Robert Millikan
American Physician and chemist
His Experiment video
His contribution to the atomic theory.
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 electrons.