Magnetism and Matter

Bending does not change the pole strength $m$; it changes the distance between the poles. The two end poles are now separated by the chord of two arms of length $\frac{L}{2}$ at $60^{\circ }$: $2\times \frac{L}{2}\sin 30^{\circ }=\frac{L}{2}$. So $M^{\prime }=m\times \frac{L}{2}=\frac{M}{2}$.