So far we have only described one way of producing electricity--namely,
by means of a dynamo-machine driven by steam or water power. The supply
of electricity so obtained is regular and constant as long as the steam
or water power is applied to the dynamo.
There is another and very different way of producing electricity, and
this is by means of a chemical process in what is called a battery.
To obtain electricity from the dynamo we must spend money for the coal
to make the steam which operates the steam-engine, or for the water
which turns the water-wheel, as well as for an engineer in both cases.
When we obtain electricity from a battery we must spend money for the
chemicals and metals which are constantly consumed in the battery.