The English physicist Sir Joseph John Thomson (1856-1940) is credited with the discovery of the electron.

On Dec. 18, 1856, J. J. Thomson was born at Cheetham Hill near Manchester. His father, a bookseller and publisher, planned a career in engineering for Joseph, but since no apprenticeship could be found for him in any engineering firm, he was sent "temporarily" to college in Manchester at the age of 14. As a result of his ability and determination, he won a scholarship in 1876 and entered Trinity College, Cambridge; he remained there for the rest of his life.

After graduation Thomson began working in the Cavendish Laboratory, which was under the direction of Lord Rayleigh. Thomson's brilliance brought him membership in the Royal Society at 27 and his appointment as Rayleigh's successor at 28. He proved to be inspiring and effective both as a teacher and as a research director, and as time passed, students came to him from all over the world. He sometimes had as many as 40 to advise at once, and for the first quarter of the 20th century the Cavendish Laboratory, where Thomson insisted that theory should be considered "a policy, not a creed," was the world center for particle research.

Thomson began his studies of the properties of "cathode rays" in 1894 and proved in 1895 that they carried a negative charge. In 1897 he passed the rays through a vacuum and showed that they are deflected in both magnetic and electric fields. He was thus able to determine the ratio of the charge to the mass of the supposed particles and showed that its mass was about 2,000 times larger than the mass of the hydrogen atom. The identification of the electron necessitated a revision of the atomic concept: Thomson visualized it as a mass of positively charged matter in which electrons were distributed like raisins in a cake.

About 1906 Thomson turned his attention to "positive rays"--positively charged ions. By 1912, using his deflection techniques and measuring the charge to mass ratio, he had shown that neon was a mixture of at least two kinds of atoms, with differing deflectibilities. Thomson had thus opened the door to the world of isotopes and had provided a beginning for the method of analysis now known as mass spectrography.

During his career Thomson published 13 books and over 200 papers. He was awarded the Nobel Prize in physics in 1906 and was knighted in 1908. In 1918 he abandoned research to become master of Trinity College, where he died on Aug. 30, 1940.

Further Reading

• Thomson's Recollections and Reflections (1936) is one of the notable scientific autobiographies. A full-length biography is Robert J. S. Rayleigh, The Life of J. J. Thomson (1942). The sketch in James G. Crowther, British Scientists of the Twentieth Century (1952), is excellent.