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.
PRIMARY BATTERIES
An electrical battery is a device in which one or more chemical substances act upon a metal and a carbon, or upon two different metals, producing thereby a current of electricity, which will continue as long as there is any action of the chemicals upon the metal and carbon, or upon the two metals.
Batteries for producing electricity may be divided into two classes, called "open circuit" batteries and "closed circuit" batteries.
Open-circuit batteries are those which are used where the electricity is not required constantly without intermission—for instance, in telephones, electric bells, burglar alarms, gas-lighting, annunciators, etc.
Closed-circuit batteries are those which are used where the effect produced must be continuous every moment, as, for instance, in electric lights and motors.
The open-circuit battery is made in many different ways, so we only describe two of the principal ones.