<Li> W and W: water inlet and outlet of the cooling device </Li> <P> The Crookes tube was improved by William Coolidge in 1913 . The Coolidge tube, also called hot cathode tube, is the most widely used . It works with a very good quality vacuum (about 10 Pa, or 10 Torr). </P> <P> In the Coolidge tube, the electrons are produced by thermionic effect from a tungsten filament heated by an electric current . The filament is the cathode of the tube . The high voltage potential is between the cathode and the anode, the electrons are thus accelerated, and then hit the anode . </P> <P> There are two designs: end - window tubes and side - window tubes . End window tubes usually have "transmission target" which is thin enough to allow X-rays to pass through the target (X-rays are emitted in the same direction as the electrons are moving .) In one common type of end - window tube, the filament is around the anode ("annular" or ring - shaped), the electrons have a curved path (half of a toroid). </P>

What is the function of the filament in the x ray tube
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