Sir Joseph John Thomson

Sir Joseph John Thomson, b. Dec. 18, 1856, d. Aug. 30, 1940, is universally recognized as the British scientist who discovered and identified the electron. At the age of 27 he succeeded (1884) Lord Rayleigh as professor of physics at Trinity College, Cambridge, and was named director of its Cavendish Laboratory in the same year, continuing in that position until 1919.

Thomson demonstrated (1897) that cathode rays were actually units of electrical current made up of negatively charged particles of subatomic size. He believed them to be an integral part of all matter and theorized a model of atomic structure in which a quantity of negatively charged electrons was embedded in a sphere of positive electricity, the two charges neutralizing each other. For these investigations he won (1906) the Nobel Prize for physics; in 1908 he was knighted.

Subsequently, Thomson turned his attention to positively charged ions. His research showed that neon gas was made up of a combination of two different types of ions, each with a different charge, or mass, or both. He did this by using magnetic and electric fields to deflect the stream of positive ions of neon gas onto two different parts of a photographic plate. This demonstration clearly pointed to the possibility that ordinary elements might exist as isotopes (varieties of atoms of the same element, which have the same atomic number but differ in mass).

Thomson was a highly gifted teacher--seven of his research assistants as well as his son, George, won Nobel Prizes for physics--and he led Great Britain to dominance in the field of subatomic particles in the early decades of the 20th century. He was accorded the honor of burial in Westminster Abbey.

James A. Booth


Bibliography: Rayleigh, Robert J., The Life of Sir J. J. Thomson (1942; repr. 1969); Thomson, George Paget, J. J. Thomson and the Cavendish Laboratory in His Day (1965).

Last modified on: Thursday, October 30, 1997.