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From: The World & I: October 2000
Demystifying Magnetism
David P. Stem
Published in 1600, De Magnete, William Gilbert's book on the properties of
magnets, bridges the divide between medieval scholarship and modern
science.
Without the magnetic compass, Columbus, da Gama, Magellan, Drake, and
other great navigators could never have conducted their voyages. The
magnetic compass was and is a simple device - a magnetized needle
suspended on a pivot-and yet up to 1600, no one had any idea what made it
point north.
Magnetism itself was a mystery. According to legend, a Greek shepherd near the city of Magnesia
found that his iron-tipped staff stuck to a peculiar outcropping of rock, a natural magnet later named
lodestone. The Chinese knew that steel rubbed against a lodestone became magnetic, that both
attracted iron, and that a magnetic needle had two different poles. Around the year 1000, some
unknown Chinese placed a lodestone on a "boat," set it afloat, and observed that it always turned to
point to a fixed direction.
Centuries passed. The familiar pivoted needle was developed and became a standard navigation tool,
but no one knew how it worked. Was it attracted by a magnetic mountain at the North Pole, a
mountain that no ship built with iron fittings or nails should ever dare to approach too close? Or was
it, as Columbus thought, the attraction of the pole star, the northern pivot around which the heavens
seemed to rotate, even the sun as it rose & and set?
Hardly anyone conducted serious experiments with magnets, in part because the tradition of science,
which today we take for granted, almost died out in the Middle Ages. History told of an ancient
golden age, when Greek philosophers knew wisdom. The books that had survived from that era were
copied, embellished, and cited, but few people studied nature itself anymore, If some ancient Roman
authority had written that the sharp smell of garlic destroyed magnetism, this was widely accepted.
And since a ship's safety might depend on its magnetic needle,
it also became a flogging offense for the helmsman to be
caught eating anything with garlic in it.
All this was greatly changed by a Latin book that appeared in
London in 1600 titled De Magnete (On the magnet). Its author
was William Gilbert , president of the Royal College of
Physicians, and it described the results of nearly 20 years of
study. Gilbert was fascinated by magnets. He collected and
read books discussing magnetism and examined the truth of
their claims, finding most of them false. Garlic had no effect,
and neither did diamonds (as was reputed).
Being a physician, he also examined medical claims, finding
them equally illusory: "The application of lodestones to all
sorts of headaches no more cures them (as some make out) than
would an iron helmet or a steel cap."
We do not know what started Gilbert's lifelong interest, but it might have been a remarkable
experiment published in 1581the year Gilbert began his studies-by Robert Norman, a British
compass maker.
In those days a craftsman would build a compass as follows: He would fashion a flat steel needle,
balance it on a pivot, and then magnetize it by stroking it with a lodestone or a strong magnet,
always in the same direction, until it, too, became magnetic. But a strange thing was noted: When
the magnetized needle was placed back on its pivot, its north pointing end always hung down, as if it
had become heavier. To restore the balance, the craftsman then had to snip a piece off that end.
The story is told that when Norman one day spoiled a compass
needle by snipping off too much, he decided to investigate. Taking a
ball of cork, he stuck a stee1 needle into it and Boated the cork ball in
a goblet of water. Next, he whittled down the cork bit by bit until the
cork and needle just hovered in the water, and then after evening out
the position of the needle in the cork, he magnetized the needle by
stroking. When Norman again floated the cork and needle, the needle
turned northward-but at a steep slant. Suddenly it became clear why
the north end of the needle always seemed heavier. The magnetic
force was not at all horizontal, but pointed downward, into the earth.
It was the first true scientific experiment in centuries.
By that time, William Gilbert was already a well-known physician in
London. Born in 1544 in Colchester, northeast of London, he studied
medicine at St. John's College in Cambridge and later on the
Continent, and he started practicing medicine in 1573. In 159 9, he became president of the Royal
College of Physicians, and in 1601 he was appointed personal physician to Queen Elizabeth I.
Gilbert never married.
London in 1600 was a thriving center of activity, with about 75,000 inhabitants, plus a comparable
number in the surrounding suburbs. England was prosperous, due in no small part to the defeat of the
Spanish Armada in 1588, which had opened the way for the British to settle North America.
Shakespeare was at his creative peak, producing in his Globe theater on the River Thames
embankment Julius Caesar in 1600, followed in 1601 by Hamlet.
Gilbert's interest in magnetism started around the time when Norman's book The Newe Attractive
appeared, and Gilbert spent a considerable fortune on it -5,000 by one account, at a time when his
annual stipend as royal physician was merely *100. He discussed his
experiments with a circle of friends who had similar interests.
Gilbert's most famous experiment involved a spherical lodestone, which
he named "terrella" -the "little earth"-because it was indeed meant to
serve as a scale model of the earth. By moving a small compass across
the surface of the terrella, Gilbert could reproduce all compass needle
behaviors observed around the earth-not just the tendency of a horizontal
needle to point north but also the inclination of the needle to the vertical,
as observed by Norman. Gilbert concluded, "that the globe of the earth is
magnetick, & a magnet, and in our hands the magnet stone has all the
primary forces of the earth."
The earth itself was a magnet! That alone was a momentous discovery, but Gilbert went much
further, trying to understand magnetism itself. Noting the way magnets attracted and repelled Gilbert
concluded that "magnetick force is animate, or imitates life and in many things surpasses human life,
. . . [which] is bound up in the organic body."
He also noted that the attraction and repulsion between magnets was mutual, with both objects
participating equally, and therefore named it "coition." He added, "Orpheus in his hymns narrates
that iron is attracted by lodestone as the bride to the arms of her espoused."
He noted that magnets could attract or repel each other not only through a sheet of paper but also
through a burning flame. Yet, he noted as well that they lost their powers when raised to red heat. On
the other hand, a long unmagnetized steel rod could be slightly magnetized if a blacksmith heated it
to a red heat, laid it in the north-south direction, and then worked on it as it cooled. Gilbert viewed
this magnetization of the rod as the iron being "reborn" and taking on the magnetic virtues of its
mother, the earth:
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 whilst it is being formed and
lengthened out, is affected by the common cause (to wit, the earth); whilst it is returning also
from its heated condition to its former temperature, it is imbued with special verticity in
accord of its position.
Verticity was Gilbert's term for what we might call "magnetic polarity," and the magnetism of the
rod was indeed "captured" from the weak magnetic field of the earth in which it was immersed.
Magnets attracted or repelled other magnets, but ordinary iron was always attracted. Why? Gilbert
correctly guessed that, near a magnet, a piece of ordinary iron temporarily became a magnet itself.
Dip a horseshoe magnet into a cupful of iron pins, and some pins will stick to its poles, as might be
expected. Other pins, however, will stick not to the poles but to pins stuck to them showing that,
those pins were themselves turned into small magnets, as long as they were in contact with the poles
of the initiating magnet.
One might propose other explanations, but Gilbert supported his point by an ingenious experiment.
He took two bars of ordinary iron and suspended them by strings, next to each other, above the pole
of a terrella. He then found that the bottom ends of the bars repelled each other. Each became a
temporary magnetic pole, of the kind opposite to the one of the terrella below it, and poles of the
same kind repelled.
Gilbert's main interest was in "magneticks," materials attracted in the manner of iron and lodestones.
However, he also noted another type of attraction, in materials such as glass, amber, and crystal.
When these were rubbed lightly with, say, a dry cloth, they attracted chaff and other light objects.
The Greek word for amber (fossilized pine pitch) is elektron, so Gilbert named such materials "electricks," and their attraction was the "electrick force." From this originated our words for electricity,
electrons, electronics, and so forth. Gilbert investigated "eleetrick" phenomena in some detail, even
devising a lightweight, pivoted "versorium" needle, like that of a compass but nonmagnetic, to point
in the direction of the "electrick force." However, he never discovered that two distinct types of
electric charge existed.
In hindsight, Gilbert's De Magnete marks the dividing line between medieval scholarship, based
primarily on the citing of ancient authorities, and modem experimental science. By 1609 Kepler
would publish his first laws of planetary motion, and by 1610 Galileo would point his telescope
skyward and discover mountains on the moon, satellites circling Jupiter, and sunspots. Western
humanity's outlook on the universe was soon to be radically altered.
In 1600 many sti11 adhered to the dogma that the earth was the center of the universe, around which
everything else revolved. Some evidence suggests that doubting the geocentric doctrine may have
been one of the reasons for which Giordano Bruno was burned at the stake that year. Gilbert devoted
the last of the six "books" in De Magnete to arguing that the earth itself revolved the view of
Copernicus, whose book had appeared in 1543 and whom Gilbert praised as "the restorer of
astronomy." De Magnete's sixth book was so radical that it was sometimes defaced or torn out.
Galileo, who praised Gilbert, wrote that he had obtained his copy of the book from a philosopher
who wanted to "rid his library of its contagion."
Interestingly, Gilbert believed that the earth rotated because it was magnetic. In 1947, P.M. Blackett,
a British Nobel Prize winner, briefly advocated an opposite view-that all large rotating masses were
magnetic and that was part of the reason why the earth had a magnetic field. Neither was right, as it
turned out.
Gilbert did not live long after the publication of his book. Shakespeare's London was also an
unsanitary, crowded city, in which the bubonic plague occasionally broke out. At such times, the
royal court found it prudent to leave the city temporarily and to suspend the performances at the
Globe.
Gilbert died of the plague in 1603. Yet, Edward Wright, in his introduction to De Magnete, quite
accurately wrote:
"… believe me . . . these books of yours on the Magnet will avail more for perpetuating the
memory of your name than the monument of any great Magnate placed upon your tomb."
David P. Stem is a physicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland
With files from http://www.phy6.org/earthmag/demagint.htm
1. Without the magnetic compass, the explorers who sailed the Atlantic might never have "discovered" the Americas.
a. How does a simple magnetic compass work?
b. When and how were magnets first employed?
c. What fallacies were passed down about magnets?
2. Although the ancient Greeks may have experimented with magnets, it wasn't until the late 1500s that texts describe
magnets.
a. Explain the process that a 1500s-era craftsman would use to create a magnet.
b. What phenomenon was supposedly corrected by cutting off a piece of the magnet at the end of this process?
What is the explanation for this phenomenon?
c. Identify and explain at least two observations that William Gilbert made using his spherical lodestone
"terrella."
d. What is meant by the term verticity?
e. Explain how Gilbert's experiments led to the coining of the word electricity.
f. What does the author mean by saying, that Gilbert's De Magnete "marks the dividing line between medieval
scholarship... and modern experimental science"?