device used to detect elementary particles and other ionizing radiation. A cloud chamber consists essentially of a closed container filled with a supersaturated vapor, e.g., water in air. When ionizing radiation passes through the vapor, it leaves a trail of charged particles (ions) that serve as condensation centers for the vapor, which condenses around them. The path of the radiation is thus indicated by tracks of tiny liquid droplets in the supersaturated vapor. The cloud chamber was invented c.1900 by C. T. R. Wilson. In the type devised by him, which is often called the Wilson cloud chamber, air or another gas is saturated with water vapor and enclosed in a cylinder fitted with a transparent window at the top and a piston or other pressure-regulating device at the bottom. When the pressure in the chamber is suddenly reduced, e.g., by lowering the piston, the gas-vapor mixture is cooled, producing supersaturation. Cloud chambers of this design are sometimes called the pulsed type, since they do not maintain a continuous state of supersaturation of the vapor. A more recent design is the diffusion cloud chamber. In this device a large temperature difference is maintained between the top and bottom of the chamber, usually by cooling the bottom of the chamber with dry ice. The gas in the chamber, usually air, is saturated with a vapor, usually alcohol; the air-vapor mixture cools as it diffuses toward the cool bottom, becoming supersaturated. If the gas is kept saturated with a fresh supply of vapor, e.g., by an alcohol-soaked pad inside the top of the chamber, the operation of the chamber can be essentially continuous. One disadvantage of the cloud chamber is the relatively low density of the gas, which limits the number of interactions between ionizing radiation and molecules of the gas. For this reason physicists have developed other particle detectors, notably the bubble chamber and the spark chamber.

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