\({B_1} = \frac{{{\mu _o}}}{{4\pi }}.\frac{{2M}}{{{l^3}}}\)
Flux linked with coil \((2)\) \({\phi _2} = {B_1}{A_2} = \frac{{{\mu _o}}}{{4\pi }}\frac{{2i\;(\pi {a^2})}}{{{l^3}}} \times (\pi {a^2})\)
Also \({\phi _2} = Mi\) \( \Rightarrow M = \frac{{{\mu _o}\pi {a^4}}}{{2{l^3}}}\)
(${e}^{-1}=0.37$ લો)