Geissler tube (gĪsˈlər) [key], gas discharge tube in which light is produced when an electric discharge passes through the rarefied gas in the tube. The color of the glow depends on the gas used. The tubes are made in a variety of shapes and are especially useful in spectroscopy. One later form of the tube is widely used in luminous signs ("neon" signs) and another in fluorescent lamps.
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