Geomagnetism Patriarchs Gilbert

Let us pause now to talk about a few of the patriarchs of geomagnetism whose brilliant scientific contributions have brought us to our present level of magnetic field knowledge. We will start with the English physician to Queen Elizabeth I, William Gilbert (Figure 1.4), who is recognized as the father of the

FIGURE 1.5 ► A dipole field pattern is superposed on Gilbert's picture of the Great Earth Magnet of 1600 in which he shows how magnets would naturally arrange about the surface.

science of magnetism. His book, De Magnete, published in 1600, is a summation of the knowledge of his time and of his own magnetic experiments. He was the first to describe the Earth's main field as a giant magnet with a horizontal field at the Earth's equator and with two oppositely directed vertical fields at the Earth's poles. That is, the Earth had the field configuration expected from a magnetized iron bar (Figure 1.5) that had a pair of north and south magnetic poles separated by a fixed distance (the arrangement that we now call a dipole).

The multiplication product of the strength of one of the poles times their separation distance is what scientists call the dipole moment of a magnet. Dipole moment values let us compare magnetized material, a small compass needle or the giant Earth itself. Measurements over the last 400 years tell us that the magnetic dipole moment of our Earth has been steadily decreasing since Gilbert's time.

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