Wilson, Charles Thomson Rees
WILSON, CHARLES THOMSON REES
(b. near Glencorse, Midlothian, Scotland, 14 February 1869; d Carlops, Peeblesshire, Scotland, 15 November 1959)
atomic physics, meteorological physics.
Wilson’s father, John Wilson, was well-known in Scotland for his experiments in sheep farming; his mother, the former Annie Clark Harper, came from a Glasgow family of thread manufacturers. John Wilson died when Charles, the youngest of his eight children by two marriages, was four years old; and Mrs. Wilson then moved to Manchester. The family was not well off, and Wilson owed his university education to the kindness and financial support of his half brother William, a successful businessman in Calcutta.
Wilson attended Greenheyes Collegiate School in Manchester; and even then he showed an interest in natural science, preparing specimens for observation under the microscope. At the age of fifteen he entered Owens College, Manchester, registering as a medical student but taking a B.Sc. degree when he was eighteen. A further year was spent studying philosophy and the classics, after which Wilson won an entrance scholarship to Sidney Sussex College, Cambridge. By this time all thought of medicine had been abandoned, and he had determined to become a physicist. The years following receipt of his degree and the death of his half brother William (1892) were difficult ones, for he had to help support his mother, yet longed to devote himself to research. Wilson taught for a short time at Bradford Grammar School in Yorkshire but was drawn again to continue his experimental work at Cambridge, making just enough to live on by serving as demonstrator for medical students. It was at this time that Rutherford, Townsend, and McClelland became research students at the Cavendish Laboratory; and Wilson joined them in the famous discussions over tea.
In 1896 Wilson was awarded the Clerk Maxwell studentship for three years. After a year of work on atmospheric electricity problems for the Meteorological Council, he was elected in 1900 a fellow of Sidney Sussex College, and was appointed a university lecturer and demonstrator. For the two university posts his annual salaries were, respectively, £100 and £50; his duties were to take charge of the teaching of advanced practical physics and to lecture to the part II physics class on light, which was a new course. Wilson’s influence on the teaching of experimental physics at Cambridge was considerable, his chief innovation being to give his students minor research problems to solve in the laboratory, rather than carry out textbook experiments. From 1925 to 1934 Wilson was Jacksonian professor of natural philosophy at Cambridge.
The Royal Society elected Wilson a fellow in 1900, and awarded him the Hughes Medal in 1911, a Royal Medal in 1922, and the Copley Medal in 1935. A Nobel Prize for physics was awarded jointly to Wilson and A. H. Compton in 1927 for their work on the scattering of high–energy photons. Wilson was appointed Companion of Honour by the king in 1937, and held honorary degrees from Aberdeen, Glasgow, Manchester, Liverpool, London, and Cambridge.
At the age of thirty-nine Wilson married Jessie Fraser Dick; the couple had a son and two daughters. Soon after his retirement from the Jacksonian chair, Wilson left Cambridge and returned to Scotland. The twenty-three years of his retirement were extremely active. He continued climbing well into his eighties, and at eighty-six traveled in an airplane for the first time. He died after a short illness at his cottage a few miles from his birthplace.
Wilson attributed the shaping of his research career to his experiences on holiday in the Highlands:
In September 1894 I spent a few weeks in the Observatory . . . on the summit of Ben Nevis. . . . The wonderful optical phenomena shown when the sun shone on the clouds surrounding the hill-top, and especially the coloured rings surrounding the sun coronas) or surrounding the shadow cast by the hilltop or observer on mist or cloud (glories), greatly excited my interest and made me wish to imitate them in the laboratory.1
Elsewhere he wrote: “. . . It is hardly necessary for me to say that these experiments might have had little result had it not been that they were made in the Cavendish Laboratory at the beginning of the wonderful years of the discovery of the electron, X-rays and radioactivity.”2 Such was the initial impetus behind Wilson’s work. J. J. Thomson assessed his achievement thus:
This work of C. T. R. Wilson, proceeding . . . since 1895, has rarely been equalled as an example of ingenuity, insight, skill in manipulation, unfailing patience and dogged determination. . . . The beautiful photographs that he published [of the tracks of atomic particles] required years of unremitting work before they were brought to the standard he obtained . . . It is to him that we owe the creation of a method which has been of inestimable value to the progress of science.3
Wilson well exemplifies the British experimental scientist whose inspiration was found not in mathematical concepts but in the observation of natural phenomena. Early in 1895 he posed himself a set of questions on cloud formation, and in March of that year he began to build the first apparatus to condense water vapor in dust-free air (see Fig. 1). By August he had established that the critical volume-ratio limit for drop formation in clean conditions was V2/V1=1.25. In February 1896, shortly after the discovery of X rays by Röntgen, Wilson used a primitive X-ray tube, made by J. J. Thomson’s assistant, to irradiate the expansion chamber. At the same volume ratio as before, a dense fog was produced by the X rays, which led Wilson to suppose that the condensation nuclei were ions, to which the conductivity of a gas exposed to X rays was attributed by Thomson and Rutherford. By the spring of 1899 he wrote a summary of his work: “General results of all the experiments. Negative ions begin to be caught about V2/v1 =1.25 and all appear to be caught when V2/V1 =1.28. Density of negative fog shows no increase from this point onwards. Positive ions begin to be visible about V2/V1 = 1.31. Fogs are constant and identical with the negative from 1 .35 upwards.”4
The phenomena discovered empirically by Wilson may, briefly, be explained as follows. When air saturated with water vapor is suddenly cooled by
an adiabatic expansion, it becomes supersaturated. In this condition, condensation into droplets will occur, provided there are nuclei present. Dust particles allow drops to form immediately, and so Wilson carefully eliminated all gross matter from his apparatus. Negative ions act as nuclei at an expansion ratio of 1.25 (fourfold supersaturation), and positive ions become nuclei at 1.31 (sixfold super-saturation). At about 1.38 (eightfold supersaturation) air molecules themselves will act as drop nuclei in the absence of all others. The vapor pressure of a spherical drop is greater than that of a plane surface in inverse proportion to the radius of the drop. So that if a very small drop forms, it will reevaporate immediately. A nucleus gives the necessary larger radius to assist the persistence of a drop. The surface tension of the liquid of the drop also is important, because it acts to contract the drop and thus reduce the radius. If a drop carries an electric charge, this acts contrary to the surface tension, tending to enlarge the drop. Because of a characteristic of the skin of a water drop, negative ions are more effective than positive ions in nucleation.
Wilson continued to experiment with ultraviolet radiation and other techniques for producing condensation effects, but soon concentrated on atmospheric electricity, not returning to the cloud chamber until December 1910. He designed an improved chamber with new methods of illumination and the possibility of photographing the results. At this time Wilson realized that it might be possible to reveal the track of an α ray by condensing water drops onto the ions produced by its passage. During March 1911 he saw this effect produced in his apparatus. Thus, the elucidation of phenomena seen in the Scottish hills led to the possibility of studying the processes of radioactivity, and the Wilson cloud chamber became an important piece of laboratory equipment. But it was in the study of cosmic rays that it achieved its full power, particularly in the refined form developed by Patrick Blackett, in which it was possible to study particles of very high energy and the production of electron-positron pairs with the chamber situated in a strong magnetic field.
The study of atmospheric electricity was dramatically thrust upon Wilson by the experience of his hair standing on end while at the summit of Ben Nevis in June 1895. The subsequent lightning flash impressed on him the magnitude of the electric field of a thundercloud. In his experiments he used captive balloons and kites to measure the strength of the electric field at various heights. He also developed a sensitive electrometer and voltameter, as well as a capillary electrometer for the measurement of the earth’s electric field and air-earth currents. In fine weather there is always an electric field directed toward the earth that has a potential gradient of 100–200 volts per meter. The total negative charge on the whole earth is about 500,000 coulombs. The current from the upper atmosphere to the earth is sufficient to discharge the earth in a matter of minutes, so the problem is to account for the maintenance of the earth’s charge. Equilibrium probably is kept by the thunderstorm, the global incidence of which is about two thousand at any given time.
The theory put forward by Wilson to explain the electric structure of a thundercloud implied that the top would be positively charged and the bottom negatively charged. He thought that larger drops would be found on negatively charged nuclei, causing them to fall faster than those positively charged. Wilson’s last scientific paper, “A Theory of Thundercloud Electricity,” was communicated to the Royal Society in 1956, when he was the oldest fellow. Although his theory is not complete, it certainly was a crucial contribution to a problem that has yet to be fully solved.
Work on the conductivity of air was done by Wilson in 1900, using very well insulated electroscopes. They always showed a residual leakage that was the same in daylight and in darkness, and for positive or negative charge. Having described his results, Wilson made the significant statement: “Experiments were now carried out to test whether the production of ions in dust-free air could be explained as being due to radiation from sources . . . outside our atmosphere, possibly radiation like Röntgen rays or like cathode rays, but of enormously greater penetrating power.”5 This ingenious hypothesis was tested in 1911 by Victor Hess, who took an electroscope up in a balloon, thereby discovering that after an initial fall, the conductivity of air increased with altitude. To explain this effect, Hess postulated the existence of “cosmic radiation.”
1. Nobel lecture, Stockholm, 12 Dec. 1927.
2. “Ben Nevis Sixty Years Ago,” in Weather,9 (1954), 310.
3. J. J. Thomson, Recollections and Reflections (London, 1936), 419–420.
4. From laboratory notebook A3 in the library of the Royal Society, cited by Dee and Wormell, “Index . . .” 57.
5. “On the Leakage of Electricity Through Dust-Free Air,” in Proceedings of the Cambridge Philosophical Society.11 (1900–1902), 32.
I. Original Works. Wilson published 45 papers between 1895 and 1956, a third of them in Proceedings of the Cambridge Philosophical Society, and the majority of the remainder in Proceedings of the Royal Society, ser. A. A complete list of the papers is printed in Blackett (below), 294 f., and in Dee and Wormell (below), 65 f. An autobiographical account is “Reminiscences of My Early Years,” in Notes and Records. Royal Society of London,14 (1960), 163–173. MS laboratory records For 1895–1940 are in the library of the Royal Society.
II. Secondary Literature. See P. M. S. Blackett, “Charles Thomson Rees Wilson 1869–1959,” in Biographical Memoirs of Fellows of the Royal Society,6 (1960), 269–295: and P. I. Dec and T. W. Wormell, “An Index to C. T. R. Wilson’s Laboratory Records and Notebooks in the Library of the Royal Society,” in Notes and Records. Royal Society of London,18 (1963), 54–66.
G. L’E. Turner
Charles Thomson Rees Wilson
Charles Thomson Rees Wilson
The Scottish physicist Charles Thomson Rees Wilson (1869-1959) was the inventor and developer of the Wilson cloud chamber.
Charles Wilson was born on Feb. 14, 1869, in Glencorse near Edinburgh. He received his first undergraduate training at Owens College, now part of the University of Manchester, and from there, at the age of 19, he went to Cambridge University with the realization that physics and not medicine was to be his life's vocation.
As Wilson himself disclosed it, two experiences determined the direction and ultimate fortunes of his interest in physics. One was his few weeks' stay in 1894 at the observatory on the top of Ben Nevis, the highest Scottish mountain. The magnificent optical phenomena observable in the interplay of sunshine, mist, and clouds "greatly excited my interest and made me wish to imitate them in the laboratory." The other experience consisted in his being exposed to an electric storm on the summit of Carn Mor Dearg in 1895. From this came Wilson's strong interest in atmospheric electricity, while the first experience inspired his efforts culminating in the construction of the first cloud chamber.
In the beginning of 1895 Wilson concluded that even after the removal of all dust particles, droplets still appeared whenever a volume of moist air was suddenly expanded. He attributed this to a residual conductivity in the air. His reasoning was fully verified a year later when his primitive cloud chamber was exposed to the newly discovered x-ray radiation. In 1904 he also proved that these droplets could be removed from the chamber by an electric field. Not until 1910, however, did he conceive the idea of making visible and of photographing the path of an ionizing particle. The chamber he designed for that purpose was simplicity itself but rested on many years of painstaking effort. It was a flat cylindrical vessel, 16.5 centimeters in diameter and 3.4 centimeters deep, with a fixed glass roof, and a glass floor that could be rapidly moved downward by a piston into an evacuated vessel.
For almost 20 years Wilson's design remained the standard form of cloud chamber by which he took his famous series of photographs of ionization tracks in 1911 and 1921-1922 respectively. The analysis of those tracks proved to be an invaluable tool for all early investigators of nuclear phenomena. Wilson himself provided the experimental evidence for Arthur Compton's theory that in x-ray scattering, the recoil electron takes up the momentum of the quantum of radiation. Fittingly enough, the two shared the Nobel Prize in physics in 1927. It was again the cloud chamber that revealed the existence of positrons and made possible the visual demonstration of "pair creation" and "annihilation" of electrons and positrons.
Upon his retirement from the Jacksonian chair at Cambridge (1925-1934) Wilson took up residence in the village of Carlops near Edinburgh. There he completed his final great study of thundercloud electricity, submitting the manuscript to the Royal Society in 1956, at the age of 87. An indomitable energy and a zest for life were the chief characteristics of Wilson, who took to airplanes for the first time at the age of 86 to observe atmospheric phenomena. He was also possibly the most serene and unassuming among the great scientists of his time. He died at Carlops on Nov. 15, 1959.
The most authoritative account of Wilson's life and work is the lengthy essay by P. M. S. Blackett in Biographical Memoirs of Fellows of the Royal Society (1960). For background see William Dampier, A History of Science (1949), and Mitchell Wilson, American Science and Invention (1954). □