CHAPTER X. GALVANIC OR VOLTAIC BATTERY. 96. Galvanic Element.-If a plate of zinc and a plate of copper dip side by side, without touching, in a vessel of water acidulated with sulphuric or hydrochloric acid, two pieces of copper wire attached one to each plate are found to differ in potential. If the wires are connected with the alternate pairs of quadrants of a quadrant-electrometer, a deflection is produced indicating a difference of potentials of about one volt, the potential of the wire from the copper plate being the higher. Similar results would be obtained if almost any two metals were substituted for copper and zinc in the above experiment, and instead of acidulated water, ordinary tap-water, or water containing a small quantity of some salt in solution, may be used. The only difference in the result would be in the magnitude of the observed difference of potentials. The material of the connecting wires is of no consequence; copper is mentioned as being the material most likely to be at hand, and, on the whole, the most convenient. Results equivalent to those just stated were observed in 1789 by Ludovico Galvani, professor of anatomy at Bologna. An arrangement such as we have described, or any modification of it producing like effects, is consequently often spoken of as a galvanic element or galvanic cell; and the study of the numerous phenomena for the discovery of which Galvani's observation formed the starting-point constitutes the science of galvanism. 97. Voltaic Pile - Galvanic Battery. The difference of potentials between the wires of a galvanic cell depends only on the materials employed, and, to some extent, on their temperature; it is not affected by the size, shape, or relative positions of the plates, nor by their absolute electrical potential. If placed on an insulating stand, a galvanic cell may be strongly electrified either positively or negatively, but however high or however low the $97.] Voltaic Pile-Galvanic Battery. 135 potential of the cell as a whole may be, the potential of the wire from the copper plate remains about one volt higher than that of the wire connected with the zinc. A consequence of this is that, if several cells are arranged one after another, and the zinc plate of the first is connected by a wire with the copper plate of the second, the zinc of that with the copper of the third, and so on throughout, the zinc of the last cell but one being joined to the copper of the last, the difference of potentials between the wire from the first copper plate and that from the last zinc plate is equal to the difference found in the case of a single cell multiplied by the number of cells. To see how this follows, let us assume for simplicity that the difference of potentials in the case of a single cell is exactly one volt, and that the wire connected with the last zinc plate is in contact with the inside of the room where the experiment is made. We may assume then (§ 36) that its potential is zero. Accordingly the potential of the copper plate of the same cell and of the wire attached to it will be one volt. But this wire is fastened to the zinc of the last cell but one, and therefore the copper of this cell will have a potential = 2 volts. In the same way that of the next preceding cell will have a potential = 3 volts, and so on to the beginning of the series, the potential rising by one volt for every cell. Proceeding in the opposite direction along the series of cells, we should find a fall of potential equal to one volt at a time as we passed from one cell to the next; so that, if the first copper plate were kept at potential zero, the second would have a potential - 1, the next -2, and so on. = The fact that the difference of potentials resulting from a single pair of metals dipping into an aqueous liquid can be thus multiplied, by arranging in order a number of such pairs connected together, was discovered in 1800 by Alessandro Volta, professor of physics at Pavia. In order to produce the effect in a marked degree, he arranged a number of alternate discs of copper and zinc, each pair being separated from the next by a round of cloth FIG. 90. soaked with acidulated water, as shown in section in Fig. 90, and in perspective in Fig. 91. This arrangement is known as Volta's pile. A corresponding combination of pairs of metal plates in separate vessels of liquid is called a voltaic battery or galvanic battery, and in general the terms voltaic and galvanic, and words formed from them, are used almost interchangeably in relation to the class of phenomena we have now to study. 98. Volta's Theory of Electrification by Contact. -In order to explain facts of the kind referred to in the last two paragraphs, Volta supposed that the surface of contact of two metals, or more generally of any two different substances, is the seat of a peculiar force, which he termed electromotive force, the effect of which is to transfer positive electricity across the surface in one direction and negative in the other, until the tendency of the two electricities to reunite balances the force which tends to separate them. He supposed the electromotive force in any given case to depend solely on the nature of the materials in contact, and not upon the size or shape of the bodies, or the extent of the surface of contact, or on their electric state. Among the numerous experiments which he devised to establish this principle the following may be cited : 1. Two plates, one of zinc and the other of copper, supported by insulating handles (Fig. 92), and at first in the neutral state, are placed in contact FIG. 92. and then separated. On applying these, each separately, to a Theory of Electrification by Contact. 137 $ 98.] delicate electroscope, it is found that they are each electrified, the zinc positively and the copper negatively. The amount of the charge is proportional to the extent of the surfaces in contact. The two plates are charged as if they were the plates of a condenser between which a difference of potentials has been established. The two electricities form on the surfaces in contact two equivalent layers, which remain separated in conformity with the electromotive force of contact of Volta, notwithstanding the absence of an insulating plate. 2. A zinc strip joined to one of copper is held in the hand, and the copper end is applied against the lower plate of a condensing electroscope, the top plate of which is touched by the other hand (Fig. 93). If the connections are broken and the top plate is raised, the gold leaves diverge, with negative electricity; this proves that the potential of the copper was negative, for as the zinc was held in the hand, it was at the potential of the earththat is, at zero. If the experiment is repeated in exactly the same manner, except that the copper is held in the hand and the zinc applied to the electroscope, the leaves do not diverge. The fall of potential from zinc to copper is evidently the same in both cases, and the potential of the plate is zero like that of the copper held in the hand. The experiments may be modified by interposing between the plate of the condenser and the strip of copper a disc of cloth or paper wetted with acidulated water. Nothing is changed in the former case; the divergence of the gold leaves is still negative. But in the second case, in which the copper is held in the hand, the leaves diverge with positive electricity, instead of remaining unaffected. 99. Objections to Volta's Theory.—While no question is raised as to the results of the fundamental experiments adduced by Volta and his supporters in confirmation of his views, there is by no means universal agreement among physicists as to the true interpretation of these results. The most serious objection to Volta's theory is that it does not afford a satisfactory account of the source of the energy the production of which it implies. Suppose S to be the capacity of the condenser used in either of the forms of experiment described in § 98, and V and V to be the potentials acquired by the two plates respectively, then the production of the difference of potentials V - V' would indicate a generation of electrical energy = } S (V – V')2. If the plates of the condenser have a radius of 5 cm. and are cm. apart, the capacity is (§§ 52, 59) S = = area 4 x distance 500 in C.G.S. units. If th of an electrostatic the difference of potentials is one volt, or unit, the energy is × 500 × 60 × 306 = 2.78 × 10 3 erg. The only source from which this energy could be derived, consistently with Volta's hypothesis, is the heat contained in the zinc and copper. It is true that the withdrawal of the quantity of heat required to furnish the quantity of energy in question,—rather less than three thousandths of an erg,--would cause an utterly imperceptible fall of temperature in a compound bar of zinc and copper large enough to be used in the experiment, so that the suggested explanation cannot be rejected on this ground. Indeed, it is known from other experiments (see Chap. xiv.), that when a much larger quantity of electricity than there is any question of in the present case, is caused to pass from copper to zinc across the common surface, an ascertainable lowering of temperature really ensues. But when a numerical comparison is made, it is found that the quantity of heat which thus disappears is inadequate to account for the difference of potentials developed in |