Robert Millikan

The Charge of an Electron (Robert Millikan)

Between 1908 and 1917, Robert Millikan measured the charge on an electron with the apparatus shown below. In these experiments, the atomizer from a perfume bottle was used to spray water or oil droplets into a sample chamber. Some of these droplets fell through a pinhole between two plates of an electric field, where they could be observed through a microscope.

A source of x-rays was then used to ionize the air in the chamber by removing electrons from the molecules in the air. Droplets that did not capture one of these electrons fell to the bottom of the chamber due to the force of gravity. Droplets that captured one or more electrons were attracted to the positive plate at the top of the viewing chamber and either fell more slowly or rose toward the top.

By carefully studying individual droplets, Millikan was able to show that the charge on a drop was always an integral multiple of a small, but finite value. When his data are converted to SI units, the charge on a drop is always some multiple of 1.59 x 10^-19 C. Combining this value for the charge on a single electron with the charge to mass ratio for the electron confirms Thomson's hypothesis. The mass of an electron is at least 1000 times smaller than the lightest atom.