Thomson, J(oseph) J(ohn) (1856–1940)

English physicist. He discovered the electron in 1897. His work inaugurated the electrical theory of the atom, and his elucidation of positive rays and their application to an analysis of neon led to the discovery of isotopes. He was awarded the Nobel Prize for Physics in 1906 for his theoretical and experimental work on the conduction of electricity by gases. He was knighted in 1908.

Using magnetic and electric fields to deflect positive rays, Thomson found in 1912 that ions of neon gas are deflected by different amounts, indicating that they consist of a mixture of ions with different charge-to-mass ratios. English chemist Frederick Soddy had earlier proposed the existence of isotopes and Thomson proved this idea correct when he identified, also in 1912, the isotope neon-22. This work was continued by his student Francis Aston.

Thomson was born near Manchester and studied there and at Cambridge, where he spent his entire career. As professor of experimental physics 1884–1918, he developed the Cavendish Laboratory into the world's leading centre for subatomic physics. His son was George Paget Thomson.

Investigating cathode rays, Thomson proved that they were particulate and found their charge-to-mass ratio to be constant and with a value nearly 1,000 times smaller than that obtained for hydrogen ions in liquid electrolysis. He also measured the charge of the cathode-ray particles and found it to be the same in the gaseous discharge as in electrolysis. Thus he demonstrated that cathode rays are fundamental, negatively charged particles; the term ‘electron’ was introduced later.