Two coaxial solenoids $1$ and $2$ of the same length are set so that one is inside the other. The number of turns per unit length are ${n_1}$ and ${n_2}$. The currents ${i_1}$ and ${i_2}$ are flowing in opposite directions. The magnetic field inside the inner coil is zero. This is possible when
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(d) ${B_{net}} = {B_1} - {B_2} \Rightarrow {B_1} - {B_2} = 0 \Rightarrow {B_1} = {B_2}$
$ \Rightarrow B \propto ni.$

So ${n_1}{i_1} = {n_2}{i_2}$ or ${n_1} = {n_2}{\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\text{and}}\,\,\,{i_1} = {i_2}$

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