The Oil Drop Experiment

Between 1909 and 1910 the American physicist Robert Millikan conducted a series of oil-drop experiments. By comparing applied electric force with changes in the motion of the oil drops, he was able to determine the electric charge on each drop. He adjusted the charge on the plates to counteract the force of gravity. Then Millikan compared the mass of the drop to the charge required to halt the fall. Through this study he found that all of the drops had charges that were simple multiples of a single number, the fundamental charge of the electron.