The Cloud Chamber
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.
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