EARTH MAGNETIC FIELD

In the year 1600, William Gilbert, an English physician, set forth the theory that Earth has the properties of a huge magnet, on whose magnetic poles nearly coincide with its geographic poles. He also suggested that Earth's magnetic field originates mainly in the planet's deep interior. These ideas have been confirmed by many investigators. 

Many explanations have been offered to account for the magnetism of our planet. One theory assumed that permanently magnetized iron was present in the deep when it was shown that Earth's core was partly fluid, and therefore could not hold permanent magnetism. It seems likely that permanent magnetism elsewhere in Earth's interior would not provide a sufficiently strong field.

In 1947, Patrick M.S. Blackett of the University of London in Britain suggested that any massive rotating body, such as Earth, generates a magnetic field solely as a consequences of the rotation. Laboratory experiments with large rotating objects, however, did not reveal any such field. A test on Earth itself carried out in deep mines also failed to support Blackett's theory.

It now seems probable that the magnetic field of Earth is generated by ordinary electric currents circulating through the planet's interior.

Sir Harold Lamb pointed out in 1893 that such currents would have to be continuously supplied from some source of energy within Earth. It is natural to suppose that this would take place in the part of Earth where there is the least electrical resistance - that is, in the fluid outer core.

In 1939, Walter Elsasser, a German-born American physicist, suggested that such a current might arise in the core when materials of different electric properties and at slightly different temperatures came into contact. This is called the THERMOELECTRIC HYPOTHESIS. Thermoelectricity is produced by the unequal heating of an electric circuit composed of 2 dissimilar metals. In 1954, Stanley K. Runcorn suggested that there might be a thermoelectric effect at the boundary between the mantle and the outer core.

The most highly developed theory of Earth's magnetism is the Dynamo Theory of Elsasser and Sir Edward Bullard. It holds that a huge natural dynamo deep within Earth converts mechanical energy would be supplied by a special type of fluid motion, called CONVECTION, carrying electric currents inside the outer core. Elsasser calculated that such a motion is possible. The dynamo theory of the origin of Earth's magnetism is considered to be the most promising of all the present time.