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Cathode Ray Tube Television

For decades, the Cathode Ray Tube was used for video displays from televisions to computer screens. 

Essentially, a cathode ray tube consists of a glass vacuum tube in which a negatively charged plate (called a cathode) is heated so it emits electrons. These electrons are focused and beamed onto a surface to create an image. While smaller CRTs used electrostatic deflection, which occurs when the electron beam is deflected as it passes through charged metal plates, most relied on electromagnetic deflection, which uses magnets to move the electron beam as seen in this tutorial.

Instructions

  1. This simplified CRT shows the cathode ray coming from the electron gun at the back of the tube. It shoots at a screen coated with phosphor, which glows when hit by the electrons.
  2. Note the magnets to the side and below the cathode ray tube.
  3. As the magnets move, notice how the electromagnetic deflection scatters the electron beam across the screen in a series of horizontal lines from top to bottom.
  4. While in auto mode, the magnets move in sequence to sweep the electrons evenly across the screen. This happens in rapid succession, dozens of times each second, to create a moving image.
  5. Adjust the speed slider to get a better sense of this sweeping pattern
  6. Press the manual button and then click on the magnets to move them yourself and see how their position impacts the direction of the electron beam. Notice that moving one magnet vertically deflects the electrons horizontally, and vice versa. This is explained by the left hand rule.
  7. If you double-click on a magnet, it will flip, reversing the magnetic field and changing the electromagnetic deflection.

The two permanent magnets shown are handy for depicting the principle of electromagnetic deflection; in real CRTs, however, two pair of electromagnetic coils are used to bend beams of electrons; the angles of deflection are altered by varying the strengths of the coils, rather than physically moving them.

This CRT demonstrates a black and white television image.

For color television, CRTs were significantly more complex, using three electron beams, multiple phosphors and a shadow mask.