Batteries are the most expensive source of current but for many reasons, as explained above, are all that is available to the average experimenter.
There are two classes of batteries, known as
1. Primary batteries; 2. Secondary or storage batteries.
*Primary Batteries* are those which generate their own current by the action of some chemical such as, for example, an acid upon a metal.
*Secondary Batteries* derive their current from a dynamo or other source of electricity and store it away in the form of *Chemical energy* until it is used up. A storage battery might be likened to a pail, which can be carried to a dynamo and filled full of electricity. Those who possess a storage battery can recharge it themselves from the 110-volt lighting or power circuit, from a dynamo or by taking it to an automobile garage where recharging is done.
Homemade batteries are not as practical as those which can be purchased ready made, but the knowledge and experience gained in making your own are so valuable that every experimenter is urged to start in this way.
Various materials such as zinc, copper, carbon, etc., can be used to make some very interesting and valuable batteries.
One of the most common mistakes made in reference to cells and batters is in calling a single cell a *battery*. One cell is a cell. More than one cell connected together is a *battery of cells* or simply a *battery*.
The Voltaic Cell.
The first practical cell was invented in 1786 by an Italian professor named Volta and it is, therefore usually called the Voltaic cell.