Degaussing

Degaussing was originally applied to reduce ships' magnetic signatures during World War II.

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 amperes flowing through it was dragged upwards on the side of the ship, starting at the waterline.

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.

[2] During World War II, the United States Navy commissioned a specialized class of degaussing ships that were capable of performing this function.

The vessel under test passed at a steady speed over loops on the seabed that were monitored from buildings on the shore.

The main advantage of the HTS Degaussing Coil system is greatly reduced weight (sometimes by as much as 80%) and increased efficiency.

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.

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 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."

[6] Color CRT displays, the technology underlying many television and computer monitors before the early 2010s, require degaussing.

Many CRT displays use a shadow mask (a perforated metal screen) near the front of the tube to ensure that each electron beam hits the corresponding phosphors of the correct color.

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.

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.

[9][10] 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.

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[11]) that is meant to be permanently recorded onto the magnetic media.

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.

[10] One magnetic media degaussers’ manufacturer, Verity Systems, has used this principle in a rotating coil technique they developed.