At the beginning of the 20th century, Charles Thomson Rees Wilson presented a quite simple apparatus. A chamber filled by alcohol and water vapours with a big flask from which air had been withdrawn. Opening the access to the flask reduces sharply the pressure in the chamber:
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(Images come from Emile Gérard)
A piston which permits to reduce the pressure
A radioactive source is introduced to the chamber. α particles are emitted.
α particles ionise the air. An electron may be torn out of an atom and pass to another atom leaving a cation and producing an anion.
The pressure is sharply reduced, the temperature decreases. Alcohol and water droplets condense on the ions making visible the path of the ionising particles .
The Wilson chamber is used to study all elementary particles. It allows - to see their trajectory - to study their interactions - to determine their caracteristics