The simplest device involving forced oscillations is the transformer.
A transformer is simply two coils arranged so that nearly all of the flux made
by one of them passes through the other one.
This means that if an AC current flows in one of the coils, the resulting
AC flux will produce an AC emf in the other one,
or to put it another way, they have mutual inductance.
The coil which is connected to the driving emf is called the primary
and the coil in which an emf is induced is called the secondary.
The primary voltage 
and the secondary voltage 
are related
by the ratio of the number of turns, 
and 
, in the two coils:
If the transformer is properly made,
the power delivered by the primary
will be very nearly the same as the power received by the secondary, requiring
that the currents in the two coils transform oppositely to the voltages:
The ability to transform voltages at will with a device consisting only of two coils, a piece of iron, and no moving parts, is the main reason that AC current is so widely used.