Illustration: Serge Bloch
Before it could flourish, electric lighting had to defeat an entrenched competitor in many towns—gas. In the mid- and late 19th century, gaslight simply meant a bare gas flame, so the electric lamp, which produced no smoke, was a clear winner. But the gas industry didn't give up easily; it developed the gas mantle during the 1880s, then greatly improved it in the 1890s. This fine mesh, made mainly of thorium oxide, became incandescent when heated by a gas flame smaller than what had been used in previous generations of gas lamps. This technology also allowed the industry to change the composition of its gas so that the flames produced more heat and less light—and less smoke. The gas mantle turned out to be a cheaper source of light than the carbon filament lamp. Score round one to gas.
Starting around 1899, electricity answered the gas mantle with the metal filament, which could be operated at a hotter temperature, and therefore more efficiently, than a carbon filament. Developers tried several different filament materials. Osmium, tantalum, and tungsten have the highest melting points in the metals family but differ in their malleability. Initially, lamp manufacturers used osmium, also seen in the tips of fountain pens and in some heavy-duty electrical contacts, and tantalum, which was first isolated in 1902. Tungsten was attractive because it has the highest melting point of all metals—just over 3400 °C. But its brittleness stymied developers who were trying to draw it into a thin wire. Then Alexander Just and Franz Hanaman, working in Vienna and Budapest, found that they could make tungsten filaments by mixing tungsten powder with a binder and then drawing that mixture into a wire and sintering it—that is, heating it until the particles adhere but do not melt. Hugo Hirst, of the (British) General Electric Co., working with Just and Hanaman, began producing tungsten lamps in 1909, in a factory in West London. William Coolidge, of the (U.S.) General Electric Co., found that if he compressed tungsten powder and hammered it, he could draw it into a wire without using any binder, which was a simpler process. (There was no connection between the two General Electric companies.) Thus was born, in 1911, the drawn-tungsten filament incandescent lamp. It continues to be the standard in incandescent bulbs to this day, 100 years later.
In early tungsten lamps, the filaments sat in near vacuums, but it turned out that a little nitrogen or argon reduced the evaporation of the metal and prolonged the filament's life. The problem was that the gas also cooled the filament, making the lamp less efficient. Winding the filament in a coil reduced the cooling, and winding the coil itself into a coil, a technique developed in the early 1930s, worked even better. And that coiled-coil filament design has never been superseded.
In 1959, General Electric (U.S.) refined the filament lamp one more time. Its researchers sealed a tungsten filament into a compact bulb containing an inert gas and a small amount of a halogen, usually iodine or bromine. (The halogens are a group of elements that react very readily and energetically with other substances.)
In a halogen bulb, the halogen gas combines with the minute particles of tungsten that evaporate from the filament, which in ordinary incandescent lamps are deposited mostly on the inner surface of the bulb and over time gradually dim the light output. The tungsten halide that forms moves around as a gas and then, when it nears the hot filament, breaks down, redepositing the tungsten back onto the filament and releasing the halogen to repeat the process.
This halogen cycle keeps the bulb clean and the light output almost constant over the life of the bulb. The bulb temperature must be higher than in conventional incandescent lamps, too high for glass at the time, so the bulb was initially made of quartz. Because the first halogen lamps used iodine as the halogen, they were known as "quartz iodine" lamps. Later, bromine replaced iodine, higher-melting-point glass replaced the expensive quartz, and the lamps became "tungsten halogen" lamps. The bulbs soon caught on for spotlights and projectors and eventually for general lighting. Right now, because they are somewhat more efficient than the standard incandescent lamp, they are not on the chopping block in any country.