Degaussing/Deperming Facility

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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. 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 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 (also devised by Goodeve, and now also called deperming), which simply dragged a large electrical cable along the side of the ship with a pulse of 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.
Control panel of the MES-device ("Magnetischer Eigenschutz" German: magnetic self-protection) in a German submarine

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.

The US Navy tested in April 2009 a prototype of its High Temperature Superconducting 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.
en.wikipedia.org/wiki/Degaussing
en.wikipedia.org/wiki/Deperming
www.submepp.navy.mil/jfmm/html/Volume%206/VI-Ch-12.htm

Used to degauss the hull of submarines to reduce their magnetic signatures. Part of the Trident Refit Facility, the MSF serves Trident, fast attack and Royal Navy submarines, as well as steel-hulled surface craft.

eyeball-series.org/deperm/deperm-eyeball.htm