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An inductor is a passive electrical component that produces a voltage proportional to the instantaneous change in current flowing through it:

V = L × dI/dt,

where V is the voltage generated, dI/dt is the rate of change of current, and L is a property of the device called inductance. The SI unit of inductance is the henry (H).

Thus an inductor resists changes in current. A pure inductor does not offer any resistance to direct current (an actual one does slightly), except when the current is switched on and off, then it makes the change more gradual.

When a sinusoidal alternating current flows through an inductor, a sinusoidal alternating voltage (or electromotive force, abbr. emf) is induced. The amplitude of the emf is related to the amplitude of the current and to the frequency of the sinusoid by the following equation.

V = I × ωL

where ω is the angular frequency of the sinusoid defined in terms of the frequency f as

ω = 2πf

The term ωL is known as inductive reactance, which is denoted by the symbol XL and is the positive imaginary component of impedance.

Table of contents
1 Construction
2 History
3 See also
4 Synonyms


An inductor is usually constructed as a coil of conducting material, usually copper wire. A core of ferrous material is sometimes used.

This effect can be understood as follows: the current produces a magnetic field; a change in current gives a change of this magnetic field; a changing magnetic field causes an electromotive force in the conductor. An induction coil is closely related to electromagnets in structure, but used for a different purpose—to store energy in a magnetic field.

Smaller inductors used for very high frequencies are sometimes made with a wire passing through a ferrite cylinder or bead.


In 1885, William Stanley, Jr built the first practical induction coil based on Lucien Gaulard and Josiah Willard Gibbs' idea. It was the precursor of the modern transformer.

See also

Electricity, Electronics, Capacitor, Transformer


coil, induction coil, choke, reactor