The magnetic field of a ship and its neutralization by coil degaussing

In November, 1939, the order was given to degauss all ships by coiling. This decision was taken after a coil had been tried on a ship at Portsmouth and proved effective against the original German magnetic mine. Mass treatment of this kind immediately showed many difficulties. In fact, study of the problem had merely begun and very much investigation was required before it was possible to make degaussing effective against the much more sensitive mines which were laid from 1941 onwards. This paper surveys briefly the study of the magnetic fields of ships made during the war, and indicates how coils were placed to neutralize them.