In this way the battery is said to be "discharged," and at the end of its discharge it can again be charged and discharged in a similar manner for a long time, until the active material is either used up or drops off the plates.
So far as the actual details of construction are concerned, lead storage batteries are made in a great many different ways, but the materials are, in general, of the same nature as those we have mentioned above.
THE ALKALINE STORAGE BATTERY
We shall now describe an entirely different type of storage battery, which contains neither lead nor acid. It is one of the many inventions of Thomas A. Edison.
In the alkaline storage battery the gas called oxygen plays a very important part, and we will try to make it clear to you what this part is.
You are well aware of the fact that if you leave your pocket-knife out in the air it will get rusty. The reason for this is that iron or steel quickly tends to combine with the oxygen of the air, and this combination of oxygen and iron is rust, otherwise called oxide of iron, or iron oxide.
This iron oxide, or rust, is therefore the result of a chemical action between the iron and the oxygen.
Now as all chemical actions require the expenditure of energy, there has been developed either heat or electricity in the process. The oxygen may be taken away from the iron oxide, chemically; but here again would be another chemical action which would require energy to be once more expended.
Iron oxide may be made chemically in many different ways. It is frequently made in the form of a powder. Therefore, we do not have to depend upon iron rust for a supply of this material.