Wellington DW.Mk I fitted with degaussing coils to trigger magnetic mines. Modern influence mines are designed to discriminate against false inputs and are much more difficult to sweep. They often contain inherent anti-sweeping mechanisms. For example, they may be programmed to respond to the unique noise of a particular ship-type, its associated magnetic signature and the typical pressure displacement of such a vessel. As a result, a mine-sweeper must accurately guess and mimic the required target signature to trigger detonation. The task is complicated by the fact that an influence mine may have one or more of a hundred different potential target signatures programmed into it. Another anti-sweeping mechanism is a ship-counter in the mine fuze. When enabled, this allows detonation only after the mine fuze has been triggered a pre-set number of times. To further complicate matters, influence mines may be programmed to arm themselves (or disarm automatically—known as self-sterilization) after a pre-set time. During the pre-set arming delay (which could be days or weeks) the mine would remain dormant and ignore any target stimulus, whether genuine or faked. When influence mines are laid in an ocean minefield, they may have various combinations of fuze settings configured. For example, some mines (with the acoustic sensor enabled) may become active within three hours of being laid, others (with the acoustic and magnetic sensors enabled) may become active after two weeks but have the ship-counter mechanism set to ignore the first two trigger events, and still others in the same minefield (with the magnetic and pressure sensors enabled) may not become armed until three weeks have passed. Groups of mines within this mine-field may have different target signatures which may or may not overlap. The fuzes on influence mines allow many different permutations, which complicates the clearance process. Mines with ship-counters, arming delays and highly specific target signatures in mine fuses can falsely convince a belligerent that a particular area is clear of mines or has been swept effectively because a succession of vessels have already passed through safely. By aircraft Aircraft can also be used for minesweeping. During the Second World War, fifteen British Vickers Wellington bombers were modified to carry a large magnetic induction loop and an electrical generator. The "Directional Wireless Installation" (DWI), a cover story for the true purpose of the magnetic loop, was used successfully on 10 May 1940 to sweep a path for the escape