A tungsten light bulb consists of a filament of tungsten wire inside a glass bulb that is filled with an inert gas such as argon. The bulbs are made from clear, coloured or frosted glass. Wires from the filament to the bulb base provide electric contact to a power supply. The electric current heats the filament to 1,704 to 2,982 degrees Celsius (3,100 to 5,400 degrees Fahrenheit) and emits visible light.
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Incandescence is the emission of visible light from a hot body. When an electric current passes through a filament – a thin thread – of material, the resistance of the material’s atoms to the current creates the heat. The filament emits visible light when white hot. The first incandescent lamps developed by inventors in Europe and the United States used platinum, iridium, and carbonised cotton filaments. However, these lamps had a poor luminosity and the filaments evaporated -- sublimed -- quickly in the heat.
At the beginning of the 20th century, Hungarians Sandor Just and Ferenc Hanaman first patented the use of a tungsten filament lamp. This was marketed by the Hungarian company Tungsram. The General Electric Company and American physicist William Coolidge followed on from this and improved and patented a tungsten filament manufacture method. These inventions made use of the properties of ductile tungsten that has a high melting point of 3,410 degrees Celsius (6,170 degrees Fahrenheit), a higher tensile strength than steel, and a low evaporation rate at high temperatures. Ductile tungsten can be formed easily into filament coils.
The tungsten filament also slowly evaporates and eventually fails. The metal condenses in black spots on the inside of the bulb. Original incandescent lamps were made using vacuum bulbs. However, the luminosity of the light and the bulb’s blackening due to filament sublimation decreases if it is filled with inert gases, such as krypton or argon.
Tungsten light bulbs are inefficient light sources. Most of the energy used in them is converted to heat rather than light. The European Union is phasing out tungsten bulbs for this reason. However, the bulbs are cheaper to buy than halogen or LED lights and remain popular with consumers because they provide a pleasant colour of light.
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