Degauss
Degaussing, named after the German scientist Carl Friedrich Gauss, is typically the process of removing magnetism from a cathode ray tube (CRT), such as a television using a degaussing coil. Undesired magnetism in these devices can often cause inaccuracies and misrepresentation of colour. Other, sometimes similar, distortions can be caused by phosphor burn in, or by a physical bending of the the aperture grille or shadow mask.
A monitor damaged by magnetism will show discoloration or darkness, usually at the edge(s) of the screen, where the monitor is more vulnerable (items are rarely placed in front of a monitor). CRT monitors are very sensitive to magnetism, since the electrons fired from the CRT are aimed using magnets. If the screen is exposed to a weak magnetic field, the distortion is temporary, as the screen is not magnetized. Stronger magnets or prolonged exposure can cause hysteresis, which makes the distortion persist until a degauss is performed. In extreme cases, the damage may be too strong for the monitor's internal degaussing coil to correct, in which case, a stronger, external degaussing coil must be used.
A degauss causes a magnetic field inside the device to oscillate rapidly, with decreasing amplitude. Visually, this causes the image to shake dramatically for a second or so. Repeated deguassings result in a weaker effect, as the monitor must charge between.
Most televisions automatically degauss their picture tube when switched on for several reasons:
- Degaussing removes distortion caused by magnets. If a monitor is distorted, it is natural for a person to turn it off and on. This will degauss the screen and fix the distortion.
- The degaussing procedure is startling to most users, so if it is done while the CRT is still charging (the image has not yet appeared), the procedure goes unseen.
Degaussing Coil
A degaussing coil is simply a coil of copper wire. When a degauss is preformed, ordinary household AC is run through the coil. Then, either the current is gradually reduced (for internal degaussing coils), or the degaussing coil is slowly pulled away from the screen (for external degaussing coils), until the degaussing is complete.
Internal degaussing coils are wound around the front glass of the screen, inside the case. External degaussing coils are circular, with a diameter appropriate for the screen. Internal degaussing coils in CRTs are generally much weaker than external degaussing coils, since a better degaussing coil takes up more space.
