Filament
The first successful light bulb filaments were made of carbon (from carbonized paper or bamboo). Early carbon filaments had a negative temperature coefficient of resistance — as they got hotter, their electrical resistance decreased. This made the lamp sensitive to fluctuations in the power supply, since a small increase of voltage would cause the filament to heat up, reducing its resistance and causing it to draw even more power and heat even further. In the "flashing" process, carbon filaments were heated by current passing through them, while in an evacuated vessel containing hydrocarbon (gasoline) vapor. The carbon deposited by this treatment improved the uniformity and strength of filaments, and their efficiency. A metallized or graphitized filament was first heated in a high-temperature oven before flashing and lamp assembly; this transformed the carbon into graphite, which further strengthened and smoothed the filament, and as a byproduct had the advantage of changing the lamp to a positive temperature coefficient like a metallic conductor. This helped stabilize power consumption, temperature and light output against minor variations in supply voltage.
In 1902, the Siemens company developed a tantalum lamp filament. These lamps were more efficient than even graphitized carbon filaments and could operate at higher temperatures. Since the metal had a lower resistivity than carbon, the tantalum lamp filament was quite long and required multiple internal supports. The metal filament had the property of gradually shortening in use; the filaments were installed with large loops that tightened in use. This made lamps in use for several hundred hours quite fragile. Metal filaments had the property of breaking and re-welding, though this would usually decrease resistance and shorten the life of the filament. General Electric bought the rights to use tantalum filaments and produced them in the US until 1913.
From 1898 to around 1905, osmium was also used as a lamp filament in Europe, but the metal was so expensive that used broken lamps could be returned for part credit. It could not be made for 110 V or 220 V so several lamps were wired in series for use on standard voltage circuits.
In 1906, the tungsten filament was introduced. Tungsten metal was initially not available in a form that allowed it to be drawn into fine wires. Filaments made from sintered tungsten powder were quite fragile. By 1910, a process was developed by William D. Coolidge at General Electric for production of a ductile form of tungsten. The process required pressing tungsten powder into bars, then several steps of sintering, swaging, and then wire drawing. It was found that very pure tungsten formed filaments that sagged in use, and that a very small "doping" treatment with potassium, silicon, and aluminium oxides at the level of a few hundred parts per million greatly improved the life and durability of the tungsten filaments.
To improve the efficiency of the lamp, the filament usually consists of coils of coiled fine wire, also known as a 'coiled coil.' For a 60-watt 120-volt lamp, the uncoiled length of the tungsten filament is usually 22.8 inches (580 mm), and the filament diameter is 0.0018 inches (0.046 mm). The advantage of the coiled coil is that evaporation of the tungsten filament is at the rate of a tungsten cylinder having a diameter equal to that of the coiled coil. The coiled-coil filament evaporates more slowly than a straight filament of the same surface area and light-emitting power. If the filament is then run hotter to bring back evaporation to the same rate, the resulting filament is a more efficient light source.
There are several different shapes of filament used in lamps, with differing characteristics. Manufacturers designate the types with codes such as C-6, CC-6, C-2V, CC-2V, C-8, CC-88, C-2F, CC-2F, C-Bar, C-Bar-6, C-8I, C-2R, CC-2R, and Axial.
Electrical filaments are also used in hot cathodes of fluorescent lamps and vacuum tubes as a source of electrons or in vacuum tubes to heat an electron-emitting electrode.
Read more about this topic: Incandescent Light Bulb