Electromagnetism

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 proved 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 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 the early 1830's. In electromagnetic induction, a changing magnetic field sets up an electric field within a 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 R. Hertz proved Maxwell correct about 20 years later when he discovered electromagnetic waves.

Contributor: Gerald Feinberg, Ph.D., Former Prof. of Physics, Columbia Univ.

See also Electric Generator; Electric Motor; Electromagnet; Electromagnetic Waves; Magnetism.

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