Wilson Cloud Chamber is used to observe the path of ionizing particles. It helps to examine the mechanism of ionization of various ionizing radiations and the product of their interaction with material inside the chamber.

It consists of a closed cylindrical chamber with transparent glass top ‘T’ and a movable piston ‘P’ on the bottom. On the sides near the top, the cylinder is provided with a glass window ‘L’ for light and an inlet ‘I’ for the ionizing particles or radiations. The piston can be moved up or down by a lever attached to it. Before making the enclosed space above the piston airtight, enough quantity of a low boiling point liquid such as water or alcohol is introduced in the space to produce its saturated vapours. A small quantity of the liquid stay on the piston.

The vapours of the liquid usually condense at its dew point but the condensation never takes place in the absence of some particles, dust particles or ions, which are essential to form the nuclei (centres) of condensation. In particle-free space the saturated vapour may cool much below the dew point. Then they are called Super Saturated Vapours. Paths, additional information about the charged and uncharged nature, the magnitude of the charge, the charge to mass ration (e/m), etc of the incident particle or the particle found by their interaction with the atoms can be obtained. By this very method a number of particles have been discovered.

It consists of a hollow metal cylinder, one end of which is closed by an insulating cap. At the centre of the cap is fixed a stiff straight wire along the axis of the cylinder. A thin mica or glass disc closes the other end which also serves as the entrance window for the ionizing particles or radiations. The sealed tube usually contains a special mixture (air, argon, alcohol etc) at a low pressure of 50 to 100 millimetres of mercury. A potential difference of the order of one thousand volts is applied between the metal cylinder and the axial wire through a suitable series resistor ‘R’ (about 10 ohms). The potential difference is only slightly less than that necessary to start a discharge between the wire and the cylinder.

When an ionizing particle enters the tube under this condition if a charged particle pass through the chamber it produces ionization along its track. The condensation of vapours takes place on ions in the form of tiny droplets of fog, which can be photographed.

The piston is pulled down suddenly with the help of the lever. The saturated vapours cool down below the dew point into super-saturated vapours. At the same time an ionizing particle or radiation is allowed to pass through the chamber. The gas molecules all along its path ionize. The super-saturated vapours condense on ions forming tiny droplets of fog, which is photographed in bright light.

b -particle is much less ionizing, its track is therefore, a thin and broken line.

g -rays are photons emitted in a widening cone of some angle. They produce ionization by photoelectric effect distributed on a wide space. Some of the photoelectrons ejected by them give tiny line tracks in random directions like the b -particles and scattered dots are produced. The g -rays do not produce well-defined line track.