Nature of the Galvanic a wire where a current of galvanic or frictional electricity is passing, there is no point which forms the •scat of positive or negative electricity, but it appears electrically homogeneous throughout. It exerts no statical inductive action on surrounding objects, neither attracting nor repelling them, for the electric action being more easily propagated along the wire than in any other direction, takes place only in it. The laws of induction and distribution applicable to frictional statical electricity hold true in current electricity only at the section of the wire or con ductor along which the action is transmitted. .As tested by the magnetic needle, there is no part of the circuit which possesses more power than another. This homogeneity gives rise to the hypothesis, that every molecule of the circuit, whether solid or liquid, acts in the transmission of the electric force, and is similarly affected in its passage. In this way the plates and connecting wires show the same molecular polarity as the liquid, only the discharge does not effect an interchange molecules, but leaves them iu the same condition as before. Each molecule of theconnecting wire may be viewed to be the seat of electric polarity and discharge with its negative faces turned towards the copper, and its positive towards the zinc; whenever, therefore, we go with the current, we meet each molecule on its negative side, and whenever we go contrary to the current, we meet each molecule on its positive side. Any portion of the circuit shows its negative face to the approaching current, and its positive face at the other extremity. A break in the connecting wire thus separates two contiguous mole cules; that ending the copper wire shows itself positive, and that ending the zinc wire negative. This is in perfect keeping with experiment, for wherever a break or change of medium is made in the circuit without stopping the current—as iu the electric light, chemical decompositions, the visible passage of electricity in vacuous tubes. and the like—the ends or poles exhibit opposite powers, from the pole meeting the current dis charging negative, and the other positive electricity. The polarity displayed at such interruptions, or visible passages of the current, is necessarily different from the polarity of frictional electricity, for the dynamical manifestation of electric force cannot be the same as the statical; in the same way that motion, for instance, the dynamical manifes tation of the force of gravity is essentially different from weight, its statical manifesta tion. Within time galvanic pair itself the same polarity is shown; the zinc plate, without the liquid or the wire connected with it, is found to act as a negative pole, and the similar copper plate and wire as a positive pole; but within the liquid of the ccli, the zinc plate shows the same chemical affinities as the exterior positive pole, and the similar copper plate acts as the exterior negative pole. Time terms positi remind negative poles are merely relative, for very molecule or series of molecules would thus appear to have its opposite poles. They serve; however, conveniently to express the relations of two consecutive parts of the circuit. Considerable confusion sometimes arises from speaking of the zinc plate as at once the positive element and negative pole, and the copper the negative element and positive pole of the galvanic pair, and such expressions seem even inconsistent. The truth is. that the zinc and copper plates must have each both poles from the very of the circuit; hut as the outer poles only of these plates are of practical importance, these are considered to he the poles.
According to the one-fluid theory of electricity, a force is developed at the seat of the action, which has the power of liberating the electric fluid. and of maintaining it in motion throughout the circuit, constituting a current in the true sense of the term. According to the two-fluid theory, two such currents, one of the positive the other of the negative fluid, are made to move in opposite directions throughout the circuit. Time propelling force is consequently termed electromotive, and the galvanic pair is called time electromotor. The terms current and electro-motive have their origin in the supposed fluidity of electricity, but being quite definite in their application, they may be used without any such admission. A current may. be taken to signify. apart from all suppo
sition, simply the peculiar electric condition of the conductor, which forms mime line of discharge between a positive and a negative source of electricity, and electro-motive force may be used simply to denote that which propagates and maintains this discharge. In the same way, when we speak of the direction of the current, we only use a con venient way of showing at which end the positive and negative electricities arise, the current being always represented as moving from the positive to the negative. The greater the electro-motive force is, the more powerfully is the discharge effected, and the more is it able to force its way through imperfect conductors.
Oeigin, of Galvanic would seem probable that the source of the electro motive force in 'the galvanie pair is the chemical action which takes place at the zinc plate. It must appear, even to the hest cursory observer, highly probable that the neat of the most active change going forward in the pair is likewise the origin of the force accompanying it. It is found, moreover, when we tax the galvanic current with electro-chemical work, that the amount of work done by it is exactly proportionate to the quantity of zinc dissolved. These and similar considerations seem to argue strongly that galvanic action has its source in chemical action. Volta, however, and several of the most eminent authorities in the science, maintain that the electro-motive force has its scat at the surface of contact of heterogeneous metals, and that chemical action is not the cause, but the manifestation of it. This view of the origin of galvanic electricity is ealled the.contact theory, as distinguished from the chemical theory, the one we have hitherto followed. The contact theory supposes that at the surfaces of contact of two heterogeneous substances, an electro-motive force,- invariable in direction and amount, is generated and subject to modification only by the offered by the conduct ing circuit. The galvanic pair is accounted for by this theory in the following way. Let us suppose, for the sake explanation, that both zinc and copper plates are con nected by copper wires. The ,eat of electro-motive force is at the junction of the cop per wire with the zinc. At this point the two metals assume opposite electrieities—the copper the negative, and the zinc the positive; and since a conducting circuit through wires, plates, and liquid is established, these eleetricities travel in opposite directions, and, meeting, neutralize each other within the liquid, to give place to succeeding sim ilar discharges of electricity. The discharge within the liquid takes place ehmtrolyti cally. The theory is,.in this case, sufficient and consistent, but it must be kept in mind, that in a circuit so perfectly homogeneous, the source of force may be placed anywhere without altering conditions. It is, however, so far wrong in assuming that the contact of the metals, where there is no force lost or transformed, maintains a never-failing development of energy in the circuit—that, in fact, force can he created from nothing. Sir William Thomson and the modern advocates of the contact theory modify Volta's theory iu this way. They admit with that the contact of the metals charges them with different electricities, but that the chemical energy of the liquid in contact with the metals is necessary to discharge them and maintain the current. Sir founds his belief of the truth of Volta's principle on an experiment like the following: A light bar of metal, AB (Fig. 1), is movable round A in the center of the compound metal ring, CZ, consisting of a semi circular band of copper, C, soldered to a like band of zinc. AB, when unchanged, lies in the line joining the junctions of the metals. When AB receives a small positive charge, it turns to the copper; if a nega tive charge to the zinc. Here, then, it is argued that contact pro duces a difference of tension or potential in the metals. It is neces sary, however, to the success of the experiment, which is, at the best, an excessively delicate one, that the zinc be bright and clear, and the copper acts more effectually when coated with a thin film of oxide.