Problem 4.81

A conductor in the shape of a square frame with side $a$ suspended by an elastic thread is located in a uniform horizontal magnetic field with induction $B$. In equilibrium the plane of the frame is parallel to the vector B (Fig. 4.25). Having been displaced from the equilibrium position, the frame performs small oscillations about a vertical axis passing through its centre. The moment of inertia of the frame relative to that axis is equal to $I,$ its electric resistance is $R$. Neglecting the inductance of the frame, find the time interval after which the amplitude of the frame's deviation angle decreases e-fold.