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7. Magnetism after Gilbert |
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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.
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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." Next Stop: 8. Oersted and Ampére link Electricity to Magnetism Back to the Master List |
Author and Curator: Dr. David P. Stern
Mail to Dr.Stern: earthmag("at" symbol)phy6.org
Last updated 25 November 2001
Re-formatted 19 March 2006
Curators: Robert Candey, Alex Young, Tamara Kovalick
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