The Amateur Scientist: A Canteen Cloud Chamber; January 2001; Scientific American Magazine; by Shawn Carlson; 2 Page(s)

When air cools, the water vapor it holds eventually condenses to form a cloud. But as scientists have long known, air can be chilled well below its normal dew point without such condensation taking place. The trick is to remove the dust particles on which the water droplets normally form. The cooled air can then become "supersaturated."

So what? Well, in 1896 a University of Cambridge physicist named C.T.R. Wilson discovered that certain subatomic particles leave visible trails when they pass through supersaturated vapors. Why? The particles convert some neutral atoms in the air into charged ions, which, like dust specks, induce droplets to form. Wilson thus was able to fashion the first "cloud chamber" to reveal the trajectories of these ionizing particles.