$\frac{\mathrm{B}}{\mathrm{M}}=\left(\frac{\mu_{0} \mathrm{N} \mathrm{I}}{2 \mathrm{R}}\right)\left(\frac{1}{\mathrm{N} \mathrm{l} \pi \mathrm{R}^{2}}\right)=\frac{\mu_{0}}{2 \pi \mathrm{R}^{3}}$
$\frac{B}{M} \propto \frac{1}{R^{3}} \quad$ (not depend on $I$)
