Wilson
thought the droplets might be condensing on charged
particles, or ions. The droplets could condense on ions
just as they did on dust, but ions could exist even
in filtered air. To test this idea Wilson tried exposing
the air in the cloud chamber to X-rays, which had been
discovered in 1895 by the German physicist Wilhelm Röntgen.
The
cloud chamber did not produce droplets until an expansion
of 1.25, as before, but with X-rays present a huge number
of droplets formed, producing a slowly falling fog.
Without X-rays this expansion produced only a few hundred
droplets per cubic centimetre, a very thin mist.
The
presence of X-rays was producing points of nucleation
in the chamber. Wilson tried an expansion thirty seconds
after the X-rays were turned off, but got the same results
as if he never used X-rays at all. Wilson concluded
that the X-rays were producing a larger number of the
nuclei which naturally occured in air, and that the
nuclei dispersed rapidly.
