11. Cloud Chamber design

Wilson described the new chamber in a paper to the Royal Society in 1912. The cloud chamber itself is a glass cylinder, about 16 centimetres across and just over 3 centimetres deep. It's walls are coated in gelatin, with the base dyed black to provide a dark background for photography. The floor of the chamber is fixed to the top of a brass plunger which can slide freely inside an outer cylinder, also made of brass. All this stands in a shallow trough of water that keeps the air in the chamber saturated with water molecules.

A diagram of Wilson's apparatus

The predominant feature of the chamber is the large spherical glass bulb, or vacuum chamber. When an expansion is required a valve is opened to connect this bulb with the air beneath the plunger. Air rushes from beneath the plunger into the vacuum, so the plunger falls producing a sudden but smooth expansion.

An electric field can be applied to the chamber to clear residual ions. The expansion is triggered by an arrangement which also fires the camera's flash bulb a moment after the expansion has taken place.

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