All ships have a permanent magnetic field or “magnetic signature” which is created during shipbuilding as a result of the hammering, riveting, and movements of the hull’s steel plates during construction in the shipyard while in the presence of the Earth’s magnetic field (see photo below, left). Changes to the strength of the magnetic field in the hull also occur at sea mainly due to the vibrations while in the Earth's magnetic field. Any steel ship is like a huge floating magnet and the Germans knew it. In November of 1939 alone more than 200,000 tons of shipping was lost off the coast of England to German mines. To protect warships and merchant vessels, the Allies needed a way to render their ships “magnetically silent” and they needed it fast.

In Britain Commander Charles F. Goodeve of the Royal Canadian Navy (later Sir Charles) worked with the Royal Navy in 1939-40  to develop ways to neutralize the inherent magnetism of steel ships. A series of experiments and sea trials was conducted at the H.M.S. Vernon naval research shore station. Work at Vernon determined that the magnetism in a hull can be read by having a ship pass over a loop of cable on the bottom of a harbor, like a miniature indicator loop. The research team developed two methods to trick the German sea mines. Since the Germans used the term “gauss” as the unit for magnetic strength when developing the triggers for mines, Goodeve named the first hull treatment “degaussing” - that is - removing the "gauss" (magnetism).
 

116. A ship such as the Pequot built and sailing at northern latitudes similar to Boston would have a "North-down" magnetisation due to the direction of the Earth's magnetic field there. The Pequot would behave like a magnet with the north pole underwater.	117. To eliminate the Pequot's magnetisation an electric current would be passed through a coil orientated like the one drawn above. The direction of the current is shown by the yellow arrow and this would produce a "North-up" magnetisation to cancel that of the ship's.

Degaussing involves the permanent installation of large copper cables around the perimeter of a ship’s hull just above the water line through which a large continuous electrical current is passed which creates a magnetic field in opposition to the field of the ship - and neutralizing it (see above right). This system was hooked up to the ship’s electrical system to easily permit degaussing at sea.

Deperming or wiping consisted of having a ship slowly move past stationary electric coils while in port, or by having large copper cables pulled across the hull through which a current of up to 2000 amps DC would be passed, to “wipe or flash” the ship to eliminate it’s magnetic signature. For most small ships deperming normally had to be repeated every 3-4 months.

Both of these countermeasures proved to be successful and permanent degaussing equipment was installed first on the largest ships in the British and American fleets. Once these techniques using large copper cables were widely adopted, the demand for copper in the US, which was already in short supply, soared,  resulting in the minting of steel pennies by the US Treasury for the remainder of the war.

Research indicates that the Pequot’s home port had the largest degaussing operation on the Atlantic coast first at the Boston Navy Yard, and beginning in 1943 at Castle Island in Boston Harbor. Between 1943 and 1945 more than 500 ships were degaussed at Castle Island. A note in the Pequot’s file at the US Coast Guard History Office shows that between May 26th and June 26th 1942 the Pequot was at the Boston Navy Yard for “conversion and installation of permanent degaussing” equipment, although none of the photographs of Pequot after 1942 show the addition of an exterior degaussing coil around her hull common with permanent systems of the time. So although speculation remains about that permanent solution, it is safe to assume that Pequot had periodic deperming treatments at the navy yard or Castle Island to camouflage her magnetic signature and greatly reduce the threat from sub-surface mines.