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Degaussing is the process of decreasing or eliminating a remnant magnetic field. It is named after the
gauss Johann Carl Friedrich Gauss (; german: Gauß ; la, Carolus Fridericus Gauss; 30 April 177723 February 1855) was a German mathematician and physicist who made significant contributions to many fields in mathematics and science. Sometimes refer ...
, a unit of magnetism, which in turn was named after
Carl Friedrich Gauss Johann Carl Friedrich Gauss (; german: Gauß ; la, Carolus Fridericus Gauss; 30 April 177723 February 1855) was a German mathematician and physicist who made significant contributions to many fields in mathematics and science. Sometimes refer ...
. 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. Degaussing was originally applied to reduce ships' magnetic signatures during World War II. Degaussing is also used to reduce magnetic fields in
cathode ray tube A cathode-ray tube (CRT) is a vacuum tube containing one or more electron guns, which emit electron beams that are manipulated to display images on a phosphorescent screen. The images may represent electrical waveforms (oscilloscope), pictu ...
monitor Monitor or monitor may refer to: Places * Monitor, Alberta * Monitor, Indiana, town in the United States * Monitor, Kentucky * Monitor, Oregon, unincorporated community in the United States * Monitor, Washington * Monitor, Logan County, West ...
s and to destroy data held on magnetic storage.


Ships' hulls

The term was first used by then-Commander Charles F. Goodeve,
Royal Canadian Naval Volunteer Reserve The Royal Canadian Navy Volunteer Reserve (RCNVR) was a naval reserve force of the Royal Canadian Navy, which replaced the Royal Navy Canadian Volunteer Reserve (RNCVR). Foundation The RCNVR was created in 1923. The organization was established ...
, during World War II while trying to counter the German magnetic naval mines that were wreaking havoc on the British fleet. The mines detected the increase in the magnetic field when the steel in a ship concentrated the Earth's magnetic field over it. Admiralty scientists, including 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 the background. Since the Germans used the gauss as the unit of the strength of the magnetic field in their mines' triggers (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 as ''coiling.'' In addition to being able to bias the ship continually, 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
cruiser A cruiser is a type of warship. Modern cruisers are generally the largest ships in a fleet after aircraft carriers and amphibious assault ships, and can usually perform several roles. The term "cruiser", which has been in use for several hu ...
s and
battleship A battleship is a large armored warship with a main battery consisting of large caliber guns. It dominated naval warfare in the late 19th and early 20th centuries. The term ''battleship'' came into use in the late 1880s to describe a type ...
s, 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,'' which Goodeve also devised, and which is now also called ''deperming''. This procedure dragged a large electrical cable along the side of the ship with a pulse of about 2000
ampere The ampere (, ; symbol: A), often shortened to amp,SI supports only the use of symbols and deprecates the use of abbreviations for units. is the unit of electric current in the International System of Units (SI). One ampere is equal to elect ...
s 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 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. To aid the Dunkirk evacuation, the British "wiped" 400 ships in four days. During World War II, the United States Navy commissioned a specialized class of degaussing ships that were capable of performing this function. One of them, USS ''Deperm'' (ADG-10), was named after the procedure. After the war, the capabilities of the magnetic fuzes 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.


High-temperature superconductivity

The US Navy tested, in April 2009, a prototype of its
High-Temperature Superconducting High-temperature superconductors (abbreviated high-c or HTS) are defined as materials that behave as superconductors at temperatures above , the boiling point of liquid nitrogen. The adjective "high temperature" is only in respect to previou ...
Degaussing Coil System, referred to as "HTS Degaussing". The system works by encircling the vessel with superconducting ceramic cables whose purpose is to neutralize the ship's magnetic signature, as in the legacy copper systems. The main advantage of the HTS Degaussing Coil system is greatly reduced weight (sometimes by as much as 80%) and increased efficiency. A sea-going metal-hulled ship or submarine, by its very nature, develops a magnetic signature as it travels, due to a magneto-mechanical interaction with Earth's magnetic field. It also picks up the magnetic orientation of the Earth's magnetic field where it is built. This signature can be exploited by magnetic mines or facilitate the detection of a submarine by ships or aircraft with magnetic anomaly detection (MAD) equipment.
Navies A navy, naval force, or maritime force is the branch of a nation's armed forces principally designated for naval and amphibious warfare; namely, lake-borne, riverine, littoral, or ocean-borne combat operations and related functions. It include ...
use the deperming procedure, in conjunction with degaussing, as a countermeasure against this. Specialized deperming facilities, such as the United States Navy's
Lambert's Point Deperming Station Lambert's Point Deperming Station is a United States Navy deperming facility located in the Elizabeth River just off Lambert's Point, Norfolk, Virginia, United States. It was built in the mid-1940s and services the U.S. Atlantic Fleet. The st ...
at
Naval Station Norfolk Naval Station Norfolk is a United States Navy base in Norfolk, Virginia, that is the headquarters and home port of the U.S. Navy's Fleet Forces Command. The installation occupies about of waterfront space and of pier and wharf space of the Hamp ...
, or Pacific Fleet Submarine Drive-In Magnetic Silencing Facility (MSF) at
Joint Base Pearl Harbor–Hickam Joint Base Pearl Harbor–Hickam (JBPHH) is a United States military base on the island of Oahu, Hawaii. It is an amalgamation of the United States Air Force's Hickam Air Force Base and the United States Navy's Naval Station Pearl Harbor, wh ...
, are used to perform the procedure. During a closed-wrap magnetic treatment, heavy-gauge copper cables encircle the hull and superstructure of the vessel, and high electrical currents (up to 4000
ampere The ampere (, ; symbol: A), often shortened to amp,SI supports only the use of symbols and deprecates the use of abbreviations for units. is the unit of electric current in the International System of Units (SI). One ampere is equal to elect ...
s) are pulsed through the cables. This has the effect of "resetting" the ship's magnetic signature to the ambient level after flashing its hull with electricity. It is also possible to assign a specific signature that is best suited to the particular area of the world in which the ship will operate. In drive-in magnetic silencing facilities, all cables are either hung above, below and on the sides, or concealed within the structural elements of facilities. Deperming is "permanent". It is only done once unless major repairs or structural modifications are done to the ship.


Early experiments

With the introduction of iron ships, the negative effect of the metal hull on steering
compass A compass is a device that shows the cardinal directions used for navigation and geographic orientation. It commonly consists of a magnetized needle or other element, such as a compass card or compass rose, which can pivot to align itself with ...
es was noted. It was also observed that lightning strikes had a significant effect on compass deviation, identified in some extreme cases as being caused by the reversal of the ship's magnetic signature. In 1866, Evan Hopkins of London registered a patent for a process "to depolarise iron vessels and leave them thenceforth free from any compass-disturbing influence whatever". The technique was described as follows: "For this purpose he employed a number of Grove's batteries and electromagnets. The latter were to be passed along the plates till the desired end had been obtained... the process must not be overdone for fear of re-polarising in the opposite direction." The invention was, however, reported to be "incapable of being carried to a successful issue", and "quickly died a natural death".


Color cathode ray tubes

Color CRT displays, the technology underlying many television and
computer monitors A computer monitor is an output device that displays information in pictorial or textual form. A discrete monitor comprises a visual display, support electronics, power supply, housing, electrical connectors, and external user controls. The d ...
before the early 2010s, use degaussing. Many CRT displays use a
metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typica ...
plate near the front of the tube to ensure that each electron beam hits the corresponding
phosphors A phosphor is a substance that exhibits the phenomenon of luminescence; it emits light when exposed to some type of radiant energy. The term is used both for fluorescent or phosphorescent substances which glow on exposure to ultraviolet or v ...
of the correct color. If this plate becomes magnetized (e.g. if someone sweeps a magnet on the screen or places loudspeakers nearby), it imparts an undesired deflection to the electron beams and the displayed image becomes distorted and discolored. To minimize this, CRTs have a copper, or often in the case of cheaper appliances, aluminum, coil wrapped around the front of the display, known as the degaussing coil. Monitors without an internal coil can be degaussed using an external handheld 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 Oscillation is the repetitive or periodic variation, typically in time, of some measure about a central value (often a point of equilibrium) or between two or more different states. Familiar examples of oscillation include a swinging pendulum ...
rapidly, with decreasing
amplitude The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of ampl ...
. This leaves the shadow mask with a field opposing any external field (such as that of the Earth), removing the discoloration. Many televisions and monitors automatically degauss their picture tube when switched on, before an image is displayed. The high current surge that takes place during this automatic degauss is the cause of an audible "thunk", a loud hum or some clicking noises, which can be heard (and felt) when televisions and CRT computer monitors are switched on, due to the capacitors discharging and injecting current into the coil. 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 degaussing coil is regulated by a simple
positive temperature coefficient A temperature coefficient describes the relative change of a physical property that is associated with a given change in temperature. For a property ''R'' that changes when the temperature changes by ''dT'', the temperature coefficient α is def ...
(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; "experimenting" with the degauss effect by repeatedly switching the device on and off may cause this component to fail. The effect will also be weaker, since the PTC will not have had time to cool off.


Magnetic data storage media

Data is stored in the magnetic media, such as
hard drive A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magneti ...
s, 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 Remanence or remanent magnetization or residual magnetism is the magnetization left behind in a ferromagnetic material (such as iron) after an external magnetic field is removed. Colloquially, when a magnet is "magnetized", it has remanence. The ...
or
remanent magnetization Remanence or remanent magnetization or residual magnetism is the magnetization left behind in a ferromagnetic material (such as iron) after an external magnetic field is removed. Colloquially, when a magnet is "magnetized", it has remanence. The ...
. Proper degaussing will ensure there is insufficient magnetic remanence to reconstruct the data. Erasure via degaussing may be accomplished in two ways: in AC erasure, the medium 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 medium 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. The magnetic field needed for degaussing magnetic data storage media is a powerful one that normal magnets cannot easily achieve and maintain.


Irreversible damage to some media types

Many forms of generic magnetic storage media can be reused after degaussing, including reel-to-reel audio tape, VHS videocassettes, and floppy disks. These older media types are simply a raw medium which are overwritten with fresh new patterns, created by fixed-alignment read/write heads. For certain forms of computer data storage, however, such as modern hard disk drives and some tape drives, degaussing renders the magnetic media completely unusable and damages the storage system. This is due to the devices having an infinitely variable read/write head positioning mechanism which relies on special servo control data (e.g. Gray Code) that is meant to be permanently recorded onto the magnetic media. This servo data is written onto the media a single time at the factory using special-purpose servo writing hardware. The servo patterns are normally never overwritten by the device for any reason and are used to precisely position the read/write heads over data tracks on the media, to compensate for sudden jarring device movements, thermal expansion, or changes in orientation. Degaussing indiscriminately removes not only the stored data but also the servo control data, and without the servo data the device is no longer able to determine where data is to be read or written on the magnetic medium. The servo data must be rewritten to become usable again; with modern hard drives, this is generally not possible without manufacturer-specific and often model-specific service equipment.


Audio tape recorders

In reel-to-reel and
compact cassette The Compact Cassette or Musicassette (MC), also commonly called the tape cassette, cassette tape, audio cassette, or simply tape or cassette, is an analog magnetic tape recording format for audio recording and playback. Invented by Lou Otten ...
audio tape recorders, remnant magnetic fields will over time gather on metal parts such as guide posts tape heads. These are points that come into contact with the magnetic tape. The remnant fields can cause an increase in audible background noise during playback. Cheap, handheld consumer degaussers can significantly reduce this effect.


Types of degaussers

Degaussers range in size from small ones used in offices for erasing magnetic data storage devices to industrial-size degaussers for use on piping, ships, submarines, and other large-sized items, equipment to vehicles. Rating and categorizing degaussers depends on the strength of the magnetic field the degausser generates, the method of generating a magnetic field in the degausser, the type of operations the degausser is suitable for, the working rate of the degausser based on whether it is a high volume degausser or a low volume degausser, and mobility of the degausser among others. From these criteria of rating and categorization, there are thus electromagnetic degaussers, permanent magnet degaussers as the main types of degaussers.


Electromagnetic degaussers

An electromagnetic degausser passes an electrical charge through a degaussing coil to generate a magnetic field. Sub-types of electromagnetic degaussers are several such as Rotating Coil Degaussers and Pulse Demagnetization Technology degaussers since the technologies used in the degaussers are often developed and patented by respective manufacturing companies such as Verity Systems and Maurer Magnetic among others, so that the degausser is suitable for its intended use. Electromagnetic degaussers generate strong magnetic fields, and have a high rate of work.


Rotating coil degausser

Performance of a degaussing machine is the major determinant of the effectiveness of degaussing magnetic data storage media. Effectiveness does not improve when the media passes through the same degaussing magnetic field more than once. Rotating the media by 90 degrees improves effectiveness of degaussing the media. One magnetic media degaussers’ manufacturer, Verity Systems, has used this principle in a rotating coil technique they developed. Their rotating coil degausser passes the magnetic data storage media being erased through a magnetic field generated using two coils in the degaussing machine with the media on a variable-speed conveyor belt. The two coils generating a magnetic field are rotating; with one coil positioned above the media and the other coil positioned below the media.


Pulse degaussing

Pulse degaussing technology involves the cyclic application of electric current for a fraction of a second to the coil being used to generate a magnetic field in the degausser. The process starts with the maximum voltage applied and held for only a fraction of a second to avoid overheating the coil, and then the voltages applied in subsequent seconds are reduced in sequence at varying differences until no current is applied to the coil. Pulse degaussing saves on energy costs, produces high magnetic field strength, is suitable for degaussing large assemblies, and is reliable due to zero-error degaussing achievement.


Permanent magnet degausser

Permanent magnet degaussers use magnets made using rare earth materials. They do not require electricity for their operation. Permanent magnet degaussers require adequate shielding of the magnetic field they constantly have to prevent unintended degaussing. The need for shielding usually results in permanent magnet degaussers being bulky. When small-sized, permanent magnet degaussers are suited for use as mobile degaussers.


See also

*
Data remanence Data remanence is the residual representation of digital data that remains even after attempts have been made to remove or erase the data. This residue may result from data being left intact by a nominal file deletion operation, by reformatting of ...


References


External links


Guide to degaussing TVs
* *{{cite web , title=A Guide to Understanding Data Remanence in Automated Information Systems , url=http://www.cerberussystems.com/INFOSEC/stds/ncsctg25.htm , website=www.cerberussystems.com , archive-url=https://web.archive.org/web/20160304103802/http://www.cerberussystems.com/INFOSEC/stds/ncsctg25.htm , archive-date=2016-03-04 , url-status=dead *https://degaussing-101.com/what-is-degaussing/ Computer storage media Magnetic hysteresis British inventions