theory that the Earth's magnetic field is produced or is best represented by a magnetic dipole, a body having poles of opposite sign, that is, positive and negative. In the first quantitative study made of the Earth's magnetic field, William Gilbert (Gilbert, William) observed that it resembled the magnetic field of a uniformly magnetized sphere. This field is the same as that of a magnetic dipole, and early theories of the origin of the geomagnetic field postulated some sort of lodestone or gigantic iron bar magnet within the Earth that gave rise to this dipolar field. Measurements and projections of pressure and temperature led to the conclusion that the Curie point (temperature at which magnetic properties are lost) of all known magnetic compounds is exceeded 10 to 40 km (6 to 25 miles) below the surface; this conclusion left the lodestone and bar-magnetic theories untenable.

Current theories postulate a self-sustaining dynamo (dynamo theory) in the Earth's core, giving rise to the magnetic dipolar field. Detailed study of the Earth's magnetic field shows the dipolar field to be the main component, although there are, in addition, irregularities caused by the superposition of a non-dipolar field, which is weaker than the dipolar field and is thought to arise in the Earth's outer core.