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What is a cathode ray tube and how does it work


A cathode ray tube is a specialized vacuum tube in which some kind of visual presentation is created when an electron beam strikes a phosphorescent surface.

The beam is generated starting at an element called a cathode. It is heated by a heating element, and this causes thermionic emission, which is a phenomenon where electrons are “driven out of” the metal into an area just around it because of the thermal energy. The electrons form what is called a space charge right there. If we apply a high positive voltage to the front of the tube (which is coated with phosphors), it will attract the negatively charged electrons. They will be accelerated across the space (and through the vacuum) between the cathode and the screen, which is the anode. The electrons are the “cathode rays” we name the tube for. Another term we sometimes use is beam current (after electron beam).

Between the two elements we’ve identified are other elements. One is close to the cathode, and it’s the control grid. It’s a kind of screen, and if we apply a negative voltage there, the electrons, which want to fly to the big positive charge, won’t be able to. The control grid “controls” the electron flow. If we apply a bit of positive voltage on the control grid, some beam current will flow. The more we drive the control grid positive, the more beam current flows. The electrons are flying to the anode in the front of the tube and striking the phosphor coating. This cause them to be excited and emit light. The phosphor coating is on the inside of the heavy glass “front” of the tube.

We’ll have things ahead of the control grid that will focus the beam, and then we’ll use one of two basic methods to “direct” or “aim” or “sweep” the beam (magnetic or electrostatic deflection). Think of the beam as a pencil of light writing on the front of the tube. It writes very quickly by scanning across in a line, turning off and “retracing” to get back to the othere side, and then scanning another line below the last one. The beam is turned on and off (modulated) a little, a lot, or something in between as it sweeps. It paints an image of some kind by scanning all the lines. Then it repeats. This is the basic operation of the cathode ray tube. You’ll want a link to do some follow up, and you can find one below.

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