Thomson, Sir Joseph John, 1856–1940, English physicist. From 1884 to 1919 he was Cavendish professor of experimental physics at Cambridge. J. J. Thomson was one of the founders of modern physics. Winner of the 1906 Nobel Prize in Physics for his study of conduction of electricity through gases, he is known also for his discovery (1897) of the electron and his investigation of its charge and mass, his development of the mathematical theory of electricity and magnetism, and his work with "positive rays" (positive ion beams), which led to a means of separating atoms and molecules according to their atomic weights. His work with F. W. Aston gave evidence of the existence of isotopes of neon; Aston was later able to show that most chemical elements have two or more different isotopes. In addition to his own research, Thomson made a significant contribution during his long tenure as director of the Cavendish Laboratory in making it a leading center for atomic research where many important developments in modern physics occurred. He was knighted (1908), served (1915–20) as president of the Royal Society, and was master of Trinity College, Cambridge, from 1918 until his death. He is buried in Westminster Abbey. His works include Elements of the Mathematical Theory of Electricity and Magnetism (1895, 5th ed. 1921), Conduction of Electricity through Gases (1903; 3d ed., with George Paget Thomson, 2 vol., 1928–33), and an autobiography, Recollections and Reflections (1936).
See biography by R. J. Rayleigh (1942); Sir George Paget Thomson, J. J. Thomson and the Cavendish Laboratory in His Day (1965).
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