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Millikan, Robert Andrews

charge physics rays electron

(1868–1953) US physicist: determined e and h accurately for the first time.

Millikan was the son of a Congregational minister and small farmer and grew up in the still romantic age of the American midwest. His talent at school was mainly in classics, and he did little physics; but in his second year at Oberlin college he was invited to teach elementary physics and was told ‘anyone who can do well in Greek can teach physics’. He learned quickly and was soon immersed in the subject, which was not then much developed in the USA. Then he went to Columbia, where he was the sole graduate student in physics there in 1893–5, working at the US Mint on the polarization of light emitted from the incandescent surface of fused gold and silver. Later he studied in Germany, and in 1896 was offered a job in Chicago . He took it, and was there until he went to the California Institute of Technology in 1921.

Between 1909 and 1913 Millikan determined the charge on an electron with considerable accuracy, not surpassed until 1928. Between two horizontal plates a cloud of fine oil droplets was introduced and irradiated with X-rays so as to introduce varying amounts of charge on some of them. By adjusting the voltage on the plates, electric force and buoyancy could be made to just counterbalance gravity for an oil-drop viewed by a microscope. Calculation then revealed its charge; a long series of measurements showed that the measured charge always occurred in multiples of a single value, the charge ( e ) on a single electron. Millikan was lucky to use a field strength (about 6000 V cm –1 ) within the narrow range in which the experiment is possible.

He then studied the photoelectric effect (1912–16) confirming deduction of 1905 that the energy E of an electron emitted from a metal by light is given by E = hvE o where v is the frequency of the incident radiation, E o is the energy required to leave the metal (the work function) and h is constant. For his accurate measurements of e and h Millikan was awarded the 1923 Nobel Prize for physics.

During the 1920s Millikan researched on cosmic rays, showing in 1925 that they come from space. Millikan argued that they are uncharged and consist of electromagnetic radiation, but showed them to consist of particles. However, Millikan was responsible for directing to view cosmic rays in a Wilson cloud chamber, which led to Anderson’s discovery of the positron.

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