Some History Thomas Edison's original electric light, introduced in 1879, was not very practical or efficient. The carbon filament in the lamp burned out easily and the lamp wasted electricity. It produced only 3.5 lumens per watt (lpw)-an efficiency of less than a quarter of a percent. Later filaments were made from metalized carbon or tantalum. These materials produced brighter illumination, but they were still very brittle.
In the early twentieth century, William Coolidge explored the metallurgy of tungsten. Tungsten is a notoriously brittle metal, but it evaporates slower than carbon when heated to high temperatures. (Higher temperatures produce brighter light and greater efficiency.) Coolidge developed a process to make "ductile" tungsten wire. In 1913, Irving Langmuir discovered that, by coiling the tungsten filament and placing an inert gas like nitrogen inside the bulb, he could increase the efficiency and the usable bulb life. Frosted bulbs and other modifications followed, but no dramatic improvements of the incandescent bulb occurred until "halogen" bulbs were developed in the 1950's.
How Bulbs "Burn Out" As electric current passes through the filament of a conventional bulb, the filament's temperature rises to about 2500° C. Although tungsten is the most temperature resistant filament material known, it is highly reactive when hot. Light bulbs are filled with an inert gas such as nitrogen or argon to prevent the filament from reacting with air. At 2500° C tungsten vapor is given off by the hot filament and is deposited on the glass, darkening it. Bulbs thus have to be made large enough so that the tungsten coating does not become opaque. Since evaporation occurs unevenly along the filament, eventually part of the filament becomes significantly thinner than the rest. When the part of the filament melts or becomes weak and breaks, the bulb burns out.
In a halogen bulb the fill gas includes traces of a highly reactive element, usually iodine or bromine, rather than the inert gas used in standard bulbs. The halogen combines with the cooling tungsten atoms on or near the glass to produce tungsten halides, which evaporate fairly easily. When these compounds reach the filament again, the intense heat breaks the halide down and deposits tungsten back on the filament, releasing halogen gas. This tungsten recycling significantly slows the filament's decay as well as the darkening of the glass. Therefore the bulb shines brighter and lasts longer.
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