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Thomson's Cathode Ray Tube
From: Science Museum
| By:
Graham Farmelo |
EDITOR'S INTRODUCTION |
'JJ' Thomson invented the cathode ray tube and with this, was able in 1899 to prove the existence of the first sub-atomic particle--the electron. His discovery was initially treated with suspicion but Thomson's critics could not have been more wrong. The electron has changed the way we understand the universe and shaped the development of modern society. Graham Farmelo, head of Interpretation and Education at the Science Museum, tells the fascinating story of a scientist who made an extraordinary breakthrough. |
 atter is made of atoms. The concept is so familiar that it is now part of general knowledge, yet a century ago some leading scientists doubted the very existence of atoms. The extraordinarily successful journey that has led to our present understanding of atomic structure began in the rather dingy laboratory of the first explorer-in-chief, John Joseph ('JJ') Thomson (1856-1940). An enthusiastic and ambitious professor at Cambridge University's Cavendish Laboratory, Thomson used results he had obtained with his cathode ray tube to prove, in 1899, the existence of the first sub-atomic particle--the electron. In many ways, this discovery marked the birth of the modern electronic age. |
Thomson was the apotheosis of the top-flight Cambridge scientist. He began his career by winning a scholarship to Trinity College  and went on to be a Fellow, Professor and Master of the College, winning en route a Nobel Prize (1906), a knighthood (1908) and the Presidency of the Royal Society (1915). He was Professor of Experimental Physics, although that may not have been the best job for him; he was very mathematically minded and was well known to be an inexpert experimenter, to the extent that his students were fearful when he approached their equipment (his wife would not let him do even minor jobs about the house). He was nonetheless a brilliant designer of scientific apparatus and equally good at interpreting the results of experiments. Both of these strengths were displayed in his conclusive contribution to the cathode ray debate which was exercising European scientists in the 1890s. |
In the late 1860s, it had been demonstrated that a column of gas at low pressure glows colourfully when electricity is passed through it from one metal plate to another. If the pressure was sufficiently low, the individual tracks of so-called 'cathode rays' could be seen between the plates. What did these rays consist of? If this question had been put to a European scientist in the mid-1890s, the answer would have depended to some extent on which side of the Rhine the scientist lived. German scientists held that the rays were mysterious waves in the ether which were supposed to pervade the whole of space, whereas French and British scientists believed that the rays were made up of individual particles.
In the race to rule out the wrong theory of cathode rays, Thomson made a key contribution in the spring of 1897. With the help of his assistant Everett, he constructed special cathode ray tubes that enabled him to study how the paths of the rays are affected by electric and magnetic fields. |
In his Royal Institution lecture of 30 April 1897. Thomson boldly--some would say rashly--suggested that cathode rays were corpuscles even smaller than hydrogen atoms. At least one member of his audience thought he was joking! But he certainly was not and two years later he went further and suggested that cathode rays were indeed particles, each with a definite charge and mass (his values for both turned out to be quite accurate). He had written his name into every book on atomic science as the discoverer of the electron. |
This may have been a great event for the scientific cognoscenti, but would it make any difference to the lives of the public at large? Some scientists evidently hoped not; for some years after Thomson's breakthrough, his colleagues at the Cavendish toasted at their annual dinner to 'The Electron: may it never be of use to anybody'. These pompous hopes have not been fulfilled--electrons are now very much a part of modern life. They illuminate the screen of every television and personal computer and are responsible for the operation of every electrical device. In laboratories, electron microscopes allow us to map the shapes of viruses and the surfaces of atoms (both far too small for ordinary optical microscopes); the electronics industry aims to exploit new and more ingenious ways of controlling electron flow; and radiologists routinely use electrons from radioactive decays to diagnose and treat cancers. |
Yet while the aspirations of the Cavendish purists have been thwarted by the technologists, others have advanced our knowledge of the sub-atomic world beyond Thomson's dreams. The electron is now known to be but one of several fundamental particles, each envisaged to exist at an unimaginably small point, with neither shape nor size. Scientists now study these basic building blocks of matter using huge particle accelerators that are each, in a sense, descendants of Thomson's little cathode ray tube. Today's particle physicists normally work in multinational teams, taking years to design and carry out experiments that consume alarmingly large chunks of national research budgets. The days in which a lone scientist could make a bench-top discovery of a new particle seem, sadly, to be long gone. |
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