The popular exhibitions of the speaking-telephone during the past six months, together with numerous newspaper articles, have created a widespread interest in the instrument; and it has been thought that a small book explanatory of its action would meet a public want.
themselves; and the liberated gases adhere to the plates, and prevent further contact with the acid; at the same time, the gases themselves act upon the plates, and generate a current of electricity in the opposite direction. This will of course interfere with the first current; and very soon the battery is useless until the plates have been withdrawn from the liquid. This physico-chemical process that takes place in such a battery is called the polarization of the plates.
[Illustration: FIG. 1.]
The accompanying figure will help one to understand the actions going on in a battery cell of the kind mentioned. Let Pt represent a plate of platinum, and Zn a plate of zinc, both placed in a vessel containing hydrochloric acid, which is also represented by the symbols HCl. As such molecules are extremely minute, there will of course be an immense number of them between the plates. The plates are now to be connected by a wire running between them through the air. As soon as these con