inductance symbol L. SI unit henry (H). An inductor is a coil of conductor. When connected in a circuit, it opposes the variation of current in the circuit. The current in the coil produces a magnetic flux. The magnetic flux changes when the current in the circuit varies, producing a self induced emf, e in the coil. As the emf is proportional to the rate of change of current,

e = -L (di / dt)

where L is a constant of the coil called the inductance (or self inductance). The inductance of a coil is 1 henry when a varying current of 1 ampere per second induces an emf of Ivolt.

Mutual inductance : When the flux from one coil (called primary) threads through another coil (called secondary), an emf can be induced by varying the current in the primary. The induced emf, es in the secondary is proportional to the rate of change in the current in the primary,

es = M (dip / dt )

where M is a constant called mutual inductance of the two coil system. The value of M remains the same if the role of primary and secondary are interchanged.