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.