[Illustration: FIG. 241.--Diagram of a simple telephone circuit.]
324. Cost of Electric Power. The water power of a stream depends
upon the quantity of water and the force with which it flows. The
electric power of a current depends upon the quantity of electricity
and the force under which it flows. The unit of electric power is
called the watt; it is the power furnished by a current of one ampere
with a voltage of one volt.