The Coolidge tube is a glass tube in which X-rays are produced. William David Coolidge, impressed by Wilhelm Conrad Rontgen's discovery of X rays, set out to build a machine that could produce the high voltages indispensable to obtaining them. Basing his mechanism on Rontgen's work, Coolidge produced a device in which air was contained at low pressure in a tube where its positive ions struck the cathode, generating electrons that in turn struck the anode, generating X rays. His machine was sufficiently successful to be purchased by a local physician for X-ray diagnosis.

A decade later, while working on ductile tungsten and molybdenum in 1909, Coolidge experimented with these two chemicals to see if they could be used as X-ray targets. Three years later, after tungsten had been used successfully to mass-produce Mazda C bulbs, Coolidge tried to use solid tungsten as the cathode and anode of an X-ray-generating tube that replaced low-pressure gas with a vacuum. Working with his colleague Irving Langmuir, Coolidge had realized that the hot metal of the cathode in a vacuum emits electrons (this effect had been studied previously by Thomas Alva Edison, who called it the "Edison effect"; today scientists use the term "thermionic emission"). By heating the cathode in his tube, Coolidge could generate X rays more reliably than was the case with the previous system using low-pressure gas.

Different Coolidge tubes were produced according to their intended uses. The first tube that was produced immediately proved itself to be useful for dental diagnosis and found a ready market. Further development of larger units extended the popularity and application of Coolidge tubes in radiology. During World War I, Coolidge was asked to create portable units to be used in field hospitals. Later, when physicians hoped to extend the use of X rays for deep therapy, Coolidge successfully developed a process to increase the voltage of his tubes while keeping the target cool by irrigating it with a flow of cold water.

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