However, the relation is far more general Note that if the inner solenoid
was much shorter than (and placed well inside) the outer solenoid, then
we could still have calculated the flux linkage N1F1 because the inner
solenoid is effectively immersed in a uniform magnetic field due to the
outer solenoid In this case, the calculation of M12 would be easy However,
it would be extremely difficult to calculate the flux linkage with the outer
solenoid as the magnetic field due to the inner solenoid would vary across
the length as well as cross section of the outer solenoid