A ball of mass $m$ is suspended from a thin horizontal rod by a thread. When the ball is at rest, the rod is moved with a constant horizontal velocity perpendicular to its length so that the ball just completes vertical circular motion about the rod. In subsequent motion of the ball, tension in the string varies between  $T_1$ at the bottom to $T_2$  at horizontal level of the string. Find $\frac{T_1}{T_2}$.