In the early $1900$s$,$ Robert Millikan used small charged droplets of oil, suspended in an electric field, to make the first quantitative measurements of the electron$$s charge. A $0\mathrm{.}70-$m$-$diameter droplet of oil, having a charge of $+$e$,$ is suspended in midair between two horizontal plates of a parallel$-$plate capacitor. The upward electric force on the droplet is exactly balanced by the downward force of gravity. The oil has a density of $860$ kg$/$m$3,$ and the capacitor plates are $5\mathrm{.}5$ mm apart.