Ships & Subs can be detected by the disturbance of the magnetic fields in it's surrounds. Magnetic Mines can measure this disturbance to know when to explode. They can even distinguish between sizes of ships, so the mine could let the escort ships pass, but explode under an oil tanker, for example. The magnetic disturbances can also be detected by planes. helicopters, and other ships using a Magnetic Anomoly Detector (MAD) to give away a otherwise invisible submerged submarine.
There are two components:-
1. PERMANENT - When Ships and Submarines are built, a permanent magnetic is built-in, which depends on the orientation of the ship yard with respect to magnetic north, and on stresses that are induced when built.
2. INDUCED - The Metal of the vessel 'channels' the surrounding Magnetic field. It depends on it's orientation with respect to magnetic north.
1. After Building, the vessel's permanent magnetic field can be reduced by DE-PERMING (wrapping large electric coils around the vessel). It reduces the field but isn't perfect
2. Electric coils can be built into the vessel, to counteract the magnetic disturbance. This is called DEGAUSSING. Put simply, the vesssel is driven back and fourth across a range (a set of MADs - see above), and various currents put in the coils, until the magnetic signature is as little as possible. These values (coupled with the direction of travel with respect to the earth's magnetic fields) are used to compute values all around the world.
Electronic device makes submarines harder to track - Article on disposable 'ranging' bouys
Polyamp - Degaussing systems - A manufacturer of degaussing systems
Lesson 19 - Some notes from the americian navy
Active shaft grounding - a related subject
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Last Updated by Steve Lawther on Saturday, 2 November, 1996 at 12:13 AM.