Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/94

 disks in the pile will always be equal to the number of pairs of metallic disks contained in it. If the pile is insulated, the sum of the numbers indicating the states of all the disks must be zero; but if the lowest disk is connected to earth, the tension of this disk will be zero, and the numbers indicating the states of all the other disks will be increased by the same amount, their mutual differences remaining unchanged.

The pile as a whole is thus similar to a Leyden, jar; when the experimenter touches the uppermost and lowest. disks, he receives the shock of its discharge, the intensity being proportional to the number of disks.

The moist layers played no part in Volta's theory beyond that of conductors. It was soon found that when the moisture is acidified, the pile is more efficient; but this was attributed solely to the superior conducting power of acids.

Volta fully understood and explained the impossibility of constructing a pile from disks of metal alone, without making use of moist substances. As he showed in 1801, if disks of various metals are placed in contact in any order, the extreme metals will be in the same state as if they touched each other directly without the intervention of the others; so that the whole is equivalent merely to a single pair. When the metals are arranged in the order silver, copper, iron, tin, lead, zinc, each of them becomes positive with respect to that which precedes it, and negative with respect to that which follows it; but the moving force from the silver to the zinc is equal to the sum of the moving forces of the metals comprehended between them in the series.

When a connexion was maintained for some time between the extreme disks of a pile by the human body, sensations. were experienced which seemed to indicate a continuous activity in the entire system. Volta inferred that the electric current persists during the whole time that communication by con-