257.]

VOLTA'S CROWN OF CUPS.

515

are experienced when the instrument is mounted in the form of a pile the liquid in the flannel soon loses the power of acting chemically on the zinc, and the activity of the combination declines rapidly.

Another more effectual arrangement adopted by Volta is shown in fig. 211; he termed it the Crown of Cups. In this form, the liquid in the cell corresponds to the moist flannel of the pile, and

FIG. 211.

the zinc of one cell being connected with the copper of the adjacent cell, the arrangement corresponds exactly with that of the pile, where the zinc is on one side of the flannel, whilst the copper in contact with the other surface of the flannel communicates with the zinc touching the flannel next above it, and so on. Other more efficient forms of the voltaic battery will be described further on.

The Conditions required to produce Voltaic Action.

(257) Electric Disturbance by Contact of Dissimilar Metals.Having in the foregoing remarks traced the voltaic phenomena to a modification of electricity, we may now proceed to examine more particularly the conditions under which they occur.

It was early observed by Volta that when two different metals, properly insulated, are brought into contact, and then separated by means of insulating handles, each plate exhibits signs of electricity which may be detected by a sensitive electroscope such as Bohnenberger's (note, § 298). The more oxidizable metal is found to be positive, while the less oxidizable metal is negative. If zinc filings be sifted through a piece of insulated copper-wire gauze upon the cap of a gold-leaf electroscope, the leaves of the instrument will diverge. On approaching the electroscope with an excited stick of sealing-wax the leaves will collapse, thus proving that the zinc filings have acquired positive electricity. If copper filings be sifted through zinc gauze, the filings will be found to be negative. The various metals may, with reference to these electric actions, be arranged in a series in which those first in order become positive by contact with all those that follow, and

516

CONDITIONS NECESSARY FOR VOLTAIC ACTION.

[257

negative with all those that precede: for example, potassium, zinc, iron, lead, tin, copper, mercury, silver, gold, platinum. This, it may be observed, is merely the order of the oxidizability of the different metals, and Delarive contends with great probability that the development of electricity in Volta's experiment is due to an excessively minute oxidation produced by the moisture of the air upon the plate which becomes positive, although the experiments by which he attempts to prove the point are not absolutely conclusive.

It was originally maintained by Wollaston that the electricity developed in the ordinary machine when an amalgam is used for exciting the action of the rubber, was produced mainly by the chemical act of oxidation exerted by the atmosphere upon the metal. But this view is shaken by the experiments of Peclet, who excited his electrical machine in a vessel filled successively with dry carbonic anhydride and dry hydrogen, and he found that in these gases as much electricity was evolved as when the same machine was surrounded, cæteris paribus, by dry air. Talc and plumbago may also be used with good effect upon the rubber as substitutes for the amalgam, though no effect of oxidation can be supposed to occur when these bodies are employed.

Volta regarded the interposed liquid of his pile in the light merely of an imperfect conductor which allowed induction to take place through it, the electrical equilibrium being perpetually disturbed by the contact of the two metals; and he overlooked the chemical changes which the liquid is constantly undergoing.

Sir William Thomson has devised an experiment for showing the charge produced on two metals by their simple contact. A light aluminium needle is supported near one end by a metallic wire connected to a Leyden jar, a counterpoise being placed on the short end of the needle so that it may rest in a horizontal position. If a disc of metal is placed beneath the needle and the Leyden jar charged positively or negatively, the needle will show no tendency to be deflected either to the left or to the right; if the disc be now replaced by two half discs of different metals, such as zinc and copper, so situated that their line of junction is vertically under the needle, then a deflection is observed when the needle is charged. If charged positively the needle will be attracted by the half disc of copper and be repelled by the zinc, showing that the zinc has become positively and the copper negatively charged. If the half discs be slightly separated and connected together by a metallic wire, the same result is obtained, but if instead of a metallic wire a drop of water be

259.]

POLARIZATION OF THE LIQUID.

517

placed between the half discs no deflection of the needle is observed. (Electrostatics and Magnetism, by Sir W. Thomson, P. 317. See also Fleming Jenkin's Electricity and Magnetism, 2nd Edit., 1874, 45.)

(258) Chemical Action essential to the Production of Voltaic Action. It is now known that chemical changes are essential to the production of the force. Contact of dissimilar substances, it is true, is necessary to the voltaic action; because without contact there can be no chemical action. Such contact produces disturbance of the electric equilibrium in the bodies which are brought together, and thus occasions a state of tension or polarity which always precedes the discharge. Chemical action, by renewing these contacts and by furnishing appropriate conductors to the electricity thus accumulated, maintains the action and accurately measures its amount; and until chemical action occurs no current is produced. The following experiment may be cited in illustration of this point:-Let an iron wire be connected with one extremity of a galvanometer of moderate sensibility, and a platinum wire with the other extremity; immerse the ends of the wires in highly concentrated nitric acid (sp. gr. about 145), without allowing them to touch each other in the liquid; no chemical action will occur upon the iron, and no movement of the magnetic needle will be produced; but the addition of a little water will determine a rapid solution of the iron in the acid, and the needle will receive a powerful impulse at the same moment that the chemical action commences.

(259) Polarization and Transfer of the Elements of the Liquid. -The simple occurrence of brisk chemical action is not alone sufficient to produce a powerful voltaic effort. The metals are all excellent conductors of electricity, and, in combining with each other to form alloys, they often give evidence of intense chemical action, but they do not produce any adequate voltaic effect. For example, if a small quantity of tin be placed in a tube bent into the form of the letter U, and be melted by the heat of a spiritlamp, and it be connected on one side with the wire of a galvanometer, which is introduced into the melted metal in one limb of the tube, whilst into the second limb of the tube a platinum wire, connected with the other extremity of the galvanometer, is plunged, the platinum will unite with the tin with incandescence, but after the first moment of contact but a slight deviation of the magnetic needle will be observed, although a brisk chemical action is continued for several seconds. A solution of the elementary bodies, chlorine or bromine, when used as the liquid between the plates,

518

VOLTAIC POLARIZATION.

[259.

although it acts powerfully on the zinc, produces by no means a proportionate effect in the circuit.

In order that the liquid shall possess any marked power of exciting voltaic action, it must be a compound susceptible of decomposition by one of the metals, such, for instance, as dilute sulphuric, hydrochloric, or hydriodic acid, or a saline substance, such as sodic chloride or potassic iodide. This necessity for the employment of a compound liquid for exciting the force, appears to arise from the necessity of a peculiar polarization in the liquid in order to enable it to transmit the voltaic action. Indeed, in all voltaic actions the transfer of power is effected by a polar influence, propagated through both the solid and the liquid particles of the circuit, and the chain of conducting material must be continuous throughout, so that the force shall circulate.

This process of polarization may be conceived to occur in the following manner, which offers an explanation of the mode in which the platinum (or the plate of metal which corresponds to platinum) may be supposed to act :-When a plate of pure zinc or of amalgamated zinc is immersed in a compound liquid, which, like a solution of hydrochloric acid (HCl), is capable of attacking it chemically, the metal at the points of contact becomes positively electrified, whilst the distant portion becomes negative. The layer of liquid in contact with the zinc undergoes polarization, which affects each molecule of its chemical constituents; the particles of chlorine (C) become negative, and the particles of hydrogen (H) positive: but in this form of the experiment there is no communication between the distant negative part of the zinc and the positively electrified particles of hydrogen; consequently, beyond the production of this state of electric tension, no change ensues. This condition is represented in fig. 212 (1). But the case is entirely altered if a plate of platinum, or of some other metal which is not easily acted upon by the acid, be introduced, and made to touch the zinc. By contact with the zinc the platinum itself becomes polarized; it imparts a certain amount of positive electricity to the zinc, and receives a portion of negative in return, and transmits the polar action to the liquid. A chain of polarized particles is thus produced, as represented in fig. 212 (2); the chlorine of the particle of HCl nearest the zinc becomes negative under the influence of the chemical attraction which exists between it and the zinc, and the hydrogen becomes positive: the second and third particles of HCl become similarly electrified by

259.]

VOLTAIC POLARIZATION.

519

induction; but the platinum, under the influence of the induction of the zinc, being negative, is in a condition to take up the positive electricity of the contiguous hydrogen. The action now rises

FIG. 212.

+ H

high enough to enable the zinc and the chlorine to combine chemically with each other: the zincic chloride thus produced is dissolved by the liquid, and is removed from further immediate action; but the particle of hydrogen nearest the zinc now seizes the oppositely electrified chlorine which lies next to it, and a new portion of hydrochloric acid is reproduced, whilst the hydrogen in the second particle of the acid is transferred to the chlorine of the adjacent particle, and the particle of hydrogen which terminates the row is electrically neutralized by its action upon the platinum, to which it imparts its excess of positive electricity, and immediately escapes in the form of gas. Fresh particles of hydrochloric acid continually supply the place of those which have undergone decomposition, and in this way a continuous action is maintained. Thus the transfer of electricity from particle to particle of the liquid is attended at the same instant by a transfer of the constituents of the liquid in opposite directions.

These changes are not successive, but are simultaneous in each cross section of the liquid, and are also attended with corresponding changes at all points of the entire circuit. These changes when continued uninterruptedly constitute what is conveniently termed a voltaic current. This term, 'current,' is in general use, but it should be borne in mind that it is in this sense employed merely to signify the continuous transmission of electrical action, not of any material substance. In every voltaic current it is assumed that a quantity of negative electricity, equal in amount to that of the positive set in motion, is proceeding along the wire in a direction opposed to that in which the positive electricity is travelling; and it is conceived that by the perpetual separation