The principle of degaussing was used during WWII to render the naval vessels undetectable (and invisible), from magnetic mines and torpedoes. The degaussing of a ship involved the generation of a powerful electromagnetic field onboard. A degaussing ship fitted a copper cable along its outer hull—projecting a “north pole up” field that neutralized the ship’s magnetic signature.
In this photo we can see RMS Queen Mary arriving New York, 20 June 1945 – note the prominent degaussing coil running around the outer hull.