Two small spheres with mass $m_1$ and $m_2$ hang from massless insulating threads of length $l_1$ and $l_2.$ The two spheres carry charges $q_1$ and $q_2$, respectively. The spheres are hung such that they are on the same horizontal level and the threads are inclined to the vertical at angles ${\theta }_1$ and ${\theta }_2.$ Which of the condition is required if ${\theta }_1={\theta }_2?$