magnets

For levitation in ancient times men knew of a special kind of rock that could pull other rocks of the same kind and pieces of iron toward themselves. They were called lodestones. Today man uses the same force exerted by electromagnets and permanent magnets to provide magnetic aide to trains and more efficient power generators. Magnetism is defined as the force exerted by a magnetic field. A magnetic field is defined as the energy exerted …

…and proceeded to dismantle parts of it. It was discovered that a small needle inside was the trigger. Once the mines landed, they were activated. Then this needle would move if anything made of metal came within 40 feet of it. Once it was upright, it completed an electric circuit that triggered the bomb. This was designed so that the needle would be upright when the ship was directly above it, at its most vulnerable position.