Thomson, Sir J(oseph) J(ohn), also called JJ 1856-1940
English physicist, discoverer of the electron and Nobel Prize winner

He was born in Cheetham Hill near Manchester, the son of a Scottish bookseller. He went to Owen's College, Manchester, at the age of 14 with the intention of becoming a railway engineer, but a scholarship took him to Trinity College, Cambridge, where he graduated Second Wrangler. In 1884 he was elected FRS and succeeded Lord Rayleigh as Cavendish Professor of Experimental Physics, and in 1919 he was himself succeeded by his brilliant student, Ernest Rutherford. Thomson's early theoretical work was concerned with the extension of James Clerk Maxwell's electromagnetic theories. This led to the study of gaseous conductors of electricity and in particular the nature of cathode rays. Using Wilhelm R�ntgen's discovery of X-Rays (1895), he showed that cathode rays were rapidly-moving particles, and by measuring their speed and specific charge, the latter by two independent methods, he deduced that these 'corpuscles' (electrons) must be nearly 2,000 times smaller in mass than the lightest known atomic particle, the hydrogen ion. This, the greatest revolution in physics since Sir Isaac Newton, was inaugurated by his lecture to the Royal Institution (1897) and published in the Philosophical Magazine. Thomson successfully studied the nature of positive rays (1911), and this work was crowned by the discovery of isotopes, which he demonstrated could be separated by deflecting positive rays in electric and magnetic fields - mass spectrometry. During World War I he was engaged in admiralty research and helped to found the Department of Scientific and Industrial Research. He made the Cavendish Laboratory the greatest research institution in the world. Although simplicity of apparatus was carried to 'string and sealing wax' extremes, seven of his research assistants subsequently won the Nobel Prize, including Niels Bohr and Ernest Rutherford. He was awarded the Nobel Prize for physics (1906), was knighted in 1908, and was the first scientist to become Master of Trinity College (1918-40). In 1936 he published Recollections and Reflections. He was one of the pioneers of nuclear physics.

Bibliography: G P Thomson, J J Thomson and the Cavendish Laboratory in his Day (1964)