He spent much time preparing textbooks and simplifying the teaching of physics. He was author or co-author of many books; A College Course in Physics, with S.W. Stratton (1898);Mechanics, Molecular Physics, and Heat (1902); The Theory of Optics, with C.R. Mann translated from the German (1903); A First Course in Physics, with H.G. Gale (1906); A Laboratory Course in Physics for Secondary Schools, with H.G. Gale (1907); Electricity, Sound, and Light, with J. Mills (1908); Practical Physics - revision of A First Course(1920); The Electron(1917; rev. eds. 1924, 1935).
As a scientist, Millikan made numerous momentous discoveries, chiefly in the fields of electricity, optics, and molecular physics and he made numerous contributions to scientific journals.
He verified experimentally Einstein's all-important photoelectric equation, and made the first direct photoelectric determination of Planck's constant h in 1912-1915.
During World War I, Millikan was Vice-Chairman of the National Research Council, and played a major part in developing anti-submarine and meteorological devices.
In 1921, he was appointed Director of the Norman Bridge Laboratory of Physics at the California Institute of Technology, Pasadena.
Millikan received numerous awards and prizes like the Comstock Prize of the National Academy of Sciences, the Edison Medal of the American Institute of Electrical Engineers, the Hughes Medal of the Royal Society of Great Britain, the Nobel Prize for Physics 1923.
His most famous success was the falling-drop method, also known as the Oil Drop.