Robert A. Millikan won the 1923 Nobel Prize in Physics for his work on the elementary charge of electricity and the photoelectric effect.
Millikan measured the charge of the electron through his famous oil-drop experiment. Tiny charged oil droplets were suspended between electrically charged plates, allowing the electric force to be compared with gravity. The results showed that charge occurs in discrete units and provided a value for the elementary charge.
His prize also recognized experimental work on the photoelectric effect. Millikan’s measurements supported Einstein’s equation for the effect, even though he initially approached Einstein’s light-quantum explanation cautiously.
The oil-drop experiment did not discover the electron itself. J. J. Thomson had established the electron’s existence and measured its charge-to-mass ratio earlier. Millikan’s work supplied a precise measurement of the elementary charge, a fundamental constant used throughout physics.