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Neon Lamps Geissler Tubes

Neon lamps used for purposes of commercial advertisement consist of long glass tubes bent into the required shape and fitted with suitable terminals at the ends. The tubes are initially evacuated and then filled with ends. The tubes are initially evacuated and then fitted with neon gas at a pressure of 0.5 cm of Hg. When a high potential of the order of 15000 volts is applied across the terminals, a current flows through the tube and causes the neon gas to glow with a bright orange-red colour. The colour of light can be made blue using a mixture of argon and neon gases with mercury vapour. Using quartz tubing instead of glass, green light can be obtained instead of blue. It is very economical to use neon tubes and the high visibility of neon light makes it useful for aeroplane beacons and traffic signals. Geissler tube is a simple form of discharge tube. C is a capillary tube of diameter about 1.5 mm. The capillary tube is connected to two wider tubes A and B at its ends. The two electrodes are fitted and the tube us filled with the required gas (neon, argon, hydrogen or helium) at low pressure and discharge is struck by applying a suitable high potential. The tube glows and the maximum brightness is observed in the capillary tube. Geissler tubes are mostly used to study the spectra of different gases at low pressure.