The Millikan oil-drop experiment enabled the charge on the electron to be deterined. Two parallel metal plates P and Q are situated in a vaccum. The plates are horizontal and separated by a distance of 5.4 mm, as illustrated in fig.
The lower plate P is earthed. The potential difference between the plates can be varied. An oil droplet of mass `7.7 xx 10^-15` kg is oserved to remain stationary between the plates , when palte Q is a t a potential of +850 suggest why plates p and Q must be parallel and horizontal and calcualte the charge with its gin, on the oil droplet.