Degaussing

At the start of the Second World War, the Germans developed a new magnetic mine that could be detonated by the mine's sensitivity to the magnetic field of a ship passing nearby.

The mechanism had a sensitivity level that could be set, and the units of the scale were in milligauss. Gauss is a measurement for the strength of a magnetic field.

Degaussing is a process in which systems of electrical cables are installed around the circumference of ship's hull, running from bow to stern on both sides. A measured electrical current is passed through these cables to cancel out the ship's magnetic field.

Degaussing equipment was installed in the hull of warships and could be turned on whenever the ship was in waters that might contain magnetic mines, usually shallow waters in combat areas. It could be said that degaussing, correctly done, makes a ship "invisible" to the sensors of magnetic mines, but the ship remains visible to the human eye, radar, and underwater listening devices.

In New Zealand during the Second World War there were degaussing ranges in Wellington Harbour off Somes Island and in Auckland in Waitemata Harbour. The ranges were in operation until the end of the war. The range consisted of a series of magnetometers under the water connected to a recording device ashore that measured the ship’s magnetic field.  The range was 120m in length.

It was the job of the Range Officer to ensure that the degaussing equipment fitted to ships was working properly. The ship was sailed over a series of sensors with its degaussing equipment working and the ship’s magnetic field signature was measured to ensure it would not set off a mine. Both warships and merchant vessels were tested frequently to ensure they were safe.  Any adjustments could be made while the ship was in port.

Degaussing successfully protected warships from magnetic mines. It remains in use to the present day. Minesweepers were developed in the post-war period that did not have magnetic hulls so they could sweep magnetic mines without having to degauss the ship. This included building small minesweepers out of fibreglass so as to be impervious to magnetic devices.