ABSTRACT
The Earth is a magnet. Its dipole character results from massive currents within the molten portion of its core, These currents, driven presumably by gravitational energy, induce, in the manner of a self-sustaining dynamo, a global dipolar magnetic field with a magnitude of roughly 0.7 gauss at the poles. Although the ancient Chinese were familiar with the polar alignment of magnetized needles, geomagnetism became science with the publication in 1600 of William Gilbert’s classic exposition De Magnete, Magneticisque Corporibus Et De Magno Magnete Tellure: Physiologia Nova, Plurimis & Argumentis & Experimentis Demonstrata, Gilbert’s predecessor, Peter Peregrinus de Maricourt in his Epistola de Magnete of 1269, had noted that a magnetized needle (compass) left free to float on water, merely rotates, coming to rest with its axis lying in the north-south plane, and is not pulled in a northward direction. He did not perceive that the source of the magnetism causing the compass deflection was the Earth itself. Other predecessors of William Gilbert had believed such magnetism was extraterrestrial or was due to some remote “magnetic mountains.” Gilbert fashioned lodestone spheres which he called terrellas or little Earths; a term indicating his suspicion that the Earth itself was a magnet, By studying the interactions between his terellas and small bits of iron wire, he arrived at a novel and experimentally based philosophy of the attractive behavior or “coition” of ferromagnets, and presented in his book the first inductive rationale for the concept of terrestrial magnetism.
