If some gas at low pressure is contained in a tube fitted with metal electrodes at each end the gas glows with a characteristic colour when a high voltage is applied to the electrodes. The electric field set up inside the tube causes ions present in the gas to move with high speeds. As a result of collisions which occur between ions and gas molecules, light is emitted of a colour which depends on the nature of the gas. Neon, for example, gives the familiar bright orange-red seen in advertising signs.
Following the successful commercial introduction of these coloured gas tubes in the 1920s, manufacturers turned their attention to the development of discharge tubes for general illumination. One of the results of their research is to be seen in the orange sodium vapour lamps and the blue-green mercury vapour lamps used for street lighting. Although these emit coloured light, they give five times more luminous energy per watt than the best filament lamps, and so are cheaper to run.