Inductors are the electronic components designed especially for introducing a required amount of inductance in the circuit. They are coils of wires, wound around some magnetic material and then packaged. The final configuration take a widely variety of forms and appearances. Coils are the only passive components in ordinary use that can show important non-linear characteristics. The behaviour of inductors is based in phenomena associated with magnetic field. The source of magnetic field is charge in motion, or current. If the current is changing with time, the magnetic field is also changing with time. A time varying magnetic field creates a voltage in a conductor connected by the field. The circuit inductance relates the induced voltage to the current. Energy can be stored in magnetic fields and inductors are capable for storing energy. Inductors are passive devices because they cannot generate energy.