The oil drop experiment proved that the electric fundamental charge exists and that it is quantized. It is also referred to as the Millikan oil drop experiment after the American physicist Robert A. Millikan.
Some researchers did not believe that subatomic particles, such as electrons, actually existed. Others did believe but were unable to find out what the charge on an electron was. Millikan, along with his graduate student Harvey Fletcher, devised an experiment. They injected oil drops into an ionized chamber and then measured the charge that appeared on the drops as they fell. They found that charge appears in discrete units, which means that it is quantized. Their experiments showed that the discrete units were in integer multiples of 1.592 x 10^-19 coulombs, which is very close to today’s fundamental measurement of 1.602 x 10^-19 coulombs.