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ArrowElectromagnetic Deflection in a Cathode Ray Tube, II

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Long before the development of plasma TVs and LCD computer monitors, there were cathode ray tubes. In fact, despite newer technologies, the cathode ray tube, or CRT, still forms the backbone of the video display industry. Invented by German scientist Karl Ferdinand Braun in 1897, it has seen many advances since then.

Essentially the device consists of a glass vacuum tube in which an image is generated when a negatively charged plate (called a cathode) is heated so it emits electrons. These electrons are focused and beamed onto a surface coated with phosphor, which glows when hit by radiation. To get this glow (and therefore the image) to appear across an entire screen, the beam of electrons must be deflected to every possible spot across the screen. Most smaller CRTs use electrostatic deflection, which occurs when the electron beam is deflected as it passes through charged metal plates; the direction of deflection depends on the amount of charge and polarity of the plates. Below is illustrated electromagnetic deflection, more common in TVs, computer screens and other larger CRTS, which uses magnets to move the electron beam.



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The tutorial, a simplified depiction of how a CRT works, can be set on automatic or adjusted manually by clicking the appropriate radio button.

When the tutorial is in manual mode, the beam of electrons emitted by the cathode in the electron gun will hit the center of the screen until one of the two external magnets is moved. Click and drag the magnets to move them closer to and farther away from the beam of electrons. Notice that moving one magnet vertically deflects the electrons horizontally, and vice versa. This is explained by the left hand rule.

Try moving the magnet oriented horizontally toward the electron beam, with its north pole (in red) facing toward the beam. The field lines outside the magnet are flowing from its north down to its south pole; so, according to the left hand rule, the electrons entering that field at a right angle to it will be deflected toward the top of the screen; the closer the magnet gets to the beam, the stronger its effect. If you now drag that same magnet away from the beam, gradually diminishing the effects of its field, you can return the beam to the center of the screen. Now, if you double-click on that same magnet, you can flip it to reverse the magnetic field. As a result (again using the left hand rule), dragging the magnet will deflect the electron beam toward the bottom of the screen and back.

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.

To observe how an entire display is scanned to produce an image, use the automatic mode of the tutorial. This triggers the rapid movement of both magnets in such a way that the electron beam sweeps across the screen in a series of horizontal lines from top to bottom in a raster pattern. The glowing areas of the phosphor screen fade soon after they have been passed over by the electron beam. For a continuous image to appear to the human eye, the scanning process repeats dozens of times each second. The process is depicted much slower in the tutorial, but as in many computer monitors, the refresh rate can be varied. To do so, adjust the speed slider. When the refresh rate is too low, flickering of the image occurs, which can cause headaches and eye strain.

In both manual and automatic modes, the intensity of the light emitted by the screen can be controlled by moving the brightness slider. This brightness is directly related to the amount of electrons hitting the screen per second. In most CRT display devices, this number is automatically adjusted according to the picture signal received and involves a variation in voltage. This enables the proper shades of black, white and gray. Brightness also affects the shades of various colors that appear on a color display device, which are significantly more complex than the black-and-white CRT display presented here, using three electron beams, multiple phosphors and a shadow mask.

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