Two small spheres of masses $M_1$ and $M_2$ are suspended by weightless insulating threads of lengths $L_1$ and $L_2$. The spheres carry charges of $Q_1$ and $Q_2$ respectively. The spheres are suspended such that they are level with one another, and the threads are inclined to the vertical at angles of $\theta_1$ and $\theta_2$ as shown. Which one of the following conditions is essential if $\theta_1 = \theta_2$?