In 1923, Robert Millikan won the Nobel Prize for discovering the mass of the ....

electron?

Charge on the electron

http://www.slac.stanford.edu/library/nobel/nobel1923.html

That's correct! In 1923, Robert Millikan won the Nobel Prize in Physics for his pioneering work in measuring the elementary electric charge and determining the mass of the electron. Millikan's famous oil drop experiment helped to determine the charge of an electron by observing the motion of charged oil droplets in an electric field. By carefully measuring the droplets' motion, he was able to calculate the charge-to-mass ratio of the electron. With this data and other known constants, he was able to determine the actual mass of an electron. This groundbreaking achievement opened new windows into the understanding of atomic structure and laid the foundation for many future advancements in physics and chemistry.