Historical introduction to degaussing, covering pre-war work and development up to the start of the campaign against the German mine

This introductory paper gives a brief account of the growth of knowledge of the magnetic state of a ship from the time of the 1914¿18 War, when mines were first made to explode by the magnetic field of the ship passing over them. The problem of altering this ship´s field as a counter-measure to such actuation was first studied experimentally in 1937, both on the model and full scale, and enabled demagnetization, or ¿degaussing, ¿ to be rapidly applied to all ships as soon as it was realized, in 1939, that the German magnetic mine was of the dipneedle type, which is most readily made less effective by this method. The battle to produce more and more perfect demagnetization to counter progressively more sensitive mines will be described in detail by other authors.