Wilson cloud chamber was a device that made the paths of electrically charged subatomic particles visible. Scientists examined these paths to determine the charge, mass, and velocity of the particles. Researchers once used Wilson cloud chambers extensively. Uses included the detection of particles sent out by radioactive materials, high-energy particles in machines called particle accelerators, and cosmic rays (high-energy particles originating in outer space).
Devices called tracking chambers, sampling calorimeters, and scintillators have replaced Wilson cloud chambers. British physicist Charles T. R. Wilson invented the Wilson cloud chamber in 1911.
A simple cloud chamber consisted of a container fitted with a piston. The chamber contained air or some other gas that was highly concentrated with water vapor, alcohol vapor, or both. Electrically charged particles passing through the gas transferred energy to electrons in molecules lying in their path. As a result, the electrons left their molecules, changing the molecules into ions–that is, giving the molecules an electric charge. Vapor then collected around the ions and formed visible streaks of droplets. These streaks, or tracks, thus represented the paths of the original charged particles. The tracks disappeared quickly, but scientists photographed them through a glass wall or window in the chamber.
For tracks to form, the gas had to be supersaturated–that is, its relative humidity had to be greater than 100 percent. Vapor in a supersaturated gas will condense to form droplets at the slightest disturbance. A saturated gas (one whose relative humidity is 100 percent) becomes supersaturated when its pressure decreases. An outward movement of the piston decreased the pressure in the cloud chamber.
