Week 7a - PHYS142/143 Rm 4.110 Ph. 02 42214798 Email:...

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PHYS142/143 Rm 4.110 Ph. 02 42214798 Email: carey_freeth@uow.edu.au bookmark J.J. Thomson and the Discovery of the Electron He also announced that they had a mass about 1000 times smaller than a hydrogen atom, and he claimed that these corpuscles were the things from which atoms were built up. J.J. Thomson used results from cathode ray tube (CRT) experiments to discover the electron. On April 30, 1897, Joseph John (J.J.) Thomson (1856-1940) announced that cathode rays were negatively charged particles which he called 'corpuscles.'
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Sir J.J. Thomson in his laboratory. He is facing the screen of a cathode ray tube. [Courtesy of Cavendish Laboratory] The two plates about midway in the CRT were connected to a powerful electric battery thereby creating a strong electrical field through which the cathode rays passed. Thomson also used magnets, which were placed on either side of the straight portion of the tube just to the right of the electrical plates. This allowed him to use either electrical or magnetic or a combination of both to cause the cathode ray to bend. The amount the cathode ray bent from the straight line using either the electric field or the magnetic field allowed Thomson to calculate the e/m ratio. This diagram appeared in J.J. Thomson’s article in 1897.
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Thomson had leaped to the conclusion that the particles in the cathode ray (which we now call electrons) were a fundamental part of all matter. This was reaching quite far beyond what he had actually discovered. As he was to recall much later: "At first there were very few who believed in the existence of these bodies smaller than atoms. I was even told long afterwards by a distinguished physicist who had been present at my [1897] lecture at the Royal Institution that he thought I had been ‘pulling their legs.’ " (J.J. Thomson (1936). Recollections and Reflections. G. Bell and Sons: London. p. 341.) Thomson's corpuscle hypothesis was not generally accepted, even by British scientists, until he spoke of it again in 1899. By this time, George Francis FitzGerald (1851- 1901), an Irish physicist, had suggested that Thomson's 'corpuscles' making up the cathode ray were actually free electrons.
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Week 7a - PHYS142/143 Rm 4.110 Ph. 02 42214798 Email:...

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