7.   Magnetism after Gilbert

    Early studies of magnetism were driven by a practical motive: ships navigating the ocean relied on the magnetic compass. Their captains had to know by how much "magnetic north" differed from "true north."

    Henry Gellibrand (soft "g") published in 1635 evidence that this difference slowly changed with time. That was an unsettling discovery. It meant that observations of the local compass bearing became inaccurate after some decades and therefore had to be repeated from time to time.

    And from a theoretical angle, how could the magnetic properties of the Earth undergo such gradual change? No known magnet behaved that way. Edmond Halley, of comet fame, came up in 1692 with an ingenious explanation. The interior of the Earth, he claimed, consisted of layers, spheres within spheres. Each sphere was independently magnetized, and each rotated slowly with respect to the others.

    (The current explanation of the Earth's magnetism involves electric currents, deep in the Earth's molten core, as discussed in later sections of this site. However, earthquake waves have shown that the Earth does consist of "spheres inside spheres," and two of these are the liquid core of the Earth and the solid "inner core" inside it. Most recently a slight difference in the rate of spin between these spheres was implicated in generating the Earth's magnetic field.

Edmond Halley

    In 1724 George Graham found that the compass needle sometimes veered off by a small angle, for a day or so; a century later Alexander von Humboldt would name such events magnetic storms. That effect was widespread: Anders Celsius in Uppsala observed one at the same time as Graham in London, and a century later it was found to be world-wide. Celsius and his student Hiorter also observed magnetic disturbances linked to the "northern lights" (polar auroras); in our time, such events are associated with "magnetic substorms."

    All that time, the only kind of magnetism known was the permanent magnetism of magnetized iron or of lodestones. The magnetic force due to the magnetic pole at the end of a magnet seemed a bit like gravity or the electric force, growing weaker in proportion to 1/r², with r the distance from the pole. This "inverse square" relationship was confirmed in 1777 by Charles Coulomb in France, through experiments with a magnetic needle suspended on a twistable string--an instrument which Coulomb introduced, the prototype of most magnetic detectors in the following 170 years.

    The main difference was that while gravity only attracted, magnetism could also repel. Jonathan Swift (1726) satirically proposed that such a repulsion could act as "anti-gravity," keeping an island floating in space, as told in the story of the 3rd voyage of "Gulliver's Travels."

Next Stop:   8. Oersted and Ampére link Electricity to Magnetism