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William Coolidge: Vacuum Tube for X-Ray Production, 1926

Improving Performance

On December 30, 1908, Coolidge married Ethel Woodward and they went on to have a daughter, Elizabeth, and a son, Lawrence. Following a serious illness, Ethel died in February, 1915. In 1916, Coolidge remarried. His second wife was Dorothy MacHaffie, the nurse who had come in to help raise his orphaned children.

On arriving at General Electric to join the group of outstanding scientists, Coolidge began working on the search to improve the power performance of the incandescent lamp. The carbon filament in Edison's first commercially produced lamp provided 3 lumens per watt and this figure had been improved by Whitney's development of the GEM (General Electric Metallic) lamp containing a filament of cellulose baked to a high temperature and "metallized."

Further competition to discover improved filament materials came from Europe where energy cost was much higher. Osmium, tantalum, and tungsten were among the more effective candidates tried. Tantalum was found useful only when used with a direct current (DC) power supply. While DC was the chosen method of Thomas Edison in his first Manhattan generating plant, it had been replaced in worldwide acceptance by Tesla's AC (alternating current system).

Coolidge began work on optimizing the use of tungsten as the lamp filament material. Tungsten has the highest melting point and lowest vapor pressure of all metals, and at temperatures over 1650C has the highest tensile strength. It has excellent corrosion resistance. Coolidge devised a method of treating the raw material before its reduction to the metallic element, and on a method of forging the metal powder to produce a pliable wire. This tungsten filament lamp came to market in 1911 and continues to be widely used today. His experience with the properties of tungsten would serve Coolidge well in his later namesake invention, the Coolidge Tube.