Millikan Oil Drop Experiment

A fine mist of oil drops is introduced into a chamber. The gas molecules inside the chamber are ionized by a beam of x-rays. The electrons adhere to the oil drops, some having one electron, others having 2 and so on. These negatively charged droplets fall under the force of gravity into the region between the electrically charged plates. By adjusting the voltage on the plates, the force of gravity on the droplet is exactly counterbalanced by the attraction of the negative droplet to the upper, positively charged plate. Analysis of these forces led to a value for the charge on the electron.