World War II made use of magnetic mines (Germans used them first). The unconnected closed SPST switch represents the magnetic needle which is attracted to metal ship hulls magnetic field. The magnetic needle is part of circuit that closes and sends current to fire detonator, boost charge, then the TNT (250 to 1600 pounds). A mechanism inside mine sets up magnetic bomb on the ocean floor.
Minesweeping for magnetic mines required a de-gaussing cable around the ship to counteract horizontal electrical field set up by metal hull and vertical to counteract vertical electrical field from metal ship’s hull. A very light current is sent through de-gaussing cables.
Behind the minesweeper is long, floating rubber- sheathed two conductor cable. A positive charge on longest cable and negative charge on cable closed to ship to activate magnetic mine a safe distance from the minesweeper.
