(1868-1953) US physicist, awarded a Nobel Prize 1923 for his determination of Planck's constant (a fundamental unit of quantum theory) 1916 and the electric charge on an electron 1913.
His experiment to determine the electronic charge, which took five years to perfect, involved observing oil droplets, charged by external radiation, falling under gravity between two horizontal metal plates connected to a high-voltage supply. By varying the voltage, he was able to make the electrostatic field between the plates balance the gravitational field so that some droplets became stationary and floated. If a droplet of weight W is held stationary between plates separated by a distance d and carrying a potential difference V, the charge, e, on the drop is equal to Wd/V.
Millikan was born in Illinois and studied at Oberlin College and Columbia University, and in Germany with Max Planck at Berlin and Hermann Nernst at Göttingen. He worked at the University of Chicago 1896–21, becoming professor 1910, and at the California Institute of Technology 1921–45.
Millikan also carried out research into cosmic rays, a term that he coined in 1925, when he proved that the rays do come from space.