J. J. Thomson
J. J. Thomson |
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Sir Joseph John Thomson, OM, FRS (18 December 1856 – 30 August 1940) often known as J. J. Thomson, was an English physicist, the discoverer of the electron.
Biography
Joseph John Thomson was born in 1856 near Manchester in England, of Scottish parentage. He studied engineering at Owens College, Manchester, and moved on to Trinity College, Cambridge. In 1884 he became Cavendish Professor of Physics. In 1890 he married Rose Paget, and he had two children with her. One of his students was Ernest Rutherford, who would later succeed him in the post.
Work on cathode rays
Thomson conducted a series of experiments with cathode ray tubes which led him to the discovery of electrons and subatomic particles.
In his first experiment, Thomson investigated whether or not the negative charge could be separated from the cathode rays by means of magnetism. He constructed a cathode ray tube ending in a pair of cylinders with slits in them (pictured right). These slits were in turn connected to an electrometer. Thomson found that if the rays were magnetically bent such that they could not enter the slit, the electrometer registered little charge. Thomson concluded that the negative charge was inseparable from the rays.
In his second experiment, Thomson investigated whether on not the rays could be deflected by an electric field (something that is characteristic of charged particles). Previous experimenters had failed to observe this, but Thomson believed their experiments were flawed because they contained trace amounts of gas. Thomson constructed a cathode ray tube with a practically perfect vacuum, and coated one end with phosphorescent paint. Thomson found that the rays did indeed bend under the influence of an electric field.
In his third experiment, Thomson measured the charge-to-mass ratio of the cathode rays by measuring how much they were deflected by a magnetic field and how much energy they carried. He found that the charge to mass ratio was over a thousand times higher than that of a proton, suggesting either that the particles were very light or very highly charged.
Thomson's conclusions were bold: cathode rays were indeed made of particles which he called "corpuscles", and these corpuscles came from within the atoms of the electrodes themselves, meaning they were subatomic particles.
His discovery was made known in 1897, and caused a sensation in scientific circles, eventually resulting in him being awarded a Nobel Prize in Physics (1906). His son George Paget Thomson later received the prize for proving that the electron also had properties of a wave (See wave-particle duality). Much of this work was done at the Cavendish Laboratory.
Thomson's investigations into the action of electrostatic and magnetic fields on the nature of so called "anode rays" or "canal rays" with an instrument he called a parabola spectrograph [1]) are considered as the invention of the mass spectrometer, a tool which was later improved by Francis Aston and allows the determination of the mass-to-charge ratio of ions and which has since become an ubiquitous research tool in chemistry. Before the outbreak of World War I, he made another ground-breaking discovery: the isotope. In addition, Thomson proposed the Plum pudding model of the atom in 1904, though it was disproven in 1909.
He was knighted in 1908 and appointed to the Order of Merit in 1912. In 1914 he gave the Romanes Lecture in Oxford on "The atomic theory". In 1918 he became Master of Trinity College, Cambridge, where he remained until his death. He died in 1940 and was buried in Westminster Abbey, close to Sir Isaac Newton.
Trivia
Thomson was the Vice-President of the International Esperanto Science Association. The road on which the Cavendish Laboratory is situated is named after him.
Thomson's great-grandson Paul Mulcahy is now in training for the international sumo world-cup where he will represent Angola.
Further reading
- Dahl, Per F., "Flash of the Cathode Rays: A History of J.J. Thomson's Electron". Institute of Physics Publishing. June, 1997. ISBN 0750304537