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Wilson is 1911, invented a cloud chamber to detect the presence of radioactive particles. To identify them and to measure their energy.

Principal

If we suddenly increase the volume of air saturated with water vapour,  the temperature  of air falls sharply and it becomes super saturated with water vapour.

The vapour is condensed in the form of small droplets on the dust particles, present in the air and a fog appear .
If their be no dust particle in the air the vapouur will not be condensed and the fog will not appear. If however we ionizing  duet free air by some ionizing radiation ( like α, β particle ). Then the vapour is condensed on the ions proiduced in the air. Thus by seen condensation of water vapour in dust free air. The presence of α and β particle can be detected.

Construction

Wilsomon’s  cloud chgamber consist of a cylindrical metallic chamber C fitted with glass top G and carrying  a movable piston P , the space between piston P and glass cover filled with dust free air saturated with water vapour intensed light is sent into chamber through a window . just above the chamber , a camera is fitted.

The α or β particle emitted by  the radioactive substance enter the chamber .
Each particle goes on producing ion by ionizing the air molecules on its path.
To mark the path of the particle the piston P is dropped down which leads to  a sudden expansion  of the saturated air in the chamber. Hence the air is called and  become  super saturated with water vapour. Now the vapour condensed in the form of drops of ions formed along the path of the particle.
Thre path of the p[article is marked as trail of the droplet which is the track of the particle . it is photographed of the camera. The process ids automatic as soon as the particle enters the chamber, the position drops down the shutter of the camera opens & track of particle is photographed by the camera.