This is a good approximation keeping
in mind that the solenoid is long, implying l  >> r2 We now consider the reverse case A current I1 is passed through the
solenoid S1 and the flux linkage with coil S2 is,
N2F2 = M21 I1
(6 10)
M21 is called the mutual inductance of solenoid S2 with respect to
solenoid S1