The invention: Metal filament used in the incandescent light bulbs that have long provided most of the world’s electrical lighting.
The people behind the invention:
William David Coolidge (1873-1975), an American electrical engineer
Thomas Alva Edison (1847-1931), an American inventor
The Incandescent Light Bulb
The electric lamp developed along with an understanding of electricity in the latter half of the nineteenth century. In 1841, the first patent for an incandescent lamp was granted in Great Britain. A patent is a legal claim that protects the patent holder for a period of time from others who might try to copy the invention and make a profit from it. Although others tried to improve upon the incandescent lamp, it was not until 1877, when Thomas Alva Edison, the famous inventor, became interested in developing a successful electric lamp, that real progress was made. The Edison Electric Light Company was founded in 1878, and in 1892, it merged with other companies to form the General Electric Company.
Early electric lamps used platinum wire as a filament. Because platinum is expensive, alternative filament materials were sought. After testing many substances, Edison finally decided to use carbon as a filament material. Although carbon is fragile, making it difficult to manufacture filaments, it was the best choice available at the time.
The Manufacture of Ductile Tungsten
Edison and others had tested tungsten as a possible material for lamp filaments but discarded it as unsuitable. Tungsten is a hard, brittle metal that is difficult to shape and easy to break, but it possesses properties that are needed for lamp filaments. It has the highest melting point (3,410 degrees Celsius) of any known metal; therefore, it can be heated to a very high temperature, giving off a relatively large amount of radiation without melting (as platinum does) or decomposing (as carbon does). The radiation it emits when heated is primarily visible light. Its resistance to the passage of electricity is relatively high, so it requires little electric current to reach its operating voltage. It also has a high boiling point (about 5,900 degrees Celsius) and therefore does not tend to boil away, or vaporize, when heated. In addition, it is mechanically strong, resisting breaking caused by mechanical shock.
William David Coolidge, an electrical engineer with the General Electric Company, was assigned in 1906 the task of transforming tungsten from its natural state into a form suitable for lamp filaments. The accepted procedure for producing fine metal wires was (and still is) to force a wire rod through successively smaller holes in a hard metal block until a wire of the proper diameter is achieved. The property that allows a metal to be drawn into a fine wire by means of this procedure is called “ductility.” Tungsten is not naturally ductile, and it was Coolidge’s assignment to make it into a ductile form. Over a period of five years, and after many failures, Coo-lidge and his workers achieved their goal. By 1911, General Electric was selling lamps that contained tungsten filaments.
Originally, Coolidge attempted to mix powdered tungsten with a suitable substance, form a paste, and squirt that paste through a die to form the wire. The paste-wire was then sintered (heated at a temperature slightly below its melting point) in an effort to fuse the powder into a solid mass. Because of its higher boiling point, the tungsten would remain after all the other components in the paste boiled away. At about 300 degrees Celsius, tungsten softens sufficiently to be hammered into an elongated form. Upon cooling, however, tungsten again becomes brittle, which prevents it from being shaped further into filaments. It was suggested that impurities in the tungsten caused the brittleness, but specially purified tungsten worked no better than the unpurified form.
Many metals can be reduced from rods to wires if the rods are passed through a series of rollers that are successively closer together. Some success was achieved with this method when the rollers were heated along with the metal, but it was still not possible to produce sufficiently fine wire. Next, Coolidge tried a procedure called “swaging,” in which a thick wire is repeatedly and rapidly struck by a series of rotating hammers as the wire is drawn past them. After numerous failures, a fine wire was successfully produced using this procedure. It was still too thick for lamp filaments, but it was ductile at room temperature.
Microscopic examination of the wire revealed a change in the crystalline structure of tungsten as a result of the various treatments. The individual crystals had elongated, taking on a fiberlike appearance. Now the wire could be drawn through a die to achieve the appropriate thickness. Again, the wire had to be heated, and if the temperature was too high, the tungsten reverted to a brittle state. The dies themselves were heated, and the reduction progressed in stages, each of which reduced the wire’s diameter by a thousandth of an inch.
Finally, Coolidge had been successful. Pressed tungsten bars measuring X x % x 6 inches were hammered and rolled into rods yg inch, or ^Xooo inch, in diameter. The unit Xooo inch is often called a “mil.” These rods were then swaged to approximately 3o mil and then passed through dies to achieve the filament size of 25 mil or smaller, depending on the power output of the lamp in which the filament was to be used. Tungsten wires of 1 mil or smaller are now readily available.
Impact
Ductile tungsten wire filaments are superior in several respects to platinum, carbon, or sintered tungsten filaments. Ductile filament lamps can withstand more mechanical shock without breaking. This means that they can be used in, for example, automobile headlights, in which jarring frequently occurs. Ductile wire can also be coiled into compact cylinders within the lamp bulb, which makes for a more concentrated source of light and easier focusing. Ductile tungsten filament lamps require less electricity than do carbon filament lamps, and they also last longer. Because the size of the filament wire can be carefully controlled, the light output from lamps of the same power rating is more reproducible. One 6o-watt bulb is therefore exactly like another in terms of light production.
Improved production techniques have greatly reduced the cost of manufacturing ductile tungsten filaments and of light-bulb manufacturing in general. The modern world is heavily dependent upon this reliable, inexpensive light source, which turns darkness into daylight.
See also Fluorescent lighting; Memory metal; Steelmaking process.
