Mercury vapour lamps, 1900.
© Science Museum / Science & Society Picture Library
The Cooper-Hewitt low presure mercury vapour lamp of 1900 (centre) had an arc drawn between electrodes, operated by switching on and tilting the lamp slowly. The arc vapourised mercury, producing light of poor colour-rendering capacity. A revival and refinement of the mercury lamp occurred in 1930s Britain. Below and to the right are high presure mercury vapour lamps with inner bulbs of quartz, 250 and 400 watt, c 1932. Lamps above and left (c 1937) combine mercury of high presure with a filament, giving better colour.