English: An early Crookes x-ray tube on display in the lobby of the Science Building at Winthrop University. These first generation 'cold cathode' tubes were used until about 1920. The electrode on the right is the cathode or negative electrode. The slanted electrode in the center of the bulb is the platinum anode or target. The electrode to the left is called the auxiliary anode. A high voltage applied between the cathode and anode ionized the small amount of residual air in the tube, creating cathode rays (electrons) at the cathode, which were accelerated and struck the anode, creating x-rays. The x-rays radiated through the side wall of the tube.
The two projections on the neck of the tube to the right is a 'softening device' to control the pressure in the tube. With time the residual gas in Wikipedia:Crookes tubes was absorbed by the walls, increasing the vacuum. This reduced the amount of x-rays produced and resulted in 'harder' more energetic x-rays, until finally the tube stopped working. The 'softeners' used various techniques to release a small amount of gas into the tube, restoring it's function.
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Photo by Y. Trottier
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Y. Trottier
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