Degaussing
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Degaussing is the process of decreasing or eliminating an unwanted magnetic field. It is named after Carl Friedrich Gauss, an early researcher in the field of magnetism. Due to magnetic hysteresis it is generally not possible to reduce a magnetic field completely to zero, so degaussing typically induces a very small "known" field referred to as bias.
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[edit] Degaussing ship's hulls
The term was first used by (then) Cmdr. Charles F. Goodeve, RCNVR, during World War II while trying to counter the German magnetic mines that were playing havoc with the British fleet. The mines detected the increase in magnetic field when the steel in a ship concentrated the Earth's magnetic field over it. Goodeve developed a number of systems to induce a small "N-pole up" field into the ship to offset this effect, meaning that the net field was the same as background. Since the Germans used the gauss as the unit of the strength of the magnetic field in their mines' triggers (this was not yet a standard measure), Goodeve referred to the various processes to counter the mines as degaussing. The term became a common word.
The original method of degaussing was to install electromagnetic coils into the ships, known simply as coiling. In addition to being able to continually bias the ship, coiling also allowed the bias field to be reversed in the southern hemisphere, where the mines were set to detect "S-pole down" fields. British ships, notably cruisers and battleships, were well protected by about 1943.
Installing such special equipment was, however, far too expensive and difficult to service all ships that would need it, so the navy developed an alternative called wiping (devised by Goodeve), which simply dragged a large electrical cable along the side of the ship with about 2000 amps flowing through it. This induced the proper field into the ship in the form of a slight bias. It was originally thought that the pounding of the sea and the ship's engines would slowly randomize this field, but in testing this was found not to be a real problem. A more serious problem was later realized: as a ship travels through the Earth's magnetic field it will slowly pick up that field, counteracting the effects of the degaussing. From then on captains were instructed to change direction as often as possible to avoid this problem. Nevertheless the bias did wear off eventually, and ships had to be degaussed on a schedule. Smaller ships continued to use wiping through the war.
After the war the capabilities of the magnetic fuses were greatly improved, by detecting not the field itself, but changes in it. This meant a degaussed ship with a magnetic "hot spot" would still set off the mine. Additionally, the precise orientation of the field was also measured, something a simple bias field could not remove, at least for all points on the ship. A series of ever-increasingly complex coils were introduced to offset these effects, with modern systems including no fewer than three separate sets of coils to reduce the field in all axes.
[edit] Degaussing monitors
Today the most common use of degaussing is in CRT-based TV sets and computer monitors. For example, many monitors use a metal plate near the front of the tube to focus the electron beams from the back. This plate, the shadow mask, can pick up strong external fields and from that point produce discoloration on the display.
To minimize this, CRTs have a copper coil wrapped around the front of the display, known as the degaussing coil. Tubes without an internal coil can be degaussed using an external hand held version. Internal degaussing coils in CRTs are generally much weaker than external degaussing coils, since a better degaussing coil takes up more space. A degauss causes a magnetic field inside the tube to oscillate rapidly, with decreasing amplitude. This leaves the shadow mask with a small and somewhat randomized field, removing the discoloration.
Many televisions and monitors automatically degauss their picture tube when switched on, before an image has been displayed. The high current surge which takes place during this automatic degauss is the cause of an audible 'thunk' or loud hum which can be heard (and felt) when televisions and CRT computer monitors are switched on. Visually, this causes the image to shake dramatically for a short period of time. A degauss option is also usually available for manual selection in the operations menu in such appliances.
In most commercial equipment the current surge to the degauss coil is regulated by a simple PTC thermistor device which initially has a low resistance but quickly changes to a high resistance due to the heating effect of the current flow. Such devices are designed for a one-off transition from cold to hot at power up, so 'experimenting' with the degauss effect by repeatedly switching the device on and off is not recommended as it may cause this component to fail. The effect will also be weaker, since the PTC won't have had time to cool off.
[edit] Degaussing magnetic data storage media
Data is stored in magnetic media, such as hard drives, floppy disks and magnetic tape, by making very small areas called magnetic domains change their magnetic alignment to be in the direction of an applied magnetic field. This phenomenon occurs in much the same way a compass needle points in the direction of the earth's magnetic field. Degaussing, commonly called erasure, leaves the domains in random patterns with no preference to orientation, thereby rendering previous data unrecoverable. There are some domains whose magnetic alignment is not randomized after degaussing. The information these domains represent is commonly called magnetic remanence since it is due to remanent magnetization. Proper degaussing will ensure there is insufficient magnetic remanence to reconstruct the data.[1]
Erasure via degaussing may be accomplished in two ways: in AC erasure, the media is degaussed by applying an alternating field that is reduced in amplitude over time from an initial high value (i.e., AC powered); in DC erasure, the media is saturated by applying a unidirectional field (i.e., DC powered or by employing a permanent magnet). A degausser is a device that can generate a magnetic field for degaussing magnetic storage media.[2]
[edit] Alternative methods
Monitors without a degauss function can be degaussed by placing a degauss-enabled monitor face-to-face with the target monitor. Activating the degauss function on the enabled monitor can help degauss the other.[citation needed]
Degaussing can also help reduce the distortion due to static electricity that often results from long-term usage of the monitor, especially in older monitors.[citation needed]
[edit] See also
[edit] Notes and references
- ^ A Guide to Understanding Data Remanence in Automated Information Systems: http://www.cerberussystems.com/INFOSEC/stds/ncsctg25.htm
- ^ National Computer Security Center TG-025.
