The typical inductor is a coil of wire, which may have a ferritic core to increase the inductance. As the electric current is increased, the changing magnetic field in the core induces a voltage in the wire opposing the change in current. As the current is reduced, the voltage opposes this change as well, acting to maintain the current. Because it resists changes in motion, the inductor's action is similar to that of mass in a mechanical system.
If two inductors share the same core, a changing current in one wire will induce a voltage in the other wire, even though they are not in electrical contact. This configuration is called a transformer, a device used widely in alternating current circuits.