Wilson's Cloud Chambers

Rutherford described the cloud chamber as 'the most original and wonderful instrument in scientific history.' In 1927 C T R Wilson was awarded the Nobel Prize for his invention of the cloud chamber. He described the origins of the work as follows: - 'In September 1894 I spend a few weeks in the Observatory which then existed on the summit of Ben Nevis. The wonderful optical phenomena shown when the sun shone of the clouds surrounding the hill top, and especially the coloured rings surrounding the sun, or surrounding the shadow cast by the hill-top or observer of mist or cloud, greatly excited my interest and made me wish to imitate them in the Laboratory.

'At the beginning of 1895 I made some experiments for this purpose, making clouds by expansion of moist air after the manner of Coulier and Aitken. Almost immediately I came across something which promised to be of more interest than the optical phenomena which I had intended to study.' He found that in dust-free air drops formed at an expansion ratio of 1.25 (corresponding, to 4-fold supersaturation), and that the nuclei on which they formed seemed to be constantly regenerated. There was a second critical expansion at 8-fold supersaturation when a dense cloud of very fine drops formed. This was condensation occurring in the absence of any nuclei other than the molecules of the vapour. But his interest was aroused by the condensation at 1.25 expansion. Could the nuclei responsible be charged atoms or ions?

In 1896 Wilson had the opportunity of experimenting with one of the new X-ray tubes. 'I can well recall my delight when I found at the first trial that while no drops were formed on expansion of the cloud chamber when exposed to X-rays if the expansion were less than 1.25, a fog which took many minutes to fall was produced when the expansion lay between the rain-like and cloud-like limits; X-rays thus produced in large numbers nuclei of the same kind as were always being produced in very small numbers in the air within the cloud chamber.' By applying an electric field to the chamber he found that these nuclei were in fact charged ions.

This found its immediate application in the determination of the charge carried by an ion by Thomson and later by H A Wilson, who partially balanced the weight of a charged drop by a known electric field and measured the change in the rate of fall. Thomson deduced the value of e to within 1% of Millikan's later, accurate value.

'Towards 1910 I began to make experiments with a view to increasing the usefulness of the condensation method... In the years which had elapsed since my earlier experiments, ideas on the corpuscular nature of alpha- and beta-rays had become much more definite, and I had in view the possibility that the track of an ionising particle might be made visible and photographed by condensing water on the ions which it liberated... The first test was made with X-rays, with little expectation of success, and on making an expansion of the proper magnitude for condensation on the ions while the air was exposed to the rays I was delighted to see the cloud chamber filled with little wisps and threads of clouds - the tracks of the electrons ejected by the action of the rays.' Wilson was delighted with the clarity of the tracks and W L Bragg recalls him exclaiming excitedly: "They are as fine as little hairs."

He used the cloud chambers to study the processes of ionisation and ejection of electrons by X-rays. By 1923 he had perfected his chamber and introduced stereoscopic photography. The photographs he obtained established the reality of the Compton effect by showing the existence of Compton recoil electrons.

The cloud chamber was further developed by Blackett who made many important discoveries with it, notably the demonstration of the creation and annihilation of electron-positron pairs.

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