Wilson Cloud Chamber
via Wikipedia of course.
The cloud chamber, also known as the Wilson chamber, is used for detecting particles of ionizing radiation. In its most basic form, a cloud chamber is a sealed environment containing a supercooled, supersaturated water vapour. When an alpha particle or beta particle interacts with the mixture, it ionises it. The resulting ions act as condensation nuclei, around which a mist will form (because the mixture is on the point of condensation). The high energies of alpha and beta particles mean that a trail is left, due to many ions being produced along the path of the charged particle. These tracks have distinctive shapes (for example an alpha particle’s track is broad and straight, while an electron’s is thinner and shows more evidence of deflection). When a vertical magnetic field is applied, positively and negatively charged particles will curve in opposite directions. This is evidenced in the photograph above showing the discovery of the positron; an electron curves the other way. It is possible (and essential) in this case to determine that the positron was actually moving upwards (presumably it had been deflected from below), because the curvature of the track is greater in the lower part of the figure (the photograph was upside down!). For more detailed track shape information see bubble chamber.