Furthermore, pieces of the same metal under different physical conditions, when combined with each other, will give a current. For instance, a piece of polished iron opposed to a rusty piece gives a current, and a plate of very rusty lead, if I may use the expression, combined with a piece of bright lead yields even more current than zinc and carbon. Unfortunately, lead does not rust sufficiently well to suit electricians, and other physical reasons prevent its being used in primary batteries.
It will thus be seen that a storage battery, when once charged, becomes nothing more or less than a primary battery. In the case before described, after charging, we have zinc and copper in a solution of zinc sulphate.
In describing the effects of electricity in passing through an electrolyte, we have assumed that the liquid contained a metal in solution. Suppose, now, that we take water, which has no metal in it, and subject it to the action of the current. The electricity can get no metal to deposit on the wire, where it passes out, and in consequence does the next "best thing and leaves one of the components of the water—viz., hydrogen gas. The other component, oxygen, collects around the entrance wire. The English physicist Grove showed that, if these two wires, around which the gases had collected, were connected together, a current of electricity would flow the same as if there were two metals instead of two gases. Now, water is cheap, and if there were not some serious technical difficulties as regards efficiency, Grove's battery would be universally employed.
It was reserved, however, for M. Gaston Planté to construct the first practical secondary battery. He considered the following points in its construction: Water is cheap; water, when subjected to the electric current, gives off oxygen and hydrogen; rusty lead, when combined with bright lead, has a high electromotive force; oxygen makes lead rusty and hydrogen makes it bright. His battery consisted, then, of two lead plates suspended in water, which contained a little sulphuric acid to assist in the conduction. When a current of electricity was passed through, hydrogen was thrown off at one plate, making it bright, and oxygen at the other plate, peroxidizing its surface. When the charging source was removed, the altered plates would send off a current, which was in a direction opposite to the one which had charged them, and this would keep up until the plates had assumed their original condition.
Planté's choice of materials was most wise, and all practical storage batteries of to-day are but modifications of his style.
In order that his battery might give a strong current, and one that would last a long time, it was found necessary that his two lead plates should be as near to each other, and that they should
