The potency of a magnetic field is measured in terms of Gauss. Accordingly, if we define Degaussing. We can say that degaussing means the procedure to diminish the potency of a magnetic field.
Any ship made from steel or a material having magnetic susceptibility is like a big floating magnet with a very high magnetic field surrounding it. This field also moves with respect to ship motion in water. And so, it counteracts the Earth’s magnetic field. As a result, the net resultant field of the Earth continuously changes and hence grows strong or weakens depending upon the position and motion of ship. This change in magnetic field of Earth can be easily detected and hence, a ship or a metal body can be used as a triggering source for equipments which have been specifically designed to sense such changes in magnetic fields. For example – the basic principle behind automatic detonation of magnetic mine when it senses a distortion in the magnetic field of the Earth caused by a nearby/moving ship (made up of steel or any magnetic susceptible metal). This property of the change in magnetic field was extensively used during the world war II. The process of degaussing the ship was first carried out during World War II as a protection against magnetic mines laid out by the enemy ships.
In addition, It affects the optimum performance of other devices which are dependent on magnetic field (for ex – Magnetic Compass). The degaussing system is installed onboard ships to lessen the ship’s effect on the Earth’s magnetic field. In order to achieve this, the ship’s own magnetic field is “zeroised” by controlling the electric current flowing through degaussing coils in precise locations within the ship’s hull. This, in turn, reduces the ship’s residual magnetism due to Earth’s magnetic field.
The degaussing is achieved by use of Direct current Coils that produce a magnetic field which is directed in the opposite direction of the original magnetic field of the ship.