In Millikan's experiment, an oil drop of radius $1\mathrm{.}82m$ and density $0\mathrm{.}866\frac{g}{c}{m}^3$ is suspended in chamber $C($see the figure$)$ when a

downward electric field of $2\mathrm{.}23{10}^5\frac{N}{C}$ is applied. Find the charge on the drop, in terms of $e.$