Teaching about the Earth's Magnetism
in Earth Sciences Class

Oersted's discovery

   He told no one, finished his demonstrations, presumably saw his visitors to the door, and then, for the next half year, he tried to make sense out of what he had seen. But he couldn't! In the end he published a short account of what he saw, and a bright Frenchman, André Marie Ampére, made the breakthrough which Oersted had missed. Magnetism was basically an electric phenomenon, a property of electric currents: two parallel currents attracted each other, two antiparallel ones (in opposing directions) repelled. Iron was only an incidental part of the story--possibly because iron atoms had little electric currents flowing around them (close, but not quite true, and it took more than 100 years to find the full explanation).

   Stories like the one outlined just now both enlighten the student and stimulate interest. How do we scientists get them across to you teachers? Several ways exist, but since we live in the age of the internet, I used the world wide web and created three large web sites.

1. "From Stargazers to Starships" with home page at http://www.phy6.org/stargaze/Sintro.htm
   A course for high school or non-calculus freshmen, covering
   • (a) Elementary astronomy
   • (b) Newtonian Mechanics
   • (c) The Sun, and applications.
   • (d) Spaceflight and spacecraft.
   • (e) A self-contained algebra and trig course.
   It also includes guidance for teachers, problems, timelines, glossary etc.

2. "The Great Magnet, the Earth, "with home page at http://www.phy6.org/earthmag/demagint.htm
   a nonmathematical historical outline of the study of the Earth's magnetism.

3. "The Exploration of the Earth's Magnetosphere" with home page at http://www.phy6.org/Education/Intro.html
   a non-mathematical but fairly detailed overview of my own field of science.

   I hope you look up all three and find them all interesting and perhaps useful. But in the short time available, we can only discuss the second one on the list. It is also the shortest, about 20 sections, and has some direct applications to the teaching of Earth sciences (and a section discussing such applications). Some specific topics to cover, using this material, are

1. The year 2000 is the 400th anniversary of "De Magnete," the first serious study of electricity and magnetism, by William Gilbert, physician to Queen Elizabeth I. He was a pioneer of the experimental method and the first to explain the magnetic compass.

2. The story of Oersted and the formation of magnets and lodestones by lightning (of which Gilbert might have had a clue!). It introduces students to the real nature of magnetism.

3. Dynamo action is introduced by Faraday's disk dynamo and by his Waterloo bridge experiment, forerunners of modern explanations of the Earth's field.

4. The magnetic evidence for moving continents and magnetic reversals fits naturally into an Earth Sciences course.

Gilbert's "De Magnete"

   The second item was already discussed: it may seem more relevant to physics than to Earth sciences, but in 9th grade you try to give students a broad preparation. Oersted's story opens the way to discussing lodestones and lightning--lightning is an electric current, so it can magnetize a rock which it strikes (if it is the right kind of rock). That, apparently, is how lodestones are produced. More can be found on the web site, including a story by Gilbert--how an iron bar atop a church steeple in Italy was found to be magnetic, and no one understood why. Today, of course, it is easy to credit lightning.

   But let me go back to item #1. When did the era of modern science begin? Some say in 1609 or 1610, when Galileo first observed the heavens through a telescope, some cite Copernicus, 70 years earlier. However, one important benchmark was certainly the book "De Magnete" or "On the Magnet," published in London in 1600 by William Gilbert, physician to Queen Elizabeth, president of the Royal College of Physicians.

Below you see the top of front page of the book's 1628 edition (to see the entire page, click here). It was written in Latin, the language of science in those days.

Front page of "De Magnete."

   Gilbert was fascinated by magnets. Britain was a major seafaring nation--remember, 1588 was when the Spanish Armada was defeated, opening the way to British settlement of America--and ships depended on the magnetic compass, yet no one understood why it worked.

• --Did the pole star attract it (as Columbus once speculated), or was there a magnetic mountain at the pole, which ships should never approach, because its pull would yank out all their iron nails and fittings?
• -- Did the smell of garlic interfere with the action of the compass--which is why helmsmen were forbidden to eat it near a ship's compass?

   For nearly 20 years Gilbert had conducted ingenious experiments--among others, making sure that garlic had no effect--to understand magnetism. Until then, experiments were not in fashion: instead, books relied on quotes of ancient authorities (that was where the myth about garlic started). But there was one important magnetic experiment before Gilbert, published by Robert Norman in 1581, and I like to think, that was what gave Gilbert his start.

   It is a good story to tell in class, not only because it teaches students about magnetic dip, but because it is an excellent example of what makes a scientific experiment.

Robert Norman's Discovery

Robert Norman was a compass maker in London. In those days, this is how you made a compass. You produced a flat steel needle, then found the place in the middle where it balanced, and made an indentation, so that the needle could be placed on top of a pivot at that point.

Norman's Goblet.

Then you rubbed the steel needle against a lodestone to magnetize it. But a strange thing was noted: when the needle was placed again on its pivot, it no longer balanced. The north-pointing end seemed heavier, and the compass builder had to snip off a bit from that end, to balance it again.

One day Robert Norman spoiled a needle by snipping off too much, and decided to investigate. He balanced a needle on a horizontal axis lined up in the east west direction, and after balancing it carefully, magnetized it. The needle could still point north-south, but now it also had the freedom to point at any angle to the horizontal. Suspended that way, it did not stay horizontal, but tilted its northward-seeking end at a steep angle downwards. This showed that the magnetic force pulling it northward was not horizontal, but slanted downward, into the solid Earth.

The picture here, from Gilbert's book, shows a magnetized needle held by a small ball of cork or wax. The buoyancy of the ball is enough to keep the needle from sinking, but not enough to make it rise to the top, and instead it hovers between sinking and floating (very hard to actually achieve!). If it is balanced before being magnetized so that the ball is exactly in its middle, after being magnetized its northward-pointing end slants down at the dip angle. If one carefully remagnetizes it in the opposite direction, without moving the ball, its other end should point northwards and downwards.

The magnetic "dip angle" or inclination.

   This pivoted needle with a scale measuring the dip angle, is nowadays known as a "dip needle"), and here is its picture published in 1600 in William Gilbert's book. Such needles were in fact used for many years, and in 1831, when a British expedition located the north magnetic pole in Northern Canada, they confirmed its location by using such a needle, which pointed straight down. For a full-page picture, click here.