Cells are said to be primary or secondary according as they generate a current of themselves, or first require to be charged from an external source, storing up a current supply which is afterwards yielded in the reverse direction to that of the charging current.
An electric current is generated mechanically by a dynamo. In either case no electricity is produced, but part of the supply already existing is simply set in motion by creating an electric pressure.
An electric current, according to the third method, is generated directly from heat energy, as will be later explained; the current thus obtained is very feeble.
Strength of Current.—It is important that the reader have a clear conception of this term, which is so often used. The exact definition of the strength of a current is as follows:
The strength of a current is the quantity of electricity which flows past any point of the circuit in one second.
Example.—If, during 10 seconds, 25 coulombs of electricity flow through a circuit, then the average strength of the current during that time is 21⁄2 coulombs per second, or 21⁄2 amperes.
Voltage Drop in an Electric Circuit.—A difference of potential exists between any two points on a conductor through which a current is flowing on account of the resistance offered to the current by the conductor.
For instance, in the electrical circuit shown in fig. 39, the potential at the point a is higher than that at m, that at m higher than that at n, etc., just as in the water circuit, shown in fig. 38, the hydrostatic pressure at a is greater than that at m′, that at m′ greater than that at n′, etc. The fall in the water pressure between m′ and n′ (fig. 38) is measured by the water head n’s.