Horseshoe magnet, 2000.
© Science Museum / Science & Society Picture Library
A magnet is a device which produces a magnetic field external to itself. All magnets have two areas where this field is strongest, the north and south poles. Like poles repel each other, while opposite poles attract. Natural magnets, or lodestones, of magnetite, a magnetic iron oxide, have been known for many centuries, but the first artificial magnets were not introduced until 1750. Permanent magnets are made by placing a piece of ferromagnetic (magnetizable) material such as iron in the centre of an electric coil and pasing a current through the coil. If the material is magnetically 'hard', it will retain the magnetism after the current is switched off. Magnets attract ferromagnetic materials by inducing a field in them in line with that in the magnet itself.