Thomson, Joseph John (1856–1940)

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THOMSON, JOSEPH JOHN (1856–1940)

The British physicist, famous for his discovery of the electron, was born in Cheetham, near Manchester, on December 18, 1856. He first entered Owens College (later Manchester University) at the early age of fourteen. In 1876 Thomson won a scholarship in Mathematics to Trinity College, Cambridge, and remained a member of the College for the rest of his life. He became a Fellow in 1880, Lecturer in 1883 and Master in 1918, a position he held with great flair until his death on August 30, 1940.

Thomson met Rose Paget in 1889 when, as one of the first women to be allowed to conduct advanced work at Cambridge, she attended some of his lectures. They married on January 22, 1890, and had two children: a son, George, and a daughter, Joan. The marriage was a long and happy one.

Many were surprised when, at the end of 1884, Thomson was appointed Cavendish Professor of Experimental Physics in succession to Lord Rayleigh. Thomson was not yet twenty-eight, and he had almost no experience in experimentation. Nevertheless he immediately began work on the conduction of electricity through gases and single-mindedly pursued this topic until he resigned the Chair in 1919. Under Thomson's inspiration and guidance the Cavendish Laboratory became the world's foremost research institution. Seven of those who started or continued their careers there went on to win Nobel Prizes, including his own son.

J.J., as Thomson was commonly called, received the 1906 Nobel Prize in Physics in "recognition of the great merits of his theoretical and experimental investigations of the conduction of electricity by gases". He was knighted in 1908, received the Order of Merit in 1912 and was successively President of the Physical Society, the Royal Society and the Institute of Physics.

The demonstration of the independent existence of the negative electron of small mass was a watershed in the long quest to understand the nature of electricity. Many scientists had contributed to this search in the 300 years prior to Thomson's discovery: Gilbert, Franklin, Coulomb, Poisson, Galvani, Volta, Davy, Oersted, Ampère, Ohm, Faraday, Maxwell, and others. In 1874 the Irish physicist George Johnstone Stoney pointed out that, on the basis of Faraday's law of electrolysis, there exists an absolute unit of electricity, associated with each chemical bond or valency. Hermann von Helmholtz, independently, drew similar conclusions, declaring that electricity, both positive and negative, was divided into elementary portions that behaved like atoms of electricity. Stoney later suggested the name "electron" for this elementary charge.

The phenomena exhibited by the electric discharge in rarefied gases had long been known to German and British physicists. Many British physicists held that cathode rays were particles of matter, similar in size to ordinary molecules, projected from the negative pole. In contrast, most German physicists maintained that cathode rays were analogous to electric waves, getting strong support from Heinrich Hertz who in 1892 showed that the rays could pass through thin sheets of metal. That fact was difficult to reconcile with the molecule-sized particle interpretation.

Thomson was convinced that whenever a gas conducted electricity, some of its molecules split up, and it was these particles that carried electricity. Originally, he thought that the molecule was split into atoms. It was not until 1897 he realized the decomposition to be quite different from ordinary atomic dissociation. At the beginning of that year Thomson performed some experiments to test his particle theory.

First he verified that cathode rays carried a negative charge of electricity and measured their deflection in magnetic and electrostatic fields. He concluded that cathode rays were charges of negative electricity carried by particles of matter. Thomson found that the ratio of the mass, m, of each of these particles to the charge, e, carried by it was independent of the gas in the discharge tube, and its value was of the order of 1/1,000 of the smallest value known at that time, namely the value for the hydrogen ion in electrolysis of solutions. He then devised a method for direct measurements of e as well as m/e, thus allowing the mass of the particles to be determined. The measurements showed that e carried the same charge as the hydrogen ion. Thus mwas of the order of 1/1000 of the mass of the hydrogen atom, the smallest mass known at that time. This numerical result was perfectly adequate for the interpretation adopted and, if not at first very accurate, was soon improved by later experiments. Thomson concluded that the negative charge carrier, its mass and charge being invariable, must represent a fundamental concept of electricity, or indeed of matter in any state. With regard to its size he estimated what is now called the "classical electron radius" to be 10^-15 m.

Thus, the search for the nature of electricity led to the discovery of the electron and the proof that it is a constituent of all atoms. These achievements gave scientists the first definite line of attack on the constitution of atoms and the structure of matter. The electron was the first of the many fundamental particles later proposed. Though Thomson was the undisputed discoverer of the electron, there were others in the hunt who came close to the prize, notably the French physicist Jean Perrin, the German physicists Emil Wiechert and Walter Kaufmann, and the Dutch physicist Pieter Zeeman. The latter had, in 1896, calculated the e/m ratio for a vibrating "ion"—in other words a bound electron—emitting light. His result was similar to Thomson's corresponding value for a free electron.

Thomson's achievement not only produced explanations for many historically puzzling observations but also opened up new fields of science. Of these the richest is surely the electronic structure of matter, a field of unceasing development that has produced, among other things, the silicon chip, the computer and the technology that provides near-instantaneous global intercommunication and access to an unprecedented amount of information.

Leif Gerward Christopher Cousins