This theory supposes that the transmission of the electricity through the moistened cloths is effected without any diminution. Such is the case with a per fect degree of conductibility. We admit, besides, that the liquids interposed between the metallic elements exerts on them a force which either amounts to no thing or is so trifling as to be entirely overlooked. Finally, to pass from one element to another, we have joined to these data a third, namely, that the excess of electricity + 1, which the zinc takes from the cop per, is the same in these two metals, whether they are in the natural state or not. This last supposition is the simplest that we can make. It is not, how ever, a supposition of which the fundamental expe riments above-mentioned afford any proof. We have heard it said by Coulomb, that he had verified this law, and that it had appeared to him exact. It is clear that it cannot well be established without the aid of the electric balance, and without measuring the quantities of free electricity at different heights in any pile ; but such observations would be affect ed by the constant imperfection in the conductibi lity of the humid conductors, and by several other causes, which we shall examine afterwards. Be this, however, as it may, let us for the present admit the equal difference in question, and endeavour to trace the consequences by calculation, though this should only lead at first to an approximation.
First, then, if we touch with one hand the base of the pile, and carry the other hand to its sum mit, all the excesses of electricity + 1, + 2, + 3, of the different pieces, must discharge themselves through the organs into the ground. Supposing the transmission of electricity in the interior of the pile perfectly free, or only very rapid compar ed with its transmission through the organs, this discharge ought to produce a shock like that of the Leyden jar; but with this remarkable differ ence, that the sensation, will appear to continue ; for the pile recharging itself at the expence of the ground much quicker than the organs of living bodies can discharge it, the superior piece will be al ways almost as highly charged as before the con tact. Experience entirely confirms these views ; and we can also produce, in the same manner, but with infinitely greater intensity, all the pheno mena of taste and of light which are excited by a single pair of pieces, and are described in the article GALVANISM, in the Encyclopedia.
If it be required to discover in this case the quan tity of electricity which forms the discharge at eve ry contact, we have only to take the sum of the quantities of electricity, which, according to the preceding deductions, exist in a state of freedom in the different parts of the apparatus. But, to simpli fy this estimate, we may suppose the moistened cloths infinitely thin, and neglect the quantity of electricity which attaches itself to their exterior boundaries ; then the preceding quantities, which are diffused over the surfaces of copper and of zinc, will be the only sums to be taken ; and the amount will be found proportional to the square of the num ber of pairs, though it will presently be seen that this result is extremely enfeebled by the imperfect con ductibility of the moistened bodies interposed be tween the pairs, and through which the transmission is effected.
We have supposed the pile fitted up in the or der—copper, moisture, zinc, copper, &c. ; the first piece of copper communicating with the ground. But we may also arrange it in a reverse order, namely, zinc, copper, moisture, zinc, &c. by form ing the communication with the ground and the first piece of zinc. In this case, the theory will be quite the same, only that the unity +1 will be come negative ; or the quantities of free electrici• ty will be of a resinous nature.
Instead of laying the metallic plates above each other, in a vertical column, we may place them ho rizontally and parallel to each other, on insulating supports ; on sticks, for example, of varnished glass ; then, instead of interposing between them pieces of cloth, which would with difficulty stand upright, we may form, from the one to the other, a series of a kind of troughs, of which they become the extreme boundaries ; and into these troughs pour the liquids which are to serve as conductors. This is called the Trough apparatus, fig. 9, Plate LXX X.* We may also solder together, and end to end, slips of copper and of zinc, bent at their point of junction, so that each metal may be plunged into a vase or cup of glass, or of porcelain, partly filled with a liquid conductor. A series of similar vessels forms an electromotive chain, of which the extremities may be brought circularly round towards each other, for the convenience of making experiments. Fig. 10. This is what Volta calls the apparatus De tames d Couronne. But in whatever way this apparatus be arranged, the prin. ciple of its action is evidently the same ; and the theory we have explained applies equally to all.
Let us now apply to the upper part of the pile, or in general to the last plate of the apparatus, a condenser, the inferior plate of which communicates with the ground. Previous to the contact, this plate, which we shall always suppose of zinc, possessed a degree of free vitreous electricity corresponding to its rank in the pile. The condenser carries off a part of this, which the zinc immediately supplies from the inferior piece, this from the following, and so on to the last, which draws the whole from the ground. This movement must continue until the superior piece has re-acquired the same quantity of free electricity which it possessed at first, and which corresponds to its situation. Thus the conductor will be charged, until the electricity spread over its collecting plate, has the same repulsive force as this plate of the pile with which it is in contact.