An Index to the Web Site

A review of "De Magnete"
Another review of "De Magnete"
    (By Stuart Malin and David Barraclough
            in Eos of 23 May, 2000).
More about "De Magnete"
Performing Gilbert's Experiment on Induced Magnetism
What was known in Gilbert's Time
London in 1600

Magnetism from Gilbert to 1820
Oersted and Ampere link Electricity to Magnetism
The Lodestone
Gauss and the Global Magnetic Field
The Sun's magnetic cycle

The Dynamo Process
The Self-Sustaining Dynamo in the Earth's Core
About Electronic Magnetometers and about Smoking
Magnetic Reversals and Moving Continents
The Magnetosphere
Planetary Magnetism
G l o s s a r y
Questions and Answers         -- and oh, do we get questions!

Of Special Interest to Science Teachers

Note to Science Teachers

Teaching Geomagnetism in an Earth Sciences Class , a short article submitted to "The Physics Teacher" with suggestions on using this material in schools.

    Teaching about the Earth's Magnetism in Earth Sciences, a one-hour illustrated talk given by the author 18 November 2000 before the regional meeting of the National Association of Science teachers (NSTA). For faster loading, this document is divided into 3 parts. Go to Part 1
Go to Part 2
Go to Part 3

•   The Exploration of the Earth's Magnetosphere
•   From Stargazers to Starships

            Happy Exploring!

Gilbert's terrella, a model       of the magnetic Earth

He started by reading and examining all existing literature, finding rather little of value. Next he designed and performed his own experiments, not just on magnetic forces but also on "electrick" ones (his term), feeling the two were somewhat related. His studies involved both naturally found magnets--"loadstones" or "lodestones"--and artificially magnetized iron. He also fully understood induced magnetism, the fact that a piece of non-magnetic iron temporarily took on all properties of a permanent magnet when placed next to one.

William Gilbert

Professionally, William Gilbert (1544-1603) was a distinguished physician, appointed in 1601 as physician to Queen Elizabeth I. The queen died two years later, and Gilbert himself perished not long afterwards from the plague, with which London was often afflicated. His life-long passion, though, was magnetism, and "De Magnete"--divided into six "books"--was certainly his greatest accomplishment. Edward Wright, who wrote an introduction to "De Magnete", accurately observed:
"... for 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. "

"De Magnete" was written in Latin, but two excellent translations exist, that of Paul Fleury Mottelay (1893) which is still in print (Dover Books, $13.95) and a more sumptuous one by Silvanus Thompson (1900), the source of the passages quoted here. Even translated, the book is a challenge, with obtuse phrasing and paragraphs that stretch for pages. Yet through it all we can see the author struggling with his material, trying to impose some sense, some logical pattern on puzzling and contradictory statements and observations. This is science in its rawest state: Newton may have stood "on the shoulders of giants," but Gilbert had to build his understanding from the ground up.

Once answers are known, you can never again recapture the fog of ignorance, the frustration of not knowing, of not being able to see a clear connection. Reading "De Magnete" is perhaps the closest we can ever come to reliving that experience. Gilbert accurately noted that cast iron was feebly magnetic, and that long iron rods had magnetic poles at their ends. Why? How? It is easy for us to nod sagely and say yes, the iron captured the surrounding magnetic field of the Earth as it cooled past the Curie point, and those field lines were channeled by its elongated shape, creating a concentrated effect at its ends. But this is now, and that was then.

Not all of Gilbert's claims have stood the test of time. Gilbert believed that the Earth's magnetism and its rotation had a common cause: the fact that magnetic north and astronomical north were so near to each other seemed too much of a coincidence. Though pure guesswork, utterly discounted now, that idea did enjoy a brief revival of sorts in the mid-1900s, due to P.M. Blackett. Concerning the rotation of the Earth Gilbert never had any doubt. Others might have viewed the Earth as the center of creation, around which stars and other luminaries whirled, but not Gilbert, who calculated the implied velocities and found them incredibly large.

Let us not forget, in this connection, that 1600 was also the year when Giordano Bruno was burned at the stake. Claims of the Earth's rotation had to be (at the very least) reconciled with religious dogma. Edward Wright, in his introduction, tried to do so in the following words:

"Nor do those things which are adduced from the sacred scriptures seem to be specially adverse to the doctrine of the mobility of the Earth; nor does it seem to have been the intention of Moses or of the Prophets to promulgate any mathematical or physical niceties, but to adapt themselves to the understanding of the common people and their manner of speech, just as nurses are accustomed to adapt themselves to infants, and not to go into every unnecessary detail... "

If rotation and magnetism went together, how come the compass needle rarely pointed to true north, but exhibited a small "variation" (today called "declination")? Gilbert noted that in the northern Atlantic Ocean, the variation was always towards the nearer continent: towards Europe near Europe, towards American near America. He then ingeniously proposed that if the Earth were a perfect sphere, the two directions would always coincide. However, the Earth is not quite spherical: the Atlantic ocean forms a gash in its surface (water apparently contributes no magnetism), while Europe and Africa to its east and America to the west rise above the average surface and may add magnetic attraction.

Gilbert's "loadstone impaired by decay"

"Let there be a loadstone somewhat imperfect in some part, and impaired by decay (such as one we had with a certain part corroded to resemble the Atlantic or great Ocean). "

Moving the compass needle around the blemished terrella, Gilbert found his guess confirmed. Away from the pitted part, and also at the center of the pit, the "versorium" pointed towards the magnetic pole. However, near the edges of the depression, the direction of the needle (faintly visible in the drawing) veered towards the unblemished parts, just as the compass needle in the oceans close to Europe or America was deflected towards the nearby continents.

Because dips and rises in the globe did not change (at least, on the scale of human history), he boldly predicted that the "variation" will stay unchanged:

"Unless there should be a great dissolution of a continent and a subsidence of the land such as there was in the region Atlantis of which Plato and the ancients tell, the variation will continue perpetually immutable... "

Unfortunately, even predictions based on experimental evidence may miss the mark. As Gellibrand discovered around 1634, the magnetic field constantly changes, which is why a new IGRF (International Geomagnetic Reference Field) must be calculated, from more recent observations, every decade or so.

There is more, much more, often embellished in colorful phrases no modern editor would ever let slip by. How stilted does modern scientific prose sound, compared to Gilbert's words! Here, for instance is how he proposed to use the dip angle (his term is "declination") to deduce latitude at sea when the skies were obscured:

"We may see how far from unproductive magnetick philosophy is, how agreeable, how helpful, how divine! Sailors when tossed about on the waves with continuous cloudy weather, and unable by means of the celestial luminaries to learn anything about the place or the region in which they are, with a very slight effort and with a small instrument are comforted, and learn the latitude of the place."

Gilbert even designed an instrument to measure the dip angle (click here for a drawing). In practice this method won't give latitude very accurately, but no matter. We now have the satellite-based global positioning system (GPS), rendering a much, much better service--but has anyone ever praised it in poetic language like the one Gilbert used?

  And those who nowadays believe in the holistic magic of magnetic bracelets may well heed Gilbert's advice

"The application of loadstones to all sorts of headaches no more cures them (as some make out) than would an iron helmet or a steel cap. "

Our understanding of the Earth's magnetism, and of magnetism in general, has come a long way in the 400 years since "De Magnete" first saw light. One has to read the book--at least, parts of it--to realize how much clearer our view is now.
    It is not because we are any wiser or more ingenious. It is just that we stand on the shoulders of giants, of which Gilbert certainly was one.

Gilbert's achievements as a doctor would have been enough to secure his fame, but he is remembered today for his investigations into magnetism and electricity, which he reported in De Magnete. These investigations were conducted from about 1581 to 1600, in parallel with his medical career. The experiments were made and discussed with like-minded friends who met in his house for this purpose; a pattern similar to that followed half a century later which led to the foundation of the Royal Society. Although magnetism was essentially a hobby, it was one that Gilbert, who never married, took very seriously and on which he expended large sums of money (£5000 according to William Harvey) for instruments and equipment.

The book itself is a handsome production, well illustrated with woodcuts and various printing devices such as illuminated letters at the start of each chapter. Marginal asterisks, of two sizes, draw attention to points of particular importance. After an author's preface, by Gilbert, and an encomiastic preface, by Edward Wright, there are 115 chapters, sometimes of only one paragraph, arranged in 6 books. Edward Wright (1558?-1615) is famous for putting Mercator's projection on a sound mathematical footing. The book is not unlike a modern PhD thesis in layout, starting with a survey of previous work, moving on to experimental results, discussing these and setting them in the broader context of worldwide results, and ending with speculation and unsolved problems.

The first book starts with an historical survey, then reviews the basic magnetic properties of a lodestone (poles, attraction and repulsion, magnetisation of iron) and ends with the famous chapter on the Earth itself being a great magnet. There is an interesting discussion of the medicinal properties of iron and lodestone, in which it is concluded that magnetism plays no part, since: "when drunk in a draught" [lodestone does not] "avail to attract or repel".

In the second book a clear distinction is drawn between the attractive properties of magnets and rubbed amber: "for it pleases us to call that an electric force". Many magnetic and static-electric experiments are reported, including investigations of the effects of interposed material, shape of lodestone and the effect of "arming" the poles with iron caps. Much folklore is refuted, not least the possibility of a perpetual motion machine: "O that the gods would at length bring to a miserable end such fictitious, crazy, deformed labours, with which the minds of the studious are blinded! "

Book three is concerned with the directive properties of a magnet, but also with details of the magnetisation of needles and the distribution of magnetism in a terrella (Gilbert's word for a spherical lodestone). The terrella experiments are of particular importance, since it was these that led Gilbert to draw the analogy between the magnetic field of the Earth and that of a terrella.

This introduction to geomagnetism is developed in more detail in book 4 (on declination, which was then known as "variation") and book 5 (on dip, which Gilbert calls declination).

The final book is more speculative. It concerns stellar and terrestrial motions, which Gilbert erroneously associates with magnetism and believes that this adds support to the Copernican theory. This was unacceptable to religious views at the time and many of the European copies of De Magnete have had Book 6 removed or defaced. Nevertheless, the idea of action at a distance for controlling the planetary motions was seminal for Hooke's and Newton's thoughts about gravity.

But a mere list of its contents can give only a hint of the book. It is constantly necessary to remind oneself while reading it of just how early it is. Here is Baconian science, based on experiment and observation rather than hearsay, being practised twenty years before the publication of Bacon's Novum Organum. And, over a hundred years before it ceased to be an offence punishable by flogging for a British naval helmsman to have garlic on his breath for fear of demagnetising the ship's compass, De Magnete dismisses as "fable and falsehood" the idea that a lodestone smeared with garlic loses its power. All this at a time when it was heresy to set experiment against the teachings of the church; indeed, it was also in 1600 that the philosopher Giordano Bruno was burnt at the stake for heresy. Admittedly, things were more relaxed in England than in Italy, but it was still courageous, perhaps even foolhardy, to publish such a book.

The quoted price of De Magnete may be a little misleading as, even without the benefit of this review, all four Latin editions, of 1600, 1628, 1633 and 1892 (a facsimile of the first edition) are sold out. While second-hand copies occasionally appear at auction, they are not cheap: a good first edition was recently (January 1998) sold for $15,000. The 1900 Chiswick Press edition in English (translated by Thompson) was limited to 250 copies and is now virtually unobtainable, but was reprinted in facsimile in 1958 by Basic Books, edited and with an introduction by Derek J. de Sola Price. A reprint of Mottelay's translation was published by Dover in 1958 and is still in print.

Once the reader has got used to its inevitably archaic style, De Magnete is a surprisingly readable book. There is no need to commend it to historians of science as they will already have read it. But any geophysicist would not only benefit from, but also enjoy reading it.

Compass directions above   different parts of Gilbert's terrella

Of his own experiments, the most important was conducted with a magnetized "terrella" ("little Earth"), a spherical magnet serving as a model for the Earth. By moving a small compass over the surface of the terrella, Gilbert reproduced the directional behavior of the compass; reputedly, he also demonstrated this in front of Queen Elizabeth and her court.

[In modern times, scientists have used magnetized terrellas inside vacuum chambers to mimic the effect of the Earth's magnetism on auroral electrons, cosmic ray particles and the solar wind. For one example, see here.]

In addition to studying magnets. Gilbert also looked into a vaguely similar phenomenon--the fact that certain materials, when lightly rubbed with cloth or fur, attracted light objects such as chaff. One such material was amber, a yellow fossilized resin called elektron by the ancient Greeks. From this Gilbert named such attraction the "electrick force," and from that came such words as electric charge, electricity, electrons and electronics. Gilbert even devised a pivoted lightweight needle--a "versorium" resembling a compass needle--to observe the direction of the electric force.

Gilbert ascribed the deviation of the compass needle from true north to the attraction of the continents, which tallied with observations in the Northern Atlantic--the needle veered eastwards near Europe, westwards near America. Noting that near the islands of Novaya Zemlaya, north of Russia, the compass needle pointed west of true north, Gilbert speculated that a "north-east passage" around Russia might exist, giving more direct access by sea to the spice islands of the Far East. Some decades earlier, Frobisher and Davis had sought in vain a similar "north-west passage" around the American continent.

Given two magnets, Gilbert knew that magnetic poles can attract or repel, depending on polarity. In addition, however, ordinary iron is always attracted to a magnet--even soft iron, which loses all its magnetism again when it is removed again.

Gilbert guessed, correctly, that near a permanent magnet such iron became a temporary magnet, of a polarity suitable for attraction. That is, the end of an iron bar stuck to an S pole of a magnet (south-seeking pole) temporarily becomes an N-pole. Because magnetic poles always come in matched pairs, the other end of the bar temporarily becomes an S-pole, and can in its turn attract more iron.

You see this if you dip (say) a horseshoe magnet into a cup with iron pins. As expected. many pins will stick to the poles, but in addition, some more pins will stick to those pins. Yet when the pins are pulled loose, they all are seen to be non-magnetic again.

Gilbert confirmed his guess of temporary ("induced") magnetism by an original experiment (see drawing). Using strings, he hung two parallel iron bars above the pole of a terrella, and noted that they repelled each other. Under the influence of the terrella, each became a temporary magnet with the same polarities, and the temporary poles of each bar repelled those of the other one.

(By the way--can you find the same experiment portrayed on the front cover of "De Magnete"?)

3. Take the remainder of the string, thread its end through one of the loops and close it in a loose loop, tying its end. (Keeping it loose allows you to cut that piece and remove it from the suspension loops; a tight knot would not let you do so.)

   Do the same with the other suspension loop, keeping distance of about 50 cm (20") between the bars. Cut off the excess.

   You should now have two short wire-bars hanging at opposite ends of a string.

4. With the wires hanging down in parallel (and not entangled!) take the middle of the string and wrap it twice around the nail or ball-point barrel. By twirling the nail with your fingers, you should be able to bring the two wires to exactly the same height.

Hold the nail still until the hanging wire-bars stop moving and are a short distance apart (a few millimeters or 1/8"). Then carry them to hang above the pole of the magnet (see drawing). Gently lower them towards the magnet, and if all goes well, you should see them move apart to about 2-3 times the distance. It is not a large effect and, contrary to Gilbert's illustration above, the wire-bars stay parallel, they do not form an angle.

When the wire-bars are removed, they slowly move together again and the experiment can be repeated

The early Chinese also knew about lodestones and about iron magnetized by them. Around the year 1000 they discovered that when a lodestone or an iron magnet was placed on a float in a bowl of water, is always pointed south. From this developed the magnetic compass, which quickly spread to the Arabs and from them to Europe. The compass helped ships navigate safely, even out of sight of land, even when clouds covered the stars. Compasses were also built into portable sundials, whose pointers had to face north to give the correct time.

The nature of magnetism and the strange directional properties of the compass were a complete mystery. For instance, no garlic was allowed on board ships, in the mistaken belief that its pungent fumes caused the compass to malfunction. Columbus felt the compass needle was somehow attracted by the pole star, which maintained a fixed position in the northern sky while the rest of the heavens rotated around it.

Two things were noted in those centuries. First, the compass needle did not point exactly north (towards the pole star) but veered off slightly to the east. As Columbus sailed across the Atlantic Ocean, he claimed changes in the direction of the needle. How much of this was the spurious effect of inaccurate observations is still being debated. One letter, written by Columbus in 1498 from Haiti, claimed the compass direction changed from east of true north to west of it, suggesting that the discrepancy might depend on location (as it indeed does).

And second, the force on the needle was not horizontal but slanted downwards into the Earth. If a compass needle was balanced evenly on its pivot before being magnetized, then afterwards its north end would be pulled downwards, and a tip had to be snipped off to restore balance. This was studied by Robert Norman of London, England, who in 1581 published his finding in a book, The Newe Attractive.

London Bridge

Elizabeth I

Under Queen Elizabeth I the city became a prosperous center of commerce. The defeat of the Spanish Armada (1588) opened the way to the British settlement of North America, where a short-lived colony had already existed 1585-7 on Roanoke Island, now in North Carolina. The second attempt, in 1606 at Jamestown Virginia, took root. The big news of 1600 was the downfall of the Earl of Essex, once Elizabeth's favorite; he was executed in 1601.

The city was congested and unsanitary, and rats carrying bubonic plague thrived in it. Outbreaks usually ocurred in the summer, and at such time the royal court sometimes prudently retreated to the countryside. Physicians such as William Gilbert had their hands full, but they could do little and were unaware of the role of the rats. Gilbert, appointed royal physician in 1601, himself died of the plague in 1603.

Shakespeare's creative genius was in full bloom in 1600. He put on his plays in the Globe theatre, a ring-shaped structure built in 1599 on the banks of the Thames from the remains of an earlier theatre. His plays of the 1599-1600 season were Julius Ceasar, 12th Night and As You Like It; in 1600-1 came Hamlet and Merry Wives of Windsor. It is quite likely that Gilbert attended at least some of those plays.

Henry Gellibrand (soft "g") published in 1635 evidence that this difference slowly changed with time. That was an unsettling discovery. It meant that observations of the local compass bearing became inaccurate after some decades and therefore had to be repeated from time to time.

And from a theoretical angle, how could the magnetic properties of the Earth undergo such gradual change? No known magnet behaved that way. Edmond Halley, of comet fame, came up in 1692 with an ingenious explanation. The interior of the Earth, he claimed, consisted of layers, spheres within spheres. Each sphere was independently magnetized, and each rotated slowly with respect to the others.

(The current explanation of the Earth's magnetism involves electric currents, deep in the Earth's molten core, as discussed in later sections of this site. However, earthquake waves have shown that the Earth does consist of "spheres inside spheres," and two of these are the liquid core of the Earth and the solid "inner core" inside it. Most recently a slight difference in the rate of spin between these spheres was implicated in generating the Earth's magnetic field.

Edmond Halley

In 1724 George Graham found that the compass needle sometimes veered off by a small angle, for a day or so; a century later Alexander von Humboldt would name such events magnetic storms. That effect was widespread: Anders Celsius in Uppsala observed one at the same time as Graham in London, and a century later it was found to be world-wide. Celsius and his student Hiorter also observed magnetic disturbances linked to the "northern lights" (polar auroras); in our time, such events are associated with "magnetic substorms."

All that time, the only kind of magnetism known was the permanent magnetism of magnetized iron or of lodestones. The magnetic force due to the magnetic pole at the end of a magnet seemed a bit like gravity or the electric force, growing weaker in proportion to 1/r², with r the distance from the pole. This "inverse square" relationship was confirmed in 1777 by Charles Coulomb in France, through experiments with a magnetic needle suspended on a twistable string--an instrument which Coulomb introduced, the prototype of most magnetic detectors in the following 170 years.

The main difference was that while gravity only attracted, magnetism could also repel. Jonathan Swift (1726) satirically proposed that such a repulsion could act as "anti-gravity," keeping an island floating in space, as told in the story of the 3rd voyage of "Gulliver's Travels."

In 1820 Oersted arranged in his home a science demonstration to friends and students. He planned to demonstrate the heating of a wire by an electric current, and also to carry out demonstrations of magnetism, for which he provided a compass needle mounted on a wooden stand.

While performing his electric demonstration, Oersted noted to his surprise that every time the electric current was switched on, the compass needle moved. He kept quiet and finished the demonstrations, but in the months that followed worked hard trying to make sense out of the new phenomenon.

Oersted's Experiment.

But he couldn't! The needle was neither attracted to the wire nor repelled from it. Instead, it tended to stand at right angles (see drawing below). In the end he published his findings (in Latin!) without any explanation.

What Oersted saw.

Here is how this can lead to the notion of magnetic poles. Bend the wires into circles with constant separation (figure below):

Parallel currents in two loops also attract

--Two circular currents in the
same direction attract each other.

--Two circular currents in
opposite directions repel each other.

Maxwell

Thus two kinds of forces were associated with electricity--electric and magnetic. In 1864 James Clerk Maxwell demonstrated a subtle connection between these two types of force, unexpectedly involving the velocity of light. From this connection sprang the idea that light was an electric phenomenon, the discovery of radio waves, the theory of relativity and a great deal of present-day physics.

•     A pocket compass.
•     A one-foot (30 cm) length of fairly thick wire, insulated or bare.
•     A 1.5 volt electric cell ("battery") of size "D" or "C". The voltage is too low to cause any risk.
  1.     Lay the compass on a table, face upwards. Wait until it points north.
  2.     Lay the middle of the wire above the compass needle, also in the north-south direction (compare to the above image "What Oersted Saw"). Bend the ends of the wire so that they are close to each other.
  3.     Grab one end of the wire in one hand and press against one end of the battery.
  4.     Grab the other end with your other hand, and press momentarily against the other terminal of the battery. The needle will swing strongly by 90 degrees.
         Quickly disconnect (it is not good for the battery to draw such a large current). The needle will swing back to the north-south direction.
  5.     Repeat with the connections of the battery reversed. Note that the needle now swings 90 degrees in the opposite direction.

  6.     Take a piece of paper 2"x4" (5x10 centimeters) and fold the longer side into pleats, about 3/8" (1 centimeter) high. Put the wire on the table, its middle in the north south direction, put the pleated paper above it so that the wire is below one of the pleats, and place the compass on top of the pleats.
         You can now repeat the experiment with the compass above the wire (if two people perform the experiment, they need no pleats or table--one can old the compass, the other the wire and battery). Note that the needle swings in the opposite direction than when the compass was below the wire.

What if no lodestones existed? The Chinese would certainly not have invented the magnetic compass. Magnetism would have been discovered much later, and one wonders how. Lacking the compass, the great voyages of discovery could hardly have taken place--Columbus, De Gama, Magellan and the rest. The history of the world might have been quite different!

Chemically and mineralogically, the lodestone is magnetite, a massive type of iron ore, an oxide of iron, a mineral related the the brown stuff coating the magnetic disks and tapes used by computers. Magnetite is quite common in nature, while lodestones are relatively rare. Why are a few rare pieces of it different from the rest?

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

"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.

Further Reading

Public interest in science owes a great deal to Alexander Von Humboldt (1769--1859). As a young man Alexander explored the jungles of South America, but much of his life was spent in Paris, where he tirelessly drew the public's attention to the achievements of the natural sciences. Late in life he assembled his scientific knowledge into a monumental set of volumes titled " Kosmos."

Carl Friedrich Gauss

Up to that time, the compass needle--and the downward-pointing "dip needle" on a horizontal axis--measured well the direction of the magnetic force, but what about measuring its strength? Gauss devised a clever method for doing so, using an auxiliary magnet; today this is a popular undergraduate lab excercise.

He also knew a method used in celestial mechanics for analyzing gravity, and applied it to the description the Earth's region of magnetic forces, its "magnetic field." That method, too, is still in use: it represents the field as the sum of a dominant north-south "dipole" (2-pole, like a bar magnet) whose strength decreases with distance r like 1/r³, plus a "4-pole" decreasing like 1/r⁴, plus an 8-pole decreasing like 1/r⁵, and so forth. The field of an isolated "monopole" would presumable decrease like 1/r², the way gravity does--but no such single pole was ever observed, they always come (at the very least) in pairs.

The new tools for better observation and description of the Earth's magnetic field led to better, world-wide observations. Gauss and Weber organized a "Magnetic Union" for setting up observatories, and Humboldt enlisted Russia's Czar to create a chain of them across Siberia. The greatest help however came from the British empire, whose "Magnetic Crusade" led by Sir Edward Sabine set up stations from Canada to Tasmania (then known as "Van Diemen's Land"). The vast network not only made possible the first global models of the field, but also demonstrated the world-wide character of magnetic storms.

One can compare today's magnetic models, some of them based on satellite observations, to the ones started by Gauss more than 150 years ago. One trend then stands out: the dominant "dipole" field is getting weaker, at about 5% per century (the rate might have increased since 1970). In the unlikely event that the trend continues unchanged, about 1500-2000 years from now the magnetic polarity of the Earth would reverse.

The magnetic field of the Earth would not disappear, because other field components (4-pole, etc.) are meanwhile growing stronger, and as the late Ned Benton has shown, the total magnetic energy remains practically unchanged. But the dominant compass direction would reverse--and "fossil magnetism" of rocks suggests that this has indeed happened many times in geological history, most recently about 700,000 years ago.