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 Second World War, German naval mines posed a major threat to British and allied shipping. Some of these mines used magnetic fuses: they detected the small disturbance to the local magnetic field made by the steel hull of a passing ship or submarine. The British Admiralty 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:

1. "Active" degaussing: passing an electric current from the ship's generator through coils wrapped around the hull
2. "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)
3. Spoofing: deliberately imitating a ship's magnetic field, in order to detonate mines (most usefully performed by an aircraft).

Both the research and the actual operation of the degaussing ranges required a high degree of organisation, calibration and applied physics. The organisation was provided by WRENS, many of whom were drafted to the Clyde and the Avon.

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

Some people say that repeated degaussing can damage a monitor.

The degaussing coil

A degaussing coil is simply a coil of copper wire. When a degauss is performed, 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.

Degaussing magnetic data storage media

Data are 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. See also: Data remanence Data recovery