An American experimental physicist, Robert Millikan graduated from Oberlin College in 1891 and received his M.A. there in 1893. He earned his Ph.D. from Columbia University in 1895. One year later, Millikan joined the faculty at the University of Chicago and remained there until 1921, with the exception of the time he spent in government and military service during World War I. From Chicago Millikan went to the California Institute of Technology, where he spent the rest of his career. Millikan made the first determination of the charge of the electron and of Planck's constant. He was awarded the 1923 Nobel Prize in physics for these contributions. The determination of the charge on the electron proved experimentally that electrons are particles of electricity. Millikan accomplished this feat by designing an experiment studying the fall of oil droplets in an electric field. He conjectured that the droplets would take up integral multiples of electrical charge. By measuring the strength of the field required to counteract the gravitational force on the droplets, he was able to compute a highly accurate unit charge for the particle. Millikan also studied the photoelectric effect experimentally in 1916, confirming Albert Einstein's equation relating the kinetic energy of a particle emitted by incident radiation to the frequency of that radiation. Until his retirement, Millikan studied cosmic rays and the ultraviolet spectra of many elements.