9. The Lodestone
4. A Gilbert Expt.
5. Before Gilbert
6. London in 1600
8. Oersted & Ampére
9. The Lodestone
11. The Magnetic Sun
12. Fluid Dynamos
13. Dynamo in the
14. Magnetometers and
15. Magnetic Reversals
& Moving Continents
16. The Magnetosphere
17. Magnetic Planets
What if no lodestones existed? The Chinese would certainly not have invented the magnetic compass. Magnetism would have been discovered much later, and one wonders how. Lacking the compass, the great voyages of discovery could hardly have taken place--Columbus, De Gama, Magellan and the rest. The history of the world might have been quite different!
Chemically and mineralogically, the lodestone is magnetite, a massive type of iron ore, an oxide of iron, a mineral related the the brown stuff coating the magnetic disks and tapes used by computers. Magnetite is quite common in nature, while lodestones are relatively rare. Why are a few rare pieces of it different from the rest?
First, he showed, not all magnetite can become a lodestone: a certain composition and crystal structure are required. But even then, just resting a few million years in the Earth's magnetic field would produce no magnetization.
For that, a strong magnetic field must be applied. The application may be very brief--as when a computer disk or video tape passes by the recording head--but the strength of the magnetic field must exceed a certain minimum.
Dr. Wasilewski believes that is what happens when a chunk of the appropriate ore is struck by lightning. Lightning is a discharge of cloud electricity, a large electric current lasting just a fraction of a second, but during that time it produces a strong transient magnetic field.
The idea was tested at a unique facility of the New Mexico Institute of Technology, the Langmuir laboratory. That lab, a center of lightning studies, was built on top of South Baldy Mountain near Soccorro, New Mexico, the location of frequent lightning strikes. By placing mineral samples where lightning would hit them, Dr. Wasilewski turned magnetite with appropriate crystalline structure into lodestones.
William Gilbert had a clue to this process, but not surprisingly, he missed its significance. In "De Magnete" he cited the following passage from a book published in Italy:
In hindsight we would guess that the church tower had been hit by lightning, which magnetized the iron. Gilbert however attributed it to long-term exposure to the Earth's magnetism, "by the turning of its extremities towards the poles for so long a time."
Magnetization by Heat-WorkingGilbert also observed that iron could acquire magnetic properties as a result of being heat-worked by a blacksmith:
"Verticity" here means magnetization. The observation is quite accurate: above a certain temperature ("Curie point"), iron loses all its magnetism; then, when it is cooled back past that temperature, it "captures" the magnetism existing in its surroundings, due to the poles of the Earth. Its "verticity" is nowhere as strong as the one produced by a stroke of lighning. Still, as Gilbert observed, in a long iron rod that magnetism was channeled to the ends, where it could be noted.
Magnetism "captured" by hot material that cools down in the presence of the Earth's magnetic influence has played an important role in the more recent discovery of plate tectonics, discussed in a later section of this web site. A similar cooling process may also be responsible for the patchy magnetization observed on the surfaces of Mars and the Moon.
Back to the Master List
Author and Curator: Dr. David P. Stern
Mail to Dr.Stern: earthmag("at" symbol)phy6.org
Last updated 23 February 2008