Robert Millikan performed the oil drop experiment to determine the charge of an electron. He did so by balancing the gravitational and electrical forces acting on a charged oil droplet so that it remained suspended in mid-air, and calculated the charge present on the oil droplet.
Robert Millikan received the Nobel Prize for quantifying the magnitude of the charge of an electron. He did this with his oil drop experiment. Millikan sprayed charged oil droplets and allowed them to fall until they reached terminal velocity. Using their terminal velocity, he determined the mass of the oil droplets and was able to use the mass to determine the force of gravity acting on the droplets to pull them down. He then applied an electric voltage to the chamber through which the oil droplets were falling. Since the charged droplets were moving through an electric field, they experienced an electric force which pushed the droplets upwards. By adjusting the strength of the electric field, Millikan was able to balance the electric and the gravitational forces acting on the droplets such that they remained suspended in mid-air. He determined the charge on each droplet. He varied the charges on the droplets and found that all the droplets had a charge whose magnitude was always a multiple of a certain basic number. He concluded that this basic number must be the charge of one electron and that higher charges must result from the presence of more than one electron on the oil droplet.