The open and closed circuit batteries we have so far described are used
to produce electricity by the action of the chemicals upon the elements
contained in them. They are called primary batteries.
[Illustration: Fig. 35]
The batteries which we will now tell you of are called secondary, or
storage, batteries, and do not of themselves make any primary current,
but simply act as reservoirs, so to speak, to hold the energy of the
electric current which is led into them from a dynamo or primary
battery. At the proper time and under proper conditions these secondary
batteries will give back a large percentage of the energy of the
electric current which has been stored in them.
This class of battery has been called by these three names: "secondary
battery," "accumulator," and "storage battery"; but as the latter name
is used almost exclusively in this country, we shall use it in the
following description.