Robert Andrews Millikan was an American experimental physicist and Nobel laureate in physics for his measurement of the charge on the electron and for his work on the photoelectric effect. He served as president of Caltech from 1921 to 1945. 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.

When Einstein published his seminal 1905 paper on the particle theory of light, Millikan was convinced that it had to be wrong, because of the vast body of evidence that had already shown that light was a wave. He undertook a decade-long experimental program to test Einstein's theory, which required building what he described as "a machine shop in vacuo" in order to prepare the very clean metal surface of the photo electrode. His results confirmed Einstein's predictions in every detail, but Millikan was not convinced of Einstein's radical interpretation, and as late as 1916 he wrote, "Einstein's photoelectric equation... cannot in my judgment be looked upon at present as resting upon any sort of a satisfactory theoretical foundation," even though "it actually represents very accurately the behavior" of the photoelectric effect. In his 1950 autobiography, however, he simply declared that his work "scarcely permits of any other interpretation than that which Einstein had originally suggested, namely that of the semi-corpuscular or photon theory of light itself."

Starting in 1909, while a professor at the University of Chicago, Millikan and Harvey Fletcher worked on an oil-drop experiment (since repeated, with varying degrees of success, by generations of physics students) in which they measured the charge on a single electron. Professor Millikan took sole credit, in return for Fletcher claiming full authorship on a related result for his dissertation. Millikan went on to win the 1923 Nobel Prize for Physics, in part for this work, and Fletcher kept the agreement a secret until his death. After a publication on his first results in 1910, contradictory observations by Felix Ehrenhaft started a controversy between the two physicists. After improving his setup he published his seminal study in 1913.

The elementary charge is one of the fundamental physical constants and accurate knowledge of its value is of great importance. His experiment measured the force on tiny charged droplets of oil suspended against gravity between two metal electrodes. Knowing the electric field, the charge on the droplet could be determined. Repeating the experiment for many droplets, Millikan showed that the results could be explained as integer multiples of a common value (1.592 × 10-19 coulomb), the charge on a single electron. That this is somewhat lower than the modern value of 1.602 176 53(14) x 10-19 coulomb is probably due to Millikan's use of an inaccurate value for the viscosity of air.

Sources

• Cleveland, Cutler (Lead Author); Peter Saundry (Topic Editor). 2007. Millikan, Robert Andrews. In: Encyclopedia of Earth. Eds. Cutler J. Cleveland (Washington, D.C.: Environmental Information Coalition, National Council for Science and the Environment). [Published in the Encyclopedia of Earth March 19, 2007; Retrieved October 16, 2008].
• Wikipedia Contributors, Robert Andrews Millikan, Wikipedia, Accessed 16 October 2008.