J. J. Thomson

Joseph John Thomson was born in Manchester on 18 December 1856, the son of a bookseller who died when the boy was sixteen. A paid engineering apprenticeship was planned for him; the money failed, so at fourteen he entered Owens College and at twenty won a scholarship to Trinity College, Cambridge. In 1884, to general surprise, the twenty-eight-year-old was chosen Cavendish Professor of Experimental Physics over older candidates. He was famously clumsy with his hands; his students built the apparatus while he supplied the questions. It proved a fair division of genius.

In the 1890s the Cavendish laboratory turned to cathode rays — the ghostly green beam that flies through a glass tube pumped almost empty when electricity is passed. German physicists insisted the rays were waves; Hertz had failed to bend them with an electric field. Thomson’s tubes, with far better vacuum, bent: the rays carried negative charge. Then, in 1897, came the measurement. Deflecting the rays with electricity and magnetism together, he worked out the ratio of their charge to their mass — and the answer barely changed whatever the cathode was made of, whatever gas filled the tube. The rays were universal, and their particles were about a thousand times lighter than the smallest atom, hydrogen. On 30 April 1897 he announced that this was matter in a new state, the substance from which all the chemical elements are built; the world settled on the name the electron, coined earlier by George Johnstone Stoney.

The atom — thought indivisible since Dalton — had parts. Thomson pictured it as a sphere of positive charge with electrons set in it like plums in a pudding, and the plum-pudding model entered the textbooks. It was wrong, and its undoing was the work of his own student: in 1911 Ernest Rutherford’s gold-foil experiment revealed a tiny nucleus, and Thomson, to his credit, welcomed the correction. He won the 1906 Nobel Prize in Physics for his studies of electricity in gases and was knighted in 1908. In 1913 he caught neon splitting into two masses, neon-20 and neon-22 — the first sighting of isotopes, a field his student Francis Aston would map. Under him the Cavendish became the nursery of modern physics: Rutherford, Wilson, Aston and others passed through its benches. His own son, G. P. Thomson, won the 1937 Nobel Prize for showing that the electron spreads as a wave — the father proved it a particle, the son proved it a wave. J. J. died in 1940, and rests in Westminster Abbey near Newton.