<P> From 1898 to around 1905, osmium was also used as a lamp filament in Europe, and the metal was so expensive that used broken lamps could be returned for partial 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 . </P> <P> 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 . </P> <P> To improve the efficiency of the lamp, the filament usually consists of multiple coils of coiled fine wire, also known as a' coiled coil' . Light bulbs using coiled coil filaments are sometimes referred to as' double - coil bulbs' . 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 . As a result, the filament can then run hotter, which results in a more efficient light source, while reducing the evaporation so that the filament will last longer than a straight filament at the same temperature . </P> <P> 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 . </P>

The intensity l of light from a light bulb