Here we see the cloud chamber apparatus. It is a glass box, of which the (black) bottom is cooled; alcohol vapour is injected in the top part of the box, which drifts down and is at the point of condensing close to the cooled bottom. When radio-active  radiation passes through, tiny drops of alcohol are formed along the track of the particle. The V-shaped image that we see is due to the two tracks of alpha-particles that result from the decay of a Radon atom.

In the beginning of the video we show the original apparatus used by Wilson. A piston is suddenly moves down in a cylinder and the expanded humid air cools down, so that water vapour is at the point of condensing and forms clouds (Wilson was a meteorologist and originally studied cloud formation, which is why he built a 'cloud chamber' ). Radiation particles leave a track of ionizations behind and on each ion a droplet is formed, so that the track becomes visible. It may be compared with condensation trails of airplanes, where droplets are formed on dust particles in the exhaust and on turbulence of the air.

In the 'continuous' type of cloud chamber, shown in the second part of the video, a flow of alcohol vapour is created from the top towards the bottom. The bottom is kept at a low temperature and in a narrow layer above the bottom the alcohol vapour is at the point of condensing. In this region the tracks of droplets can be seen in a strong beam of light. The droplets evaporate again after a fraction of a second, so that a new track can be seen.

We see the 'background' radiation, a 'cosmic' burst, various examples of radioactive decay and we see a nuclear reaction (This is in fact the first nuclear rection ever seen, by Wilson in the  apparatus shown in the beginning of the video.) 

Andries Hummel

hummela@xs4all.nl