9. Particle track photographs

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.

Wilson 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.

In 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.

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