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Thomson, Sir Joseph John

rays cathode electric field

(1856–1940) British physicist: discovered the electron.

J J Thomson was a bookseller’s son who first studied at Owens College (later Manchester University), hoping to become an engineer. Poverty caused by his father’s death in 1872 led him to study mathematics, physics and chemistry instead, as he could not afford the charge then made to become an apprentice engineer. He did well and won a scholarship to Trinity College, Cambridge (1876). As a mathematician he graduated Second Wrangler (1880), and subsequently became a Fellow of Trinity College, Cavendish Professor (1884–1919, succeeding ) and Master of Trinity in 1918. In modern terms ‘JJ’ was an experimentalist; but his hands were clumsy and his best work was actually performed by assistants. He was exceptionally well-liked.

Thomson had carried out an excellent mathematical analysis of vortex rings in 1883 and speculation that atoms might be vortex rings in the imagined electromagnetic ‘ether’ led him to investigate cathode rays (the electrical discharge emitted from an electrode under high fields in a gas at low pressure). Several German physicists believed that cathode rays were waves, and had tried to show that they could not be particles, because in his experiments the cathode rays were not deflected by an electric field. However, Thomson repeated the experiment in a better vacuum, in which there was no polarizable air to mask the electric field, and demonstrated that electric fields would deflect cathode rays (1897). Having shown that the rays were made up of negatively charged particles, he proceeded to use their deflection under combined electric and magnetic fields to find the charge to mass ratio ( e / m ) of the particles, which did not vary from one cathode material to another. In April 1897 he revealed this discovery of a new particle. Developing this classic series of experiments, Thomson then measured the charge e by allowing the particles to strike water droplets and observing the droplet’s rate of fall in an electric field . He obtained the same value as the charge on a hydrogen atom, but using both results he found a mass m for the new particle about 1000 times lighter than hydrogen. Shortly afterwards Thomson’s particle was named the ‘electron’ by . Its discovery opened the way for the study of atomic structure by , who succeeded him as Cavendish Professor. His device for measuring e / m is essentially the cathode ray oscillograph, so much used afterwards in both research and in television receivers.

Thomson also examined E Goldstein’s (1850–1930) positive rays obtained when a perforated anode was used in the discharge tube, whose nature depended on the gas in the discharge tube; the cathode rays were the same whatever gas was present, and in 1912 Thomson showed how to use positive rays to separate atoms of different mass. This was done by deflecting the positive rays in electric and magnetic fields (a method now called mass spectrometry). The method allowed him to discover that neon had two isotopes, neon-20 and   neon-22, and then developed the technique. Thomson was prone to attack some chemical problems when he felt (as in 1923) that physics was becoming too non-classical for his taste. His chemical knowledge and instincts were limited, and such ventures were rather unfruitful – in contrast to his early work on positive rays which, via and mass spectrometry, proved so valuable to chemists.

J J Thomson’s apparatus for finding the ratio of charge to mass ( e / m ) for the electron. In the evacuated tube, a high voltage applied to the cathode and anode causes cathode rays (a stream of electrons) to be emitted from the cathode. A narrow beam is selected by two small holes, and forms a bright spot on the screen. The beam is deflected by a magnetic field, but can be restored to its normal position by an electric field applied to the two horizontal plates. From the field strengths, e / m can be calculated.


Thomson also showed that electrons are emitted from a hot, negatively charged metal wire, and from a negatively charged zinc plate if it is exposed to ultraviolet radiation.

Thomson received the 1906 Nobel Prize for physics for research on conduction through gases. One of his achievements was to have built up the Cavendish Laboratory as the foremost in experimental physics, with seven of his research assistants subsequently winning Nobel Prizes. To his great pleasure his son was also a Nobel Prize winner, for demonstrating that the electron possessed both particle-like and wave-like behaviour.

Thomson, William, 1st Baron Kelvin (of Largs) [next] [back] Thomson, Sir George Paget

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