Degaussing
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Degaussing is the process of reducing or eliminating an unwanted magnetic field. It is named after Carl Friedrich Gauss, an early reseacher 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 ships
The term was first used by Charles F. Goodeve 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 gauss as the measure of 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 wiping, 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.
A more permanent solution was found in the form of electromagnets built into the ships in the form of a coil, 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. 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 that 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 that 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 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 combat this, CRTs add 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.
The high current surge which takes place during this automatic degauss is the cause of the audible 'thunk' 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 second or so. Many televisions and monitors automatically degauss their picture tube when switched on, before an image has been displayed.
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 hasn't 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 that 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 that these domains represent is commonly called magnetic remanence. Proper degaussing will ensure that there is insufficient magnetic remanence to reconstruct the data.
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.
Source National Computer Security Center TG-025.
[edit] Other information
Monitors without a degauss option, can be degaussed by placing a degauss-able monitor face-to-face (facing the target degauss monitor) for the monitors to degauss themselves. Degaussing is a method that is also able to fix distortation because of static electricity due to long term usage of the monitor (usually on older monitor models). Magnetic entities are re-defaulted by this method, thus fixing the problem that occurs.
