Gas Filled Incandescent Lamp

Gas Filled Incandescent Lamp :

Construction :
         Construction is same as vacuum lamps.

Working :

• A metal filament can work in an evacuated bulb upto 2,000°C without oxidation and if it is worked beyond this temperature it vaporizes quickly and blackens the lamp.
• For higher efficiency it is necessary to use working temperature more than 2,000°C keeping down the evaporation, which is possible by filling the bulb with an inert gas-argon with a small percentage of nitrogen.
• Nitrogen is added to reduce the possibility of arcing. Krypton is the best gas for this purpose but it is so expensive that it is used only in special purpose lamps, such as miner lamps.
• The tungsten filament temperature of 2,400°C to 2,750°C according to the size of the lamp.
• However, due to presence of gas there is heat loss due to convection currents. This loss depends upon the surface area of filament.
• As such, coiled coil filaments, which take much less space compared with coiled filaments, are used with such lamps.
• A coiled coil filament is made by winding tungsten wire on a fine iron wire to form a spiral which is again wound on to a thick iron wire to form a coiled coil. (Iron is later on dissolved out by acid).
• Helical filaments also have a slower rate of tungsten evaporation. This evaporation eventually causes bulb blackening because the tungsten vapour condenses as a black film on the inner surface of the bulb.
• In a gas-filled lamp, the hot gas carries the tungsten vapour upward, Therefore, a black spot forms at the top of the bulb instead of spreading over the entire inner surface, as in a high-vacuum bulb. Chemicals called 'getters' are often placed inside the bulb to capture tungsten vapour and thereby reduce the rate of blackening.

• A piece of wire mesh called a collector grid may also be attached to each lead in wire to attract the particles of tungsten vapour. Efficiency of coiled coil lamp is about 12 lumen/watt.
• For low wattage lamps, however, the heat loss due to introduction of gas is more than in medium wattage lamps, so for low wattage (upto 40 watts) vacuum type lamps are used.