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Q.
Rate of change of torque $\tau$ with deflection $\theta$ is maximum for a magnet suspended freely in a uniform magnetic field of induction $B$, when
Magnetism and Matter
Solution:
$\tau=M B \sin \theta$
or $\frac{d \tau}{d \theta}=M B \cos \theta$
This will be maximum when $\theta=0^{\circ}$