Joseph John Thomson (18 December 1856–30 August 1940) was a British physicist whose investigations of electrical conduction in gases established the electron as a constituent of matter. His 1897 experiments showed that cathode rays consist of negatively charged particles much lighter than atoms. He received the 1906 Nobel Prize in Physics for his theoretical and experimental research on the conduction of electricity through gases. His subsequent work included models of atomic structure and methods for distinguishing charged atoms by their mass-to-charge ratios. (nobelprize.org)
Education and Cambridge career
Thomson was born in Cheetham Hill, near Manchester, England. He entered Owens College in 1870 and Trinity College, Cambridge, in 1876. In 1880 he graduated as Second Wrangler, the second-ranked candidate in Cambridge’s Mathematical Tripos, and became a fellow of Trinity. His early research combined mathematics with theoretical physics, including an investigation of vortex rings that received the Adams Prize in 1884. (nobelprize.org)
In 1884, aged twenty-seven, Thomson succeeded Lord Rayleigh as Cavendish Professor of Experimental Physics at the University of Cambridge. He held the professorship until 1918, when he became Master of Trinity College. His career at the Cavendish Laboratory spanned the transition from nineteenth-century studies of electricity and magnetism to experimental investigations of subatomic matter. Cambridge’s institutional history identifies his discovery of the electron in 1897 as a central event in this development. (nobelprize.org)
Cathode rays and the electron
Cathode rays were produced by electrical discharges in glass tubes containing gas at low pressure. Their nature was disputed: some explanations treated them as a form of radiation, while others regarded them as streams of particles. Thomson investigated their relationship to negative electric charge, their deflection by an electric field, and their response to a magnetic field. His apparatus directed a narrow beam through openings in metal components, allowing its path and the charge it carried to be examined. (webserver.lemoyne.edu)
An important difficulty was that residual gas became electrically conducting under the influence of the rays and could screen the applied electric field. By evacuating the tube more thoroughly, Thomson obtained a clear electrical deflection. The direction of that deflection was consistent with negatively charged particles. Combining electrical and magnetic measurements allowed him to determine the particles’ speed and their mass-to-charge ratio, rather than measuring mass and charge separately. (webserver.lemoyne.edu)
The measured ratio was much smaller than that of the hydrogen ion and remained approximately the same when different gases or electrode materials were used. Thomson interpreted these results as evidence for a common constituent of matter, smaller than the ordinary chemical atom. He called the particles “corpuscles.” The crucial claim was therefore not merely that cathode rays carried charge, but that their carriers were subatomic and shared across different substances. He publicly announced his conclusions at the Royal Institution on 30 April 1897. (webserver.lemoyne.edu)
Electrical conduction in gases
The electron experiments formed part of a wider investigation of how gases become conducting. Thomson studied charged carriers produced through several mechanisms, including X-ray irradiation and other processes that generate ionization. His Nobel lecture distinguished the exceptionally light carriers of negative electricity from the much heavier charged particles encountered in other gas-discharge phenomena. It also emphasized that the same light negative carriers could be obtained from different sources. (nobelprize.org)
The Nobel award consequently recognized a broader research programme than a single experiment. Thomson’s work joined measurements of particle motion, charge, and electrical discharge to a theoretical interpretation of matter. His book Conduction of Electricity through Gases, first published in 1903, presented this field systematically and later appeared in an expanded edition written with his son. (nobelprize.org)
Atomic structure
In 1904 Thomson published a mathematical model in which negatively charged corpuscles moved within a sphere of uniformly distributed positive charge. Usually called the plum pudding model, it represented an electrically neutral atom without a concentrated central nucleus. Thomson examined the stability and oscillations of particle arrangements, including rings, and sought connections between those arrangements and the properties of chemical elements. The model was more dynamic than the familiar illustration of stationary electrons embedded in a positive substance suggests. (gilles.montambaux.com)
Its distributed positive charge was subsequently displaced by Ernest Rutherford’s nuclear interpretation of particle scattering. Measurements involving energetic alpha particles indicated that atomic charge and much of atomic mass were concentrated in a very small atomic nucleus. Thomson’s model nevertheless provided an explicit framework for calculating how subatomic particles might produce atomic properties, preceding the models associated with Rutherford and Niels Bohr. (arxiv.org)
Positive rays and isotopes
Thomson also investigated beams of positive ions. Deflection by electric and magnetic fields distinguished particles with different mass-to-charge ratios, contributing to the development of mass spectrometry. Work with Francis Aston produced evidence that neon contained components with masses near 20 and 22, despite their shared chemical identity. These observations became important evidence for isotopes of a nonradioactive element. Aston subsequently developed a more precise mass spectrograph and extended isotope measurements to numerous elements. (nobelprize.org)
Honours and family
Thomson was elected a fellow of the Royal Society in 1884 and served as its president from 1915 to 1920. He was knighted in 1908. In 1890 he married Rose Elisabeth Paget; they had a son and a daughter. Their son, George Paget Thomson, shared the 1937 physics Nobel Prize with Clinton Davisson for the experimental discovery of electron diffraction by crystals. Joseph John Thomson died in Cambridge on 30 August 1940. (pictures.royalsociety.org)