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Teaching about the Earth's Magnetism
in Earth Sciences Class

by David P. Stern
Lab. for Extraterrestrial Physics, Goddard Space Flight Center (NASA)
Greenbelt, MD 20771     earthmag("at" symbol)phy6.org

Talk presented at the
Baltimore Meeting of the Natl. Science Teacher Assoc., 18 November 2000


This is Part 2 of three
Go to Part 1
Go to Part 3

The Terrella

 Gilbert' Terrella
   Gilbert carried out many experiments, but the most famous one was his explanation, why the compass pointed north. He shaped a big lodestone into a sphere. Since it was meant be a model of the Earth, he named it "Terrella," meaning, "little Earth."

   Over the surface of the terrella he moved a magnetic compass. More accurately--since the compass had to stay horizontal--he moved the terrella, placing the compass next to various points on it.

When the compass was level with the surface of the terrella, the needle always pointed to the magnetic north pole of the terrella.

   The picture here shows what happened when the compass was perpendicular to the terrella--the poles are on the left and right, the equator on top, and the needle in general slants down, just the way Robert Norman observed. At the poles, it points straight down, on the equator it is horizontal. This experiment convinced him that the directionality of the compass was caused by the Earth itself being a great magnet. Supposedly, he demonstrated his terrella to Queen Elizabeth--at least, a painting of such a demonstration exists.

 Gilbert's Experiment on
Induced Magnetism
   Here is another experiment. Why are pins and nails attracted to magnets? Gilbert correctly argued that the influence of the magnet turned them into temporary magnets, too. Here is how he demonstrated it--by hanging two bars of iron above the pole of a terrella, and noting that they repelled each other. (By the way, the same experiment appears on the front page of "De Magnete", held by the soldier on the right.)

   Trying to duplicate this experiment (in class or otherwise) is hard, the force is small and the bars do not seem to angle the way they do in the drawing. However, a different way is possible, even on top of a "vu-graph" projector. It is essentially a magnetic analog of the well known electroscope for detecting static electricity. Instructions for performing this experiment are in the section Performing One of Gilbert's Experiments:
Induced Magnetism..

   Gilbert's interest in magnets made him examine other kinds of attraction as well. Glass, amber, crystals and some other substances, in dry air, could attract little bits of straw and paper if they were rubbed by cloth or fur. The Greek word for amber is elektron (spelled with a K), so Gilbert named such materials "electricks" and their attraction the electrick force. From that came our modern words of "electricity," "electron, " "electronics" and the rest.

   The figure below shows a lightweight pivoted needle which he designed, to indicate the direction of the electric force. Clearly, it was modeled after the magnetic compass.

 Gilbert's "Versorium" for
observing the Electric Force

The Geomagnetism Web Site

   In the panels below is the part of the home page of the web site, with a picture of Gilbert, based on an old one kept in Oxford. Gilbert was a contemporary of Shakespeare and probably attended first runs of his plays-- in 1600 "Julius Caesar," was staged, followed by "Hamlet" in 1601. London was a crowded, unsanitary city of 75,000, where the bubonic plague often broke out in the summer. Gilbert died of the plague in 1603, and one can wonder whether he was infected by one of his patients.


The Great Magnet, the Earth

by David P. Stern

Commemorating the 400th anniversary of "De Magnete" by William Gilbert of Colchester


(Best viewed in font #14,
but print in #10 or #12)


      William Gilbert
       In 1600, four hundred years ago William Gilbert, later physician to Queen Elizabeth I of England, published his great study of magnetism, "De Magnete"--"On the Magnet". It gave the first rational explanation to the mysterious ability of the compass needle to point north-south: the Earth itself was magnetic. "De Magnete" opened the era of modern physics and astronomy and started a century marked by the great achievements of Galileo, Kepler, Newton and others.

       If you lived in London in 1600, you could have purchased "De Magnete" for seven shillings and sixpence. To read it, of course, you would have to know Latin, the language of science in 1600. You might have had the rare privilege of attending first runs of Shakespeare's plays in the "Globe" theatre--sitting in the balcony if you could afford it, standing in front of the stage if not. However, you might have had to weigh this pleasure against the peril of bubonic plague, which usually spread in the city during summer months.

       This web site tells the story of Gilbert and his book--with glimpses of London in 1600, and with studies of magnetism before Gilbert. It then recounts the later history of the Earth's magnetism, including...



                        Table of Contents

The Gilbert anniversary provided the original motivation for assembling the a site, and the first 5 sections after the home page are related to this and include two reviews of his book. You can still buy from Dover books, for $14.00, a paperback copy of an English translation first published in 1893. All the links on the list below are active.


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:

  1. --Why is the Earth magnetic?

  2. --Why does the magnetism of the Earth slowly change--unlike that of a bar magnet?

  3. --Why are sunspots intensely magnetic--when their temperature is so high that they must consist of hot gas?

  4. --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?

  5. --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:

  6. --How do satellites measure magnetic fields--even ones 100,000 weaker than those observed on the ground?

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

Last updated 25 November 2001
Re-formatted 9-27-2004