Robert Andrews Millikan (1868–1953), an American physicist, was awarded the 1923 Nobel Prize in Physics for his measurement of the charge on the electron and for his work on the photoelectric effect. Millikan's earliest major success was the accurate determination of the charge carried by an electron, using the elegant "falling-drop method". He also proved that this quantity was a constant for all electrons in 1910, thus demonstrating the atomic structure of electricity. Millikan verified experimentally Einstein’s essential photoelectric equation, and made the first direct photoelectric determination of Planck's constant, 'h' (1912-1915). Millikan conducted important studies of cosmic rays (which he named), X-rays, and physical and electric constants.