also called Vapour Lamp, lighting device consisting of a transparent container within which a gas is energized by an applied voltage and thereby made to glow. The French astronomer Jean Picard observed (1675) a faint glow in a mercury-barometer tube when it was agitated, but the cause of the glow (static electricity) was not then understood. The Geissler tube of 1855, in which gas at low pressure glowed when subjected to an electrical voltage, demonstrated the principle of the electric discharge lamp. After practical generators were devised in the 19th century, many experimenters applied electric power to tubes of gas. From about 1900, practical electric discharge lamps were in use in Europe and the United States. The French inventor Georges Claude was the first to use neon gas, about 1910. Mercury vapour in a neon lamp gives a bluish light; mercury is used also in fluorescent lamps and some ultraviolet lamps. Helium in amber glass glows gold; blue light in yellow glass shows green; combinations of gases give white light. The sodium-vapour lamp, developed about 1931 in Europe, is a good illuminant if the yellow colour of its light is acceptable. The glow lamp, used as an indicator or a night-light, contains a high-resistance filament in a small bulb. The voltage difference between plates at the ends of this filament causes the enclosed gas, usually neon or argon, to glow faintly. It uses little power and lasts a long time. Because the glow discharge tends to keep the voltage across the lamp constant, it is sometimes used as a voltage regulator. See also arc lamp; fluorescent lamp.

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