sulphuric acid in the porous cup, and the carbon (several rods of electric-light carbon will answer) outside the cup in a saturated solution of potassium nitrate acidulated with one tenth its volume of strong sulphuric acid; sodium or ammonium nitrate may be used in place of the potassium salt. $103. BICHROMATE OR GRENET'S CELL. A Bunsen cell without the porous cup, and in which bichromate of potassium or sodium is added to the sulphuric acid as a depolarizer, becomes a Grenet cell. The bichromate in the presence of sulphuric acid forms chromic acid, which gives up its oxygen readily to the free hydrogen. The sodium salt is to be preferred, since it is more soluble. Fig. 109 is a common form of the bichromate cell, in which the zinc plate can be raised when the cell is not in use. In this and other forms of the cells where the zinc is in an acid solution, it should be amalgamated; commercial zinc contains impurities which form local closed circuits in the plate itself, when placed in the acid. These cause a wasting away of the zinc and add nothing to the E. M. F. of the cell. The zinc is amalgamated by cleaning it in sulphuric acid and rubbing mercury over its surface; the mercury readily forms an alloy with the zinc, dissolving Fig. 109. the pure zinc only; hence the acid is in contact with the amalgam or pure zinc dissolved in mercury.

104. THE LECLANCHÉ CELL. The Leclanché cell (Fig. 110) may be taken as a type of a large number of cells, in which the zinc and carbon are used as the plates, and a strong solution of ammonium chloride as the electrolyte. The carbon is packed in a porous cup with granulated manganese dioxide and carbon; the granulated carbon is added to decrease the resistance of the cell, and the manganese dioxide serves as a depolarizer.

Fig. 110.

This, however, acts slowly; hence, if the current is kept closed, these cells polarize in a a few minutes, but rapidly recover when the circuit is opened. The chemical action practically ceases when the circuit is opened; hence this class of cells is well suited for all kinds of light intermittent work, such as door-bells, signals, etc. The cells are often made portable by replacing the glass jar with a zinc cup, which serves also as the zinc plate; the electrolyte is made into a stiff paste by adding dextrine, starch, chloride of lime and other ingredients; it is then known as a dry cell. When the circuit is closed in this class of cells, the zinc displaces the ammonium from the ammonium

chloride, forming zinc chloride. The ammonium breaks up into ammonia and hydrogen; the former is dissolved by the liquid of the cell, or escapes when the cell is worked hard, while the hydrogen is oxidized by the manganese dioxide. In a number of these cells the porous cup is omitted, the manganese dioxide being incorporated in the carbon plate or omitted altogether.

0

$105. COPPER OXIDE CELL. The copper oxide cell was invented by Lalande and Chaperon. A zinc spiral or plate is immersed in a strong solution of caustic potash or soda, containing thirty to forty parts of alkali to one hundred parts of water. The positive plate is either copper or iron, in contact with copper oxide. The zinc replaces hydrogen in the alkali, forming sodium zincate (Na, ZnO,); the hydrogen, following the current, takes oxygen from the copper oxide, leaving metallic copper. Fig. 111 is a common form of the copper oxide cell, known as the EdisonLalande cell. The copper oxide is in the form of a compressed plate held in a copper frame, C. A layer of paraffin oil, O, is placed on the solution. of caustic potash to prevent the absorption of carbon dioxide from the air. The cell has a very low internal resistance, about 0.03 of an ohm; hence it is capable of giving a large current. is only 0.7 volts.

Fig. 111.

The E.M.F.

$106. SILVER CHLORIDE CELL. In a silver chloride cell the elements are zinc and silver; silver chloride is cast on the silver, which acts as a depolarizer. The electrolyte is dilute ammonium chloride containing 23 grams of the salt to one liter of water; a denser solution dissolves the silver chloride. The cell polarizes but slightly and recovers quickly, but can be used for small currents only.

Seal

Cork

Solution

Cast Zn

$107. CLARK'S STANDARD CELL. The Clark standard cell, one form of which is shown in Fig. III, has for its negative electrode pure zinc (or a ten-per-cent amalgam) in a neutral saturated solution of zinc sulphate, with an excess Zn SO. of the zinc sulphate crystals. The positive Asbestos electrode is pure mercury in contact with a paste of mercurous sulphate. In the form shown, due to Carhart, the zinc sulphate solution and the paste are kept from mixing by a layer of asbestos. When

+ Pt.wire

Fig. 112.

Hg, SO.

Paste

Hg.

this cell is carefully prepared from pure materials, the E. M. F. is constant, and is 1.434 volts at 15° C. It diminishes by about 0.001 volt per degree rise in temperature between 10° and 25° C. The cell has been adopted as the international standard of E. M. F. Helmholtz suggested the substitution of the chlorides of zinc and mercury for their sulphates. The E. M. F. is then lower, and may be made exactly one volt by adjusting the density of the zinc chloride solution. Cadmium and cadmium sulphate have been substituted for the zinc and zinc sulphate; the E. M. F. is then nearly one volt, and the variation with temperature is small.

$108. DATA RELATING TO PRIMARY CELLS.

1171

secondary cell the active materials are prepared in the cell by means of a current sent through it. When the current is continued, the materials revert to their original state, and during this process the secondary cell is precisely the same as the primary cell. While the fact that a reverse current could be obtained in the case of oxygen and hydrogen under the conditions stated was known as early as 1803, it was not until 1860 that Planté discovered the exceptional efficiency of lead for this purpose, and constructed the first secondary batteries which were commercially useful.

Planté's cell consisted of two large sheets of lead rolled up together, but without metallic con

A

CELL.

B

$109. SECONDARY CELLS, OR ACCUMULATORS. If two pieces of platinum, A and B (Fig. 113), are immersed in dilute sulphuric acid, and an electric current sent through the cell in the direction indicated, oxygen and hydrogen are liberated, oxygen collecting at the plate, A, or anode, and hydrogen at B, the cathode (see ELECTROLYSIS, in these Supplements). Electrical enFig. 113. ergy has been used up in separating the oxygen and hydrogen, and they possess a certain amount of chemical energy because of this separation. If, now, the source of current be removed, and the wires leading from A and B are connected through a galvanometer, it shows a deflection, and indicates a current in the reverse direction; for since the E. M. F., which was applied to produce the separation, has been removed, the oxygen and hydrogen are free to combine, which they do under these conditions, and give back the chemical energy in the form of electrical energy.

Grove constructed a gas battery working upon this principle. The action just described may be taken as the type of all secondary batteries, accumu lators, or storage-batteries; but it is to be noted that they do not store electrical, but chemical, energy, the only difference between a primary and secondary battery cell being that in the former the active materials are taken from some outside source, and brought together under the proper conditions, and in uniting they produce a current; but in the

To

tact, and immersed in dilute sulphuric acid. "form" or prepare the lead, a current is sent through the cell; the oxygen liberated at the anode oxidizes the lead, forming a dark brown coating of peroxide of lead. The cell is then discharged and a current is sent through in the reverse direction, coating the other plate with peroxide, while the hydrogen which collects at the plate first coated reduces the peroxide on it to metallic lead again, but leaving it in a finely divided state. The current is reversed several times in this manner until the plates are covered with a coating of finely divided lead. The cell is finally charged by sending a current through it until the electrolyte begins to be decomposed, showing that the action is complete; it will remain in this condition for many days, and will furnish current until the two plates are reduced to a chemically inactive state. The E. M. F. of the cell is approximately two volts. Faure, in 1881, modified the Planté cell by supplying the two plates with a coating of active material. The plates are cast or rolled in the form of grids, the holes or spaces being filled with the lead oxides which compose the active material, thus dispensing with the tedious and expensive process of forming. During the operation of charging, the oxides are changed to the peroxide at the anode or positive plate, and spongy lead at the cathode or negative plate. The chemical reactions of the cells are not thoroughly understood; sulphuric acid is formed during the charging and disappears. when the cell is discharged. An almost endless variety of storage-cells have been devised, but they are generally some modification of one or the other of the two types described.

A commercial form of the Planté cell is shown in Fig. 114. Fig. 115 shows a section of the lead plates, which are grooved in order that they may present as much surface as possible. The active material is formed on the surface of the plates by an electrochemical process which greatly reduces the time and expense of forming.

Fig. 116 is a common form of cell known as the chloride accumulator. The elements are Fig. 114 made of half-tablets or pastilles of a salt of lead, inclosed in a dense frame of metal cast around them under heavy pressure; this plate of lead-salt so framed is then reduced electro-chemically to pure metallic lead. This gives a plate entirely composed of metallic lead, partly in compact form, partly in minute crystalline subdivision, differing only from a plate of cast or rolled lead in that some of its parts are of a granular character. The plates to be oxidized are then put, with alternate lead plates, in an electrolytic cell, and a current is passed through them for a sufficient time to convert the pure crystalline metallic lead into peroxide of lead.

mmmmmmm

Fig. 115.

Fig. 116.

In cells of the Faure type the plates are cast full of square holes, the holes in the positive plate being filled with a stiff paste, prepared by mixing red lead (Pb,O) with dilute sulphuric acid. This hardens as it dries, and during the process of charging it is partly changed to the peroxide. The paste for the negative plates is made of yellow lead, or litharge (PbO) and dilute sulphuric acid, which is covered with spongy metallic lead when the cell is charged. The voltage of both types is approximately the same.

Secondary batteries are usually rated at a certain number of ampere-hours. A 150-ampere cell should provide a current of 15 amperes for 10 hours, or one ampere for 150 hours. There is a limit, however, to both the charging and dischar ging rate, for if the current in either case is too great, the cell is not efficient. A change in the nature of the active material causes, also, a change in its volume; hence if the change in volume. takes place too rapidly, the active material crumbles and falls off the plate. Secondary cells have, in general, a high electromotive force and low internal resistance, and hence are better suited to give large currents than primary cells; they give back from 70 per cent to 85 per cent of the energy used in charging. For many purposes they possess the disadvantage of great weight, and many attempts have been made to replace the lead by some lighter material, but as yet none have been entirely successful. The negative plate has been, in some cases, replaced by a zinc plate, well amalgamated; the solution used is dilute sulphuric acid containing zinc sulphate. On charging, zinc is deposited electrolytically on the zinc plate, while the action on the positive plate is the same as in other cells. The voltage is somewhat higher, being 2.5 volts; but the chief defect in the cell is its tendency to local action, necessitating the keeping of the zinc plate well amalgamated.

SIIO. ELECTRICAL UNITS. An absolute sys

tem of electrical units is one in which the units are independent of any arbitrary quantities, other than the fundamental units of mass, length and time. Two such systems are in use, and since they both take the centimeter, the gram and the second as the units of length, mass and time, they are called "centimeter-gram-second" systems, or, briefly, C. G. S. systems. The units of these systems have no special names, and are convenient only for mathematical investigation, being, in most cases, too large or too small for practical use.

The first of these systems is built up from the phenomenon of attraction between two charged bodies, and is called the electrostatic system. Unitcharge, or quantity of electrification, in this system has already been defined (13). The second system depends upon the magnetic field about a conductor carrying a current (§ 50), and is therefore called the electromagnetic system of

units.

Electrostatic Units.

Unit Quantity. The unit of quantity is that quantity of electricity which, when placed at a distance of one centimeter (in air) from a similar and equal quantity, repels it with a force of one dyne (§ 13).

Unit Potential. Potential being measured by work done in moving a unit of positive electricity against the electric forces, the unit of potential will be measured by the unit of work, the erg.

Unit Difference of Potential. Unit difference of potential exists between two points when it requires the expenditure of one erg of work to

bring a positive unit of electricity from one point to the other against the electric force (§ 25).

Unit Capacity. A conductor has unit capacity when it requires a charge of one unit of electricity to bring it up to unit potential. A sphere A sphere of one centimeter radius possesses unit capacity (§ 31).

Specific Inductive Capacity. This is defined in $36 as the ratio between two quantities of electricity. The specific inductive capacity of air is taken as unity.

Electromagnetic Units.

Unit Magnet-Pole. Unit magnet-pole is one of such strength that, when placed at a distance of one centimeter (in air) from a similar pole of equal strength, repels it with a force of one dyne.

Unit-Strength of Current. A current has unitstrength when one centimeter length of its circuit, bent into an arc of one centimeter radius (so as to be always one centimeter away from the magnet-pole), exerts a force of one dyne on a unit magnet-pole placed at the center (§ 50).

Unit Quantity of Electricity is that quantity which is conveyed by unit-current in one second. Unit Difference of Potential (or E.M.F.). tential is work done on a unit of electricity, hence unit difference of potential exists between two points when it requires expenditure of one erg of work to bring a unit of positive electricity from one point to the other against the electric force.

Unit Capacity. A conductor of unit capacity requires unit quantity to charge it to unit potential.

Unit Induction. Unit induction is such that unit E. M. F. is introduced by the variation of the current at the rate of one unit of current per second.

Practical Units and Standards.

The practical units are derived by taking convenient multiples of the units in the C. G. S. electromagnetic system.

A Standard is a quantity of the same kind as that to be measured, and with which comparisons are actually made. A standard is conveniently a unit, but not necessarily so; for example, the standard of E. M. F. is that of a Clark cell, but this is known to be 1.434 practical units, or volts.

=

Resistance. The Ohm 10° absolute electromagnetic units, and is represented by the resistance offered to an unvarying electric current by a column of mercury at o° C., 14.4521 grams in mass, of a constant cross-sectional area, and a length of 106.3 centimeters.

Current. The Ampere 101 absolute electromagnetic units, and is represented by the current which deposits silver at the rate of 0.001118 grams per second.

Electromotive Force. The Volt 103 absolute electromagnetic units, and is that E. M. F. which, applied to one ohm, will produce in it a current of one ampere; being

standard cell at go of the E. M. F. of a Clark

C.

1173

Quantity. The Coulomb = 101 absolute electromagnetic units of quantity, being the quantity of electricity conveyed by one ampere in one second. Capacity. The Farad 10 (or one thousandmillionth) absolute electromagnetic units of capacity, being the capacity of a condenser such as to be charged to a potential of one volt by one coulomb. The microfarad (or millionth part of one farad) = 10-15 absolute units.

=

Induction. The Henry 10° absolute electromagnetic units of induction, is the induction in a circuit when the E. M. F. induced in this circuit is one volt, while the inducing current varies at the rate of one ampere per second.

The prefixes mega- and micro-, when used in connection with the above units, signify, respectively, "one million" and "one millionth" part. Thus a megohm is a resistance of one million ohms and a microfarad is one millionth of a farad. The prefix kilo is used for "one thousand," and milli for "one thousandth" part.

The following units of work and power are used in connection with the practical units: Work. The Joule The Joule = 10' ergs. It is represented by the energy expended per second by one ampere in one ohm.

Power. The Watt 10' ergs per second; it is equivalent to the power of a current of one ampere due to an E. M. F. of one volt, or one joule per second; approximately, of a horse-power.

INDEX.

S. W. STRATTON.

Accumulators, or Secondary Cells, § 109.
Alternating Currents, § 86; Electrodynamic Action be-
tween, $ 90.

Armature Drum, § 91; Ring, § 80.
Bichromate or Grenet's Cell, § 103.
Bunsen's Cell, § 102.
Choking Coil, $78.

Circuit, Magnetic, § 63.

Clark's Standard Cell, § 107.

Condensers, 34; in Parallel and Series, § 35.
Conductivity, $44.

Conductors and Insulators, § 6.

Conductors and Non-Conductors, SS 10, 11; Capacity of a Conductor, § 30; Energy Expended in Charging a Conductor, 29; Force or Unit-Area of a Charged Conductor, 22; Intensity at the Surface of a Charged Conductor, 21; Resistance of Conductors placed in Parallel, 46; Resistance of Conductors placed in Series, $ 45.

Constant-Current Machines, § 83.
Constant-Potential Machines, § 84.
Continuous Potential Difference, § 39.
Copper Oxide Cell, § 105.

Currents, 38; Action of Currents upon Each Other, § 54;
Strength of Current, § 41; Unit Current, § 50.
Daniell Cell, $ 99; Calculation of E. M. F. of, § 100.
Density, Surface, § 15.

Discharge, § 14; at High Potential, § 94; In High Vacua (Cathode Rays), § 95.

Dynamos, § 79; Alternating Current, § 85.
Electrical Units, § 110.
Electric and Dielectric, § 7.

Electricity Defined, § 1.

Electrification, § 2; Both Kinds Always Produced, § 5; By Induction or Influence, § 12; Other Sources of, 4; Two Kinds of, § 3.

Electromagnet, Poles of an, § 67.

Electromotive Force, $40; Direction of the Induced Electromotive, § 69; Value of the Induced Electromotive, $ 70.

1174

Electrophorous, 12a.

Equipotential Surfaces, § 26.

Gauss's Theorem, § 19.

Grove's Cell, § 101.

Hysteresis, § 62.

Induced Current, Value of, § 73.

Induced Electromotive Force, Value of, § 70.

Induction Coils,

74; Tesla's, $ 75.

Induction, Mutual, § 71.

Induction or Influence, Electrification by, § 12. Induction over a Surface, Total Normal, $18. Intensity, 20; at the Surface of a Charged Conductor, 21; of the Field, Effect of the Dielectric on the, $ 37; Electromagnetic Intensity, § 55; of the Electric Field, 16; of the Field at the Center of a Circle, § 49; Relation between E. M. F. Intensity of Current and Resistance, § 42.

Insulators and Conductors, § 6.

Leclanché Cell, § 104.

Magnetic Circuit, Law of. § 66.

Magnetic Properties, Methods of Representing, § 61.

Magnetic Susceptibility, § 58.

Magnetomotive Force, § 65.

Motion in an Electromagnetic System, § 68.

Oscillatory Electric Discharge, § 92.

Parallel Plates, Cases of Two, § 33.

Paramagnetic and Diamagnetic Substances, § 60.
Permeability, $ 57.

Poles of an Electromagnetic, § 67.
Polyphase Currents, § 88.

Potential, 23; Due to a Charged Sphere, § 27; of a

Charged Conductor, § 28; Difference of, between Two

Points, § 25.

Primary Cells, § 97; Data Relating to Primary, § 108. Resistance of Conductors in Series, § 45; of Conductors Placed in Parallel, § 46; Energy Expended by the Current in Overcoming, $ 43.

Relation between μ and k, $ 59.

Reluctance, § 64.

Resonance, § 92.

Roentgen Rays and Photography, S 96.

Secondary Cells, or Accumulators, § 109.

Self-Induction, $ 72.

Silver Chloride Cell, § 106.

Specific Inductive Capacity, § 36.

Spheres, Capacity of, § 31; Capacity of Two Concentric, 32; Potential Due to a Charged Sphere, § 27. Tesla's Induction Coil, $ 75.

Transformers, § 76; Rotary,

77.

Tubes of Force, § 9a; Representation of the Electric Field

ELECTROCUTION. See CAPITAL PUNISHMENT, in these Supplements.

ELECTRODYNAMIC ACTION. See ELECTRICITY, Vol. VIII, pp. 10, 66; and ELECTRICITY, § 90, in these Supplements.

*ELECTROLYSIS. While many new facts have been learned about electrolysis, or the decomposi tion of chemical substances by the agency of the electric current, the chief material additions to our

knowledge of the subject have lain in the interpretation of the facts already known, and in the confirmation of the new theories. Electrolysis takes place when a current of electricity is passed through a fused substance, or a solution in water, and the latter is by far the more usual method. A very clear account of the phenomena is given in the main article (Vol. VIII, p. 106), but the conflicting nature of the views held in regard to them is made equally plain. The difficulty lay in our ignorance of the condition of the substance in solution, and of the nature of the process of solution in the case of acids, bases and salts, which are the only bodies whose solutions show electrolytic conduction.

The distinction between conductors, like copper and iron, which remain unchanged by the passage of the current, and solutions in which decomposition takes place, and the constituents are set free partly at one pole, and partly at the other, leads to the perfectly correct idea that the latter process is in some sense a transportation of the electricity with the substance, rather than a simple conduction. through it. Thus when a current of electricity passes through a solution of hydrochloric acid, hydrogen is set free at the negative pole, and chlorine at the positive. It is found that the amount of each set free is proportional to the quantity of elec tricity passing round the circuit. It seems, at first sight, plain that the compound has been torn apart. by the electricity. But this is inconsistent with the observation that the weakest current is capable of

*Copyright, 1897, by The Werner Company.