Cloud chambers [local] were first developed by Charles T.R. Wilson around 1911 for experiments on the formation of rain clouds. Wilson knew that water vapor condensed around ions, atoms which have become charged by gaining or losing electrons. However, an alpha particle, for example, which had an electric charge, would leave a trail of ions as it passed through a gas. If water vapor condensed on these ions, the track of the alpha particle would become visible as a line of water droplets.

Building a cloud chamber [local] can be either a classroom demonstration [local] or experiment activity [local].