A tiny spherical oil drop carrying a net charge q is balanced in still air with a vertical uniform electric field of strength $\frac{81\mathrm{\pi }}{7}\times {10}^5{\mathrm{Vm}}^{-1}$. When the field is switched off, the drop is observed to fall with terminal velocity $2\times {10}^{-3}{\mathrm{ms}}^{-1}$. Given $\mathrm{g}=9.8{\mathrm{ms}}^{-2}$, viscosity of the air $=1.8\times {10}^{-5}{\mathrm{Nsm}}^{-2}$ and the density of oil $=900{ \mathrm{kg} \mathrm{m}}^{-1}$, the magnitude of q is $x \mathrm{x} {10}^{-19}\mathrm{C}.$ The value of $x$ is___