A lodestone, a naturally magnetized piece of magnetite, hanging from a hook so it can act as a magnetic compass. The magnetic field of the Earth exerts a torque on the stone so its poles align with the magnetic poles of the Earth, pointing north and south. Lodestones were the first magnetic compasses. This stone has been coated with iron filings to show the locations of its magnetic poles. The filings align parallel to the magnetic field lines, so the places where the filings stand straight up indicates the locations where the field lines enter and leave the stone, its magnetic poles. Caption: ""A piece of lodestone swinging freely""