The simplest form of a coil is wound copper wire, which is coated with enamel insulation so that there is no electrical contact between the windings. When the copper wire conducts current, it is surrounded by an electric and a magnetic field. When alternating current is sent through the coil, the coil will build up a varying magnetic field. The field opposes changes in the current according to Lenz's Law. Most often, the copper wire is wound around an iron core or bobbin to strengthen the fields. The more turns wound, the higher the inductance. Inductance is measured in Henry, which is a large unit, therefore mH and μH are most often used. A coil's impedance is measured in ohms, and is a measure of the resistance to alternating current. The higher the frequency of the alternating current, the higher the impedance will be.
