The Old Style Cathode Ray Tube

The cathode ray tube is a very common device used in television sets and computer screens. Inside the tube is a vacuum, so the electrons are free to move and do not hit any gas molecules. Electrons are emitted from a negative cathode at A, which is heated to aid the emission of electrons by thermionic emission, and accelerated and focused into a beam by the anode, cooled so as not to emit electrons by thermionic magnets at B. Having passed through the focusing magnets at B, they enter an electric field between C and D. These electric fields can be changed so the beam of electrons is directed towards certain spots on the screen. When they hit the screen, which is coated with a fluorescent material, they emit light of a colour which depends of the applied voltage.

Television and computer screens work by moving the beam of electrons across and down the screen many times a second. Then the applied voltage is changed so that light of the required colour is emitted from the required spot on the screen. This is done for every spot on the screen many time a second to give the appearance of a moving picture.

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