An inductor usually consists of a coil of conducting material, typically insulated copper wire, wrapped around a core either of plastic or of a ferromagnetic material; the latter is called an "iron core" inductor. When the current flowing through an inductor changes, the time-varying magnetic field induces an electromotive force. Small value inductors can also be built on integrated circuits using the same processes that are used to make transistors. Aluminum interconnect is typically used, laid out in a spiral coil pattern. However, the small dimensions limit the inductance, and it is far more common to use a circuit called a gyrator that uses a capacitor and active components to behave similarly to an inductor.
Inductors used to block very high frequencies are sometimes made by stringing a ferrite bead on a wire.