Gilbert' Terrella

   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

   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

Geomagnetism since Gilbert

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.