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Get a Straight Answer

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.

Index of Questions arranged by Subject



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
  16. Inner Core Rotation
  17. How does the Earth's field vary with location?
  18. Effect of magnetism on water

  19. "Why does this happen?" (electromagnetic induction)

  20. What would a Compass on the Moon point to?
  21. Why do iron filings outline magnetic field lines?
  22. Is Earth held in its orbit by magnetic forces?
  23. All magnetism due to different arrangements of magnetic poles?
  24. Magnetism to replace gravity in a space station?
  25. Magnetic reversal due soon? And are volcanoes a factor?
  26. Can magnetic reversals affect the human mind?
  27. When and where can I see "Northern Lights"?
  28. Magnetic reversals due to comet impact?

  29. Space Radiation and our weakening magnetic field

  30. Can the Sun trigger magnetic reversals?
  31. What is the smallest magnet?
  32. Isn't the Sun too hot to be magnetic?
  33. "Artificial magnetic shields" for astronauts?
  34. The movie "The Core"
  35. Can we tell if a symmetric magnetic field rotates around its axis?
  36. What causes permanent magnetism?
  37. What types of metal are attracted to magnets?
  38. "If the earth is a giant magnet, why doesn't all iron stick to it?"

  39. Risks from stormy "Space Weather"
  40. Does our magnetic field stop the atmosphere from getting blown away?
  41. Dynamos triggered by the sun?
  42. Could generated electricity affect Earth's magnetic field?
  43. "Magneto-therapy"
  44. Curie Point
  45. Blocking of magnetic fields
  46. Earth magnetism from rotating electric charges?
  47. Teacher seeks easy experiments
  48. Local field does not always decrease!

  49. Loss of magnetic energy from Earth
  50. Tesla's patents, and ball lightning
  51. Can electricity be generated from the Earth's magnetic field?
  52. Decay of magnetism in a magnet
  53. Magnetizing glass by a radio wave?
  54. Magnetization of materials
  55. Induction by non-fluctuating magnetic fields?
  56. Good "magnetic insulators"
  57. Creating magnetic pottery
  58. Shielding magnetic fields (2 messages)
  59. Conductivity and Transparency
  60. Heat sources inside the Earth
  61. Geomancy
  62. Are we approaching a polarity reversal?
  63. Magnetic Levitation
  64. Why does the magnetic field stop particles but not EM radiation?
  65. Earth's rotation and magnetism
  66. A career in geomagnetism?
  67. The movie "The Core"
  68. Telling the 6th grade about polarity reversals

  69. Magnetic Flux
  70. Why do moving electric charges create a magnetic field?
  71. Weakening of the Earth's Field (2 questions)
  72. Focusing magnetic fields
  73. Is gravity related to magnetism?
  74. Observing Magnetic Planets
  75. How does magnetism spin aluminum disks in power meters?
  76. Magnetic Poles in Druid times?
  77. Magnetism linked to Global Warming?
  78. Uses of Magnetic Energy
  79. Can sparks generate magnetic fields
  80. Can a magnetometer detect cracks in an oil well?
  81. Telling about magnetism
  82. Does North-South orientation slow down iron corrosion?
  83. Why two magnetic poles and not more?

  84. Why no inverse-square law for magnetism?
  85. Sources of magnetic fields in space near Earth
  86. Force and Energy
  87. Technical questions on magnetic energy and heating rate
  88. Complex (non-dipole) parts of the Earth's Field
  89. What causes sunspots?
  90. Magnetic shielding
  91. Can a lightning surge clean-wipe your hard disk?
  92. A billion-Tesla field on Earth?
  93. Measuring the Earth's Magnetic Field
  94. Orientation of ancient magnetized rocks
  95. Why is southern end of compass needle heavier?
  96. Dynamo theory
  97. How can an intensely hot Sun be magnetic?
  98. Building one's own hybrid car
  99. Is volcanism related to magnetic changes?
  100. Nuclear reactor at the Earth's center?



  101. Protecting Magnetically encoded Tickets
  102. Location of the Magnetic Pole
  103. Currents that Generate the Earth's Magnetism
  104. "Dead Zones" for radio signals
  105. Deriving Dynamo models from Equations?
  106. Taking Hard Disks across the Magnetic Equator
  107. Human effects on Earth Magnetism
  108. Harry Paul Sprain's machine
  109. Reversal of Magnetic Poles

  110. Magnetometers and MRI
  111. Earth--conductor or insulator?
  112. Effects of Earth's magnetic field on electronic gadgets
  113. Rotation of magnetic field lines (1)
        Rotation of magnetic field lines (2)
  114. Magnetism of the human body
  115. Rapidly reversing magnet
  116. Earth's core of frozen magnetic oxygen?
  117. Heating the inside of Earth
  118. Magnetism inside the Earth
  119. Electric field due to electromagnetic induction
If you have a relevant question of your own, you can send it to
earthmag("at"symbol)phy6.org
Before you do, though, please read the
instructions
     

    19.     "Why does this happen?" (electromagnetic induction)

        Hi

        I'm 17 years old and studying physics and recently learned about electromagnetic induction. It's pretty basic and I understand that when a magnetic field or flux cuts a wire at 90 degrees it generates a current and the same when a wire cuts the field lines. My question is why does this happen? could you please explain or just point me in the right direction of some good web sites.
          thanks                   Ziad

        ["Electromagnetic Induction" means the creation of an electric current in a conductor moving across a magnetic field, a phenomenon discovered and studied by Faraday--see http://www.phy6.org/earthmag/dynamos.htm.]

    Reply

       Dear Ziad

        The question "why" is usually the hardest and the last to be answered. People observe phenomena, use them to deduce laws, and only afterwards (maybe) see the underlying pattern and how it all fits together. I will not fully answer your question here--it's too long since I studied these things, and I mainly remember that the explanation is complicated. I will only try to point you in the right direction.

       First: it makes no difference if the wire moves and the source of the magnetic field is fixed, or vice versa. Only relative motion matters. That is one of the things relativity established, and I discussed it briefly at

        http://www.phy6.org/stargaze/Srelativ.htm

        Second: the electric field E and the magnetic field B (or "magnetic induction"; I won't quibble here about preferring B to the related quantity H) are two tightly coupled quantities. These letters are printed in bold face, to show they are vectors, with direction in space as well as intensity. Maxwell's equations of electromagnetism

        (see     http://www.phy6.org/stargaze/Sun5wave.htm )

    and Einstein's relativity involve both of them jointly. Here B at some point is the force on a magnetic pole of unit strength, IF it were placed at the point, and E is the force on an electric charge of unit intensity, at the same point. Faraday speculated--and Maxwell showed this made mathematical sense--that in both cases this was best viewed not as a "force-at-a-distance," exerted by distant magnetic sources or electric charges, but rather as a modification of space, and that the pole or charge was reacting to the modification at that point. Such modified space was gradually named "field."

        In this formulation, what a changing magnetic field or flux creates is not a current but an electric field. If an electric conductor occupies the location, the electric field causes a current to flow, because an electric field in a conductor drives a current. But the basic relation is not between magnetic fields and electric currents, it is between magnetic and electric FIELDS.

        Still with me? It turns out that the components of magnetic and electric fields belong to a single symmetric pattern, and Faraday's law of induction is necessary to maintain that pattern. In relativity, they express different components of the same 4-dimensional tensor (that's one level up from a vector), and transforming fields from one moving frame to another, magnetic fields in one frame contribute part of the electric field in the other, and vice versa.

        Why that pattern and not another one? I don't know, maybe because it has a pretty symmetry. Ask the guy upstairs.

    20.     What would a Compass on the Moon point to?

    Dear Dr. Stern,

        I am student teaching in an 8th grade science class. We have been studying the earth's magnetic force, and the question was asked by one of the students, "What would a compass on the moon point to?" I know that the moon has no internal magnetic force, but does have some areas on the surface that are magnetized. Would a compass point to these? Thank you for your help with this question.

    Reply

        Your information about the moon is correct, but your students should bear in mind two points. One, we are used to regarding the compass needle as a sensitive instrument. It may be so, compared to other everyday indicators of magnetic forces, but space-borne electronic magnetometers, such as have mapped magnetism in space, are many thousands of times more sensitive. I would suspect that the weak magnetization of some areas on the moon is not nearly strong enough to move the needle of an ordinary compass, even in the weaker gravity.

        You can find more about such magnetometers in         http://www.phy6.org/earthmag/magmeter.htm

        Even if the technical description of such instruments is beyond your students, they may be interested in the way they were used by Dr. David Cohen at MIT.

        And secondly, the magnetic force, on Earth or on the moon, is usually not horizontal. That is discussed very briefly in
            http://www.phy6.org/earthmag/upto1600.htm
    and in more detail at the end of
            http://www.phy6.org/earthmag/NSTA1A.htm

        An ordinary compass only responds to the horizontal part of the force, and in a complicated magnetic field, this can be highly variable--towards magnetized patches, away from them, or neither. I do not have a magnetic map of the moon, but your students may look at one of Mars, which is also magnetized in patches, and appreciably more strongly than the Moon (though it lacks a core field like the Earth's). See map at the very end of
            http://www.phy6.org/earthmag/planetmg.htm
    and the link given there (since then that map has been made more accurate).  

    21.     Why do iron filings outline magnetic field lines?

        Could you please answer me one question. Why do the iron fillings line up in the same field lines all the time? What makes them line up in the same field lines?

    Reply

    Near a magnet, each piece of iron filings becomes a temporary magnet too.(see http://www.phy6.org/earthmag/inducemg.htm)

    It then acts very much like a little compass needle, and like the needle, it turns to point along the local magnetic field line (see 2nd figure in http://www.phy6.org/Education/wfldline.html). If you draw a collection of such needles, all pointing along the field line, you will see that the N end of one needle is close to the S end ot another. Since opposite ends attract, the needles--and likewise, the iron filings--will stick to each other, creating a continuous chain which outlines the field line.  

    22.     Is Earth held in its orbit by magnetic forces?

        I am a civil engineer and have become fascinated with the research on magnetism. I have had some crazy thoughts like
      ...What if the sun and the planets are like an atom with electrons circling. but the force holding the planets is really magnetism not gravity?
    Question:..do each of the planets have a net charge? Does the sun have a net charge?

    Reply

    If you are indeed fascinated by magnetism, I recommend going over "The Great Magnet, the Earth" and "The Exploration of the Earth's Magnetosphere," both non-mathematical, both linked to
                    http://www.phy6.org/prospect.htm
    which also has about 100 detailed links. If you seek detailed and practical knowledge beyond this level, find a good text on physics and/or a course at a university.

    Your idea is interesting but FAR off the mark:

        The electric charge and magnetic field needed to keep the Earth orbiting the Sun are very different from what is observed and MUCH stronger. The electric charge needed to generate a force comparable to gravity would be ENORMOUS and would immediately attract ions or electrons from space, which would cancel it out (and bombard us with penetrating radiation). Also, the observed magnetic field is deformed by the solar wind to where it could not give forces for orbiting the Sun in a plane. Furthermore, magnetic forces act very differently from gravity. And what about the motion of the Moon?

    It just won't work.
     

    23.     All magnetism due to different arrangements of magnetic poles?

    Dear Dr. Stern Instead of describing magnetism as the force between two currents, it might be easier to understand if it was described as the force between two charge pairs.

        When I say charge pair, I'm referring a positive and negative charge of equal magnitude separated by a distance. (+)...(-)

        When you are far away from a charge pair, they look like they are together to any other charges out there. So there is little force if any. When far away from a charge pair, the static electric force is 0.

        When another charge get closer to a charge pair, one side is closer to it than the other. This slight difference in static electric force from the difference in distance is called the magnetic force.

        If these two pairs spin together into the page with their poles like this, they attract.

            (+)...(-)    pair one

            (-)....(+)    pair two

        Like this they repel. One will try to flip around to line up with the other.

            (-)...(+)   pair one

            (-)...(+)   pair two I think like this there is a slight attractive force if they are spinning into the page. It's not stable though.

        If the charges are yo-yoing through each other in the pair, then there is a strong attraction and it's stable.

            (-)...(+)    (-)...(+)

        When you pass a current through a wire, the electrons move away from the protons to jump from atom to atom. This gives the separation.

        A good example of magnets in action are water molecules in snowflakes. Water molecules have a plus side, and a negative side, separated by a slight distance. Science calls them polar bonds, but in reality they are just magnetic.

    Reply

    Dear Jim

        What you are describing seems very close to the theory of magnetism which existed up to 1820, though you might be confusing electric charge and magnetic charge. If you have (+) and (-) electric charges and connect them with a wire, an electric current briefly flows. If you have (N) and (S) magnetic poles (north- and south-seeking, on a compass needle) and connect them with a conducting wire, nothing happens. Charles Augustin Coulomb proved by a clever experiment, in the years after 1777, that two electric charges attract or repel with a force proportional to 1/r^2, and so do two magnetic charges. See the end of http://www.phy6.org/earthmag/to1820.htm

        For gravity the "inverse square" law is always one of attraction, as Newton showed from the motion of the Moon and planets, and Henry Cavendish proved in the lab in 1796, using a very sensitive version of Coulomb's instrument. Thus scientists around 1800 believed that nature had 3 basic forces, very similar to each other, originating in "charges" and decreasing with distance r as 1/r^2.

        The way you superpose magnetic charges is also very similar to spherical harmonic analysis of the magnetic field of Earth, as described briefly in http://www.phy6.org/earthmag/gauss.htm

        Two electric charges close to each other (as you describe) are an electric dipole, and do exert forces on an isolated electric charge very much like the forces a magnetic "dipole" would exert force on an isolated magnetic pole. However, that force is electric, not magnetic.

        In any case, the theory of magnetic poles completely failed to explain Oersted's experiment http://www.phy6.org/earthmag/oersted.htm

        and that was what brought on the interpretation of magnetism as a force exerted by electric currents. That view still holds, and nicely fits the fact that isolated magnetic poles (unlike isolated electric poles) can never be produced, also with the theory of electromagnetic waves http://www.phy6.org/stargaze/Sun5wave.htm

        and with relativity. The use of magnetic poles and spherical harmonic analysis (strictly as mathematical devices) is still OK in regions where no electric currents flow, e.g. for the external magnetic field of Earth.  

    24.     Magnetism to replace gravity in a space station?

        Do you have any idea if the magnetic lose its magnetic property in the space. I am developing virtual space station and I would like to so calculations of how can I have some furniture fixed in their position such as chairs, tables in this station and if there is any technique that can be applied to gives the spaceman the ability to walk in the space station instead of flying.

        I would really be very happy by getting any information.

    Reply

    Magnets operate in space just as well as on Earth. However using magnets to keep people in position is questionable, because they attract all sorts of iron objects, can confuse instruments and an astronaut with magnetic shoes encounters resistance at each step. For many uses, weightless "flying" is preferred. And you need no chairs! On the space station "Skylab (a unit of which can be visited in the Smithsonian Museum in Washington--see picture in
    http://www.phy6.org/stargaze/Sskylab.htm), the astronauts walked on a special grid-floor, and could twist attachments in their shoes into the holes of the grid when they needed to be held down. A spinning space station can use centrifugal force to imitate gravity (as in the movie "2001") but some problems remain (see http://www.phy6.org/stargaze/Srotfram.htm).  

    25.     Magnetic reversal due soon? And are volcanoes a factor?

        Question 6-B (above) on reversals of the magnetosphere interest me. Well, I don't really worry about any effects of a reversal, but I think it's quite interesting what this Ricky wrote: "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."

        Mmh, other scientists have studied those large volcanos like that Yellowstone national park-giant. They say, that its caldera was formed ca. 630.000 years ago and before this ca. 1,2 million years ago. They also say that its eruption was overdue.

        Well, there could possibly be a relation between those eruptions of a super volcano and those reversals (if Ricky and those volcano-scientists are right). I mean what are 630,000 or 500,000 years? I don't think, anyone can define the time so precisely... Maybe a large volcano can cause this effect?

        Give it a try ... maybe you can find out more of this if you brainstorm with a volcano specialist. Me, I'm too busy (unfortunately) as I am not a scientist - only a technician.

        But reading your work is quite interesting, though.

    Reply

    Your message reminds me of what a friend once told me about Las Vegas (a place I have yet to visit). In a casino he saw a lit tabulation of how many times each roulette wheel had come up (say, in the past week) with each of its numbers. The interesting thing he noted was that, if, say, the frequency of the number "15" was unusually low, people did NOT avoid betting on 15, having concluded some uneven feature of the wheel made the ball less likely to stop there. No, MORE money was bet on 15, because players argued "it was overdue to be hit."

        I do not think volcanism is a factor. It is a relatively shallow process, while the Earth's magnetic field originates deep down.

        Gary Glatzmeier and his colleagues, in California, have used a computer to model variations of the magnetic field (Glatzmaier, Gary A., Robert S. Coe, Lionel Hongre and Paul H. Roberts, The role of the Earth's mantle controlling the frequency of geomagnetic reversals, Nature, 401, 885-90, October 28, 1999). They believe (like most geophysicists) that the magnetic field in the core is produced by flows in the liquid iron there, and those flows are driven (as are volcanoes, true) by heat released in the Earth's interior, from radioactivity and from other processes. One big question is how heat flows out to the surface--does more come out near the poles or more near the equator, etc. By changing these and other conditions, they sometimes obtained frequent reversals, sometimes infrequent ones, sometimes none at all. Furthermore, they also obtained "excursions" which almost ended in reversals, before the magnetic polarity decided to stay in its original direction. Paleomagnetic data have found evidence for similar events.

        So... it seems complicated. Paleomagnetic data even found "superchrons" in the distant past when NO reversals occured for tens of millions of years. Go figure.  

    26.     Can magnetic reversals affect the human mind?

        I have a master of arts in philosophy and am interested in the Earth's magnetic field for a romantic science-fiction novella with a surrealistic angle, which I am thinking I may write. I have heard from someone the following theory, whose origin unfortunately I could not track. It claims:
    1. The earth has a magnetic field and every human has a magnetic field likewise.
    2. Since the earth magnetic field is declining, the rational capacities of humans with a "weak personality" are diminished.
    3. A general effect of the diminishing earth magnetic field is that the population is turning more and more psychological unstable, slightly turning bit by bit more and more crazy.

        Do these claims have any validity?

    Reply

    You may have an idea for an interesting story, but magnetism would not explain it. As far as I know, magnetic fields do not affect either mind or body. If they did, we would have a hard time finding the reason, since the body has neither appreciable electric currents nor magnetic materials.

        Also, it is not true that humans generate a magnetic field. I remember an experiment (never mind how it came about, it's a long story) when the psychic Uri Geller and a friend visited my NASA center to give a lecture. They were invited to a facility used for testing satellites in very weak magnetic fields, like those in space, to detect magnetic fields produced by currents or magnetic materials on the spacecraft, which could later distort magnetic observations in space. In that facility (it still exists) large current-carrying coils reduced the magnetic field in the center to 1/10000 its usual value, as indicated by a magnetometer. Geller and his friends shook hands at it and tried their best to change the magnetic field registered by it, but nothing happened, while starting a car out on the parking lot did create a clear signal.

        I am afraid that if you want to have in your story a "population turning more and more psychological unstable, slightly turning bit by bit more and more crazy," magnetism is not a believable cause. A US researcher named Calhoun once ran an interesting experiment, by growing laboratory mice in a large cage where they had everything--food, clean water, clean padded places to live etc.--everything but unlimited space. At first the population grew at a steep rate, but as it filled the cage and became crowded, the mice indeed turned "more and more psychological unstable, slightly turning bit by bit more and more crazy." Not having any quiet place to retreat, living in constant encounters with other mice, some stopped breeding, some attacked each other...and in the end the population "crashed." I am afraid, however, that this is too realistic and scary for a work of fiction.  

    27.     When and where can I see "Northern Lights"?

        When and where am I most likely to see Northern Lights?

    Reply

        What matters most is "where," or as they say in the real estate business--location, location, location. Your best bet is Fairbanks, Alaska; you may also see northern lights in Winnipeg, Canada, or even International Falls, Minnesota, but they could only be near the northern horizon.

        Scientists call the phenomenon the "polar aurora." Earlier it was named "aurora borealis" meaning "northern dawn" in Latin, since in Europe it was mostly seen as a glow near the northern horizon. However, it also occurs near the south pole, so "polar aurora" is now preferred. Aurora is caused by electrons energized in the Earth's magnetic environment, the magnetosphere, and guided earthward along magnetic field lines (lines of force) along which they slide, a bit like beads sliding along a wire. They move at about 1/5 the speed of light and in many ways they resemble electrons beamed at the screen inside the picture tube of a TV receiver. Where picture-tube electrons produce light when they hit the screen, auroral electrons do so when they hit the fringes of the atmosphere, about 100 kilometers (60 miles) up.

        The field lines on which this happens are the ones coming down along a circle centered near the magnetic pole ("auroral circle" or "auroral oval"), and that is where the accelerated electrons end up. The circle can expand and contract, but it usually passes near Fairbanks.

        When? Don't expect to see aurora in the Alaskan tourist season, during the summer, it just does not get dark enough. Just as near-polar winter nights are long and dark, summer days are long and bright, and even after the Sun sets, twilight persists. September has some darkness, October much more--March and April are also OK, and so is winter, if you do not mind the cold. When? The brightest auroras come from the night side of the magnetosphere, so you should see them around midnight, or later because of Alaska's time zone. If you stay in a hotel, ask the desk clerk to wake you when a good display occurs.

        What about the 11-year sunspot cycle? Occasionally, but especially in years of peak sunspot activity, the Sun sends out blobs of hot gas, which hit the magnetosphere and agitate it, causing "magnetic storms." At such times the "auroral circle" expands and reaches the lower 48 states of the US and mid-latitude Europe. The agitation also produces fine auroras, as happened on 5 November 2001, when the US was favorably located. However, since Fairbanks is located near the normal auroral circle, auroras are observed there throughout the 11-year cycle. The main factor is the slanting of interplanetary magnetic field lines (which come from the Sun)--southward slant, auroras likely, northward slant, not so much. That slant varies randomly, though satellites monitor it, and you can read their latest reports on the web.

        You find much more in the various chapters and links of "The Exploration of the Earth's Magnetosphere", http://www.phy6.org/Education/Intro.html. That collection includes sections on the aurora, magnetic field lines, the event of 5 November 2001 and one about "space weather," with links telling where to find predictions of the likelihood of aurora.  

    28.     Magnetic reversals due to comet impact?

        I just recently studied geology in my science class. What caught my attention was why the Earth's magnetic polarity has changed so much and no one knows why. My teacher told us when you hit a magnet with a hammer it changes polarity.

        I believe it possible that, like the impact of a hammer to a magnet, a comet hits the Earth and changes the Earth's polarity, by either dispersing the liquid iron core of the Earth enough to change the polarity, or by hitting so hard that the poles just change.

        (from Australia)

    Reply

    I do not know in which of my files you found my e-mail address, but I would encourage you to go to "The Great Magnet, the Earth," home page
    http://www.phy6.org/earthmag/demagint.htm and read all of its sections. You will see that the Earth is NOT magnetized the way an iron magnet is, but by a more complicated "dynamo process," a process which also creates reversals.

        By the way, suggest to your teacher to read it too, because I do not think hitting a magnet with a hammer reverses its polarity. It may destroy some of it, by disorienting magnetic domains, but that's all. Your teacher may like the suggested lessons on the Earth's magnetism, starting in http://www.phy6.org/earthmag/NSTA1A.htm

        About a comet hitting... a comet or meteorite may have been involved in creating the magnetism of Mars, which is very irregular and whose sources do not seem to be far below the surface. See end of the last section in "The Great Magnet."


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Author and Curator:   Dr. David P. Stern
     Mail to Dr.Stern:   earthmag("at" symbol)phy6.org .
Last updated 23 February 2008