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Thereason why alternating current is often used in place
of direct current is that it can be sent over the Wires for
long distances more economically than direct current. This
is more fully explained farther on in the chapter dealing
with a step-down transformer.

The number of cycles taking place in one second is known
as the frequency of the current. The usual frequency of
commercial alternating currents is 60 cycles per second or
7200 alternations per minute.

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