An electric current i flowing round a circuit produces a magnetic field and hence a magnetic flux Φ through the circuit. The ratio of the magnetic flux to the current is called the inductance, or more accurately self-inductance of the circuit. The term was coined by Oliver Heaviside in February 1886. It is customary to use the symbol L for inductance, possibly in honour of the physicist Heinrich Lenz. The quantitative definition of the inductance is therefore
It follows that the SI units for inductance are webers per ampere. In honour of Joseph Henry, the unit of inductance has been given the name henry (H): 1H = 1Wb/A.
In the above definition, the magnet flux φ is that caused by the current flowing through the circuit concerned. There may, however, be contributions from other circuits. Consider for example two circuits C1, C2, carrying the currents i1, i2. The magnetic fluxes Φ1 and Φ2 in C1 and C2, repectively, are given by
According to the above definition, L11 an L22 are the self-inductances of C1 and C2, repectively. It can be shown (see below) that the other two coefficients are equal: L12 = L21 = M, where M is called the mutual inductance of the pair or circuits.