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.
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
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.
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.
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."
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.
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.
here to return to the Museum