Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/93

 cate electroscope, which indicated by the divergence of its straws that the disks were now electrified—the zinc had, in fact, acquired a positive and the copper a negative electric charge. Thus the mere contact of two different metals, such as those employed in the pile, was shown to be sufficient for the production of effects undoubtedly electrical in character.

On the basis of this result Volta in the same year (1800) put forward a definite theory of the action of the pile. Suppose first that a disk of zinc is laid on a disk of copper, which in turn rests on an insulating support. The experiment just described shows that the electric fluid will be driven from the copper to the zinc. We may then, according to Volta, represent the state or "tension" of the copper by the number -$1⁄2$, and that of the zinc by the number +$1⁄2$, the difference being arbitrarily taken as unity, and the sum being (on account of the insulation) zero. It will be seen that Volta's idea of "tension" was a mingling of two ideas, which in modern electric theory are clearly distinguished from each other-namely, electric charge and electric potential.

Now let a disk of moistened pasteboard be laid on the zinc, and a disk of copper on this again. Since the uppermost copper is not in contact with the zinc, the contact-action does not take place between them; but since the moist pasteboard is a conductor, the copper will receive a charge from the zinc. Thus the states will now be represented by -$2⁄3$ for the lower copper, +$1⁄3$ for the zinc, and +$1⁄3$ for the upper copper, giving a zero sum as before.

If, now, another zinc disk is placed on the top, the states will be represented by -1 for the lower copper, 0 for the lower zine and upper copper, and +1 for the upper zinc.

In this way it is evident that the difference between the numbers indicating the tensions of the uppermost and lowest