Cloud chamber
A device that makes the path of a charged particle visible as a trail of fine water droplets.
Expand moist air suddenly so that it cools, and the vapour is ready to condense. A charged particle passing through leaves the air molecules along its path electrically charged, and droplets form on them. The resulting streak, like an aircraft's vapour trail, is the particle's footprint. C. T. R. Wilson, who had been studying how clouds form, succeeded in photographing particle tracks in 1911[1].
In a strong magnet the track curves one way or the other according to the sign of the particle's charge, and the amount of curvature gives its momentum. The positron and many of the other new particles of the first half of the twentieth century were found in cloud chambers. Wilson received the 1927 Nobel Prize in Physics. Electronic detectors have replaced it in today's experiments, but the idea of recording particle tracks and studying them is unchanged.
Related terms
- PositronA particle with the electron's mass and a positive charge. It is the electron's antiparticle; when the two meet, both vanish into light.
- Cosmic rayHigh-energy particles raining continuously on Earth from space. Most are protons, coming from the Sun and from far across the galaxy.
- Alpha particleA heavy, positively charged particle made of two protons and two neutrons: the same thing as a helium nucleus. It is thrown out by radioactive substances.