But this property enables liquids to transmit the reciprocal action between the copper and zinc with out weakening it by their contact. Thus, for exam ple, if we take the second experiment (fig. 5), where the zinc was placed between two pieces of copper, we have seen, that, in that case, the electromotive forces, exerted upon the zinc, being equal and con trary, there was no developement of electricity, and the condenser was not charged. But, if between the zinc and the collecting plate, which is of copper, we interpose a stratum of conducting liquid, such, for instance, as a drop of water, or a piece of paper moistened with some saline solution, then the con denser will be charged. This intermediate body is now then sufficient to prevent the electromotive action of the plate upon the zinc, which only becomes manifest during contact ; it cannot, at the same time, supply this action itself, as its own electromotive force is so very weak and insensible; and, lastly, in consequence of its conducting quality, it is enabled to transmit the electricity of the zinc, if the latter acquires any thing above its natural share. The zinc, therefore, is now in a condition peculiarly adapted for this deve lopement, being interposed between two bodies which touch it, and of which the one, namely the copper, exerts on it a sensible electromotive action, while that of the other, namely, the liquid, is but ex tremely feeble. The production of electricity then will go on nearly as well as if the zinc were insulat ed in the air • and, from the communication which is formed by air; humid conductor, it must, besides, necessarily happen that this conductor, and the plate of the condenser on which it lies, will divide between them the superabundant electricity of the zinc, until they acquire a repulsive force exactly equal to its own.
Hence, if we solder together two thin circular plates, the one of zinc and the other of copper, and if, after having laid this compound plate with the copper side on the hand, we cover the zinc side with a humid conductor, whose electromotive force is insensible, with a piece of cloth, for example, soaked in water, or some saline solution, all the conducting bodies which we place above this system will share in the excess of the vitreous electricity of the zinc side, and of the humid body which covers it. If then, on this first system, we place another similar one, so that its copper side may lie on the moist ened cloth, this second system will then, as a conduct ing body, share the excess of the vitreous electricity of the first zinc side ; and the second piece of zinc will, besides, take a new excess of electricity equal ly vitreous, produced by the electromotive force of the copper to which it is soldered. In thus adding successively several similar systems on each other, we obtain an apparatus in which the electric state of the successive pieces will go on augmenting from the bottom to the top, according to the number of pairs superimposed.
Such is the admirable instrument now universally known under the name of the Voltaic Pile, and by which both Natural Philosophy and Chemistry have obtained such astonishing results. To comprehend rightly its effects, we must analyse, with precision, the electrical state which the different pieces that compose it assume, as well as the changes that arise when we make any of them communicate with the ground or with a conductor.
To present this analysis in the simplest form, we shall first suppose that the humid bodies interposed between the pairs of metallic slates, serve absolutely no other purpose than to conduct from the one to the other the free electricity which is 'developed on the surface of the pieces, and that those liquids them selves do not in the least contribute to the produc tion of the electricity with which the column is charged. This supposition, which we do not offer as
at all definitive, but merely as the first case which we submit to examination, will have the advantage of showing distinctly, the phenomena which may be produced by the mutual contact alone of the metallic plates, and by the circulation of the electricity which results from it. We shall first then examine if this be sufficient to represent all the phenomena, and, se condly, what modifications must be applied to make it embrace them.
Let us consider, first, a single pair of metallic pieces formed of a plate of zinc soldered or firmly fixed to a plate of copper of equal dimensions ; and place the copper side in communication with the ground. This side will then be in the natural state, but the zinc side will be covered with a stratum of free vitreous electricity, the total amount of which we shall represent by + 1. The value of this unity depends on the extent of the two plates, and will be proportional to their surface.
The copper side communicating always with the ground, we place on the zinc side a piece of cloth soaked with saline water, or with any other liquid conductor whose electromotive action is insensible.
Then the free electricity of the zinc side will spread itself over the surface of the conductor; but as the zinc must necessarily always possess the excess of vitreous electricity which its contact with the copper requires, it will draw a new supply from the copper, and the latter from the ground. All this is but a simple consequence of the experiment of Volta, relat ed above.
Take now a new piece of copper and zinc, similar to the first, and after having touched its copper side and insulated it, place this side upon the moistened cloth, as is represented in fig. 7. According to Volta, two actions now begin ;, first, The zinc side of this second piece preserves the excess of vitreous electricity + 1, which it acquires from its contact with the copper. Second, The whole system of the piece shares the free electricity of the cloth, as every other conducting body would do. The cloth renews this electri city, by drawing a supply from the inferior piece of zinc, this latter from the copper, and the copper from the ground ; so that, after a certain time, which, if the conductibility be perfect, must joe infinitely short, there arises a state of electrical equilibrium where the quantities of free electricity are such as is represented in the following table : On this system, lay a second cloth, then a third piece of copper and zinc in the same manner, fig. 8 ; the zinc side of this new piece will preserve its excess of vitreous electricity -I- 1 ; but, besides this, it will share, like every conducting body, the free electricity of the inferior pieces, which will be supplied at the expence of the ground; and when the electrical state will have become permanent, we shall have By continuing always the superposition of pairs in the same manner, the quantities of free vitreous elec tricity will increase from the bottom to the top, in an arithmetical progression.