Electromagnetism is the branch of physics that studies the relationship between electricity and magnetism. Electromagnetism is based on two facts. The first fact is that an electric current or a changing electric field produces a magnetic field. The second fact is that a changing magnetic field produces an electric field.
In 1820, the Danish scientist Hans Oersted made an important discovery. He found 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. 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. 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. Electromagnets were used in many inventions, such as the telegraph and the electric bell.
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 magnetic field can be used to create electric current in 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 Scottish physicist, studied the earlier experiments. He deduced 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.
