Electromagnetism is the branch of physics that studies the relationship between electricity and magnetism. Electromagnetism is based on the fact that (1) an electric current or a changing electric field produces a magnetic field, and (2) a changing magnetic field produces an electric field.

In 1820, the Danish scientist Hans Oersted discovered that a conductor carrying an electric current is surrounded by a magnetic field. When he brought a magnetized needle near a wire in which an electric current was flowing, the needle moved. Because a magnetized needle is moved by magnetic forces, the experiment demonstrated that an electric current produces magnetism.

During the 1820's, the French physicist Andre Marie Ampere declared that electric currents produce all magnetism. He concluded that a permanent bar magnet has tiny currents flowing in it. The work of Oersted and Ampere led to the development of the electromagnet, which is used in such devices as the telegraph and the electric bell. Most electromagnets consist of a coil of wire wound around an iron core. The electromagnet becomes temporarily magnetized when an electric current flows through the wire. If the direction of the current changes, the north and south poles of the electromagnet switch places.

Magnetism produces an electric current by means of electromagnetic induction. The English scientist Michael Faraday and the American physicist Joseph Henry discovered electromagnetic induction independently in 1831. In electromagnetic induction, a changing magnetic field sets up an electric field within the conductor. For example, a magnet moving through a coil of wire causes the voltage to vary from point to point along the wire. An electric current flows along the wire as long as the magnetic field passing through the wire is changing. Electromagnetic induction is the basis of the electric generator. An electric motor reverses the process. A current sent through the wire causes the wire to move in a magnetic field.

In 1864, James Clerk Maxwell, a British scientist, used the earlier experiments to deduce that electric and magnetic fields act together to produce electromagnetic waves of radiant energy. The German physicist Heinrich Hertz proved Maxwell correct about 20 years later when he discovered electromagnetic waves.

Electromagnetic waves are related patterns of electric and magnetic force. They are generated by the oscillation (movement back and forth) of electric charges. Electromagnetic waves travel through space at the speed of light, which is 299,792 kilometres per second. The simplest electromagnetic waves are plane waves. They move through space in straight lines. The strength of the wave varies in space and time with alternating crests and troughs. The distance from crest to crest is called the wavelength.

The electromagnetic spectrum consists of bands of different wavelengths. The chief kinds of electromagnetic waves are--in order of increasing wavelength--gamma rays, X rays, ultraviolet light, visible light, infrared rays, microwaves, and radio waves. Gamma rays are only about 10-trillionths of a metre in length, whereas some long radio waves measure more than 10,000 kilometres.

All types of electromagnetic waves have the properties of visible light. They can be reflected, diffracted (spread), and refracted (bent). The direction of magnetic force in all electromagnetic waves is perpendicular to the direction in which the wave is moving. The direction of electric force is perpendicular to both the direction of magnetic force and the direction of wave motion. The strength of magnetic force always equals the strength of electric force.