THOMSON, JOSEPH JOHN

Joseph John Thomson is remembered for his recognition of the electron in 1897, which was the basis of elementary particle physics. But this was just one aspect of his prolific study of the relationship between the electromagnetic ether and matter: he was the first to suggest electromagnetic mass; his theory of gaseous discharge by ionization is still broadly accepted; his atomic theories succored both the nuclear atom and ideas of ionic bonding; his work on the structure of light expedited acceptance of the quantum theory of radiation in Britain. Finally Thomson was a leading spokesperson for science and a renowned teacher; his students held influential posts throughout the English-speaking world, and eight of them won Nobel Prizes.

Early Life

Joseph John Thomas was born December 18, 1856, at Cheetham Hill near Manchester, England. Thomson's father, Joseph James Thomson, was a Manchester bookseller; his mother, Emma Swindells, came from a textile manufacturing family. His younger brother, Frederick Vernon Thomson, joined a firm of calico merchants. As a child Thomson developed a lifelong interest in botany. His parents encouraged his scientific interests, and entered him at Owens College, Manchester, at the age of

fourteen, to begin engineering training. When his father died two years later, Thomson made his way by scholarships in mathematics and physics (taught by Thomas Barker and Balfour Stewart, respectively) at which he excelled.

In 1876 Thomson obtained a scholarship at Trinity College, Cambridge, to study mathematics. Cambridge mathematics at that time was dominated by an emphasis on physical analogies and a mechanical worldview, elucidated by analytical dynamics (the use of Lagrange's equations and Hamilton's principle of least action). Thomson's coach, Edward Routh, gave him a thorough grounding in these methods, and in 1880 he graduated as Second Wrangler (second place).

Thomson remained in Cambridge, working for a College Fellowship. He used analytical dynamics to explore Maxwell's electrodynamics, which he had encountered at Owens College, and then learnt from William Niven at Cambridge. In 1881 he showed that the mass of a charged particle increases as it moves, suggesting that the particle drags some ether with it. In 1882 he won Cambridge's Adams Prize for "A Treatise on Vortex Motion," which investigated the stability of interlocked vortex rings, and developed the then-popular idea that atoms were ethereal vortices into a theory that could account for the periodic table. This work laid the foundations of all his subsequent atomic models.

Thus Thomson was working in the mainstream of Cambridge mathematical physics. In college also he identified himself with Cambridge values and social mores. In 1884, his conventionality and scientific accomplishments established, Thomson was elected Cavendish Professor of Experimental Physics at Cambridge at the age of twenty-eight.

Cavendish Professorship

Thomson became, overnight, a leader of British science. He held an increasing number of positions in scientific administration, was on the Board for Invention and Research during World War 1, President of the Royal Society from 1915 to 1920, and from 1919 to 1927 was an active member of the Advisory Council to the Department for Scientific and Industrial Research. His social position was strengthened by his marriage, in 1890, to Rose Paget, daughter of Cambridge's Professor of Physic (that is, Medicine). He sent his children (George and Joan) to private schools and joined the Athenaeum and Saville clubs. He received a knighthood in 1908, the Order of Merit in 1912, and in 1918 was appointed Master of Trinity College, a Crown appointment.

Under Thomson's leadership the Cavendish Laboratory became a place of lively debate, at the forefront of physics, with a colloquium and a dynamic social life. But it was also a place of financial stringency where space and equipment were bitterly fought over.

Gaseous Discharge

As Cavendish Professor, Thomson had free choice of scientific direction and a duty to undertake experimental physics, which coincided with a realization of the limitations of analytical dynamics. He chose the academically unpopular subject of discharge of electricity through gases that appealed to him for its visual effects. By 1890 he had developed the concept of a discrete electric charge, modeled by the terminus of a vortex tube in the ether, which guided his later work.

The discovery of X rays in 1895 proved crucial for they ionized a gas in a controllable manner, allowing the effects of ionization and secondary radiation to be distinguished. Within a year, working with his student Ernest Rutherford, Thomson had convincing evidence for his theory of discharge by ionization of gas molecules.

X rays also rekindled interest in the cathode rays that caused them. With new confidence in his apparatus and theories, Thomson, in 1897, suggested that the properties of cathode rays could be explained by assuming that they were subatomic charged particles, which were a universal constituent of matter. He called these "corpuscles," but they soon became known as "electrons." Thomson unified his ionization and corpuscle ideas into a widely applicable theory of gaseous discharge for which he won the Nobel Prize in Physics in 1906.

Thomson next investigated the role of corpuscles in matter. His "plum pudding" atomic model, in which thousands of corpuscles orbited in a sphere of positive electrification, was highly sophisticated, giving a qualitative explanation of the periodic table and ionic bonding. In 1906 Thomson pioneered the use of scattering calculations in atomic theory, using scattering of X rays and beta rays to show that the number of corpuscles in the atom was comparable with the atomic weight. His methods proved invaluable, leading Rutherford to the nuclear atom, and promoting analysis of the structure of light. But his first result, that there were only hundreds rather than thousands of corpuscles in the atom, was fatal for his own model which lost both its mass and its stability. Thomson began experiments with positive ions to investigate the mass of the atom. This work led to recognition of the H₃⁺ ion and the discovery of the first non-radioactive isotopes, those of neon, in 1913, a discovery which prompted the invention of the mass spectrograph by Thomson's collaborator, Francis Aston, in 1919.

Later Life

In 1919 Thomson resigned the Cavendish Professorship. As Master of Trinity College, he now had a major social and administrative role. But he continued to experiment until a few years before his death laying, among other things, the foundations of plasma physics. He died on August 30, 1940, and was buried in Westminster Abbey.

See also:Electron, Discovery of