Question

Write the principle of Millikan's oil drop experiment.

Answer 1

Millikan's experiment is used for the measurement of charge of an electron.
This method is based on the study of the motion of uncharged oil drop under free fail due to gravity and charged oil drop in a uniform electric field.
By adjusting uniform electric field suitably; a charged oil drop can be made to move up or down or even kept balanced in the field of view for sufficiently long time and as series of observations can be made.