The cloud chamber is filled with supercooled steam, propanol in this case. Electrically charged particles ionize gas molecules as they pass through the cloud chamber, triggering condensation. Thus tiny drops of liquid are generated along the particle tracks – similar to the condensation trails left by airplanes.

Anderson was working with a cloud chamber in 1932 when he discovered anti-matter: the positron as the anti-particle of the electron. The electron’s heavier partner, the muon, was also discovered in a cloud chamber.