One of the first photographs to show the tracks of subatomic particles in a cloud chamber. The picture was taken by the cloud chamber's inventor, English physicist C. T.R. Wilson, at the Cavendish Laboratory in Cambridge in 1911. The tracks are due to alpha particles emitted by a small amount of radium on the top of a metal tongue inserted into the cloud chamber. As an electrically charged particle moves through the water vapour in a cloud chamber, it ionises the gas & water drops condense on the ions, thus producing a track of droplets where the particle has passed. Wilson was awarded the physics Nobel Prize for his invention in 1927.