Inert gas discharge tubes, c 1960s.
© Science Museum / Science & Society Picture Library
When a high voltage is applied to electrodes in a gas at a low presure, an electric current is carried ('discharged') by charged atoms of the gas. A characteristic light is emitted. In addition to modern discharge tubes, the photograph also shows some of the discharge tubes used by William Ramsay (1852-1916) in his research on the inert (noble) gases. The modern coloured tubes above show the characteristic colours produced by the noble gases in a discharge tube. The red of neon was the first of these to be exploited commercially.