(1869-1959) Scottish Experimentalist, Atomic Physicist, Particle Physicist
Charles Thomas Rees Wilson is famous for his invention of the Wilson cloud chamber, the first instrument to detect the tracks of atomic particles. For this work, which initiated the age of modern experimental particle physics, he was awarded the 1927 Nobel Prize in physics.
He was born on February 14, 1869, in the parish of Glencorse, just outside Edinburgh, into a family who had farmed in the south of Scotland for many generations. His father died when he was four and the family moved to Manchester, where he received his early education at a private school. He went on to Owens College (now the University of Manchester), where he majored in biology, intending to become a physician. After receiving his B.Sc. in 1887, he won a scholarship to Cambridge University’s Sidney Sussex College, where he became interested in physics and chemistry and earned a degree in natural sciences. After graduation, he taught for four years at Bradford Grammar School, then returned to Cambridge, where he was the Clerk Maxwell scholar from 1896 to 1899.
Wilson invented the cloud chamber at the outset of his career and, like many great discoveries, Wilson’s was made inadvertently, while he was looking for something else. While vacationing in 1894, he climbed Ben Nevis, Scotland’s highest mountain, where he was intrigued by the marvelous optical effects of coronas and “glories” (i.e., colored rings cast by surrounding shadows on mist and clouds). The experience sparked a desire to study atmospheric cloud formation, and when he returned to Cambridge, from 1895 to 1899, he carried out a series of groundbreaking experiments designed to produce “artificial clouds” in the laboratory. He succeeded in doing this by causing the adiabatic (i.e., with no heat loss) expansion of moist air. At the time, physicists thought that each water droplet must form around a dust particle, but in the supersaturated air of Wilson’s cloud chamber microscopic droplets formed in the absence of dust particles. When Wilson exposed the cloud of water vapor to X rays, which had been discovered that same year by wilhelm conrad rontgen, the process of droplet formation intensified. From this observation, he concluded that water vapor condensed around ions, atoms that have become charged by gaining or losing electrons. Thus, he reasoned that the track of a positively charged alpha particle, for example, would become visible in a line of water droplets. Wilson then used illumination to make the track stand out clearly, enabling the cloud chamber both to detect ions in gases and to record them photographically.
Having achieved this much, Wilson abandoned the cloud chamber to pursue another passion spurred by the dramatic weather conditions of Ben Nevis. In 1895, while on the mountain, he heard distant thunder and, suddenly feeling his hair stand up, fled without waiting for the storm to break. The experience led to an intense interest in atmospheric electricity, which he pursued from 1900 to 1910, while working as a lecturer at Sidney Sussex. He devised the gold-leaf electroscope, a sensitive device for measuring electric charge, which enabled him to demonstrate that some electrical charge always occurs in air and that the conductivity of air inside the electroscope is the same in daylight as in darkness and is independent of the sign of the charge for leaf potential. Wilson was at a loss to explain his results; many years later they would be understood in terms of the existence of cosmic rays: radiation emitted everywhere in the universe.
The year 1911 was a pivotal one for Wilson: he married Jessie Frasier of Glasgow, with whom he would have two sons and two daughters, and he developed a working model for a more advanced cloud chamber. Since the track of a charged particle was detectable because water droplets condensed along the particle’s path, he reasoned, on the basis of james clerk maxwell’s laws of electrodynamics, that if he applied a magnetic field to the chamber, the track would curve and give a measure of the charge and mass of the particle. He built his new chamber in the form of a short cylinder in which saturation was achieved and controlled by the movement of a piston through a fixed distance. The condensation effects were monitored through the other end. The instrument immediately became essential to the study of radioactivity; it was used to confirm the classic alpha particle scattering and transmutation experiments first performed by ernest rutherford. However, the immense value of the Wilson cloud chamber and subsequent variations developed by others only became apparent in the early 1920s, when modifications by lord patrick maynard stuart blackett and carl david anderson led to the discovery of the positron in 1932 and the pi meson in 1936.
In 1925, Wilson was appointed Jacksonian Professor of Natural Philosophy at Cambridge, a post he held until 1934. Two years later, he retired and moved to the village of Carlops, near his birthplace. C. T. R., as his friends called him, continued to meet with friends and colleagues, continued his research, and wrote his last paper, a long-promised manuscript on the theory of thundercloud electricity, when he was 87. He died at Carlops on November 15, 1959, surrounded by his family.
Wilson’s cloud chamber, celebrated by Rutherford as “the most original and wonderful instrument in scientific history,” represented the beginning of experimental particle physics. Although it is not used today, the principle underlying it was incorporated by donald arthur glaser in 1952 into the bubble chamber, an extremely sensitive detector that uses supersaturated liquid helium and is a component of today’s giant particle accelerators.
