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


In 1895, Thomas Edison took up the investigation of X-ray fluorescence and after his laboratory's typical exhaustive research, produced a fluoroscope in which the target material producing the X-rays was calcium tungstate. William Coolidge redesigned and reinvented the X-ray generator tube by alteration and substitutions of its characteristics.

Earlier tubes contained electrodes and gases which were ionized by high voltage, creating positive ions that impacted the cathode and resulted in a cathode ray beam. The beam was directed at a metal anode with the high energy impact, creating X-rays.

His previous experience and knowledge of the properties of tungsten in light bulb filaments led Coolidge to use it in his improvements to the X-ray generator tube. Tungsten, a transition metal of high atomic weight, has a low vapor pressure, the highest melting point in the periodic table, and good electrical conductivity. In his tube, Coolidge replaced the cathode material with a tungsten filament, substituted tungsten as the target anode material, created a high vacuum in the tube, and cooled the anode.

The new tungsten filament cathode produced energy via the thermionic effect first observed by Thomas Edison during his experiments with light bulb filaments. On being heated, the filament emitted a rapidly increasing number of thermions that adopted the negative charge applied. In the vacuum environment, the stream of thermions, a cathode ray, was attracted to the anode, also made from tungsten. The bombarded anode yielded the X-rays.

These combined modifications gained reliability from the properties of tungsten, controllable intensity from the ability to control the cathode filament temperature, and controllable penetrating power from adjustment of the anode voltage. This marked a great advantage in medical diagnostics and assured the Coolidge Tube's long-lived success.