\(\mathrm{M}=\frac{\mathrm{N}_{2} \mathrm{~B}_{1} \mathrm{~A}_{2}}{\mathrm{I}_{1}}\)
\(\mathrm{M} \propto \mathrm{B}_{1} \mathrm{~A}_{2}\)
\(\mathrm{M} \propto \frac{\mu_{0} \mathrm{I}_{1}}{2 \pi \mathrm{R}_{1}} \times \pi \mathrm{R}_{2}^{2}\)
\(\mathrm{M} \propto \frac{\mathrm{R}_{2}^{2}}{\mathrm{R}_{1}}\)