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Television would never have developed without the cathode ray tube (CRT). It was invented by a German, Karl Ferdinand Braun, in 1897. A CRT is a glass vacuum tube with a narrow neck that flares into a flat "screen" at one end. The inside surface of the screen is covered with a phosphorescent layer. Electrons are fired at high voltage along the tube, illuminating when they hit the phosphorescent layer . Refined versions of this basic tube were at the heart of millions of TVs, radars and computer monitors manufactured in the 20th century.