Wilson had shown
that normal air was slightly ionised. At first he thought
this a normal property of the air, and didn't think
the nuclei were produced by radiation. It was not until
1912 that Hess showed the atmospheric charge was caused
by 'cosmic rays', radiation from outside our atmosphere.
was very interested in the atmospheric charge, and continued
to study it between 1904 and 1910. During this time
charged particles, gamma-rays and X-rays were being
studied by physicists around the world, and they were
beginning to be understood.
1910 Wilson decided to try using his condensation apparatus
to see the tracks of moving ionised particles. An ionising
particle would leave a track of ions in its path, so
that an expansion would leave a track of water droplets
showing the route the particle had taken. Wilson built
a new cloud chamber and in the spring of 1911 presented
his first photographs to the Royal Society.