Problem 3.332

A small cylindrical magnet $M$ (Fig. 3.95 ) is placed in the centre of a thin coil of radius $a$ consisting of $N$ turns. The coil is connected to a ballistic galvanometer. The active resistance of the whole circuit is equal to $R$. Find the magnetic moment of the magnet if its removal from the coil results in a charge $q$ flowing through the galvanometer.