Define mutual inductance between a pair of coil. On what factors does it depend ?

Explanation

The phenomenon in which a change of current in one coil induces an emf in a neighbouring coil is called mutual induction. Mutual inductance between a pair of coils is numerically equal to the emf induced in the secondary coil when the current through the primary coil changes at unit rate.
Mutual inductance depends on,
(i) the number of turns in both the coils
(ii) The are of the inner coil,
(iii) the length of primary coil and
(iv) the magnetic permeability of the core on which the primary is would.
`M=(mu_0mu_rN_1N_2A)/l`