A more serious problem was later realized: as a ship travels through Earth's magnetic field, it will slowly pick up that field, counteracting the effects of the degaussing. From then on captains were instructed to change direction as often as possible to avoid this problem. Nevertheless, the bias did wear off eventually, and ships had to be degaussed on a schedule. Smaller ships continued to use wiping through the war. To aid the Dunkirk evacuation, the British "wiped" 400 ships in four days. During World War II, the United States Navy commissioned a specialized class of degaussing ships that were capable of performing this function. One of them, USS Deperm (ADG-10), was named after the procedure. After the war, the capabilities of the magnetic fuzes were greatly improved, by detecting not the field itself, but changes in it. This meant a degaussed ship with a magnetic "hot spot" would still set off the mine. Additionally, the precise orientation of the field was also measured, something a simple bias field could not remove, at least for all points on the ship. A series of ever-increasingly complex coils were introduced to offset these effects, with modern systems including no fewer than three separate sets of coils to reduce the field in all axes. High-temperature superconductivity The US Navy tested, in April 2009, a prototype of its High-Temperature Superconducting Degaussing Coil System, referred to as "HTS Degaussing". The system works by encircling the vessel with superconducting ceramic cables whose purpose is to neutralize the ship's magnetic signature, as in the legacy copper systems. The main advantage of the HTS Degaussing Coil system is greatly reduced weight (sometimes by as much as 80%) and increased efficiency. A sea-going metal-hulled ship or submarine, by its very nature, develops a magnetic signature as it travels, due to a magneto-mechanical interaction with Earth's magnetic field. It also picks up the magnetic orientation of the Earth's magnetic field where it is built. This signature can be exploited by magnetic mines or facilitate the detection of a submarine by ships or aircraft with magnetic anomaly detection (MAD) equipment. Navies use the deperming procedure, in conjunction with degaussing, as a countermeasure against this. Specialized deperming facilities, such as the United States Navy's Lambert's Point Deperming Station at Naval Station Norfolk, or Pacific Fleet Submarine Drive-In Magnetic Silencing Facility (MSF) at Joint Base Pearl Harbor–Hickam, are used to perform the procedure.