Instead, if a medium of relative permeability mr had been present,
the mutual inductance would be
M =mr m0 n1n2p r2
1 l
It is also important to know that the mutual inductance of a pair of
coils, solenoids, etc , depends on their separation as well as their relative
orientation Example 6 8 Two concentric circular coils, one of small radius r1 and
the other of large radius r2, such that r1 << r2,  are placed co-axially
with centres coinciding