This is the cloud chamber with which Rochester and Butler of the University of Manchester obtained conclusive evidence for the existence of new unstable elementary particles in 1947. It was operated at an altitude of 2900 metres in the French Pyrenees at the Observatoire du Pic du Midi. Cloud chambers are vesels filled with a gas, supersaturated with water vapour. When a piston is moved outwards, the volume of the vesel expands, the vapour cools and a cloud of tiny water droplets forms on any nuclei, dust or ions present. Any fast-moving ionizing subatomic particles colliding with the gas molecules show up as visible tracks, which can be photographed.
© Science Museum / Science & Society Picture Library