magnetic field

The Earth's magnetic field is similar to that of a bar magnet with poles near, but not exactly at, the geographic poles. Compass needles align themselves with the magnetic field, which is horizontal near the equator and vertical at the magnetic poles.

Maxwell's screw rule, named after the physicist James Clerk Maxwell, predicts the direction of the magnetic field produced around a wire carrying electric current. If a right-handed screw is turned so that it moves forward in the same direction as the current, its direction of rotation will give the direction of the magnetic field.

Fleming's rules give the direction of the magnetic field, motion, and current in electrical machines. The left hand is used for motors, and the right hand for generators and dynamos.

A magnet is an object that forms a magnetic field. It has a north pole and a south pole. As iron is a magnetic material, iron filings shaken around a magnet will form along the lines of force and produce the pattern of the magnetic field.

Region around a permanent magnet, or around a conductor carrying an electric current, in which a force acts on a moving charge or on a magnet placed in the field. The field can be represented by lines of force parallel at each point to the direction of a small compass needle placed on them at that point. These invisible lines of force are called the magnetic field lines or flux lines. A magnetic field's magnitude is given by the magnetic flux density (the number of flux lines per unit area), expressed in teslas. See also polar reversal. Experiments have confirmed that homing pigeons and some other animals rely on their perception of the Earth's magnetic field, together with other cues, for their sense of direction.