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Summary Article: Millikan, Robert Andrews (1868-1953) from The Hutchinson Dictionary of Scientific Biography

Place: United States of America

Subject: biography, physics

US physicist who made the first determination of the charge of the electron and of Planck's constant. For these achievements, he was awarded the 1923 Nobel Prize for Physics.

Millikan was born in Morrison, Illinois, on 22 March 1868. He showed no interest in science as a child and became interested in physics only after entering Oberlin College in 1886, where he obtained his BA in 1891 and MA in 1893. Millikan then went to Columbia University to continue his studies in physics, gaining his PhD in 1895. He then went to Germany to study with Max Planck at Berlin and Hermann Nernst at Göttingen before taking up an assistantship in physics at the University of Chicago in 1896.

Millikan remained at Chicago until 1921. At first he concentrated on teaching but from 1907, when he became associate professor of physics, he took more interest in research and began his experiments to find the electronic charge, completing this work in 1913. In 1910, Millikan became professor of physics at Chicago and during World War I was director of research for the National Research Council, which was concerned with defence research. In 1921, Millikan moved to Pasadena to become director of the Norman Bridge Laboratory at the California Institute of Technology. He retained this position until 1945, when he retired. In addition to the 1923 Nobel Prize for Physics, Millikan received many honours, including the award of the Royal Society's Hughes Medal in 1923 and the presidency of the American Physical Society 1916-18. Millikan died in Pasadena on 19 December 1953.

Millikan employed a method to determine the electronic charge that was simple in concept but difficult in practice, and a satisfactory result took him the five years 1908-13 to achieve. He began by studying the rate of fall of water droplets under the influence of an electric field. Millikan conjectured correctly that the droplets would take up integral multiples of the electronic charge, which he would be able to compute from the strength of field required to counteract the gravitational force on the droplets. By 1909, Millikan had arrived at an approximate value for the electronic charge. However, the droplets evaporated too quickly to make precise determination possible and Millikan switched to oil droplets. These were far less volatile and furthermore Millikan was able to irradiate the suspended droplets to vary their charge. In 1913, Millikan finally announced a highly accurate value for the electronic charge that was not bettered for many years.

Millikan also worked on a study of the photoelectric effect during this period, investigating the interpretation of Albert Einstein that the kinetic energy of an electron emitted by incident radiation is proportional to the frequency of the radiation multiplied by Planck's constant. Millikan took great pains to improve the sensitivity of his apparatus and announced in 1916 that Einstein's equation was valid, thereby obtaining an accurate value for Planck's constant.

After World War I, Millikan moved into two new areas of research. In the 1920s, he investigated the ultraviolet spectra of many elements, extending the frequency range and identifying many new lines. Millikan also undertook a thorough programme of research into cosmic rays, a term that he coined in 1925, when he proved that the rays do come from space. Millikan did this by comparing the intensity of ionization in two lakes at different altitudes. He found that the intensity was the same at different depths, the absorptive power of the difference in the depth of water being equal to the absorptive power of the depth of atmosphere between the two altitudes. This proved that the rays producing the ionization must have passed through the atmosphere from above and could not have a terrestrial origin.

Millikan went on to assert that cosmic rays were electromagnetic waves, a theory disproved by Arthur Compton in 1934, when he demonstrated that they consist of charged particles. However, in the course of his research, Millikan directed Carl Anderson to study cosmic rays in a cloud chamber and, as a result, Anderson discovered the positron.

Millikan's achievements are of fundamental value in the history of science. His determination of the charge on the electron was very important because it proved experimentally that electrons are particles of electricity, while the determination of Planck's constant was vital to the development of quantum theory.

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