Note to Teachers


  1. The science and stories of the "De Magnete" site are quite appropriate for science classes, especially in this anniversary year of 2000. Since no math is involved, they can be presented in a 9th grade Earth Sciences course, and even to younger students. The main points stressed would be

    • Permanent magnetism and the magnetism of the Earth.
    • Magnetism caused by electric currents
    • Sunspots and their cycles
    • Magnetic reversals and the spreading sea floor
    • Magnetism in space and on planets.

  2. Your best approach is to read the material through--it is not too long, and may match your own interests. Then decide what of it can be applied in class, and how.

    You may send comments, questions and corrections to my home address, audavstern@erols.com. I will try to answer--but no promises, my schedule is already quite crowded!

  3. Among the subjects covered, one is somewhat more difficult from the rest, and is therefore best left out when teaching younger students. That is the notion of dynamos and of dynamo circuits. The teacher may skirt this area when the subject of the origin and reversal of the Earth's magnetic field comes up, by simply stating the bare facts, namely

    • The slow variation of the Earth's magnetism, its high internal heat and the magnetism of sunspots suggest permanent magnetism is probably not involved in those phenomena.

    • Theories exits, showing how flows in the fluid core of the Earth, caused by the heat generated there, can generate the electric currents responsible for the Earth's field.

    • When volcanic lavas cool and harden, they take on (weakly) the local existing magnetization which exists at their location. The examination of the magnetization of such ancient lava flows suggests that in the past the north-south polarity of the Earth's field reversed directions, on the everage every half-million years or so.

  4. Final comment: translations to languages other than English are encouraged. Please contact me (audavstern@erols.com) to work out the details. Your name will be credited with the translation, and as long as its use is free and non-commercial, you may place it on your own computer and/or on this one. We will try to help make the existence of translations known as widely as possible.

    Dr. David P. Stern
    Code 695, Lab. for Extraterrestr. Phys.
    Goddard Space Flight Center
    Greenbelt, MD 20771, USA

################################

Teaching about the Earth's Magnetism in High School


David P. Stern, Code 695, Laboratory for Extraterrestrial Physics,
Goddard Space Flight Center, Greenbelt, MD. 20771
audavstern@erols.com or u5dps@lepvax.gsfc.nasa.gov

Submitted to The Physics Teacher


    Covering the Earth's magnetism in a high school course on Earth sciences addresses two important problems of the science curriculum.

    On one hand, the customary sequence allocates to physics just one year (and that as an elective!), not nearly enough to sample all areas of physics. In particular, the coverage of electromagnetism, deferred to the end of that course, often ends up short.

    On the other hand, while physics in high school suffers from lack of time, Earth science could use more substance. It involves rather little math and only limited experimentation, and so all too often ends up mainly as rote memorization. A better strategy may be to qualitatively describe the physical and historical foundations of Earth sciences, making the student appreciate the way understanding comes from observations and the way science evolves. The challenge is for the teacher to assemble such a course, with meaningful and memorable threads tying together the many topics covered.

    The Earth's magnetism and the historical evolution of its study are one such thread. Appropriate topics include:

  •     The magnetic compass and its evolution. Robert Norman (1581) and William Gilbert (1600) demonstrated that the force on the compass needle came from the globe of the Earth (evident to us, but not to early mariners!), and their experiments are neat examples of the scientific method. Through them students can also sense how our era of science began.

  •     The story of Oersted and his unexpected discovery of a link between magnetism and electricity prepares the student for later study of electromagnetism in physics class. It may also introduce the subject of lodestones and of magnetization by lightning.

  •     The slow changes of the Earth's magnetic field, which mystified scientists for a long time. Today they are seen as a signature of the electric currents which produce this field, deep inside the Earth. The core of the Earth is molten iron, slowly circulating, and motion of a conductor through a magnetic field can generate an electric current, which in turn can maintain that field. A "fluid dynamo" for producing currents this way (though not its own magnetic field) was proposed by Faraday, who in 1832 tried to demonstrate it, using the flow of water in a river and the Earth's magnetism. However, only after 1908 was the process taken seriously as a possible source of magnetism in nature, after sunspots were found to be intensely magnetic.

  •     The magnetization of ancient lava flows, which suggested that in the distant past, the north-south polarity of the Earth was sometimes reversed. Between 1962 and 1965 seafloor magnetization finally solved the puzzle, providing a detailed chronology of past reversals as well as evidence that continents slowly moved about the Earth's surface. Riding on top of "tectonic plates," their motion resembled the "continental drift" unsuccessfully proposed by Alfred Wegener early in the century. Dynamo theory later explained how such reversals might occur.

The Web Site

    The material for such a course, in clear plain language, is available for free on the world-wide web, at a site named "The Great Magnet, the Earth." Its home page is http://www.phy6.org/earthmag/demagint.htm and it contains about 20 files. Some of it goes beyond the items listed above (e.g. the link between magnetometers and research on smoking), though such extensions may be appreciated by students who want to explore further. One file gives instructions for performing an experiment of Gilberts', and two are addressed to teachers. Three separate files contain the full text of an hour-long talk "Teaching about the Earth's Magnetism in Earth Sciences Class" given 11.18.2000 as an AAPT lecture at the Baltimore meeting of the National Association of Science Teachers. The site has a Spanish translation (a French one has been started), a large glossary and a list of questions by users (with answers), and it can be downloaded to one's own computer in a choice of compressed formats.

History of Science

    Like two earlier educational web sites by the same author1,2 , this one, too, stresses the history of science. History is a framework logically relating different parts of the subject to each other, and it also adds human interest and stories of discovery, so important in keeping the attention of the class. Some examples;
  •     Students may be intrigued to hear that Gilbert was the queen's physician and that some copies of his book were defaced, for voicing the heresy that the Earth was not the fixed center of the universe, but rotated around its axis.

  •     The unpredictable path of discovery is demonstrated by Oersted, who had the key to a major scientific breakthrough, but was unable to interpret it. They may also enjoy Gilbert's story of an iron bar atop a steeple which strangely became magnetized--by a stroke of lightning, we might guess, though Gilbert missed that clue.

  •     They may be interested in the parallels between Faraday's experiment at Waterloo bridge and the space tether experiment on the space shuttle. And...

  •     They may sense the drama of Wegener's quest for acceptance of his theory of "continental drift," of Larry Morley's unsuccessful attempts to publish his theory of seafloor magnetization, and of the way plate tectonics provided the final synthesis.
    The four subjects listed earlier can be covered in about one month, a week for each topic. The one dealing with Gilbert's work deals with familiar concepts--permanent magnets and compass needles. It may perhaps be wise to introduce this part with a bit of astronomy, refreshing the students' memory about what "north" means. The teacher may then tell about Robert Norman's experiment--perhaps even demonstrate it, hints for doing so are given in the NSTA talk. The class could then discuss Gilbert's terrella experiments, including the one using a damaged terrella, which seemed to explain to him why the needle deviated slightly from true north, an incorrect explanation as it turned out. Students can prepare short presentations based on different parts of the web site, including the one on London in 1600, and the class might also discuss the confrontation in Gilbert's era between emerging science and dogmatic religion.

    The second week would be devoted to electromagnetism. The teacher should start with a very brief overview of electricity and electric currents, topics about which students may know little at this stage. Tell about electrons, static electricity and the flow of electrons through wires, e.g. in a flashlight. The flow of electricity may be compared to the more familiar flow of water through pipes.

    Then introduce the story of Oersted and Ampere, and of how Faraday rose from apprentice bookbinder to leading scientist. Oersted showed how electric currents created magnetism, while Faraday looked for the opposite effect, for magnetism to create electric current. That turned out to be harder, requiring magnetic fields that varied, or in cases of interest here, motion of electric conductors through fields. Faraday's Waterloo bridge experiment can be viewed as the forerunner of the tether experiment on the space shuttle.

    The third week, on the origin of magnetic fields on Earth and Sun, may be the hardest. How come the observed field varies, decade by decade? Could it come from magnets inside Earth which are slowly moving? Halley, of comet fame, thought so. But sunspot magnetism left little doubt: at least some magnetic fields in nature must be produced by electric currents, created by the flow of electrically conducting fluids through the very same magnetic fields. On the Sun, this process is probably related to the 11-year sunspot cycle, and to the fact the Sun's equator rotates faster than the rest. This part of the course brings the student in touch with present-day science, with problems still not completely solved.

    The Sun's polar field reverses every 11 years or so, and this forms a bridge to the last section, on geomagnetic reversals and plate tectonics. It is a large subject, and the teacher might well shorten coverage of the 3rd part by 1-2 days to allow more time here. This story starts with magnetic reversals of the Earth's poles, and the way it was deduced from ancient lava flows. Reversals are a topic that seems to intrigue students. Do they expose life on Earth to deadly radiation? No, the atmosphere protects us. Will the trend of the last 150 years continue, until the field reverses somewhere around the year 3500? Possible--but by the past record, not likely.

    Next come plate tectonics and the story of Alfred Wegener. To start with, students should be made aware that the elevations on our globe are not distributed smoothly, but cluster around two levels--continents (including continental shelves) and ocean floors. This suggests continents are distinct entities. Did they fit together like jig-saw puzzle pieces? Wegener believed they did, and ended as outcast among scientists. Only 30 years after his death, thanks to a new awareness of magnetic reversals and to new electronic magnetometers, which allowed seafloor magnetization to be mapped, did a clear picture emerge. This too is still an area of active research, as satellite systems allow the motions of different parts of continents to be tracked as well.

    It all adds up to an exciting voyage of discovery, for students and teachers alike. Who ever said Earth Sciences had to be dull, or was unrelated to high school physics?


References

  1. David P. Stern and Mauricio Peredo, Space Physics for Poets, The Physics Teacher, 35, 38-39, Jan. 1997
  2. David P. Stern, Using Space to Teach Physics, The Physics Teacher, 37, 102-3, Feb. 1999
################################

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     audavstern@erols.com

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


Contents


  1. Introduction: Why and how to make the study of science interesting.
  2. Example: How Oersted discovered a link between electricity and magnetism, but left the interpretation to Ampére.
  3. Three web sites useful for science education:
    • (a) "From Stargazers to Starships" with home page at http://www.phy6.org/stargaze/Sintro.htm
    • (b) "The Great Magnet, the Earth, " discussed here, with home page at http://www.phy6.org/earthmag/demagint.htm
    • (c) "The Exploration of the Earth's Magnetosphere" with home page at http://www.phy6.org/Education/Intro.html
  4. Four areas in geomagnetism suitable for a non-math course on Earth sciences:
    • --William Gilbert's "De Magnete" ("On the Magnet", publ. 1600)
    • --Oersted's story (see above) and magnetization by lightning.
    • --Fluid dynamos and the reason Earth and Sun are magnetic.
    • --Reversals of the Earth's magnetic polarity, and how they provided the clue to "Continental Drift" (now known as "plate tectonics").
  5. Robert Norman's experiment (1581): magnetic force not horizontal.
  6. William Gilbert's many experiments: the "Terrella" or "little Earth," temporary magnetism, etc.
  7. "Dynamos" and Faraday's experiment at Waterloo bridge (1832)
  8. How volcanic lavas recorded reversals of the Earth's polarity. Current trends in the Earth's magnetism, and do we need it to protect us from space radiation?
  9. Alfred Wegener and his "Theory of Continental Drift" (1918)
  10. How magnetization of the sea floor confirmed Wegener's ideas (1962-5)


    Problems of Teaching Science

    I am not a teacher, but a scientist with NASA, studying the Earth's magnetic environment in space, or as it is called, the magnetosphere. However, during my entire scientific career I have been concerned with science education, for several reasons.

One reason: I am appalled to see how unfamiliar the average public is with basic science. Electricity, TV and nowadays computers are vital to modern civilization--yet most people know little about electricity, and magnetism is to them a deep mystery. Just consider all those magnetic bracelets worn for better health, or magnets attached to a car's fuel line, to get extra miles per gallon! For most people, magnetism is only associated with iron, or at most, with an electric current in a coil wrapped around an iron core. I'll come back to this.

Another appalling observation is the way students lack interest in science. It is considered hard, and above all, "BO-RING!" And when you look at some dry and formal textbooks (or some others, fancy and shallow) you can understand why. Teachers deserve better ammunition. To the scientist, science is not just logical and intuitive, it is also full of stories, of interesting personalities (some oddball, some inspired), and full of interesting twists and turns.

The history of science, in particular, is rich in such material. Some of it also involves space, and we know how some students are turned on by space exploration. It is up to us, scientists to make teachers aware of this view of science, and through them reach out to their students. A scientifically literate public is in our interest--not just to keep this nation's technology the best in the world, but also because much of the support of science comes from the public.

Hans Christian Oersted

Let me start here an example to such a story from history--please bear with me if you already know it. Up to 1820, everyone believed that magnetism was the property of specially treated iron, or of natural magnets, iron-rich rocks known as lodestones. That year a Danish professor named Hans Christian Oersted (by the way--a good friend of Hans Christian Andersen, the writer of popular folk tales) conducted in his living room, before a group of visitors, a number of science demonstrations.

Oersted's discovery
    He demonstrated magnetism, using a compass on a stand, and he demonstrated the heating of a wire, connected to an electric battery and carrying an electric current. With the second demonstration, he noted from the corner of his eye that every time the electric current was connected, the compass needle moved. Electric currents creating magnetism? That was something completely new!

    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) attracted. 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.

Three educational 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.

Each of these has the size of a book, the first two also contain Spanish translations (the third will have one soon) and "Stargazers" has in addition a set of 42 lesson plans.

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 took a round needle and stuck it in a cork, so that it floated with the cork in its middle.

With a knife he then whittled away the cork, until the needle was close to sinking. Then he moved the cork so that it was exactly in the middle and the needle was balanced and horizontal, after which he whittled it some more, so that it was just hovering in the water.

    (You can repeat the experiment, too, by using a trick--drop some salt into the water. It will sink to the bottom and dissolve, and for a minute or two at least, the bottom water will be saltier and denser, so that the needle sinks in the unsalted water on top, but is stopped by the denser salty water near the bottom. After a while the salt mixes evenly and the trick no longer helps.)

Then Norman magnetized the needle and put it back. Of course, it pointed north, like any compass needle. But it was no longer horizontal! Instead, it slanted downwards, showing that the magnetic force was not horizontal but was directed into the Earth, at an angle we now call the dip angle. By the way, the needle did not try to move to the north end of the container, showing that the magnetic force did not attract the needle, it just rotated it.


################################

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 Dip Needle

He also devised a pivoted needle to measure the dip angle, and here is its picture from his 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.

 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.)
    Note It is tempting to try perform this experiment before a class, but it is not an easy one. The bars cannot be of steel (e.g. needles, so easy to suspend!) otherwise they get permanently magnetized, and once they start repelling, they continue doing so even when the main magnet is removed again. Also, the effect is fairly small--bars 1/8" apart may be pushed to 1/2", no more.

    Instructions for performing the experiment can be found in the section Performing One of Gilbert's Experiments:
    Induced Magnetism    ..

His interest in magnets made Gilbert look into 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 top of the home pageof 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

New! Mr Jesus Mendez of Algorta, near Bilbao, has produced a Spanish translation of this site. You can reach the Spanish home page here.


    New as of July 2000: You can now copy The Great Magnet, the Earth and its sister sites onto your own computer. Click here

(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 outlines, 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 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.


################################


Dynamos in Nature

How can you create electric currents? One way is a battery, which does so by chemical processes. However, the current which we use every day in our home is generated in other ways--by electric generators, also known as "dynamos."

Faraday's Waterloo Bridge Experiment

All dynamos are based on principles found by Michael Faraday a few years after Oersted's discovery--basically, an electric current is created in a wire or electric conductor that moves through the region of magnetic forces (also known as "magnetic field"). In the generators of the power station, the wires rotate near electromagnets (or vice versa, the magnets rotate and the wires do not move).

But the conductor can also be a conducting fluid flowing through a magnetic field. Faraday himself in 1832 tried to measure electric currents created this way, by the flow of London's river Thames across the magnetic field of the earth. The circuit was completed by a copper wire, strung across Waterloo Bridge in London, its ends dipping into the water. The current was too weak to be measured, but the principle was correct.

We believe that the uneven rotation of the hot gases on the Sun--fastest near the equator, as the motion of sunspots shows--helps create electric currents in this manner. Not all details of the process are understood, because we can only guess what goes on under the surface of the Sun, in regions we cannot see, but the web site explains some of the ideas.

The Earth's Core
Neither can we see how the molten iron of the core flows (earthquake waves tell that the innermost part is solid again), even though its flows may be very slow. They may be driven by the heat released by radioactivity, or by the solidification of the inner core--again, we can only guess. The theory of the process is very mathematical and intricate, because to produce the currents, you need not only a flow of liquid iron, you also need a magnetic field, and this field is none other, but the one created by the currents themselves! The web site gives some general ideas, but the details are too hard for this level.


Reversals of the Earth's Magnetic Poles


The drawing below is again taken of Gilbert's book. "Auster" is south--remember "Australia"--and the smith is hammering a bar of hot iron, as it cools down while lined up in the north-south direction. Gilbert noticed that such a bar became weakly magnetized, with the polarity appropriate to its direction, and viewed it as the iron being "reborn" and in the process (like a baby that starts breathing) acquiring the magnetism of its mother, the Earth.

Gilbert's Blacksmith
We now know that other materials also acquire a weak magnetization as they cool down in the direction of the prevailing field of the Earth. Among them is basalt, a black rock created when a common type of volcanic lava solidifies. Its magnetism is weak, but lava in general comes out in huge masses, so that it shows up in the total field.

(Bricks and pottery, by the way, also do so. Magnetic observatories often keep sensitive instruments in wooden sheds, because brick walls may affect observations.)


Early in this century scientists realized that ancient lava flows may tell about variations of the Earth's magnetic field in the past. They were in for a surprise: some old lavas had the same north-south polarity as the Earth has now, but others had the opposite polarity. For a while the issue was confused by certain chemical processes, which can reverse the polarity of some magnetic minerals. But in 1952 Jan Hospers, studying basalts in Iceland, came with convincing proof that the first impression was the right one--that there existed long periods in the past, when the north-south magnetic polarity of the Earth was reversed.

The gradual weakening of the Earth's Dipolar Field
That discovery echoed an interesting trend. The first person to devise a method of analyzing the Earth's field mathematically and estimating its strength was the German mathematician Carl Friedrich Gauss, around 1838 (details on the web site).

Comparing his observations with recent ones--and with those made in-between--shows that the north-south field of the Earth is weakening by about 5% per century, maybe even a bit faster. No one of course guarantees the trend will continue for 2000 years, until the poles are reversed. In fact, the field has twists and features that are added on top of the simple two-pole structure, and these seem to be getting stronger.

What it all suggests is that inside the core of the Earth, where flows of liquid iron create the currents responsible for the magnetism of the Earth, the flow pattern can be quite complicated. There may be several independent eddies, and some grow stronger, others weaker, changing the pattern. One can construct systems where the currents of eddy A produce the magnetic field of eddy B, and vice versa, and such systems can in principle create magnetic reversals.

The Sun also has a polar magnetic field, and its polarity reverses in each sunspot cycle, every 11 years or so. More about that, again, on the web site.

    By the way--I get a fair amount of e-mail related to the web site. One question that comes up again and again (and you too may get it from a student) is--what happens to life during magnetic reversals? When the magnetic field of the Earth no longer protects us from space radiation, is our life in danger?

    The answer is no. It is true that the magnetic field deflects space radiation, but the protection is very weak near the magnetic poles, e.g. in parts of Alaska. Yet people live and work there. Our true protection is the atmosphere, equivalent to about 10-12 feet of concrete. Future astronauts on Mars--which only has a weak magnetic field and a very thin atmosphere--may have to get into underground shelters when the Sun lets go, but we here are safe.

    (Also, the magnetic field apparently does not vanish during reversals--it just gets weaker and more complex. If it did completely vanish for a really long time, it is just possible that the solar wind would indeed erode our atmosphere.)


Ocean Floor Magnetization

  Alfred Wegener
I don't know how much you teach your classes about plate tectonics, the modern name to what was once called continental drift. It is well known that the coastlines of Africa and South America fit together, and that the edges of other continents also show such fits. Here you have a picture of the German scientist Alfred Wegener--polar explorer, balloonist and climate specialist--who published in 1918 a book, proposing that the continents at one time were in fact together, and later drifted apart.

He formulated his ideas in a military hospital, while recovering from wounds suffered as an army officer in World War I. He based them, not just on the similarity of coastlines but also on the matching of geological formations on opposite sides of the ocean, even matching animal and plant life. He viewed continents as slabs of lighter rock, floating like ice floes on denser minerals below them. That was an accepted idea in geology, and still is.

But he also imagined them drifting, like icebergs in the ocean, and with that most geologists disagreed. Those lower rocks are tough stuff, they argued, and unlike the ocean, they would strongly resist any motion through them.

For about 50 years only a few supporters were found. Wegener himself died in 1931, caught by bad weather while traveling by dogsled across Greenland. What gave his ideas new credibility were reversals of the Earth's magnetic field and measurements of the magnetization of the ocean floor.

The Mid-Atlantic Ridge
Oceanographers and in particular seismologists--scientists studying earthquakes--had long noticed an interesting feature in the middle of the Atlantic Ocean, about halfway between America and Africa or Europe. Earthquakes traced to the Atlantic Ocean almost always originated there, and some volcanic islands--the Azores, St. Helena, sections of Iceland--also straddled that line. On the sea bottom that line contained a long volcanic ridge, and Harry Hess of Princeton proposed this was a major crack in the crust of the Earth, from which lava flowed in both directions.

Up to the 1950s, magnetic fields were measured by magnetic needles suspended in one way or another. Then some quite sensitive electronic instruments were developed, and these could be put on a wooden boat, towed behind a ship, or in a package suspended from an airplane. Oil companies started mapping magnetic irregularities from airplanes, hoping to find clues to oil deposits: on land, all the patterns they found were irregular.

The First Mapping of Sea-Floor
  Magnetization near the Mid-Atlantic Ridge
But oceanographers, mapping the effect of the ocean floor on the overall magnetic field, found an orderly pattern. The sea-floor was magnetized in long stripes lined up (in the Atlantic) with the mid-ocean ridge, stripes of opposite magnetic polarity. When these were finally mapped in 1965--the picture you see here--they turned out to be amazingly symmetric.

Maps like this one led to a new view, by which continents were indeed moving. America was moving away from Europe and Africa, though not exactly the way Wegener had proposed. Rather, the deeper layers on which they floated--the "lithospheric plates"-- were pulled away to both sides of the mid-Atlantic ridge, at about one inch per year, carrying the continents with them. As they pulled apart, volcanic lava oozed up into the crack between them, hardened into basalt, and became part of one plate or the other.

And here is where magnetism entered the picture: as the lava hardened into basalt, it acquired a magnetic polarity, due to the magnetic field of the Earth. However, every half-million years or so, the polarity of the Earth's field reversed, and the basalt, too, became magnetized reversely. The Atlantic seafloor was like a giant magnetic tape (or rather, twin tapes moving in opposite directions), unreeling at a rate of one inch a year and recording each polarity reversal of the magnetic field as long stripe of magnetization.


  Schematic View of Sea-Floor Spreading
There is much more to the story, but you will have to look it up on the web site.

  • About Larry Morley, a Canadian, who in 1962 first proposed this idea, but had his article rejected by the journals as being too far out, too speculative.
  • About Fred Vine and Drummond Matthews, who had better luck in publishing.
  • About the ocean trenches, the deepest parts of the ocean floor, where tectonic plates again descend into deeper parts of the Earth.
  • About plates in other oceans, and in particular, the Pacific Plate at the western edge of the US, which does not move towards or away the land, but rotates. As it does, it caries with it a sliver of California, and the break between it and the rest of the US continent is the famous San Andreas fault.

All these and more you will find on the web site "The Great Magnet, the Earth." By all means, read it.

    And if what you heard in the past hour seems interesting and relevant to Earth Sciences and to magnetism--then your students may feel so, too, when you retell these stories to them.



################################

The Answer Man

Please note!

    Listed below are questions submitted by e-mail to the author of "The Great Magnet, the Earth." Some of them (marked ***) came in response to an earlier site "The Exploration of the Earth's Magnetosphere" and are also found there in the question-and-answer section. Only some of the questions that arrive are listed, either because they keep coming up again and again--on the reversal of the Earth's magnetic field, for instance--or because the answers add extra details, which might interest other users.

AND OH!   Please note--this is a fairly long file and may take a while to load.

Items covered:

  1. What is "Magnetic Flux" and what are "Flux Lines"?
  2. Is the surface of the Earth expanding?
  3. Will a Compass work inside a Car?
  4. Pole shifts? What Pole Shifts?
  5. What was it that Ned Benton did?
  6. Reversals of the Earth's field (4 queries)
  7. Can Magnetism propel Spaceships?
  8. Reversal of the Sun's Magnetic Poles
  9. Measuring Earth's magnetic field
  10. The strength of the Earth's mgnetic field
  11. Magnetic Shielding
  12. Building an electromagnet
  13. How do Magnetic Reversals affect Animal Migrations?
  14. Which is the "True" North Magnetic Pole?
  15. Magnetic intensity at Singapore

If you have a relevant question of your own, you can send it to
u5dps@lepvax.gsfc.nasa.gov.
Before you do, though, please read the instructions

  1. Question #1

    What is "Magnetic Flux" and what are "Flux Lines"?

    I am retired from IBM. I love to read about space and astrophysics. When reading about the magnetic force, flux lines are mentioned. My question is: what are these flux lines made of. Does the magnetic force field have an exchange particle? Or are magnetic flux lines absent of any particles? This area never seems to be covered in any articles that I have read.

    REPLY

    Dear Paul "Flux lines" is another name for magnetic field lines, originally named "magnetic lines of force" by Michael Faraday, who introduced them.

    What is "flux"? You may be aware that the mathematical description of magnetic fields is very similar to that of a fluid like water, which cannot be compressed. Where magnetic fields have "field lines", flowing water has "streamlines" (we talk about the interior flow, ignore the surface) and both field lines and flux lines are closed loops.

    In describing "fluid motion" the "flux" through an area--or the one carried by a bundle of streamlines--is the amount of water crossing it per second, or crossing each second any cross section of the bundle. Magnetic flux of a bundle of lines is similarly the cross sectional area of a bundle of field lines, TIMES the average field intensity on it. Mathematically, both are similar. Flux is important in designing transformers, etc. Some engineers call the flux through a cross section "the number of magnetic field lines" through it, but it's the same thing. You can see from that how the word "flux line" arose.

    Back to the main list

  2. Question #2

    Is the surface of the Earth expanding?

    I am a university student taking business, I often have a few interests during a week of abstract ideas. I am currently in two geology courses as electives, and that is how I stumbled across your homepage, while looking for information on the movement/change reversal of our magnetic poles.

    Part of our geology course was obviously the expanding and contracting of the continental plates. On average, using your knowledge, does the surface of the earth increase or decrease within terms of expanding from plate movement, and if so (I'm going to try to phrase this intelligently) does the atmosphere end up spreading itself thinner and therefore covering the new surface of the earth?

    REPLY

    Dear David

    It is difficult to reconstruct what went on millions of years ago. However, one can be pretty sure that AT THE PRESENT TIME (1) The crustal plates of the Earth ARE moving and (2) The Earth IS NOT expanding or shrinking.

    Point (1) was confirmed by accurate distance measurements based on signals from radio stars, the VLBI experiment (Very Large Baseline Interferometer), and more recently by the satellite-based Global Positioning System (GPS) which (as you might know) is capable of greater accuracy than the public is allowed to use. Point (2) is derived by comparing the observed rotation period of the Earth, measured by telescopes, with very accurate clocks. Even a tiny expansion or contraction would slow down or speed up the Earth's rotation. The Earth's rotation is observed to slow down, but from all I know this is explained by loss rotational energy to ocean tides, which transfer angular momentum to the Moon.

    Back to the main list

  3. Question #3

    Will a compass work inside a car?

    Dear Dr. Stern,

    I was recently told that a compass will not work inside of a metal building. Is that true? Doesn't the field still propagate through the concrete floor, even though surrounded by a metal roof and walls? Also, I'm told that compasses that are available in automobiles must be specifically calibrated to compensate for their metal enclosure.

    REPLY

    Dear Gordon

    Here is what I can write off the top of my head: I will ask around and if any of what follows is not correct, you will hear from me again.

    It is true that iron channels magnetic fields. Magnetic field lines of the Earth crowd into any iron oriented north-south, and since they are denser inside the iron, in the surrounding space they are more spread out, that is, the magnetic intensity there is weaker. Iron walls (if sufficiently thick) can therefore shield out magnetic fields, as described in http://www.phy6.org/earthmag/magmeter.htm , "About Electronic Magnetometers and about Smoking" (last section there).

    However, I don't think a reinforced concrete building has enough iron to make much of a difference. Cars may be somewhat different, and their structural iron may be the reason why a car compass is usually mounted high on the windshield, next to the rear-view mirror. The accuracy demanded of car compasses is in any case so small--they only need enough to tell a right road from a wrong one-- that small deviations may be tolerated.

    It is not so with ships and airplanes, whose navigation demands precision. Thanks to today's global positioning system, and before that, to gyrocompasses and radio beacons, this may be a moot point, but at least up to WW II the magnetic correction to a magnetic compass on a ship was quite important. The compass was placed on a pedestal called the binnacle, under an ellipsoidal cover of brass (brass has no magnetic influence) with spheres and bars of soft iron attached on the side, their positions calibrated to cancel the ship's effect. The binnacle stood in the open, since the enclosed bridge was surrounded by iron, but an electric repeater provided a helmsman inside the bridge with the compass heading.

    I hope this tells you all you wanted to know.

    Back to the main list

  4. Question #4

    Pole Shifts? What Pole Shifts?

    Given current knowledge on the subject of a pole shifts, where would the exact pivot point be and nearest large city in such an event. How does Antarctic ice buildup effect this process? What is the likelihood of such an event in our lifetime? Best guess?

    REPLY

    Dear Gary

    I am not sure what you mean. The poles of the Earth wobble in a small circle, a fraction of a mile-- due, I believe, to the attraction of the Moon on the equatorial bulge (I may be wrong here, have not checked). However, that is apparently not what you have in mind.

    Before 1965, there existed a theory of "polar wandering" by which the poles wandered all over the Earth's surface--or to be more accurate, the crust of the Earth slid around the interior which, containing most of the angular momentum, rotated more or less without change. That theory was motivated by the magnetic record in lavas: when lava from a volcano hardens, it records the direction of the Earth's magnetism at that time. Geologists, examining ancient lavas, found that at times north and south were interchanged, and for India, it was 90 degrees from either direction.

    From the magnetization of the sea floor we now know better (see http://www.phy6.org/earthmag/reversal.htm). Those observations led to "plate tectonics," suggesting that pieces of the crust indeed move slowly, but not at all in unison--different plates move differently. India has moved from south of the equator to where it is now (and the collision continues, raising the Himalaya mountains). We also understand that in the past the Earth's magnetic poles exchanged polarity now and then, typically half a million years apart (on the Sun this happens every 11 years or so).

    The poles thus don's shift. Land masses may shift--one half of San Francisco is sliding past the other half, for instance, with the San Andreas fault in between. But the rate is only about one inch per year.

    Dr. David P. Stern

    Back to the main list

  5. Question #5

    What was it that Ned Benton did?

    Dear Dr. Stern,

    I was reading a section 'Gauss and Global Magnetic Field ' from http://istp.gsfc.nasa.gov/earthmag/gauss.htm and encountered your reference late Ned Benton. Excuse my ignorance - who was Ned Benton. Could you give me reference of his work saying that the Earth's magnetic field may not reach zero but at some stage the polarity of the geomagnetic field may flip as observed from paleomagnetic studies.

    REPLY

    Dear Nalin

    The article by Benton et al appeared in Geophysical Research letters--I think in the mid 1980s. Ned (Edward) Benton died not long afterwards, from cancer, still quite young. If I find the reference I will send it to you, but meanwhile, let me explain what Benton and his two co-workers did.

    The Earth's field slowly changes, all the time. As I explained in the section on Gauss, the field can be resolved into terms that depend on distance R like 1 over the 3rd, 4th, 5th etc. power of R: the dipole goes like 1/R3 and increasing powers are associated with parts of increasing complexity.

    The complex parts--e.g. those that go like 1/R7 are very small at the surface of the Earth, but clearly they grow much faster than the dipole as you move inwards: at a distance of half the radius of the Earth, for instance, the dipole field is 8 times stronger, but the 1/R7 Actually, of course, you can only do so up to the boundary of the core: after that you are in a region of electric currents, and the description introduced by Gauss must be replaced with a different one, which does not grow so fast.

    The Earth's dipole field is declining at a rate that suggests it will cross zero before the year 4000. Benton however knew from theory, that (under appropriate restrictions) the core field cannot change its energy that fast. He therefore proposed that what is actually happening is a transfer of energy from the dipole to more complicated parts of the field. Others have voiced the idea, but Benton and his colleagues found a way to test it. You can calculate the energy of the field outside the core, and also (from the observed change rates of field parts with various complexity) the rate at which this energy changes. The unknown quantity here is the radius of the core, at which the calculation stops. Benton assumed that the total energy did not change, and from that calculated the radius of the core. He got a value very close to the one obtained in earthquake studies.

    That suggests that dipole reversals are not associated with general decay of the field--it is just its dipole part that goes through zero, and other parts actually grow stronger. During reversals you will see a weaker field (the complex parts decrease faster with distance from the core) and perhaps 4 or more magnetic poles.

    You can now see why I was reluctant to go into details! They can be much more complicated. But that is what physics usually involves.

    Back to the main list

  6. Question #6

    Four questions about Magnetic Reversals       ***

    Question #6-A

    I need to know as much as possible about the reversal of the magnetic field:

    • how it was noticed
    • who discovered the reversal
    • how long ago did it reverse
    • how many times did it reverse
    • more information about the radiation from the sun if the magnetic field reverses.

    REPLY

    ABOUT GEOMAGNETIC REVERSALS: This is a huge subject and I cannot do quick justice to it: look up in the index volume of the Britannica under: Geomagnetism, Plate tectonics, Reversals of the Earth's magnetic field.

    HOW IT WAS NOTICED: When lava pours from a volcano, it solidifies to a black rock called basalt. Basalt is slightly magnetic, and it takes on the direction of the surrounding magnetic field at the time it solidifies. Scientists examined lavas for their magnetism early in this century (I believe) to see how consistent the direction of ancient magnetic fields was with the direction we observe now (would compasses point in the same direction?). The directions generally agreed, but there existed reversals of directions which suggested that there were times in the past when the poles were roughly interchanged.

    No one knew what to make of it. Some suggested "polar wandering", that the whole surface of the Earth slid around the interior like a loose shell.

    WHO DISCOVERED: I don't remember. Check a book by Allan Cox, a collection of historic articles.

    But a big change happened in 1963. People noted that while rocks on Earth were magnetized in a disordered way, the sea bottom was magnetized in long strips. Larry Morley (whose article was regarded so speculative that journals would not publish it) and then Matthews and Vine (who managed to publish) suggested that molten rock was spreading out like a conveyer belt from volcanic cracks in the middle of the ocean floor, e.g. the one in the middle of the Atlantic (Azores islands sit on it). Or rather like 2 belts, one moving towards Europe, one towards America, carrying on them the continental plates, so that Europe and America gradually drift apart. As each belt comes out of the crack, its lava solidifies to basalt, causing it to become magnetized, and when the field reverses, its magnetization reverses too. So the bottom of the ocean records the field like the tape of a tape recorder, containing perhaps 50 million years of record.

    HOW LONG AGO: about 700,000 years, according to the "tape recorder"

    HOW MANY TIMES: Many, about half a million years apart on the average.

    RADIATION FROM THE SUN: Sunlight of course is undisturbed. High-energy protons from the Sun are usually diverted by the magnetic field. During the reversal the field probably does not disappear, but becomes complex and weaker, and protons can more easily reach the atmosphere, as they do now within 1000 miles or so of the magnetic pole. On the ground it makes no difference because the thick atmosphere shields us very well, and none of the protons penetrates far into it.

    David P. Stern

    Question #6-B


    Reversal of magnetosphere

    We have been studying the magnetosphere and the Van Allen radiation belts in a high school physical science class. It has been brought to our attention that the magnetic poles of the earth reverse on an average of about every 500,000 years. The last change was about 700,000 years ago, so it would appear that we are long overdue.

    What are the implications of this? How significant would the fluctuation of the magnetic field during such a change be on our protection from solar wind?

    Ricky

    REPLY

    Dear Ricky

    Only yesterday a similar question was submitted, so as a shortcut a copy of it [next item below] and its answer are attached below.

    Some people worry that during magnetic reversals the Earth would receive a higher dosage of high-energy ions and electrons ("radiation" in common terms), which might affect us and any living creatures on Earth. This is not so. Even today, the magnetic shield is not effective near the magnetic poles, yet the radiation received there on the ground is only slightly higher than anywhere else. The reason is that our main shield against such particles is not the magnetic field of the Earth but the atmosphere, equivalent to some 10 feet of concrete.

    In any case, during reversal the magnetic field does not go away, it only gets weaker and develops several more magnetic poles, at unpredictable locations.

    Question #6-C

    Could you tell me when the earth's magnetic poles will change, and what will happen when it does? Will it happen fast (seconds) or slowly? Thank you!
    Sarah

    REPLY

    Dear Sarah

    No one knows when the next field reversal will occur: in the past, they have occurred on the average about once in 700,000 years. The change, whenever it occurs, will be gradual and the field will not drop to zero in between--doing so would mean that the magnetic energy of the Earth was somehow converted or dissipated, and all processes we know for this tend to run on scales of thousands of year, if not more. Right now the main (dipole) field is getting weaker at a rate of about 7% per century, and if you draw a straight line through the points you find it reversing between 1000 and 2000 years from now. It might happen, although the trend may well change. The energy of the field, however, has hardly changed. What seems to have happened is that the more complicated parts of the field (equivalent to several magnets in different directions) have got stronger while the main two-pole ("dipole") field lost strength. The complex field is somewhat weaker (it drops off faster with distance from the source, which is the core of the Earth), but we should not expect the field to be ever greatly weakened.

    The polar field of the Sun seems to reverse every 11 years or so, taking about a year or more. But the Sun's magnetism is different, it has foci right on the surface, in sunspots.

    Hope this answers it.

    David Stern

    Question #6-D


    Earth's magnetic field weakening--leading to a pole shift?

    I am just a tax paying citizen, interested in astronomy all of my life. I am very interested in the physics of our Earth which I believe is related to astronomy as it is our home and a part of this solar system.

    My question is: Is the Earth"s magnetic field weakening, heading to zero point? With this, is the base pulse frequency of the Earth speeding up causing the magnetic fields to fluctuate so that it interferes with the pilots navigational equipment, so that the navigational charts have to be redrawn periodically and the air strips renumbered? Are the magnetic poles fluctuating? My experience is that they are. I have a quality, liquid filled compass secured to my desk. It has been very still now for the past month but the six weeks or so prior to that, there were consistent fluctuations in its direction, up to as much as 2 1/2 degrees, always to the west.

    My understanding is: I have seen photographs of the sun taken from satellites, showing the sun going through major activity. Repolarizing itself? Causing the earth to repolarize itself? Going through a natural cycle as it has many times in the past with pole shifts? On a scale from 1 to 10, with 1 being the weakest and 10 the strongest, 2,000 years ago it was a 10, today it is a 1. Is it heading for a zero point when a pole shift will occur? The closer it gets to the zero point, the more fluctuations will occur?

    Are the change in the magnetic frequencies causing at times a confusion in migratory animals? Causing cells to mutate, changing the DNA pattern within the cell? Causing certain strains of bacteria such as staph infections to become resistant to our antibiotics and causing new viruses to appear that we have never seen before, being able to survive in a new magnetic frequency?

    I believe these are very fascinating times in which we live. The science of all of this intrigues me to no end. I have some taped interviews of scientists and geologists relating to this subject and I read all that I can get my hands on, on the subject also. Your straightforward comments and answers will be most welcomed to help me to understand more, what is taking place. Thank you so very much.

    Michael

    REPLY

    Dear Michael

    Is the Earth's field getting weaker? Yes and no. That field is often viewed as being a two-pole ("dipole") structure similar to that of a small bar-magnet at the center of the Earth, inclined by about 11 degrees to the rotation axis of the Earth, so that the magnetic poles are not the same as the geographic ones. But the actual situation is more complicated, and magnetic charts note the fact by mapping deviations between magnetic north and the direction to the magnetic pole, which fit no simple pattern.

    Why? Because the magnetic field is actually more complicated, and it contains additional fields, of more complex nature. All this originates in the Earth's core, about half the radius of the Earth. If we could go to the surface of the core, all the complicated parts would be much bigger. But they weaken more rapidly with distance, so at the surface of the Earth they are already quite weak, while the "dipole" part stands out more (in addition of actually BEING the biggest chunk of the field).

    Are you still with me?

    The magnetic field of the Earth changes all the time, and yes, magnetic charts have to be redrawn from time to time (this was first found in 1641, by an Englishman named Gellibrand). And yes, in the century and a half since the first careful mapping of the Earth's field, the dipole has become weaker by about 8% (the rate may have speeded up in 1970). If you draw a straight line through the points, you will find that perhaps 1200 years from now, the line goes through zero.

    Extending straight lines too far beyond the present, however, is risky business, as noted by no less a scientific authority than Mark Twain. In "Life on the Mississippi" Twain noted that the Mississippi river was getting progressively shorter (mainly by floods--and by people--creating shortcuts through bends in the river) and he wrote:

      "Now, if I wanted to be one of those scientific people, and "let on" to prove what had occurred in the remote past by what had occurred in a given time in the recent past, or what will occur in the far future by what has occurred in late years, what an opportunity is here! ... Please observe:

      In the space of one hundred and seventy six years the lower Mississippi has shortened itself two hundred and forty-two miles. That is an average over a mile and a third per year. Therefore, any calm person, who is not blind or idiotic, can see that in the lower Oolitic Silurian Period, just a million years ago next November, the lower Mississippi was upward of one million three hundred thousand miles long, and stuck out over the Gulf of Mexico like a fishing rod. And by the same token any person can see that seven hundred and forty years from now the lower Mississippi will be only a mile and three quarters long, and Cairo and New Orleans will have joined their streets together, and will be plodding comfortably along under a single mayor... There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment in fact."

    It is not impossible that the magnetic field will go through zero 1200 years from now, but (judging by the past record of reversals) not likely. In any case, the field is not going away: when one uses observations on the surface to reconstruct fields at the core, one finds that while the dipole field is getting weaker, the complicated parts are getting stronger, and the total magnetic energy does not change, within our observational accuracy. That's why I wrote "yes and no."

    I don't know about migrating animals (they may have magnetic organs, sort of built-in compasses), but there seem to exist no magnetic effects on DNA, resistance to antibiotics and so on; those changes seem more related to chemistry.

    Finally, be cautious with compass readings in your house. Houses do contain electric currents and machinery, and these may affect the readings of a magnetic compass. On NASA's satellites the magnetic sensor usually sits at the end of a long boom, to keep it away from interfering electric currents in the satellite's circuits.

    Keep up your interest in science!
    David

    Back to the main list

  7. Question #7

    Can Magnetism propel Spaceships?       ***

    Dear Mr. Stern:

    I am an Industrial Technology teacher at a middle school and one of my students is dreaming of a space propulsion system based on magnetic repulsion of the earth's magnetic field. Could you possibly squeeze in a moment for us and provide some information on the strength of this field and how it has been measured and maybe a relative comparison? Tyson, my student, is really excited about the Internet and will be enthralled to have an answer from a NASA scientist. Perhaps you could steer him to other references as I certainly will explain to him how busy a schedule you must have. Thank you.

    REPLY

    I am afraid it won't work. First of all, the magnetic field is very weak. Compared to fields in electric machinery, where appreciable forces are exerted, it is a few thousand times weaker.

    But there is a more fundamental reason. Magnetic poles always come in pairs, equal and opposite: if a field attracts an N pole, it repels the attached S pole. Similarly, if we generate the field by a current in a loop of wire --say, shaped like a rectangle--for each side in which the current flows in one direction, there exists a side where it flows in the opposite direction, and the magnetic field exerts opposite forces of equal strength on the two sides.

    From the preceding one would guess that magnetic forces always cancel, and no net force is exerted. So how come magnets are attracted to each other, or pins to a magnet (same thing, really, since each pin in the magnetic field turns into a small magnet)?

    The answer is that the forces on the N and S poles (or on the opposing currents) are not exactly equal, if one pole, or one wire, is closer to the source of the field than the other. This can be put into a mathematical formulation and the bottom line is that a suitably oriented magnet may be attracted by a magnetic field, moving towards the greatest strength of that field. But the force is proportional to the rate at which the field changes with distance, which in the case of the Earth, is very small.

    The idea of magnetism as anti-gravity has come up before. Your student may look up "Gulliver's Travels" by Swift, where in the third voyage, in a spoof on science and learned societies, Gulliver arrives at an island floating in the air, held there by the repulsion of a large magnet. Swift even gives an explanation, except it's all gibberish gobbledygook, as befits a book of satire

    Back to the main list

  8. Question #8

    Reversal of the Sun's Magnetic Poles       ***

    Dear Mr. Stern:

    I have a question about the sun that I was hoping you might be able to answer for me. A friend of mine recently returned from a new-age conference where it was presented that the magnetic poles of the sun were about to reverse, and cause a number of changes.

    The idea of the sun having magnetic poles seemed counter to what I remember learning about the sun, and your web page seems to dispel the idea that the sun has actual poles. My guess is that the presenter was taking a dose of creative license with the 11 year cycle of sunspot activity.

    Is it true then, that:

      1.) There are no magnetic poles on the sun.
      2.) Is the change in sunspots related at all to a reverse of polarity of magnetic fields?
    Thank you.
    If you can provide reference to a college-level text as a reference, it would be appreciated.

    REPLY

    Actually, your friend was right: the Sun does have polar fields, and they do seem to reverse their polarity each sunspot cycle.

    The Sun's most concentrated magnetic fields are of course in sunspots, but people have long suspected there might also exist polar fields, because during a total eclipse of the Sun one often sees streamers coming out from the polar regions, looking very much like the pattern of iron filings near the poles of a magnet.

    But there was no good way of measuring such diffuse magnetic fields: the field of sunspots affects the light emitted from them ("Zeeman splitting") but the effect elsewhere is very weak. Then in the 1950s (if memory serves me) the Babcocks pushed the technique to its limits and found the polar field. This revealed the reversal of the polar magnetic field and suggested this field was somehow coupled to that of sunspots (which also reverse each cycle--they come in pairs, and the leading spot, in the direction of the Sun's rotation, has north or south polarity, in alternate cycles), a sort of a cumulative effect of the distant field of many spots. Theories exist by Horace Babcock and Robert Leighton, though they are somewhat qualitative.

    The fact the magnetic field lines at the poles stick straight out means they do not hinder the escape of the solar wind in any way, and indeed the Ulysses spacecraft which recently passed above the Sun's poles confirmed (as was predicted) that the solar wind there is faster. There seems to exist no great effect of the reversal on Earth, though one might expect a bit more magnetic storminess when the polarity is opposite to that of the Earth.

    For more on the Sun, see:

    Back to the main list

  9. Question #9

    Measuring Earth's Magnetic Field       ***

    I am doing a sixth year studies project on magnetism in and was delighted to find the question and reply page with topics similar to what I had thought of studying.

    I was wondering if there was a practical method for measuring the strength and direction of the Earth's magnetic field at different geographical locations. Any help or inspiration would be greatly appreciated.

    REPLY

    Is your "sixth year" in school or 6th year in college? It is not easy to tailor an answer to fit either level!

    In any case, the electronic gizmos nowadays used in space are too complicated for a quick discussion, so let me instead describe earlier, simpler methods.

    The direction of the magnetic field is of course given by the compass needle: but that is just the horizontal part of the force, Actually the magnetic force also points i n t o the Earth (or out of it, in the southern hemisphere).

    To find the angle at which the force points down ("dip angle") people used a needle similar to a compass needle, but on a horizontal axis, allowing it to swing in the various directions to which the hands of a wall clock might point.

    That is a bit harder to arrange than a compass needle: if one end of such a needle points at an angle downwards, how is one to know whether the magnetic force is responsible, and not, say, that the needle is not quite balanced on its pivot, but that one end is slightly heavier and therefore points downwards? To avoid this problem one starts with an unmagnetized needle, balances it very carefully, and only then magnetizes it. When in 1831 the expedition of John Ross searched for the north magnetic pole, it carried along a dip needle, and when it pointed straight down (while the regular magnetic needle showed no preference for any direction), that was it .

    Measuring the strength of the field is harder. Take a thin long bar magnet and hang it by a thin thread, then wait until it points north-south. After it does, push one tip slightly left or right and let go: it will swing back to north-south, but will overshoot to the other side, then turn back to the right direction, swinging back and forth like a pendulum, gradually quieting down to point steadily. The average length of each swing depends on two things: the strength of the bar magnet and the strength of the magnetic force. With a stopwatch, measure 20 swings or so and figure out how long each swing takes.

    Then put a small compass needle on a table, and put the small magnet nearby, in such a position that it tries to line up the compass to point east-west.The small magnet and the Earth's magnetic force obviously compete fordetermining which way the needle points, and by looking at the actual angle of the needle, and its distance from the small magnet, we again get an observation that depends on how strong are (1) the small magnet and (2) the magnetic attraction of the Earth. Using these two observations and some calculation, the physicist can find both these unknown quantities.

    This method was proposed by Carl Friedrich Gauss in Germany around 1835. It obviously won't work on an orbiting satellite--but how measurements are made there is another story altogether.

    Back to the main list

  10. Question #10

    The Strength of the Earth's Magnetic Field       ***

    Could you please send me any information regarding the current field strength of the Earth's electromagnetic field? My data is current as of 1975 which is by far outdated. My reading from that time were 30,000 gammas at the equator. If possible could you please send information on the current decay of the earth's magnetic field.

    Any information would be greatly appreciated.

    REPLY

    I am not sure at what type of information you need, or to what use you put it. The most complete information on the Earth's internal magnetic field is in form of a set of coefficients, to be plugged into a mathematical representation--the so-called spherical harmonic expansion. The coefficients generally used are the so-called IGRF set (International Geomagnetic Reference Field) chosen by a committee every 5-10 years and based on the "best available" observations. You can find them on the world-wide web at

    http://fdd.gsfc.nasa.gov/IGRF.html

    Some of these models also include the annual change of the field (but not in the above files). You might like to search the web using (say) the Altavista or Yahoo search engine, on the term IGRF.

    If you just want maps of the field, for instance those describing, the variation of its strength over the globe, try

    http://swdcdb.kugi.kyoto-u.ac.jp/igrf/index-j.html

    The text seems to be in Japanese, for on my computer it does not give anything readable, but the maps are in English. Clicking on the first will show you that the magnetic intensity around the equator varies quite a bit. but 30,000 gamma (or nanotesla, same thing) is a reasonable value.

    The field has been weakening since Carl Friedrich Gauss measured it around 1836, by about 5% per century, recently accelerating to 7%/century. The total energy of the field however is nearly constant, as shown by the late Ned Benton. This means that the field is not really weakening, only reshuffling its energy, reducing the "main dipole" (=north-south bar-magnet pattern, declining as noted by about 7% per century) and reinforcing the more complicated parts.

    These tend to contribute a weaker field, because the magnetism originates in the Earth's core, about half an Earth-radius down: all magnetic fields at the surface are weaker than those in the core, because of the distance, but the more complicated fields decrease faster.

    Whether the main dipole will reverse in about 1300 years is anyone's guess. Geological evidence suggests it has happened in the past, but odds are against it, because the mean frequency of such reversals in the past seems to be about once in 500,000 years.

    Back to the main list

  11. Question #11

    Magnetic Shielding       ***

    My question is about induced magnetism and magnetic shielding.

    I understand that we can screen out magnetic fields from a region by wrapping a piece of soft iron around the region. However, I also understand that soft iron can easily receive induced magnetism when placed near a permanent magnet.

    So now my question is that: How is it possible to shield a region that near a permanent magnet by using a piece of soft iron? Won't this piece of soft iron eventually get induced magnetization and have the ability to attract any magnetic material that is nearby.?

    Ong

    REPLY

    Magnetic shielding is not my specialty and you might get a better answer from an engineer familiar with magnetic design, but I will try.

    Soft iron--especially the kind used in shielding (mumetal, etc.) does not take permanent magnetization. Steel does, but even there, the magnetic intensity must be high enough for that to occur.

    In shielding (e.g. a video tube) you wrap a sheet of soft iron around the shielded object, and the magnetic field lines which would have closed through the interior are diverted and close through the shield instead. Therefore any magnetic field that existed in the interior is greatly weakened. The field inside the iron sheet is stronger, but that is no problem--in fact, that is what we wanted to do, take the magnetic field from the inside volume and put it elsewhere (you can't just get rid of it, for all magnetic field lines have to close somewhere).

    I hope this answers your question

    Back to the main list

  12. Question #12

    Building an electromagnet       ***

    My name is Jon and I am a 6th grader. I have an invention using magnetism to prevent cars from being stolen and to keep them from bumping into each other. I tried making an electromagnet with a 9Volt battery, but it wasn't very strong. Can you tell me how to make a stronger magnet? Can I use a larger battery or real electricity? Thank you,

    REPLY

    Dear Jon

    I don't know what your invention is, what the magnet is supposed to do. If you want it to close an electric circuit, you are essentially building a device known as a relay. You can probably get old relays from a radio repair shop, or any place which has junked electric devices (cars have relays, too). Or ask your science teacher for help.

    Building electromagnets without calculating and measuring is not simple: you must match the size of the wire and its length to the source of current (manufacturers of relays do so, of course). In particular be cautious about using house current (you call it "real electricity", but anything you use is real electricity). A small battery is limited in what it can do--usually, not much. House current is backed by big power stations, which can pour a LOT of "juice" into whatever you attach. If your wire is short and thick, it will try to draw a big electric current: a battery will be unable to provide it, but the power station can and will, enough electricity to perhaps melt a wire and cause a fire, or at least blow the fuses or trip the circuit breakers which are meant to protect houses against just this.

    Also, house current is backed by a relatively high "electric pressure" (voltage) and can cause a nasty shock. Finally, even if you got the magnet working on this, it would hum and jitter, because houses have an "alternating current", which goes down to zero and up again more than 100 times each second. If you ever heard an electric device humming (old fluorescent lights sometimes do), that is the reason.

    So my advice--stick to batteries, get a relay (you can also disassemble it and use just its magnet, if that's what you want), and most important, read and learn. You are just at the very beginning of an interesting adventure.

    David Stern

    Back to the main list

  13. Question #13

    Effect of magnetic reversals on animal migrations       ***

    I teach 9th grade Earth Science and my class would appreciate the answer to the following question. What is the effect of the magnetic reversals of the poles on the migratory paths of sea turtles and certain birds and fish?

    Thank you..... Janet

    REPLY

    Dear Janet

    I have no idea how to answer your question. How could one find out? The last reversal was 700,000 years ago!

    I heard that some bacteria, suspended in water, find the "down" direction using magnetic materials embedded in their bodies. When they are moved to the opposite hemisphere, they tend to orient themselves in the opposite direction. That's as far as I know

    Back to the main list

  14. Question #14

    Which is the "True" North Magnetic Pole?       ***

    Hi!

    I'm a teacher from Sweden. I'm also studying science and I have a question that I would like to ask you, about magnetism. I found your e-mail at http://www.phy6.org/Education/ The Earth's geographic north pole is near the magnetic north pole. But if the Earth's magnetic north pole is up north, why does a compass point up north? Then the magnetic north pole has to be a magnetic south pole, because south and north attract each other. So, my question is: Why isn't the Earth's magnetic north pole a "true" magnetic north pole?

    REPLY

    Dear Asa

    Your question has come up before and it is not really about science, but about language. The needle of the compass--or of any bar magnet-- has two ends, the N end tends to point to the north magnetic pole of Earth and the S end tends to point to the south magnetic pole.

    So, if the source of the Earth's magnetism were a very powerful bar magnet somewhere deep inside, where would its N pole be and where its S pole? The answer, of course, is--the S pole would be at the northern end and the N pole at the southern end. How confusing!

    Teachers and students have struggled with this ambiguity since times immemorial. One popular solution has been to call the N pole of a bar magnet, not its "north pole" but its "north-seeking pole", and the other end its "south-seeking pole," marking them N and S for short. You might try doing so with your students, too.

    Sincerely

    David

  15. Question #1

    Magnetic Intensity at Singapore

    I would like to know the value of the horizontal component of the earth's magnetic field. I am living near Singapore (at the equator). By the way does it vary a lot from place to place?

    REPLY

    The horizontal intensity at Singapore is about 0.4 gauss, among the highest along the equator. The lowest is about 0.27 gauss in southern Venezuela.

    One reason for the variation is that if we represent the Earth's magnetism by a bar magnet of small size but strong intensity ("dipole"), the best description is obtained by placing that magnet NOT at the center of Earth but some distance away from it, more or less in the direction of Singapore. As a result, over South America the field is relatively weak, and if you ever heard about the "Brazilian Anomaly" (or "South Atlantic Anomaly") where trapped particles of the radiation belt are most likely to hit the upper atmosphere--the weakness of the field there is the reason.

    Of course, this is just approximate. The field near Earth has other sources of uneven structure. As one moves upwards, into space, these become smaller and smaller and the field appears smoother.

    David

Author and curator: David P. Stern, audavstern@erols.com
Last updated 19 February 2001