A tiny spherical oil drop carrying a net charge $q$ is balanced in still air, with a vertical uniform electric field of strength $\frac{81}{7} \pi \times 10^5 \mathrm{~V} / \mathrm{m}$. When the field is switched OFF, the drop is observed to fall with terminal velocity $2 \times 10^{-3} \mathrm{~ms}^{-1}$. Here $g=9.8 \mathrm{~m} / \mathrm{s}^2$, viscosity of air is $1.8 \times 10^{-5} \mathrm{Ns} / \mathrm{m}^2$ and density of oil is $900 \mathrm{~kg} \mathrm{~m}^{-3}$. The magnitude of $q$ is