Cloud Chamber, a device used by nuclear physicists to detect and study elementary particles Among the many particles that can be detected are alpha particles, protons, electrons, positrons, and various types of mesons. These particles are emitted by unstable (radioactive) atomic nuclei and produced during nuclear collisions. Some of them reach the earth as cosmic rays.

In a cloud chamber, the path of a particle appears as a track of mist called a cloud track. By studying the track, or a photograph of it, a physicist can determine the energy and electric charge of the particle that produced it. Every kind of particle has a characteristic cloud track, which varies in shape, length, and width. Many elementary particles have been discovered through their characteristic cloud track.

The cloud chamber was invented by an English physicist, C. T. R. Wilson, in 1911. It is based on the same principles that determine the formation of clouds in the sky. If air is saturated with water vapor and then cooled, tiny droplets of mist form around floating bits of dust or other material. They also form readily around ionselectrically charged atoms or groups of atoms. When a charged particle, such as a proton, passes through the chamber it leaves behind a trail of ions as it strikes molecules of air along its path. Mist droplets form around these ions, creating a cloud track.