<P> Neon lighting consists of brightly glowing, electrified glass tubes or bulbs that contain rarefied neon or other gases . Neon lights are a type of cold cathode gas - discharge light . A neon tube is a sealed glass tube with a metal electrode at each end, filled with one of a number of gases at low pressure . A high potential of several thousand volts applied to the electrodes ionizes the gas in the tube, causing it to emit colored light . The color of the light depends on the gas in the tube . Neon lights were named for neon, a noble gas which gives off a popular orange light, but other gases and chemicals are used to produce other colors, such as hydrogen (red), helium (yellow), carbon dioxide (white), and mercury (blue). Neon tubes can be fabricated in curving artistic shapes, to form letters or pictures . They are mainly used to make dramatic, multicolored glowing signage for advertising, called neon signs, which were popular from the 1920s to the 1950s . </P> <P> The term can also refer to the miniature neon glow lamp, developed in 1917, about seven years after neon tube lighting . While neon tube lights are typically meters long, the neon lamps can be less than one centimeter in length and glow much more dimly than the tube lights . They are still in use as small indicator lights . Through the 1970s, neon glow lamps were widely used for numerical displays in electronics, for small decorative lamps, and as signal processing devices in circuity . While these lamps are now antiques, the technology of the neon glow lamp developed into contemporary plasma displays and televisions . </P>

When is light given off in a neon light
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