the zinc lowers the E. M. F.; oxidation of the copper raises it. With solutions of equal sp. gr. the E. M. F. is 1.104 volts. If the copper sulphate solution is 1.100 sp. gr. and the zinc sulphate solution 1.400 sp. gr., both at 15 C. (59F.), the E. M. F. will be 1.074 volt. Clean pure zinc and freshly electrolyzed copper should be used.



Cell, Standard Voltaic, Latimer Clark's. A mercury and zinc electrode couple with mercurous sulphate as excitant and depolarizer. The positive element is an amalgam of zinc, the negative is pure mercury. Each element, in a representative form, the H form, is contained in a separate vessel which communicate by a tube. Over the pure mercury some mercurous sulphate is placed. Both vessels are filled to above the level of the connecting tube with zinc sulphate solution, and kept saturated. It is tightly closed or corked. The E. M. F. at 15 C (59 F.) is 1.438. Temperature correction

(1 - (.00077 *(t - 15 C) ) )

t being expressed in degrees centigrade (Rayleigh). A diminution in specific gravity of the zinc solution increases the E. M. F. The cell polarizes rapidly and the temperature coefficient is considered too high.



Cements, Electrical. A few cements find their use in electrical work. Marine glue, Chatterton's compound, and sealing wax may be cited.

Centi-. Employed as a prefix to indicate one-hundredth, as centimeter, the one-hundredth of a meter; centi-ampere, the one-hundredth of an ampere.

Centigrade-scale. A thermometer scale in use by scientists of all countries and in general use in many. The temperature of melting ice is 0; the temperature of condensing steam is 100 ; the degrees are all of equal length. To reduce to Fahrenheit degrees multiply by 9 and divide by 5, and add 32 algebraically, treating all readings below 0 as minus quantities. For its relations to the Reamur scale, see Reamur Scale. Its abbreviation is C., as 10 C., meaning ten degrees centigrade.

Centimeter. A metric system unit of length; one-hundredth of a meter; 0.3937 inch. The absolute or c. g. s. unit of length.

Centimeter-gram-second System. The accepted fundamental or absolute system of units, called the C. G. S. system. It embraces units of size, weight, time, in mechanics, physics, electricity and other branches. It is also called the absolute system of units. It admits of the formation of new units as required by increased scope or classification. The following are basic units of the system :

Of length, centimeter; of mass, gram; of time, second: of force, dyne: of work or energy, erg.

See Dyne, Erg., and other units in general.


Central Station Distribution or Supply. The system of supplying electric energy in current form from a main generating plant to a district of a number of houses, factories, etc. It is in contrast with the isolated plant system in which each house or factory has its own separate generating installment, batteries or dynamos.

Centre of Gravity. A point so situated with respect to any particular body, that the resultant of the parallel attracting forces between the earth and the several molecules of the body always passes through it. These are resultants of the relative moments of the molecules. If a body is suspended, as by a string, the centre of gravity always lies vertically under its point of suspension. By two trials the point of intersection of plumb lines from the point of suspension being determined the centre of gravity is known. The vertical from the point of support coincides with the line of direction.

Centre of Gyration. The centre of gyration with respect to the axis of a rotating body is a point at which if the entire mass of the body were concentrated its moment of inertia would remain unchanged. The distance of this point from the axis is the radius of gyration.

Centre of Oscillation. The point referred to in a body, suspended or mounted to swing like a pendulum, at which if all the mass were concentrated, 1t would complete its oscillations in the same time. The distance from the axis of support to this point gives the virtual length of the pendulum which the body represents.

Centre of Percussion. The point in a suspended body, one free to swing like a pendulum, at which an impulse may be applied, perpendicular to the plane through the axis of the body and through the axis of support without shock to the axis. It is identical with the centre of oscillation, q. v., when such lies within the body.

Centrifugal Force. The force which draws a body constrained to move in a curved path away from the centre of rotation. It is really due to a tangential impulse and by some physicists is called the centrifugal component of tangential velocity. It has to be provided against in generator and motor armatures, by winding them with wire or bands to prevent the coils of wire from spreading or leaving their bed upon the core.


Centrifugal Governor. The usual type of steam-engine governor. The motion of the engine rotates a system of weights, which are forced outward by centrifugal force, and are drawn inwards by gravity or by springs. Moving outwards they shut off steam, and moving inwards they admit it, thus keeping the engine at approximately a constant speed. The connections between them and the steam supply and the general construction vary widely in different governors.

C. G. S. Abbreviation or symbol for Centimeter-gram-second, as the C. G. S. system. (See Centimeter-gram-second System.) It is sometimes expressed in capitals, as above, and sometimes in small letters, as the c. g. s. unit of resistance.

Chamber of Incandescent Lamp. The interior of the bulb of an incandescent lamp. (See Lamp, Incandescent.)



Characteristic Curve. A curve indicating the variations in electro-motive force developed during the rotations of the armature of a dynamo or other generator of E. M. F. The term as used in the electrical sense is thus applied, although the indicator diagram of a steam engine may be termed its characteristic curve, and so in many other cases. As the amperes taken from a series generator are increased in number, the E. M. F. rises, it may be very rapidly up to a certain point, and thereafter more slowly. To construct the curve coordinates, q. v., are employed. The resistance of the dynamo and of the outer circuit being known, the current intensity is measured. To obtain variations in electro-motive force the external resistance is changed. Thus a number of ampere readings with varying known resistance are obtained, and for each one an electro-motive force is calculated by Ohm's law. From these data a curve is plotted, usually with volts laid off on the ordinate and amperes on the abscissa.

By other methods other characteristic curves may be obtained, for which the titles under Curve may be consulted.


Characteristic, Drooping. A characteristic curve of a dynamo which indicates a fall in voltage when an excessive current is taken from the dynamo in question. It is shown strongly in some Brush machines, and is partly due to the arrangements for cutting out two of the coils as they approach the neutral line. It is an advantage, as it protects from overheating on short circuit.

Characteristic, External. In a dynamo the characteristic curve in which the relations of volts between terminals to amperes in the outer circuit are plotted. (See Curve, External Characteristic.)

Characteristic, Internal. A characteristic curve of a shunt dynamo, in which the relations of volts to amperes in the shunt circuit is plotted.

Characteristics of Sound. Of interest, electrically, as affecting the telephone, they comprise:

(1) Pitch, due to frequency of vibrations.

(2) Intensity or loudness, due to amplitude of waves of sound.

(3) Quality or timbre, the distinguishing characteristics of any specific sound due to overtones, discords, etc., by which the sound is recognizable from others. The telephone is held by the U. S. courts to be capable of reproducing the voice by means of the undulatory current. (See Current, Undulatory.)

Charge. The quantity of electricity that is present on the surface of a body or conductor. If no electricity is supplied, and the conductor is connected to the earth, it is quickly discharged. A charge is measured by the units of quantity, such as the coulomb. The charge that a conductor can retain at a given rise of potential gives its capacity, expressible in units of capacity, such as the farad. A charge implies the stretching or straining between the surface of the charged body, and some complimentary charged surface or surfaces, near or far, of large or small area, of even or uneven distribution.

Charge. v. (a) To introduce an electrostatic charge, as to charge a condenser.

(b) To decompose the elements of a secondary battery, q. v., so as to render it capable of producing a current. Thus, a spent battery is charged or recharged to enable it to do more work.



Charge, Bound. A charge of electricity borne by the surface of a body so situated with reference to another oppositely charged body, that the charge is imperceptible to ordinary test, will not affect an electroscope nor leave the surface if the latter is connected to the earth. To discharge such a body it must be connected to its complimentarily charged body. The bound charge was formerly called dissimulated or latent electricity. (See Charge, Free.)

The charge or portion of a charge of a surface which is neutralized inductively by a neighboring charge of opposite kind. The degree of neutralization or of binding will depend on the distance of the two charged surfaces from one another and on the electro-static nature of the medium intervening, which must of necessity be a dielectric. A charge not so held or neutralized is termed a free charge. Thus a surface may be charged and by the approach of a surface less highly charged may have part of its charge bound. Then if connected to earth. it will part with its unbound or free charge, but will retain the other until the binding surface is removed, or until the electricity of such surface is itself bound, or discharged, or until connection is made between the two surfaces. Thus a body may have both a bound and a free charge at the same time.

Charge, Density of. The relative quantity of electricity upon a given surface. Thus a charged surface may have an evenly distributed charge or one of even density, or an unevenly distributed charge or one of uneven density. In a thunderstorm the earth has a denser charge under the clouds than elsewhere.

Synonym—Electrical Density.

Charge, Dissipation of. As every body known conducts electricity, it is impossible so to insulate a surface that it will not lose its charge by leakage. An absolute vacuum might answer, and Crookes in a high vacuum has retained a charge against dissipation for years. The gradual loss is termed as above.

Charge, Distribution of. The relation of densities of charge on different parts of a charged body. On a spherical conductor the charge is normally of even distribution; on other conductors it is unevenly distributed, being of greatest density at points, edges, and parts of smallest radius of curvature. Even distribution can also be disturbed by local induction, due to the presence of oppositely charged bodies.


Charge, Free. The charge borne by an insulated body, independent of surrounding objects. Theoretically it is an impossibility. A charge always has its compliment somewhere in surrounding objects. As a matter of convenience and convention, where the complimentary charge is so distributed that its influence is not perceptible the charge is called a free charge. If connected to earth the free charge will leave the body. If the body is connected with an electroscope the free charge will affect the same. (See Charge, Bound.)

Charge, Residual. When a Leyden jar or other condenser is discharged by the ordinary method, after a few minutes standing a second discharge of less amount can be obtained from it. This is due to what is known as the residual charge. It seems to be connected in some way with the mechanical or molecular distortion of the dielectric. The jarring of the dielectric after discharge favors the rapidity of the action, diminishing the time required for the appearance of the residual charge. The phenomenon, it will be seen, is analogous to residual magnetism. This charge is the reciprocal of electric absorption and depends for its amount upon the nature of the dielectric. (See Absorption, Electric, and Capacity, Residual.)

Synonym—Electric Residue.

Chatterton's Compound. A cement used for cementing together layers or sheets of gutta percha, and for similar purposes in splicing telegraph cables. Its formula is: Stockholm Tar, 1 part. Resin, 1 part. Gutta Percha, 3 parts. All parts by weight.

Chemical Change. When bodies unite in the ratio of their chemical equivalents, so as to represent the satisfying of affinity or the setting free of thermal or other energy, which uniting is generally accompanied by sensible heat and often by light, as in the ignition of a match, burning of a candle, and, when the new compound exhibits new properties distinct from those of its components, a chemical combination is indicated. More definitely it is a change of relation of the atoms. Another form of chemical change is decomposition, the reverse of combination, and requiring or absorbing energy and producing several bodies of properties distinct from those of the original compound. Thus in a voltaic battery chemical combination and decomposition take place, with evolution of electric instead of thermal energy.

Chemical Equivalent. The quotient obtained by dividing the atomic weight, q. v., of an element by its valency, q. v. Thus the atomic weight of oxygen is 16, its valency is 2. its chemical equivalent is 8. It is the weight of the element corresponding to a unit weight of hydrogen, either as replacing it, or combining with it. In electro-chemical calculations the chemical equivalent is often conveniently used to avoid the necessity of dividing by the valency when atomic weights are used. The latter is really the better practice. The atomic weights in the old system of chemical nomenclature were chemical equivalents.


Chemical Recorder. A form of telegraphic recorder in which the characters, often of the Morse alphabet or some similar one, are inscribed on chemically prepared paper by decomposition affecting the compound with which the paper is charged. In the original chemical recorder of Bain, the instrument was somewhat similar to the Morse recorder, except that the motionless stylus, S, always pressing against the paper was incapable of making any mark, but being of iron, and the paper strip being impregnated with potassium ferrocyanide, on the passage of a current a stain of Prussian blue was produced where the stylus touched the paper. The current passes from the line by way of the iron stylus, through the paper, and by way of a brass surface, M, against which the paper is held and is pressed by the stylus, to the earth. This recorder is extremely simple and has no part to be moved by the current. The solution in which the paper is dipped contains a mixture of potassium ferrocyanide and ammonium nitrate. The object of the latter is to keep the paper moist. In recent recorders a solution of potassium iodide has been used, which gives a brown stain of free iodine, when the current passes. This stain disappears in a few days.


In the cut, R is the roll of paper, B is a tank of solution with roll, W1, for moistening the paper; M is the brass surface against which the stylus, S, presses the paper, P P; W, W are feed rollers; T is the transmitting key, and zk the battery; Pl, Pl are earth plates. The apparatus is shown duplicated for each end.


Chemistry. The science treating of atomic and molecular relations of the elements and of chemical compounds of the same.

Chimes, Electric. An apparatus employed to illustrate the principles of the electrostatic charge, involving the ringing of bells by electrostatic attraction and repulsion. It is used in connection with a frictional, or influence electric machine. Two bells are employed with a button or clapper suspended between them. One bell is connected to one of the prime conductors, q. v., of the machine. The other insulated therefrom is connected to earth, or if an influence machine is used, to the other prime conductor. The clappers are hung by a silk thread, so as to be entirely insulated. On working the machine the bells become oppositely excited. A clapper is attracted to one, then when charged is repelled and attracted to the other, it gives up its charge and becoming charged with similar electricity to that of the bell it touches, is repelled and attracted to the other, and this action is kept up as long as the excitement continues, the bells ringing continuously.


Chronograph, Electric. An apparatus for indicating electrically, and thereby measuring, the lapse of time. The periods measured may be exceedingly short, such as the time a photographic shutter takes to close, the time required by a projectile to go a certain distance, and similar periods.

A drum rotated with even and known velocity may be marked by a stylus pressed upon it by the action of an electro-magnet when a key is touched, or other disturbance. Then the space between two marks would give the period elapsing between the two disturbances of the circuit. As it is practically impossible to secure even rotation of a drum, it is necessary to constantly measure its rate of rotation. This is effected by causing a tuning-fork of known rate of vibration to be maintained in vibration electrically. A fine point or bristle attached to one of its arms, marks a sinuous line upon the smoked surface of the cylinder. This gives the basis for most accurately determining the smallest intervals. Each wave drawn by the fork corresponds to a known fraction of a second.

For projectiles, the cutting of a wire opens a circuit, and the opening is recorded instead of the closing. By firing so as to cut two wires at a known distance apart the rate is obtained by the chronograph.



Chutaux's Solution. A solution for bichromate batteries. It is composed as follows: Water, 1,500 parts Potassium bichromate, 100 parts mercury bisulphate, 100 parts 66 sulphuric acid, 50 parts.

Circle, Galvanic or Voltaic. A term for the voltaic circuit; obsolete.


Circle, Magic. A form of electro-magnet. It is a thick circle of round iron and is used in connection with a magnetizing coil, as shown, to illustrate electro-magnetic attraction.


Circuit. A conducting path for electric currents properly forming a complete path with ends joined and including generally a generating device of some kind. Part of the conduction may be true and part electrolytic. (See Electrolytic Conduction.) The term has become extended, so that the term is often applied to any portion of a circuit conveniently considered by itself. The simplest example of a complete circuit would be a circular conductor. If rotated in the earth's field so as to cut its lines of force a current would go through it, and it would be an electric circuit. Another example is a galvanic battery with its ends connected by a wire. Here the battery generates the current which, by electrolytic conduction, goes through the battery and by true conduction through the wire. For an example of a portion of a circuit spoken of as "a circuit" see Circuit, Astatic.

Circuit, Astatic. A circuit so wound with reference to the direction of the currents passing through it that the terrestrial or other lines of force have no directive effect upon it, one member counteracting the other. It may be produced by making the wire lie in two closed curves, A and B, each enclosing an equal area, one of identical shape and disposition with the other, and with the current circulating in opposite directions in each one. Thus each circuit represents a magnetizing turn of opposite polarity and counteracting each other's directive tendency exhibited in a field of force with reference to an axis a c. Another form of astatic circuit is shown in Fig. 86. The portions C, D, lying on opposite sides of the axis of rotation a c, are oppositely acted on by the earth's directive force as regards the direction of their rotation.

Figs. 86 and 87. ASTATIC CIRCUITS.

Circuit, Branch. A circuit dividing into two or more parts in parallel with each other.


Circuit Breaker. Any apparatus for opening and closing a circuit is thus termed, but it is generally applied to automatic apparatus. A typical circuit breaker is the hammer and anvil of the induction coil. (See Induction Coil; Anvil.) Again a pendulum connected to one terminal of a circuit may swing so as to carry a point on its lower end through a globule of mercury as it swings, which globule is connected to the other terminal. A great many arrangements of this character have been devised.

Synonym.—Contact Breaker.

Circuit Breaker, Automatic. A circuit breaker worked by the apparatus to which it is attached, or otherwise automatically. (See Induction Coil; Anvil; Bell, Electric.)

Circuit Breaker, File. A coarsely cut file, forms one terminal of an electric circuit, with a straight piece of copper or steel for the other terminal. The latter terminal drawn along the teeth makes and breaks the contact once for every tooth. The movable piece should have an insulated handle.

Circuit Breaker, Mercury. A circuit breaker which may be identical in principle, with the automatic circuit breaker of an induction coil, but in which in place of the anvil, q. v., a mercury cup is used, into which the end of a wire dips and emerges as it is actuated by the impulses of the current. Each dip makes the contact, which is broken as the wire springs back. The mercury should be covered with alcohol to protect it from oxidation.

Circuit Breaker, Pendulum. A circuit breaker in which a pendulum in its swing makes and breaks a contact. It may be kept in motion by clockwork, or by an electro-magnet, attracting intermittently an armature attached to its rod, the magnet circuit being opened and closed by the pendulum or circuit breaker itself. A mercury contact may be used with it.



Circuit Breaker, Tuning Fork. A circuit breaker in which a tuning fork makes and breaks the circuit. Each vibration of one of the prongs in one direction makes a contact, and the reverse vibration breaks a contact. The adjustment is necessarily delicate, owing to the limited amplitude of the motion of the fork. The fork is kept in vibration sometimes by an electro-magnet, which is excited as the circuit is closed by the fork. One leg of the fork acts as the armature of the magnet, and is attracted according to its own natural period.

Circuit Breaker, Wheel. A toothed wheel with a spring bearing against its teeth. One terminal of a circuit connects with the wheel through its axle, the other connects with the spring. When the wheel is turned the circuit is opened and closed once for each tooth. The interstices between teeth on such a wheel may be filled with insulating material, giving a cylindrical surface for the contact spring to rub on.


Circuit, Closed. A circuit whose electric continuity is complete; to make an open circuit complete by closing a switch or otherwise is to close, complete, or make a circuit.

Synonyms—Completed Circuit—Made Circuit.

Circuit, Compound. A circuit characterized by compounding of generating or receiving devices, as including several separate batteries, or several motors, or other receiving devices. It is sometimes used to indicate a circuit having its battery arranged in series. It should be restricted to the first definition.


Circuit, Derived. A partial circuit connected to two points of another circuit, so as to be in parallel with the portion thereof between such two points; a shunt circuit.

Synonyms—Shunt Circuit—Derivative Circuit—Parallel Circuit.

Circuit, Electric, Active. A circuit through which a current passes. The circuit itself need only be a conducting ring, or endless wire. Generally it includes, as part of the circuit, a generator of electro-motive force, and through which generator by conduction, ordinary or electrolytic, the same current goes that passes through the rest of the circuit. One and the same current passes through all parts of a series circuit when such current is constant.

A current being produced by electro-motive force, and electromotive force disappearing in its production in an active circuit, there must be some source of energy which will maintain electromotive force against the drain made upon it by the current.

The simplest conception of an active electric circuit is a ring or endless conductor swept through a field of force so as to cut lines of force. A simple ring dropped over a magnet pole represents the simplification of this process. In such a ring a current, exceedingly slight, of course, will be produced. In this case there is no generator in the circuit. An earth coil (see Coil, Earth,) represents such a circuit, with the addition, when experimented with, of a galvanometer in the circuit.

In practice, a circuit includes a generator such as a battery or dynamo, and by conductors is led through a continuous path. Electric lamps, electrolytic cells, motors and the like may be included in it.

The term "circuit" is also applied to portions of a true circuit, as the internal circuit, or external circuit. A certain amount of elasticity is allowed in its use. It by no means necessarily indicates a complete through circuit.

Circuit, Electrostatic. (a) A circuit through which an electrostatic or high tension discharge takes place. It is virtually an electric circuit.

(b) The term is applied also to the closed paths of electrostatic lines of force.

Circuit, External. The portion of a circuit not included within the generator.

Circuit, Grounded. A circuit, one of whose members, the return circuit, is represented by the earth, so that the earth completes the circuit. In telegraphy each end of the line is grounded or connected to an earth-plate, q. v., or to the water or gas-pipes, and the current is assumed to go through the earth on its return. It really amounts to a discharging at one end, and charging at the other end of the line. The resistance of the earth is zero, but the resistance of the grounding or connection with the earth may be considerable.

Synonyms—Ground Circuit—Earth Circuit—Single Wire Circuit.

[Transcriber's note: The resistance of the earth is high enough that large power system return currents may produce dangerous voltage gradients when a power line is shorted to the ground. Don't walk near downed lines!]


Circuit Indicator. A pocket compass, decomposition apparatus, galvanometer or other device for indicating the condition of a wire, whether carrying a current or not, and, if carrying one, its direction, and sometimes roughly indicating its strength.

Circuit, Internal. The portion of an electric circuit included within the generator.

Circuit, Line. The portion of a circuit embracing the main line or conductor, as in a telegraph circuit the line carried on the poles; distinguished from the local circuit (see Circuit, Local,) in telegraphy.

Circuit, Local. In telegraphy, a short circuit with local generator or battery included, contained within the limits of the office or station and operated by a relay, q. v. This was the original local circuit; the term is applicable to any similar arrangement in other systems. Referring to the cut, the main line circuit includes the main battery, E, Key, P, Relay, R, ground plates, G, G1. The relay magnet opens and closes the local circuit with its local battery, L, and sounder magnet, H, with its armature, B. The minor parts, such as switches, are omitted.


Circuit, Local Battery. A local circuit worked by and including a local battery in its course.


Circuit, Loop. A minor circuit introduced in series into another circuit by a cut-out, or other device, so as to become a portion of the main circuit.

Circuit Loop Break. A supporter or bracket with two arms for carrying insulators. Its use is to enable a loop connection to be introduced into a line which is cut, so as to enable the connection of the ends of the loop to be made, one to each end of the through wire, which ends are attached, one to each of the two insulators.

Circuit, Main. The circuit including the main line and apparatus supplied by the main battery, as distinguished from the local circuit. (See Circuit, Local.)

Circuit, Main Battery. The main circuit, including the main or principal battery in its course.

Circuit, Metallic. A circuit in which the current outside the generator, or similar parts, is carried on a metallic conductor; a circuit without any ground circuit. The including of a galvanic battery or electro plating bath would not prevent the application of the term; its essential meaning is the omission of the earth as the return circuit.

Circuit, Negative Side of. The side of a circuit opposite to the positive side. (See Circuit, Positive Side of) It is defined as the half of a circuit leading to the positive terminal of the generator.

Circuit, Open. A circuit with its continuity broken, as by disconnecting a wire from the battery, or opening a switch; a broken circuit is its synonym. To open a switch or disconnect or cut the wire is termed opening or breaking the circuit.

Synonyms—Incomplete Circuit—Broken Circuit.

Circuit, Positive Side of. This side is such that an observer standing girdled by the current with his head in the positive side or region, would see the current pass around him from his right toward his left hand. It is also defined as the half of the circuit leading to the negative terminal of the generator.

Circuit, Recoil. The portion of a parallel circuit presenting an alternative path, q. v., for a disruptive discharge.

Circuit, Return. (a) The part of a circuit extending from the generator to the extreme point in general, upon which no apparatus is placed. In telegraph systems the ground generally forms the return circuit. The distinction of return and working circuit cannot always be made.

(b) It may also be defined as the portion of a circuit leading to the negative terminal of the generator.


Circuits, Forked. Circuits starting in different paths or directions from one and the same point.

Circuit, Simple. A circuit containing a single generator, and single receiver of any kind, such as a motor or sounder, with a single connecting conductor. It is also used to indicate arrangement in multiple arc, but not generally, or with approval.

Circuits, Parallel. Two or more conductors starting from a common point and ending at another common point are termed, parallel circuits, although really but parts of circuits. If of equal resistance their joint resistance is obtained by dividing the resistance of one by the number of parallel circuits. If of unequal resistance r, r', r" , etc., the formula for joint resistance, R, of two is

R = ( r * r' ) / ( r + r' )

This resistance may then be combined with a third one by the same formula, and thus any number may be calculated.

Synonym—Shunt Circuit.

Circuit, Voltaic. Properly a circuit including a conductor and voltaic couple.

It is also applied to the electric circuit, q. v., or to any circuit considered as a bearer of current electricity.

Circular Units. Units of area, usually applied to cross sectional area of conductors, by whose use area is expressed in terms of circle of unit diameter, usually a circular mil, which is the area of a circle of one-thousandth of an inch diameter, or a circular millimeter, which is the area of a circle of one millimeter diameter. Thus a wire one-quarter of an inch in diameter has an area of 250 circular mils; a bar one centimeter in diameter has an area of ten circular millimeters.

[Transcriber's Note: Area is the diameter squared. A 1/4 inch wire has 62500 circular mils of area. A one centimeter (10 millimeter) wire has 100 circular millimeters of area. Actual area = circular mils * (PI/4).]

Circumflux. The product of the total number of conductor turns on the armature of a dynamo or motor, into the current carried thereby. For two pole machines it is equal to twice the armature ampere-turns; for four pole machines to four times such quantity, and so on.

Clamp. The appliance for grasping and retaining the end of the rod that holds a carbon in the arc lamp.

Clark's Compound. A cement used for the outside of the sheath of telegraph cables. Its formula is: Mineral Pitch, 65 parts. Silica, 30 parts. Tar, 5 parts. All parts by weight.


Cleats. A support; a short block of wood, grooved transversely, for holding electric wires against a wall. For the three wire system three grooves are used. The entire wiring of apartments is sometimes done by the "cleat system," using cleats instead of battens, q. v., or mouldings. The cleats are secured against the wall with the grooves facing it, and the wires are introduced therein.



Cleat, Crossing. A cleat with grooves or apertures to support wires which cross each other. Two or three grooves are transverse, and on the under side, as above; one groove is longitudinal and on the upper side.

Cleavage, Electrification by. If a mass of mica is rapidly split in the dark a slight flash is perceived. Becquerel found that in such separation the two pieces came away oppositely charged with electricity. The splitting of mica is its cleavage.

Clock, Controlled. In a system of electric clocks, the clocks whose movements are controlled by the current, regulated by the master or controlling clock.

Synonym—Secondary Clock.

Clock, Controlling. In a system of electric clocks the master clock which controls the movements of the others, by regulating the current.

Synonym—Master Clock.

Clock, Electric Annunciator. A clock operating any form of electric annunciator, as dropping shutters, ringing bells, and the like. It operates by the machinery closing circuits as required at any desired hour or intervals.


Clock, Electrolytic. A clock worked by the electrolytic deposition and resolution of a deposit of metal upon a disc. It is the invention of Nikola Tesla. A metallic disc is mounted on a transverse axis, so as to readily rotate. It is immersed in a vessel of copper sulphate. A current is passed through the bath, the terminals or electrodes being near to and facing the opposite edges of the disc, so that the line connecting the electrodes lies in the plane of the disc. If a current is passed through the solution by the electrodes, copper is deposited on one side of the disc, and as it rotates under the influence of the weight thus accumulated on one side, the same metal as it is brought to the other side of the disc is redissolved. Thus a continuous rotation is maintained. The cause of the deposition and solution is the position of the disc; one-half becomes negative and the other positive in their mutual relations.

Clock, Self-winding Electric. A clock which is wound periodically by an electric motor and battery.

Clockwork, Feed. In arc-lamps the system of feeding the carbon or carbons by clockwork whose movements are controlled by the resistance of the arc. This system is employed in the Serrin, and in the Gramme regulators, among others. The carbons, if they approach, move clockwork. The movement of this is stopped or freed by an electro-magnet placed in shunt around the arc and carbons.

Cloisons. Partitions or divisions; applied to the winding of electro-magnets and coils where the winding is put on to the full depth, over single sections of the core, one section at a time, until the whole core is filled up.

Closure. The closing or completion of a circuit by depressing a key or moving a switch.

Clutch. In arc lamps a device for the feed of the upper carbons. In its simplest form it is simply a plate or bar pierced with a hole through which the carbon passes loosely. The action of the mechanism raises or lowers one end of the plate or bar. As it rises it binds and clutches the carbon, and if the action continues it lifts it a little. When the same end is lowered the carbon and clutch descend together until the opposite end of the clutch being prevented from further descent, the clutch approaches the horizontal position and the rod drops bodily through the aperture. The cut shows the clutches of the Brush double carbon lamp. In practice the lifting and releasing as regulated by an electro-magnet are so very slight that practically an almost absolutely steady feed is secured. A similar clutch is used in the Weston lamp.


Clutch, Electro-magnetic. A clutch or appliance for connecting a shaft to a source of rotary motion while the latter is in action. In one form a disc, in whose face a groove has been formed, which groove is filled with a coil of wire, is attached to the loose wheel, while the shaft carries a flat plate to act as armature. On turning on the current the flat plate is attached, adheres, and causes its wheel to partake of the motion of the shaft. Contact is made by brushes and collecting rings.

In the cut, A A is the attracted disc; the brushes, B B, take current to the collecting rings, C. The magnetizing coil is embedded in the body of the pulley, as shown.




Coatings of a Condenser or Prime Conductor. The thin conducting coatings of tinfoil, gold leaf or other conducting substance, enabling the surface to receive and part with the electric charge readily. Without such a coating the charge and discharge would be very slow, and would operate by degrees only, as one part of a non-conducting surface might be densely charged and another part be quite devoid of sensible charge.

Code, Cipher. A code of arbitrary words to designate prearranged or predetermined words, figures or sentences. The systems used in commerce have single words to represent whole sentences or a number of words of a sentence. This not only imparts a degree of secrecy, but makes the messages much shorter. Codes are used a great deal in cable transmission.

Code, Telegraphic. A telegraphic alphabet. (See Alphabets, Telegraphic.)

Coefficient. In algebra, the numerical multiplier of a symbol, as in the expression "5x," 5 is the coefficient. In physics, generally a number expressing the ratio or relation between quantities, one of which is often unity, as a standard or base of the set of coefficients. Thus the coefficient of expansion by heat of any substance is obtained by dividing its volume for a given degree of temperature by its volume at the standard temperature as 0 C., or 32 F. This gives a fraction by which if any volume of a substance, taken at 0 C., or at whatever may be taken as the basic temperature, is multiplied, the expanded volume for the given change of temperature will be obtained as the product. A coefficient always in some form implies the idea of a multiplier. Thus the coefficient of an inch referred to a foot would be 1/12 or .833+, because any number of inches multiplied by that fraction would give the corresponding number of feet.

[Transcriber's note: 1/12 is 0.0833+]

Coefficient, Economic. In machinery, electric generators, prime motors and similar structures, the number expressing the ratio between energy absorbed by the device, and useful, not necessarily available, work obtained from it. It is equal to work obtained divided by energy absorbed, and is necessarily a fraction. If it exceeded unity the doctrine of the conservation of energy would not be true. The economic coefficient expresses the efficiency, q. v., of any machine, and of efficiencies there are several kinds, to express any one of which the economic coefficient may be used. Thus, let W—energy absorbed, and w = work produced ; then w/W is the economic coefficient, and for each case would be expressed numerically. (See Efficiency, Commercial—Efficiency, Electrical—Efficiency of Conversion.)

The distinction between useful and available work in a dynamo is as follows: The useful work would include the work expended by the field, and the work taken from the armature by the belt or other mechanical connection. Only the latter would be the available work.


Coercive or Coercitive Force. The property of steel or hard iron, in virtue of which it slowly takes up or parts with magnetic force, is thus termed ("traditionally"; Daniell). It seems to have to do with the positions of the molecules, as jarring a bar of steel facilitates its magnetization or accelerates its parting, when not in a magnetic field, with its permanent or residual magnetism. For this reason a permanent magnet should never be jarred, and permitting the armature to be suddenly attracted and to strike against it with a jar injures its attracting power.

Coercive force is defined also as the amount of negative magnetizing force required to reduce remnant magnetism to zero.

By some authorities the term is entirely rejected, as the phenomenon does not seem directly a manifestation of force.

Coil and Coil Plunger. A device resembling the coil and plunge, q. v., except that for the plunger of iron there is substituted a coil of wire of such diameter as to enter the axial aperture of the other, and wound or excited in the same or in the opposite sense, according to whether attraction or repulsion is desired.

Coil and Plunger. A coil provided with a core which is free to enter or leave the central aperture. When the coil is excited, the core is drawn into it. Various forms of this device have been used in arc lamp regulators.

Synonym—Sucking coil.




Coil and Plunger, Differential. An arrangement of coil and plunger in which two plungers or one plunger are acted on by two coils, wound so as to act oppositely or differentially on the plunger or plungers. Thus one coil may be in parallel with the other, and the action on the plunger will then depend on the relative currents passing through the coils.

Coil, Choking. A coil of high self-induction, used to resist the intensity of or "choke" alternating currents. Any coil of insulated wire wound around upon a laminated or divided iron core forms a choking coil. The iron coil is usually so shaped as to afford a closed magnetic circuit.

A converter or transformer acts as a choking coil as long as its secondary is left open. In alternating current work special choking coils are used. Thus for theatrical work, a choking coil with a movable iron core is used to change the intensity of the lights. It is in circuit with the lamp leads. By thrusting in the core the self-induction is increased and the current diminishes, lowering the lamps; by withdrawing it the self-induction diminishes, and the current increases. Thus the lamps can be made to gradually vary in illuminating power like gas lights, when turned up or down.

Synonyms—Kicking Coil—Reaction Coil.




Coils, Bisected. Resistance coils with connections at their centers, as shown in the diagram. They are used for comparing the resistances of two conductors. The connections are arranged as shown in the coil, each coil being bisected. For the wires, movable knife-edge contacts are employed. The principle of the Wheatstone bridge is used in the method and calculations.

Coil, Earth. A coil of wire mounted with commutator to be rotated so as to cut the lines of force of the earth's magnetic field, thereby generating potential difference. The axis of rotation may be horizontal, when the potential will be due to the vertical component of the earth's field, or the axis may be horizontal, when the potential will be due to the vertical component, or it may be set at an intermediate angle.

Synonym—Delezenne's Circle.


Coil, Electric. A coil of wire used to establish a magnetic field by passing a current through it. The wire is either insulated, or so spaced that its convolutions do not touch.

Coil, Flat. A coil whose windings all lie in one plane, making a sort of disc, or an incomplete or perforated disc.

Coil, Induction. A coil in which by mutual induction the electromotive force of a portion of a circuit is made to produce higher or lower electro-motive force, in an adjoining circuit, or in a circuit, part of which adjoins the original circuit, or adjoins part of it.

An induction coil comprises three principal parts, the core, the primary coil and the secondary coil. If it is to be operated by a steady current, means must be provided for varying it or opening and closing the primary circuit. A typical coil will be described.


The core is a mass of soft iron preferably divided to prevent extensive Foucault currents. A cylindrical bundle of soft iron wires is generally used. Upon this the primary coil of reasonably heavy wire, and of one or two layers in depth, is wrapped, all being carefully insulated with shellac and paper where necessary. The secondary coil is wrapped upon or over the primary. It consists of very fine wire; No. 30 to 36 is about the ordinary range. A great many turns of this are made. In general terms the electro-motive force developed by the secondary stands to that of the primary terminals in the ratio of the windings. This is only approximate.

The greatest care is required in the insulating. The secondary is sometimes wound in sections so as to keep those parts differing greatly in potential far from each other. This prevents sparking, which would destroy the insulation.

A make and break, often of the hammer and anvil type, is operated by the coil. (See Circuit Breaker, Automatic.) As the current passes through the primary it magnetizes the core. This attracts a little hammer which normally resting on an anvil completes the circuit. The hammer as attracted is lifted from the anvil and breaks the circuit. The soft iron core at once parts with its magnetism and the hammer falls upon the anvil again completing the circuit. This operation goes on rapidly, the circuit being opened and closed in quick succession.

Every closing of the primary circuit tends to produce a reverse current in the secondary, and every opening of the primary circuit tends to produce a direct current in the secondary. Both are of extremely short duration, and the potential difference of the two terminals of the secondary may be very high if there are many times more turns in the secondary than in the primary.

The extra currents interfere with the action of an induction coil. To avoid their interference a condenser is used. This consists of two series of sheets of tin foil. Leaves of paper alternate with the sheets of tin-foil, the whole being built up into a little book. Each sheet of tin-foil connects electrically with the sheet next but one to it. Thus each leaf of a set is in connection with all others of the same set, but is insulated from the others. One set of leaves of tin-foil connects with the hammer, the other with the anvil. In large coils there may be 75 square feet of tin-foil in the condenser.

The action of the condenser is to dispose of the direct extra current. When the primary circuit is opened this current passes into the condenser, which at once discharges itself in the other direction through the coil. This demagnetizes the core, and the action intensifies and shortens the induced current. The condenser prevents sparking, and in general improves the action of the coil.

Many details enter into the construction of coils, and many variations in their construction obtain. Thus a mercury cup into which a plunger dips often replaces the anvil and hammer.


The induction coil produces a rapid succession of sparks, which may spring across an interval of forty inches. The secondary generally ends in special terminals or electrodes between which the sparking takes place. A plate of glass, two inches in thickness, can be pierced by them. In the great Spottiswoode coil there are 280 miles of wire in the secondary, and the wire is about No. 36 A.W.G.



Induction coils have quite extended use in electrical work. They are used in telephone transmitters, their primary being in circuit with the microphone, and their secondary with the line and receiving telephone. In electric welding, and in the alternating current system they have extended application. In all these cases they have no automatic circuit breaker, the actuating current being of intermittent or alternating type.


In the cuts the general construction of an induction coil is shown. In the sectional elevation, Fig. 100, A, is the iron core; B is the primary of coarse wire; C is a separating tube, which may be of pasteboard; D is the secondary of fine wire; E, E are the binding posts connected to the secondary; H, H are the heads or standards; K, K are the terminals of the primary; F is the vibrating contact spring; G, a standard carrying the contact screw; J is the condenser with wires, L, M, leading to it.

Referring to the plan, Fig. 101, H represents the primary coil; B and A are two of the separate sheets of the condenser, each sheet with projecting ears; G, G are the heads of the coil; the dark lines are connections to the condenser. One set of sheets connects with the primary coil at C, and also with the vibrating spring shown in plan and in the elevation at F. The other set of sheets connects with the post, carrying the contact screw. The other terminal of the primary runs to a binding post E. F, in the plan is a binding post in connection with the standard and contact screw.

Coil, Induction, Inverted. An induction coil arranged to have a lower electro-motive force in the secondary than in the primary. This is effected by having more convolutions in the primary wire than in the secondary. Such coils in practice are used with the alternating current and then do not include a circuit breaker or condenser. They are employed in alternating current system and in electric welding. (See Welding, Electric—Converter.)

In the cut an inverted coil, as constructed for electric welding is shown. In it the primary coil is marked P; the secondary, merely a bar of metal, is marked E, with terminals S, S; the heavy coils, I, of iron wire are the core; K is a screw for regulating the clamps; J, Z is a second one for the same purpose, while between D and D' the heat is produced for welding the bars, B, B', held in the clamps, C, C'. It will be seen how great may be the difference in turns between the single circle of heavy copper rod or bar which is the secondary of the coil, and the long coil of wire forming the primary.



Coil, Induction, Telephone. An induction coil used in telephone circuits. It is placed in the box or case near the transmitter. The primary is in circuit with the microphone. The secondary is in circuit with the line and receiving telephone. In the Bell telephone apparatus the primary of the induction coil is wound with No. 18 to 24 A. W. G. wire to a resistance of 1/2 ohm; the secondary, with No. 36 wire to a resistance of 80 ohms. The Edison telephone induction coil was wound with similar wires to a resistance of 3 to 4 ohms and of 250 ohms respectively.

Coil, Magnetizing. A coil of insulated wire for making magnets; and for experimental uses; it has a short axis and central aperture of as small size as consistent with the diameter of the bar to be magnetized, which has to pass through it readily. The wire may be quite heavy, 2 or 3 millimeters (.08—.12 inch) thick, and is cemented together with carpenter's glue, or with shellac or ethereal solution of gum copal. In use it is passed over the bar a few times while a heavy current is going through it. It is used for magic circles also. (See Circle, Magic.)


Coil, Resistance. A coil constructed for the purpose of offering a certain resistance to a steady current. This resistance may be for the purpose of carrying out quantitative tests, as in Wheatstone bridge work (see Wheatstone's Bridge), or simply to reduce the intensity of a current. For the first class of work the coils are wound so as to prevent the creation of a magnetic field. This is effected by first doubling the wire without breaking it, and then starting at the bend the doubled wire, which is insulated, is wound on a bobbin or otherwise until a proper resistance is shown by actual measurement. The coils are generally contained or set in closed boxes with ebonite tops. Blocks of brass are placed on the top, and one end from one coil and one end from the next connect with the same block. By inserting a plug, P, so as to connect any two blocks, which have grooves reamed out for the purpose, the coil beneath will be short circuited. German silver, platinoid or other alloy, q. v., is generally the material of the wire. A great object is to have a wire whose resistance will be unaffected by heat.



Coil, Rhumkorff. The ordinary induction coil with circuit breaker, for use with original direct and constant current, is thus termed. (See Coil, Induction.)


Coil, Ribbon. A coil made of copper ribbon wound flatwise, often into a disc-like shape, and insulated by tape or strips of other material intervening between the successive turns.

Coils, Compensating. Extra coils on the field magnets of dynamos or motors, which coils are in series with the armature windings for the purpose of keeping the voltage constant. In compound wound machines the regular series-wound coil is thus termed. In a separately excited dynamo a coil of the same kind in circuit with the armature may be used as a compensator.

Coils, Henry's. An apparatus used in repeating a classic experiment in electro-magnetic induction, due to Prof. Henry. It consists in a number of coils, the first and last ones single, the intermediate ones connected in pairs, and one of one pair placed on the top of one of the next pair. On opening or closing the circuit of an end coil the induced effect goes through the series and is felt in the circuit of the other end coil. Prof. Henry extended the series so as to include seven successive inductions, sometimes called inductions of the first, second, third and other orders. Frequently ribbon coils (see Coil, Ribbon,) are used in these experiments.

Coils, Sectioned. A device for prolonging the range of magnetic attraction. It consists of a series of magnetizing coils traversed by an iron plunger. As it passes through them, the current is turned off the one in the rear or passing to the rear and turned into the next one in advance. The principle was utilized in one of Page's electric motors about 1850, and later by others. The port-electric railroad, q. v., utilizes the same principle.


Collecting Ring. In some kinds of generators instead of the commutator a pair of collecting rings of metal, insulated from the machine and from each other, are carried on the armature shaft. A brush, q. v., presses on each, and the circuit terminals connect to these two brushes. Such rings are employed often on alternating current generators, where the current does not have to be changed or commuted. Collecting rings with their brushes are used also where a current has to be communicated to a revolving coil or circuit as in the magnetic car wheel, the cut of which is repeated here. The coil of wire surrounding the wheel and rotating with it has to receive current. This it receives through the two stationary brushes which press upon two insulated metallic rings, surrounding the shaft. The terminals of the coil connect one to each ring. Thus while the coil rotates it constantly receives current, the brushes being connected to the actuating circuit.


Collector. (a) A name for the brush, q. v., in mechanical electric generators, such as dynamos, a pair of which collectors or brushes press on the commutator or collecting rings, and take off the current.

(b) The pointed connections leading to the prime conductor on a static machine for collecting the electricity; often called combs. The points of the combs or collectors face the statically charged rotating glass plate or cylinder of the machine.

Colombin. The insulating material between the carbons in a Jablochkoff candle or other candle of that type. Kaolin was originally used. Later a mixture of two parts calcium sulphate (plaster of Paris) and one part barium sulphate (barytes) was substituted.

The colombin was three millimeters (.12 inch) wide, and two millimeters (.08 inch) thick. (See Candle, Jablochkoff.)

Column, Electric. An old name for the voltaic pile, made up of a pile of discs of copper and zinc, with flannel discs, wet with salt solution or dilute acid, between each pair of plates.


Comb. A bar from which a number of teeth project, like the teeth of a comb. It is used as a collector of electricity from the plate of a frictional or influence electric machine; it is also used in a lightning arrester to define a path of very high resistance but of low self-induction, for the lightning to follow to earth.

Communicator. The instrument by which telegraph signals are transmitted is sometimes thus termed.

Commutator. In general an apparatus for changing. It is used on electric current generators, and motors, and on induction coils, and elsewhere, for changing the direction of currents, and is of a great variety of types.

Synonym—Commuter (but little used).


Commutator Bars. The metallic segments of a dynamo or motor commutator.

Commutator, Flats in. A wearing away or lowering in level of one or more metallic segments of a commutator. They are probably due in many cases to sparking, set up by periodic springing in the armature mounting, or by defective commutator connections.

Commutator of Current Generators and Motors. In general a cylinder, formed of alternate sections of conducting and non-conducting material, running longitudinally or parallel with the axis. Its place is on the shaft of the machine, so that it rotates therewith. Two brushes, q. v., or pieces of conducting material, press upon its surface.


As a part of electric motors and generators, its function is to collect the currents produced by the cutting of lines of force so as to cause them all to concur to a desired result. The cut shows the simplest form of commutator, one with but two divisions. Its object may be to enable a current of constant direction to be taken from a rotating armature, in which the currents alternate or change direction once in each rotation. It is carried by the shaft A of the armature and rotates with it. It consists of two leaves, S S, to which the terminals of the armature are connected. Two springs, W W, the terminals of the outer circuit, press against the leaves. The springs which do this take off the current. It is so placed, with reference to the springs and armature, that just as the current changes in direction, each leaf changes from one spring to the other. Thus the springs receive constant direction currents. The changing action of this commutator appears in its changing the character of the current from alternating to constant. Were two insulated collecting rings used instead of a commutator, the current in the outer circuit would be an alternating one. On some dynamos the commutator has a very large number of leaves.

Taking the Gramme ring armature, there must be as many divisions of the commutator as there are connections to the coils. In this case the function of the commutator is simply to lessen friction, for the brushes could be made to take current from the coils directly outside of the periphery of the ring.

Commutator, Split Ring. A two-division commutator for a motor; it consists of two segments of brass or copper plate, bent to arcs of a circle, and attached to an insulating cylinder. They are mounted on the revolving spindle, which carries the armature, and acts as a two part commutator. For an example of its application, see Armature, Revolving, Page's. (See also Fig. 107.)


Compass. An apparatus for utilizing the directive force of the earth upon the magnetic needle. It consists of a circular case, within which is poised a magnetized bar of steel. This points approximately to the north, and is used on ships and elsewhere to constantly show the direction of the magnetic meridian. Two general types are used. In one the needle is mounted above a fixed "card" or dial, on which degrees or points of the compass, q. v., are inscribed. In the other the card is attached to the needle and rotates with it. The latter represents especially the type known as the mariner's compass. (See Compass, Mariner's—Compass, Spirit, and other titles under compass, also Magnetic Axis—Magnetic Elements.) The needle in good compasses carries for a bearing at its centre, a little agate cup, and a sharp brass pin is the point of support.

Compass, Azimuth. A compass with sights on one of its diameters; used in determining the magnetic bearing of objects.


Compass Card. The card in a compass; it is circular in shape, and its centre coincides with the axis of rotation of the magnetic needle; on it are marked the points of the compass, at the ends generally of star points. (See Compass, Points of the.) It may be fixed, and the needle may be poised above it, or it may be attached to the needle and rotate with it.

Compass, Declination. An instrument by which the magnetic declination of any place may be determined. It is virtually a transit instrument and compass combined, the telescope surmounting the latter. In the instrument shown in the cut, L is a telescope mounted by its axis, X, in raised journals with vernier, K, and arc x, for reading its vertical angle, with level n. The azimuth circle, Q, R, is fixed. A vernier, V is carried by the box, A, E, and both turn with the telescope. A very light lozenge-shaped magnetic needle, a, b, is pivoted in the exact centre of the graduated circles, Q R, and M. The true meridian is determined by any convenient astronomical method, and the telescope is used for the purpose. The variation of the needle from the meridian thus determined gives the magnetic declination.


Compass, Inclination. A magnetic needle mounted on a horizontal axis at its centre of gravity, so as to be free to assume the dip, or magnetic inclination, when placed in the magnetic meridian. It moves over the face of a vertical graduated circle, and the frame also carries a spirit level and graduated horizontal circle. In use the frame is turned until the needle is vertical. Then the axis of suspension of the needle is in the magnetic meridian. The vertical circle is then turned through 90 of the horizon, which brings the plane of rotation of the needle into the magnetic meridian, when it assumes the inclination of the place.


Compass, Mariner's. A compass distinguished by the card being attached to and rotating with the needle. A mark, the "lubber's mark" of the sailors is made upon the case. This is placed so that the line connecting it, and the axis of rotation of the card is exactly in a plane, passing through the keel of the ship. Thus however the ship may be going, the point of the card under or in line with the "lubber's mark," shows how the ship is pointing. The case of the mariner's compass is often bowl-shaped and mounted in gimbals, a species of universal joint, so as to bc always horizontal. (See Compass, Spirit-Gimbals.)


Compass, Points of the. The circle of the horizon may bc and is best referred to angular degrees. It has also been divided into thirty-two equiangular and named points. A point is 11.25. The names of the points are as follows: North, North by East, North North-east, North-east by North, North-east, North-east by East, East North-east, East by North, East, East by South, East South-east, South-east by East, South-east, South-east by South, South South-east, South by East, South, South by West, South South-west, South-west by South, South-west, South-west by West, West South-west, West by South, West, West by North, West North-west, North-west by West, North-west, North West by North, North North-west, North by West. They are indicated by their initials as N. N. W., North North-west, N. by W., North by West.

Compass, Spirit. A form of mariner's compass. The bowl or case is hermetically sealed and filled with alcohol or other nonfreezing liquid. The compass card is made with hollow compartments so as nearly to float. In this way the friction of the pivot or point of support is greatly diminished, and the compass is far more sensitive.

Compass, Surveyor's. A species of theodolite; a telescope with collimation lines, mounted above a compass, so as to be applicable for magnetic surveys. Its use is to be discouraged on account of the inaccuracy and changes in declination of the magnetic needle.


Compensating Resistances. In using a galvanometer shunt the total resistance of the circuit is diminished so that in some cases too much current flows through it; in such case additional resistance, termed as above, is sometimes introduced in series. The shunt in parallel with the galvanometer is thus compensated for, and the experimental or trial circuit does not take too much current.

Complementary Distribution. Every distribution of electricity has somewhere a corresponding distribution, exactly equal to it of opposite electricity; the latter is the complimentary distribution to the first, and the first distribution is also complimentary to it.

Component. A force may always be represented diagrammatically by a straight line, terminating in an arrow-head to indicate the direction, and of length to represent the intensity of the force. The line may always be assumed to represent the diagonal of a parallelogram, two of whose sides are represented by lines starting from the base of the arrow, and of length fixed by the condition that the original force shall be the diagonal of the parallelogram of which they are two contiguous sides; such lines are called components, and actually represent forces into which the original force may always be resolved. The components can have any direction. Thus the vertical component of a horizontal force is zero; its horizontal component is equal to itself. Its 450 component is equal to the square root of one-half of its square.

Condenser. An appliance for storing up electrostatic charges: it is also called a static accumulator. The telegraphic condenser consists of a box packed full of sheets of tinfoil. Between every two sheets is a sheet of paraffined paper, or of mica. The alternate sheets of tinfoil are connected together, and each set has its own binding post. (See Accumulator, Electrostatic.)

Condenser, Sliding. An apparatus representing a Leyden jar whose coatings can be slid past each other. This diminishes or increases the facing area, and consequently in almost exactly similar ratio diminishes or increases the capacity of the condenser.

Conductance. The conducting power of a given mass of specified material of specified shape and connections. Conductance varies in cylindrical or prismatic conductors, inversely as the length, directly as the cross-section, and with the conductivity of the material. Conductance is an attribute of any specified conductor, and refers to its shape, length and other factors. Conductivity is an attribute of any specified material without direct reference to its shape, or other factors.

Conduction. The process or act of conducting a current.


Conductivity. The relative power of conducting the electric current possessed by different substances. A path for the current through the ether is opened by the presence of a body of proper quality, and this quality, probably correlated to opacity, is termed conductivity. There is no perfect conductor, all offer some resistance, q. v., and there is hardly any perfect non-conductor. It is the reverse and reciprocal of resistance.

Conductivity, Specific. The reciprocal of specific resistance. (See Resistance—Specific.)

Conductivity, Unit of. The reciprocal of the ohm; it is a more logical unit, but has never been generally adopted; as a name the title mho (or ohm written backwards) has been suggested by Sir William Thomson, and provisionally adopted.

Conductivity, Variable. The conductivity for electric currents of conductors varies with their temperature, with varying magnetization, tension, torsion and compression.

Conductor. In electricity, anything that permits the passage of an electric current. Any disturbance in the ether takes the form of waves because the ether has restitutive force or elasticity. In a conductor, on the other hand, this force is wanting; it opens a path through the ether and a disturbance advances through it from end to end with a wave front, but with no succession of waves. This advance is the beginning of what is termed a current. It is, by some theorists, attributed to impulses given at all points along the conductor through the surrounding ether, so that a current is not merely due to an end thrust. If ether waves preclude a current on account of their restitutive force, ether waves cannot be maintained in a conductor, hence conductors should be opaque to light, for the latter is due to ether waves. This is one of the more practical every day facts brought out in Clerk Maxwell's electromagnetic theory of light. The term conductor is a relative one, as except a vacuum there is probably no substance that has not some conducting power. For relative conducting power, tables of conductivity, q. v., should be consulted. The metals beginning with silver are the best conductors, glass is one of the worst.

[Transcriber's note: See "ether" for contemporary comments on this now discarded concept.]

Conductor, Anti-Induction. A current conductor arranged to avoid induction from other lines. Many kinds have been invented and made the subject of patents. A fair approximation may be attained by using a through metallic circuit and twisting the wires composing it around each other. Sometimes concentric conductors, one a wire and the other a tube, are used, insulated, one acting as return circuit for the other.

Conductor, Conical. A prime conductor of approximately conical shape, but rounded on all points and angles. Its potential is highest at the point.


Conductor, Imbricated. A conductor used in dynamo armatures for avoiding eddy currents, made by twisting together two or more strips of copper.

Conductor, Prime. A body often cylindrical or spherical in shape, in any case with no points or angles, but rounded everywhere, whose surface, if the conductor itself is not metallic, is made conducting by tinfoil or gold leaf pasted over it. It is supported on an insulating stand and is used to collect or receive and retain static charges of electricity.

Conductors, Equivalent. Conductors of identical resistance. The quotient of the length divided by the product of the conductivity and cross-section must be the same in each, if each is of uniform diameter.

Conjugate. adj. Conjugate coils or conductors are coils placed in such relation that the lines of force established by one do not pass through the coils of the other. Hence variations of current in one produce no induced currents in the other.

Connect. v. To bring two ends of a conductor together, or to bring one end of a conductor in connection with another, or in any way to bring about an electrical connection.

Connector. A sleeve with screws or other equivalent device for securing the ends of wires in electrical contact. A binding-post, q. v., is an example. Sometimes wire spring-catches are used, the general idea being a device that enables wires to be connected or released at will without breaking off or marring their ends. The latter troubles result from twisting wires together.

Consequent Poles. A bar magnet is often purposely or accidentally magnetized so as to have both ends of the same polarity, and the center of opposite polarity. The center is said to comprise two consequent poles. (See Magnet, Anomalous.)

Conservation of Electricity. As every charge of electricity has its equal and opposite charge somewhere, near or far, more or less distributed, the sum of negative is equal always to the sum of positive electrical charges. For this doctrine the above title was proposed by Lippman.

Contact Breaker. Any contrivance for closing a circuit, and generally for opening and closing in quick succession. An old and primitive form consisted of a very coarsely cut file. This was connected to one terminal, and the other terminal was drawn over its face, making and breaking contact as it jumped from tooth to tooth. (See Circuit Breaker—do. Automatic, etc.—do. Wheel-do. Pendulum.)


Contact, Electric. A contact between two conductors, such that a current can flow through it. It may be brought about by simple touch or impact between the ends or terminals of a circuit, sometimes called a dotting contact, or by a sliding or rubbing of one terminal on another, or by a wheel rolling on a surface, the wheel and surface representing the two terminals.

There are various descriptions of contact, whose names are self-explanatory. The term is applied to telegraph line faults also, and under this, includes different descriptions of contact with neighboring lines, or with the earth.

Contact Electricity. When two dissimilar substances are touched they assume different electric potentials. If conductors, their entire surfaces are affected; if dielectrics, only the surfaces which touch each other. (See Contact Theory.)

Contact Faults. A class of faults often called contacts, due to contact of the conductor of a circuit with another conductor. A full or metallic contact is where practically perfect contact is established; a partial contact and intermittent contact are self-explanatory.

Contact Point. A point, pin or stud, often of platinum, arranged to come in contact with a contact spring, q. v., or another contact point or surface, under any determined conditions.

Contact Potential Difference. The potential difference established by the contact of two dissimilar substances according to the contact theory, q. v.

Contact Series. An arrangement or tabulation of substances in pairs, each intermediate substance appearing in two pairs, as the last member of the first, and first member of the succeeding pair, with the statement of the potential difference due to their contact, the positively electrified substance coming first. The following table of some contact potentials is due to Ayrton and Perry: CONTACT SERIES. Difference of Potential in Volts. Zinc—Lead .210 Lead—Tin .069 Tin—Iron .313 Iron—Copper .146 Copper—Platinum .238 Platinum-Carbon .113

The sum of these differences is 1.089, which is the contact potential between zinc and carbon.

Volta's Law refers to this and states that— The difference of potential produced by the contact of any two substances is equal to the sum of the differences of potentials between the intervening substances in the contact series.

It is to be remarked that the law should no longer be restricted to or stated only for metals.


Contact-spring. A spring connected to one lead of an electric circuit, arranged to press against another spring, or contact point, q. v., under any conditions determined by the construction of the apparatus. (See Bell, Electric—Coil, Induction.)

Contact Theory. A theory devised to explain electrification, the charging of bodies by friction, or rubbing, and the production of current by the voltaic battery. It holds that two bodies, by mere contact become oppositely electrified. If such contact is increased in extent by rubbing together, the intensity of their electrification is increased. This electrification is accounted for by the assumption of different kinetic energy, or energy of molecular motion, possessed by the two bodies; there being a loss and gain of energy, on the two sides respectively, the opposite electrifications are the result. Then when separated, the two bodies come apart oppositely electrified.

The above accounts for the frictional production of electricity. In the voltaic battery, a separation of the atoms of hydrogen and oxygen, and their consolidation into molecules occurs, and to such separation and the opposite electrification of the electrodes by the oxygen and hydrogen, the current is attributed, because the hydrogen goes to one electrode, and the oxygen to the other, each giving up or sharing its own charge with the electrodes to which it goes. If zinc is touched to copper, the zinc is positively and the copper negatively electrified. In the separation of hydrogen and oxygen, the hydrogen is positively and the oxygen negatively electrified. In the battery, the current is due to the higher contact difference of oxygen and hydrogen compared to that between zinc and copper. It will be seen that the two contact actions in a battery work against each other, and that the current is due to a differential contact action. The zinc in a battery is electrified negatively because the negative electrification of the oxygen is greater in amount than its own positive electrification due to contact with the copper.

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