Degaussing

Degaussing, named after the German scientist Carl Friedrich Gauss, is the process of removing permanent magnetism (magnetic hysteresis) from an object. It is accomplished by passing the object through a magnetic field that oscillates with diminishing amplitude.

= Degaussing of ships =

At the start of the researched methods of counteracting these mines.

Major successful techniques were based on cancelling out the residual magnetism inevitably remaining in a ship s hull from manufacture:

# Active degaussing: passing an electric current from the ship s generator through coils wrapped around the hull # Passive degaussing: passing the ship one or more times through a strong magnetic field created by coils placed on the sea bed (degaussing ranges, still to be found on UK charts) #Spoofing: deliberately imitating a ship s magnetic field, in order to induce mines to detonate without damaging matériel.

Both the research and the actual operation of the degaussing ranges required a high degree of organization, calibration and applied Physics. The organization was provided by Women s Royal Naval Service, many of whom were drafted to the Clyde and the Avon.

For more information, please visit [http://www.fas.org/man/dod-101/navy/docs/swos/eng/62B-303.html this page].

= Degaussing of cathode-ray tubes =

Undesired magnetism in a cathode-ray tube device 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 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. The shadow mask (or aperture grill) of a color monitor is made of a Permeability_(electromagnetism) material to shield the front of the mask from the deflection fields. If the screen is exposed to a weak magnetic field, the distortion is temporary, as the shadow mask 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. Monochrome CRTs don t have a shadow mask, and don t require degaussing.

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. 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; doing it while the CRT is still charging (no image has appeared yet) allows the procedure to go unseen.

In addition to practical uses, degaussing is used by many bored high school students as a way of entertaining themselves during class, who enjoy the turbulent display of colors.

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. 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.

== The degaussing coil ==

A degaussing coil is simply a coil of copper wire. When a degauss is performed, ordinary household Alternating current 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.

=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 Alternating current 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. See also: Data remanence Data recovery