Question

explain millikan's oil drop experiment

Answer 1

it measured the charge of an electron before this experiment , existence of subatomic particles was not accepted. 
they suspended tiny charged oil droplets between two metal electrodes by balancing downward gravitational force with upward drag and electric forces when the charged drop fells at a constant rate ,the gravitational and electric forces on it were equal .
charge of an electron was measured by 
q.E=m.g
q=m.g/E
charge was always multiple of -1.6*10^-19C ,the charge on a single electron

Answer 2

$\textbf{Answer is in Attachment !}$