Editing Degaussing (section)

==Ships' hulls==
{{Refimprove section | date=December 2011}}
[[File:USS Jimmy Carter being depermed.jpg|thumb|right|[[USS Jimmy Carter|USS ''Jimmy Carter'']] in the magnetic silencing facility at [[Naval Base Kitsap]] for her first deperming treatment]]
[[Image:RMS Queen Mary 20Jun1945 NewYork.jpeg|thumb|right|[[RMS Queen Mary|RMS ''Queen Mary'']] arriving in New York Harbor, 20 June 1945, with thousands of U.S. soldiers{{spaced ndash}}note the prominent degaussing coil running around the hull]]
[[Image:MES U10.jpg|thumb|Control panel of the ''MES''-device (''"Magnetischer Eigenschutz"'' German: [[magnetic self-protection]]) in a German [[Type 205 submarine]]]]
[[File:Ivan Gren landing ship (1).jpg|thumb|Close-wrap deperming of the [[Ivan Gren-class landing ship|''Ivan Gren''-class]] [[landing ship]] [[Russian landing ship Ivan Gren|''Ivan Gren'']], 2016. The cables are floated into position before wrapping around the vessel.]]

The term was first used by then-Commander [[Charles F. Goodeve]], [[Royal Canadian Naval Volunteer Reserve]], during World War II while trying to counter the German magnetic [[naval mine]]s 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 "N-pole down" fields. British ships, notably [[cruiser]]s and [[battleship]]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. In this procedure, a large electrical cable with a pulse of about 2000 [[ampere]]s flowing through it was dragged upwards on the side of the ship, starting at the waterline. For submarines, the current came from the vessels' own propulsion batteries. 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.<ref>{{cite book |last1=Rowland |first1=Burford |last2=Boyd |first2=Wiliam B. |title=U. S. Navy Bureau of Ordnance in World War II |date=1953 |publisher=Bureau of Ordnance, Dept. of the Navy |location=Washington, D.C. |oclc=7833847 |page=84}}</ref> 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.<ref>PBS Nova "Great Escape at Dunkirk" https://www.pbs.org/video/great-escape-at-dunkirk-qb5qcr/</ref>

During World War II, the [[United States Navy]] commissioned a [[List of mine warfare vessels of the United States Navy#Degaussing Ships (ADG)|specialized class of degaussing ships]] that were capable of performing this function. One of them, [[USS Deperm (ADG-10)|USS ''Deperm'' (ADG-10)]], was named after the procedure.

After the war, the capabilities of the [[Magnetic proximity fuze|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 not for all points on the ship. A series of ever-increasingly complex coils were introduced to offset these fuze improvements, with modern systems including no fewer than three separate sets of coils to cancel the field in all axes.

===Degaussing range===
The effectiveness of ships' degaussing was monitored by shore-based ''degaussing ranges'' (or ''degaussing stations, magnetic ranges'') installed beside shipping channels outside ports. The vessel under test passed at a steady speed over loops on the seabed that were monitored from buildings on the shore. The installation was used both to establish the magnetic characteristics of a hull to establish the correct value of degaussing equipment to be installed, or as a "spot check" on vessels to confirm that degaussing equipment was performing correctly. Some stations had active coils that provided magnetic treatment, offering to un-equipped ships some limited protection against future encounters with magnetic mines.<ref>{{cite book |title=Shipboard Electrical Systems |date=1962 |publisher=[[Bureau of Naval Personnel]] |location=Washington DC |page=240 |edition=1966|oclc= 2164435}}</ref>

===High-temperature superconductivity===
The US Navy tested, in April 2009, a prototype of its [[High-temperature superconductivity|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.<ref>{{cite web  | last = Stimak  | first = George  | title = Degaussing Coil April 2009  | url = https://www.onr.navy.mil/Media-Center/Press-Releases/2009/New-Counter-Mine-Technology-Ships.aspx  | access-date = 2017-01-09  | archive-date = 18 January 2021  | archive-url = https://web.archive.org/web/20210118141340/https://www.onr.navy.mil/Media-Center/Press-Releases/2009/New-Counter-Mine-Technology-Ships.aspx  | url-status = dead  }}</ref>

A ferrous-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 detector|magnetic anomaly detection (MAD)]] equipment. [[Navy|Navies]] 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]] at [[Naval Station Norfolk]], or Pacific Fleet Submarine Drive-In Magnetic Silencing Facility (MSF) at [[Joint Base Pearl Harbor–Hickam]], are used to perform the procedure. During a close-wrap magnetic treatment, heavy-gauge [[copper]] cables encircle the hull and superstructure of the vessel, and high electrical currents (up to 4000 [[ampere]]s) are pulsed through the cables.<ref>{{cite book |title=Reduction of a Ship's Magnetic Field Signatures |volume=23 |series=Synthesis lectures on computational electromagnetics |last=Holmes |first=John J. |year=2008 |publisher=Morgan & Claypool |isbn=978-1-59829-248-0 |page=19 |url=https://books.google.com/books?id=28PCgAGjXCwC&q=Deperming&pg=PA19}}</ref> 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 adverse effect of the metal hull on steering [[compass]]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 cell|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".<ref>{{cite book |first=Commander S.T.S. |last=Lecky |title=Wrinkles in Practical Navigation |publisher=George Philip & Son |location=London |date=1917 |isbn= |pages=36 |url= |edition=19th |orig-year=1881}}</ref>
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