Geomagnetism since Gilbert
The rest of the site contains a great deal of more recent developments--even about the Sun, on magnetism in space and on the magnetic fields of planets, some of them much stronger than the Earth's. You might enjoy, for instance, the section "About Electronic Magnetometers and about Smoking. " For and Earth Sciences class, some of the basic questions are:
- --Why is the Earth magnetic?
- --Why does the magnetism of the Earth slowly change--unlike that of a bar magnet?
- --Why are sunspots intensely magnetic--when their temperature is so high that they must consist of hot gas?
- --Does the faint magnetization of volcanic lavas faithfuly record the magnetic field at the time they cooled? And if so, what about lavas which seemed to tell that the north and south magnetic poles were once reversed?
- --Is it just a coincidence that South America and Africa fit together, like jigsaw puzzle pieces? How can magnetism help find the answer?
And some questions you will find on the web site, but won't be discussed here:
- --How do satellites measure magnetic fields--even ones 100,000 weaker than those observed on the ground?
- --Are any other planets of the solar system magnetic?
Here only some ideas can be outlined, but you will find much more material on the web site, and many more stories.
Why is the Earth magnetic? Gilbert thought magnetism was an inherent property of the Earth, permanent magnetism like that of lodestones.
(He also thought it was related to the rotation of the Earth around its axis. At that time the church still maintained the Earth was the center of the universe and everything revolved around it. In the same year 1600 Giordano Bruno was burned at the stake, in part because he claimed the Earth rotated, and some copies of Gilbert's book had the pages on the Earth's rotation torn out or defaced. Galileo--who praised the book--got his copy as a gift from a "philosopher who wanted to rid his library of its contagion.")
But a few decades later it turned out that the direction of the compass needle slowly varied: a permanent magnet would not do so. Halley--the astronomer who predicted the return of the comet now named after him--came up with an ingenious explanation. The inside of the Earth contained spheres within spheres, each magnetized with its own poles, and the field changed because each sphere slowly rotated in relation to the others.
We now know the Earth is too hot to be magnetic. As Gilbert discovered, magnetized iron lost its power when it was made red-hot-- although when it cooled again, it "captured" the surrounding magnetic field of the Earth, and became weakly magnetized in that direction. (Please remember that, we will come back to it! ) Although the core of the Earth is probably mostly iron, it is hot enough for that iron to be molten, far too hot for permanent magnetism.
In 1908 the astronomer George Ellery Hale--founder of the great observatories on Mt. Wilson and Palomar in California--analyzed the light of sunspots, and found evidence that they were all strongly magnetized. The Sun is even hotter than the Earth's core--it is gas, hot enough to conduct electricity. Whatever made sunspots magnetic could not be permanent magnetism. That only left electric currents, as Oersted had shown.
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