Deperming [dee-purm-ing] is a procedure for erasing the permanent magnetism from ships and submarines to camouflage them against magnetic detection vessels and enemy marine mines. 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 the Earth’s magnetic field. 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.

Specialized deperming facilities, such as the United States Navy’s Lambert’s Point Deperming Station are used to perform the procedure. Heavy gauge copper cables are wrapped around the hull and superstructure of the vessel, and very high electrical currents (as high as 4000 amps) are pulsed through the cables. This has the effect of ‘resetting’ the ship’s magnetic signature. 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. Over time the deperm will begin to degrade and the procedure must be redone periodically to maintain the desired effect.

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