What did Robert Millikan discover

Millikan discovered the charge of an electron for which he received the Noble Prize. He designed the Oil Drop experiment. He found that charge is quantized.

Millikan repeated the experiment with varying strength of xrays and found the charge on the oil drop to be a multiple of - 1.6 x 10-19 C. He concluded that the smallest unit of charge is - 1.6 x 10-19 C, which is the charge of one electron.

Starting in 1908, while a professor at the University of Chicago, Millikan worked on an oil-drop experiment in which he measured the charge on a single electron. J. J. Thomson had already discovered the charge-to-mass ratio of the electron.