A steel wire of length l has a magnetic moment M It is then bent into a semicircular arc. The new magnetic moment is

Poles strength,
$\mathrm{m}=\frac{\mathrm{magnetic} \mathrm{moment}}{\mathrm{seperation} \mathrm{between} \mathrm{poles}}=\frac{\mathrm{M}}{\mathrm{l}}$
when the wire is bent, the pole strength remains the same. In semi-circular arc, the separation between poles is 2r, where r is the radius of semi-circle. Given $\mathrm{\pi }$r = I New magnetic moment, M' = m.2r = m(2l/$\mathrm{\pi }$)
$\therefore {\mathrm{M}}^{\mathrm{\prime }}=\left(\frac{\mathrm{M}}{\mathrm{l}}\right)\times \left(\frac{2\mathrm{l}}{\mathrm{\pi }}\right)=\frac{2\mathrm{M}}{\mathrm{\pi }}$