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9.   The Lodestone

(That is the modern spelling, even if a translation
of "De Magnete" spells it "loadstone")
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  Index

4. A Gilbert Expt.

5. Before Gilbert

6. London in 1600

7. 1600-1820

8. Oersted & Ampére

9. The Lodestone

10. Gauss

11. The Magnetic Sun

12. Fluid Dynamos

13. Dynamo in the
    Earth's Core

14. Magnetometers and
    Tobacco Smoking

15. Magnetic Reversals
    & Moving Continents

16. The Magnetosphere

17. Magnetic Planets

    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:

    "A druggist of Mantua showed me a piece of iron entirely changed into a magnet, drawing another piece of iron in such a way that it could be compared to a loadstone. Now this piece of iron, when it had for a long time held up a brick ornament on the top of the tower of St. Augustine in Rimini, had been at length bent by the force of the winds, and remained so for a period of ten years. When the monks wished to bend it back to its former shape, and had handed it over to a blacksmith, a surgeon named Maestro Giulio Caesare discovered that it was like a magnet and attracted iron. "

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-Working

Gilbert also observed that iron could acquire magnetic properties as a result of being heat-worked by a blacksmith:

    "For as when a babe is brought forth into the light from its mother's womb, and acquires respiration and certain animal activities.... so that piece of iron ... while it is returning also from its heated condition to its former temperature, it is imbued with a certain verticity in accord with its position."

"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.


Questions from Users:         What causes permanent magnetism?
          ***     Decay of magnetism in a magnet
                    ***     What is the smallest magnet?
                          ***     Can a lightning surge clean-wipe your hard disk?
                                ***     Protecting Magnetically encoded Tickets
                      ***     Taking Hard Disks across the Magnetic Equator
  Next Stop:   Gauss and the Global Magnetic Field

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Author and Curator:   Dr. David P. Stern
     Mail to Dr.Stern:   earthmag("at" symbol)phy6.org

Last updated 23 February 2008