10. The Cloud Chamber

The Cloud ChamberWilson was careful to ensure that the cloud tracks accurately matched the paths followed by the ionising particles. The cloud tracks would be distorted if the gas in the chamber were stirred up, so the expansion had to take place soon after the ionisation took place. Wilson's new cloud chamber was carefully designed to ensure that expansions occured quickly without producing air turbulence.

The other important factor was that the chamber should be free of any other ions, like those that exist in the air around us. Wilson put an electric field around his chamber, so that any ions which had been in the chamber longer than a fraction of a second were swept away. The tracks in his photographs show only ions that were formed one fortieth of a second before the photograph was taken!

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