Thomson, Sir Joseph John (1856-1940), a British physicist, received the 1906 Nobel Prize in physics for his discovery of the electron. In 1937, his son and pupil, Sir George Paget Thomson, shared the Nobel Prize in physics with Clinton Davisson, an American physicist.
Thomson began in 1895 to investigate the mysterious rays that occurred when electricity was passed through a vacuum in a glass tube. Because the rays seemed to come from the cathode (negative electrical pole in the tube), they were called cathode rays. No one had succeeded in deflecting them by an electric force. Some scientists therefore assumed that cathode rays were like light waves. Thomson believed that they were really tiny particles of matter.
Thomson built a special cathode-ray tube in which the rays passed through electric and magnetic fields that were perpendicular to each other. The rays became visible as a dot on the opposite end of the tube. By measuring the deflections of the dot as he changed the strength of the electric and magnetic fields, Thomson determined the ratio of the charge to the mass of the particle (symbolized as e/m). From the direction of their deflection, he decided that the particles were negatively charged. Because their e/m was always the same, he felt sure that they were a fundamental part of all atoms. These particles were later called electrons.
Thomson was also the first to separate isotopes of the chemical elements. This accomplishment spurred the invention of the mass spectrograph by his assistant, Francis W. Aston (see Mass spectrometry).
Thomson was born near Manchester on Dec. 18, 1856, and was educated at Manchester and Cambridge. Thomson’s experimental work was invaluable to physics. His theoretical model of the atom, however, became obsolete after new models were proposed by Ernest Rutherford in 1911 and Niels Bohr in 1913. Thomson died on Aug. 30, 1940.
