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Tungsten Lightbulb

Fall 2010 | Volume 25 |  Issue 3
Inventor:
Era:
1900s

While Thomas Edison’s 1879 lightbulb represented an epochal advance, it remained far from perfect: its carbonized cellulose filament gulped power. In 1905 managers at General Electric’s pioneering research laboratory in Schenectady, New York, decided to figure out a way to improve filament performance. They hired 32-year-old William Coolidge, a research assistant to Arthur Noyes at MIT’s Department of Chemistry.

Coolidge picked up on the work of European bulb makers who had had some success using metal filaments. They found that tungsten, which has the highest melting point of any metal, looked promising but was brittle and exceedingly difficult to draw into wire. (GE already had tungsten bulbs on the market, made with a licensed German process, but they were expensive and so fragile that they broke when shaken, precluding them from universal use.)

Coolidge also took note of GE’s early work on creating ductile tungsten. Typically the engineers would add starch paste or other sticky organic material to tungsten powder, then squirt it through a die to form a fine thread. Heating burned off the temporary binder and sintered the particles of tungsten together. But there was a problem. “The organic binder, “Coolidge wrote, “left a trace of carbon in the tungsten filaments; and this carbon, in the lamps, vaporized out and blackened the bulbs.” He started to evaluate binders that didn’t contain carbon.

He found his answer from an unlikely source. “Some little time before coming to Schenectady,” Coolidge explained, “I had watched my dentist prepare silver amalgam for one of my teeth, and had been impressed by its plasticity.” Dental fillings are prepared by dissolving, or amalgamating, silver in mercury, usually with smaller amounts of other metals. The result is a conveniently moldable substance that conforms to the shape of a cavity and quickly hardens in place. Was it possible that some amalgam might serve as a temporary binder for the tungsten powder?

After much experimentation, Coolidge made a breakthrough. He dissolved tungsten powder in a cadmium amalgam and then forced the resulting material through a small hole to form a wire. Heating this wire in a vacuum led to the evaporation of the cadmium mercury, which left pure tungsten. This could be rolled and pressed mechanically and then drawn into strong, ultrathin wire.

Shortly thereafter GE used a modified version of this method to mass-produce inexpensive lightbulbs that burned one-third less energy than conventional filaments. Ductile tungsten also gave a great boost to the emerging technologies of radio and electronics. Coolidge went on to develop an X-ray tube that used tungsten wire for the cathode and a piece of sintered tungsten for the target. These advances made X-ray tubes cheap and reliable enough to be used in doctors’ offices.

In 1923 things came full circle when the Victor X-Ray Corporation of Chicago, which would soon became part of GE, brought out the first compact, safe, and easy-to-use dental X-ray machine. As today’s patients sit in dentists’ chairs, they benefit from an inspiration that another dental patient had in the same situation nearly a century ago.

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