target+cathode

SUBJECT AREA: Electricity, Metallurgy

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Coolidge obtained his BS from the Massachusetts Institute of Technology (MIT) in 1896, and his PhD (physics) from the University of Leipzig in 1899. He was appointed Assistant Professor of Physics at MIT in 1904, and in 1905 he joined the staff of the General Electric Company's research laboratory at Schenectady. In 1905 Schenectady was trying to make tungsten-filament lamps to counter the competition of the tantalum-filament lamps then being produced by their German rival Siemens. The first tungsten lamps made by Just and Hanaman in Vienna in 1904 had been too fragile for general use. Coolidge and his life-long collaborator, Colin G. Fink, succeeded in 1910 by hot-working directly dense sintered tungsten compacts into wire. This success was the result of a flash of insight by Coolidge, who first perceived that fully recrystallized tungsten wire was always brittle and that only partially work-hardened wire retained a measure of ductility. This grasped, a process was developed which induced ductility into the wire by hot-working at temperatures below those required for full recrystallization, so that an elongated fibrous grain structure was progressively developed. Sintered tungsten ingots were swaged to bar at temperatures around 1,500°C and at the end of the process ductile tungsten filament wire was drawn through diamond dies around 550°C. This process allowed General Electric to dominate the world lamp market. Tungsten lamps consumed only one-third the energy of carbon lamps, and for the first time the cost of electric lighting was reduced to that of gas. Between 1911 and 1914, manufacturing licences for the General Electric patents had been granted for most of the developed work. The validity of the General Electric monopoly was bitterly contested, though in all the litigation that followed, Coolidge's fibering principle was upheld. Commercial arrangements between General Electric and European producers such as Siemens led to the name "Osram" being commonly applied to any lamp with a drawn tungsten filament. In 1910 Coolidge patented the use of thoria as a particular additive that greatly improved the high-temperature strength of tungsten filaments. From this development sprang the technique of "dispersion strengthening", still being widely used in the development of high-temperature alloys in the 1990s. In 1913 Coolidge introduced the first controllable hot-cathode X-ray tube, which had a tungsten target and operated in vacuo rather than in a gaseous atmosphere. With this equipment, medical radiography could for the first time be safely practised on a routine basis. During the First World War, Coolidge developed portable X-ray units for use in field hospitals, and between the First and Second World Wars he introduced between 1 and 2 million X-ray machines for cancer treatment and for industrial radiography. He became Director of the Schenectady laboratory in 1932, and from 1940 until 1944 he was Vice-President and Director of Research. After retirement he was retained as an X-ray consultant, and in this capacity he attended the Bikini atom bomb trials in 1946. Throughout the Second World War he was a member of the National Defence Research Committee.

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