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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. magnetometers

  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
  120. Length of compass needle
  121. Teaching the dynamo process of the Earth's core
  122. Electricity from the Earth's core?
  123. What makes iron magnetic?
  124. Magnetism and gravity don't mix!
  125. Retracing ancient magnetism
  126. "Magnetic Anomalies"
  127. Magnetic pole reversals

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


 

120.     Length of compass needle

Dear Sir;
    In the course of a history research project I have learned that the compass needles used by the nineteenth century surveyors who laid out the townships and sections of the United States were limited to about six inches in length because longer needles can not be magnetized or magnetize poorly.  Why is this?  My guess (in lay terms) is that after a certain length the “attraction” of the north pole for the south pole is not strong enough to reach the south pole and instead begins to “attract” the middle of the magnet, but that doesn’t explain why a long needle wouldn’t be as strong as a shorter one or function as well.  Can you help me out? Jim

Reply

Dear Jim
        I see no reason why a long needle cannot keep the same magnetic polarity. No, the reason was probably that surveyors' magnetic needles were balanced on a single pivot, and the magnetic force in general has a downwards component, trying to give the needle a downward angle. A needle on a pivot may compensate for this by being slightly unbalanced, but that compensation only holds when the magnetic inclination angle is constant.  In fact, it changes from place to place, and a long needle balanced at one place may start dragging at one end at another.

      Gauss overcame this by suspending compass magnets at two points near the center of mass, and some of his magnets were quite long.  Magnets used aboard ships are attached to a compass card floating in a fluid, and I guess that keeps them horizontal anywhere on Earth.

Response

Thank you very much for your response.  

    Actually the compass needles had a small twist of wire on them to counter-balance dip.  This wire could be moved in (toward the pivot) or out as necessary like a weight on a beam balance.  The typical surveyor's compass was about six inches in diameter with needles about five inches or a bit longer.  They were graduated in half degree (thirty minute) increments and equipped with a vernier which made it theoretically possible to obtain an accuracy of one minute but as a practical matter they couldn't consistently achieve accuracies better than about six to ten minutes because of needle quiver and an inability to adjust the vernier without moving the compass.  These limitations could have been partly overcome with longer needles which would have allowed a larger circumference that could have been subdivided into smaller increments.  According to contemporary text books, however, longer needles simply could not be magnetized as well as needles equal to or less than about six inches.  They were made of soft iron and lost magnetism naturally so occasionally the compass man had to re-magnetize them by stroking them with bar magnets carried for the purpose.

    Incidentally, both the pivots (steel or iridium) needed to be sharpened occasionally and eventually replaced as did the agate or ruby caps from which the needles hung on the pivots because of wear.  To replace and repair them in the wilderness required quite a bit of skill because it was necessary to adjust the pivots so the needle pivoted exactly in the center of the compass.  It was difficult to sharpen a pin so its tip was exactly in the same place as the worn tip and no two caps or replacement pivots were identical.  A further problem with replacement needles was that the magnetic axis of the replacement needle was frequently different from the old needle. (the geometric and magnetic axes of needles are not the same.)  I mention all this to let you know that the compassmen were really technically skilled.

    The foregoing reminds me of a second question:  What causes needle quiver?

    With respect to nautical compasses, liquid is used to dampen the compass card (no pun intended) but this was not done until the latter part of the nineteenth century when manufacturers were able to overcome certain technical difficulties. Until then the compass cards were simply hung on a pivot like a 'land' compass.  Even with liquid present, the cards were hung on pivots.  These pivots and caps wore out much more rapidly than those on 'land' compasses because the compasses were in constant motion about the cards so they required frequent maintenance or replacement. The needles of 'land' compasses, on the other hand were locked when not in use.  The cards were magnetized by attaching a magnetized needle (usually several lined up in parallel) to the bottom of the card.  Similar compasses were sometimes used for land navigation but they seem to have been exceptions.

Reply

You obviously know a lot about historical compasses, maybe more than I do. Yes, long needles may be less efficient as magnets, because some magnetic flux leaks out of their sides, but that should not affect the direction in which they point. I do not know why the magnetic and geometric axes of a needle should differ, but a problem of compass needles aboard iron ships was the presence of so much steel around them, often magnetized slightly during production.

    I don't know about quivering, but a finely balanced needle needs very little force to make it quiver. I think that is the reason compasses were filled with liquid, and still are, or else needles were suspended from quartz fibers.

    And by the way, here is something if you are interested in surveyor compasses. In 1997 Herman Heyn from Baltimore gave a talk at the American Geophysical Union, showing us that while the older streets of his city formed a rectangular grid aligned with the main compass points, the grid was rotated by about 3.5 degrees relative to astronomical north etc. His guess was that they were laid out using a magnetic compass, aligned with the magnetic north (of that time) rather than true north.
 

121.     Teaching the dynamo process of the Earth's core

Dear Dr. Stern,
        I am trying to understand the origin of planetary magnetic fields in order to create a series of lessons for middle school students.  My question stems from the following statement on your website:
        "So the molten metal is believed to be circulating. By moving through the existing magnetic field, it creates a system of electric currents, spread out through the core, somewhat like Faraday's disk dynamo, discussed earlier. Currents create a magnetic field--a distribution of magnetic forces--and the essence of the self-sustaining dynamo problem is to find solutions such that the resulting magnetic field is also the input field required for generating the current in the first place."
Is this a "chicken and egg" problem? I was under the impression that by rotating a liquid metal, the current is created and that this produces the magnetic field around earth.

Reply

Dear Douglas

    The problem you posed is mathematically very complicated. By "circulating" I meant flowing in closed loops of some shape--not necessarily "rotating" in a circle. In fact, Thomas Cowling in 1933 showed that the fluid flow associated with a dynamo cannot have axial symmetry, and it was only in 1964 that the first solutions were found. I you have the patience, see long review article linked from
http://www.phy6.org/earthmag/demagint.htm .

    As a crude analogy to the forces producing the dynamo, take weather. The atmosphere circulates because heat energy is constantly deposited in it from the Sun, and to help return that heat to space (heat conduction is not enough), air must constantly circulate between the ground and the stratosphere (where it is radiated), and also between the tropics, where the input is largest, and the rest of Earth. Atmospheric motions are certainly affected by the rotation of Earth (e.g. see sections 24b, S-1A and S-1B in "Stargazers") but, the circulation itself is due to heat trying to escape.

    Something like that (only in a differently shaped region--in the interior of a sphere rather than in a layer surrounding it) is believed to occur inside the Earth.. The heat--that's the heat generated deep inside the Earth--may be at least in part from long-lived radioactivity of uranium, thorium and potassium.

    There does exist a chicken-and-egg analogy, too, as in many self-starting or self-maintaining processes. A commercial dynamo produces electricity by whirling a "rotor" which is essentially an electromagnet, in the cylindrical space inside another electromagnet, the "stator". The rotation of the rotor supplies electric current to power both sets of coils. Initially the field which it senses may be very weak--just the tiny one originating at the Sun, or in little traces of magnetized iron--so it only causes a small current, but that small current amplifies the field and increases its production, and grows exponentially. The growth ends when the resisting magnetic force, which the rotation must overcome to produce the current, equals the force provided by the power source (car engine, steam turbine, hydro-electric turbine etc.) driving the dynamo.

    I hope all this is reasonably clear. Look at "The Great Magnet, the Earth" and its attached review. It is not an easy subject to get across in middle school, but at the end of that collection--especially the 3-part lecture given to NSTA--are attempts to help teachers do so. Please let me know if you succeed in getting the ideas across!  

122.     Electricity from the Earth's core?

    Does earth's core emit electricity and if so could it be collected to help solve energy problem?

Reply

There probably exist electric currents in the Earth's core, inferred from the Earth's magnetism. However they are very spread out, and of low voltage and local density. Anyway, I know of no way of connecting to them through two thousand miles of rock.

    Humanity needs a source more available and more powerful to help solve its energy problem. Sunlight, perhaps?
 

123.     What makes iron magnetic?

What changes inside iron take place that causes iron to be magnetic?

Reply

  Electrons are also small magnets, associated with a property known as "electron spin." The arrangement of electrons in an iron atom makes it quite magnetic. However the magnetic axes of atoms in unmagnetized iron point in random directions, and as many point in any direction as in the opposite one, so the effect cancels (as it does in most non-iron materials).  When you magnetize iron, the magnetic axes of atoms get lined up and their magnetic properties reinforce each other.

          In soft iron, the alignment lasts only as long as the cause of magnetization persists, e.g. the electric current in a coil around a soft-iron electromagnet. Such iron is useful in cores of electric transformers for AC, which must reverse their magnetization 120 times each second, to track the ups and downs of AC.

          Steel and some other materials, on the other hand, stay magnetic after being magnetized, either by an electric current or by a permanent magnet held close.  That is useful for compass needles. The alignment is not of individual atoms but of "magnetic domains", relatively large chunks of iron crystals which are already magnetized naturally in the same direction. In unmagnetized steel, the magnetization directions of domains are oriented in random directions; running a current in a coil around them, or stroking the iron with a permanent magnet, lines them up  in a new direction, as long as the magnetism sensed locally by each domain exceeds a certain minimum intensity (which depends on the material). See http://en.wikipedia.org/wiki/Magnetic_domain

Response

Thanks for getting back to me.  When a permanent magnet is moved next to iron, do the electrons orbiting the iron nucleus get attracted to one side of the iron nucleus, thus resulting in magnetic waves pointing in the same direction?

Reply

Dear James
        A compass needle is not attracted to either magnetic pole (except for a negligible force due to the variation of the magnetic force with location, its "gradient"). One end is pulled by the north pole, the other is repelled by an equal and opposite force, and the result is zero.  What those forces do, however, is rotate the needle until is is aligned with the direction of the magnetic force.

        Similarly, electrons are not attracted or repelled.  
 

124.     Magnetism and gravity don't mix!

            Frankly, I'm no scientist, but I have enough ancient history, physics, and astronomical influence to understand what could/might/can happen if the Earths field fluxuated.

      I thought about it very simply, what keeps us on the ground...Gravity(duh). Then, what 'creates' or causes Gravity?...The earth's magnetic field(basically). Now, what would happen to gravity as the Earth's magnetic field weakened/Strengthened? I would expect us to have a bit of a ''lightness'' to things, if not catastrophy (the great lakes draining into the gulf, mountains falling to the sea, Mass earthquakes, Doomsday etc etc)....
    .... How does someone prepare for that kind of thing, and if this is old news to you, what do you think of it?

Reply

    Gravity and magnetism are two different types of force. Gravity originates in mass and ALWAYS attracts. Magnetism originates from electric currents (or particle spin, sometimes) and always creates two OPPOSITE magnetic poles with opposite forces, or else more complicated forces which cannot be described by magnetic poles. Any object which is not magnetized (like you or me) is not attracted or repelled by it.
    The Moon, for instance, has some very weak magnetism (in spots), maybe from meteorites or ancient lavas, but has enough gravity to let astronauts walk over its surface.
    A reversal of magnetic polarity is possible (and has happened many times in the geological past), because the electric currents in the Earth's core, which create it, are constantly shifting. See "The Great Magnet, the Earth" which I wrote. They shift all the time, so magnetic charts need to be corrected every 10-20 years. But it has nothing to do with gravity, and the atmosphere does not change either.
 

125.     Retracing ancient magnetism

    I am from Romania, first officer on commercial vessels and also I've graduated the history university.
     I have a huge curiosity regarding of evolution of our world. I am interested to calculate the annual variation of the geographical position of north magnetic pole. From the reading of your site, I understand that is not a regular movement from the point of location. But I want at least to determinate the position (area where was located) in the past, going back in steps of 500 years up to year 10 000 b.c. and even more. For this kind of steps there is any mathematical model to follow? If yes, can you provide me?

Reply

Your plan is good, but unfortunately not possible. Our accurate measurements of the Earth's magnetic field--before 1836, only its direction, not strength--are mainly taken from ships' logbooks, they only go back some additional 300 years and do not cover the entire Earth's surface. Volcanic lava records magnetism over long periods but their coverage is local: one attempt to use such data is shown at http://paleomag.coas.oregonstate.edu/research/projects/arctic/arcticfig2.html
    We also have observations of sea-floor magnetization, going back millions of years, but it is not enough for reconstructing the position of the poles. Please also realize that there exists more than one definition of the north magnetic pole. See   http://www.phy6.org/earthmag/magnQ&A6.htm#q102
    If you have not yet done so, I would recommend going over the web collection "The Great Magnet, the Earth", home page   http://www.phy6.org/earthmag/demagint.htm especially the review article "A Millennium of Geomagnetism" at its end. You may also enjoy the one-hour lecture at http://www.youtube.com/watch?v=1SOSmHPTods
 

126.     "Magnetic Anomalies"

    I have a problem about magnetic anomalies. I found this on an internet site about magnetic declination (http://earthsci.org/education/fieldsk/declin.htm)
    "Local anomalies originating in the upper mantle, crust, or surface, distort the WMM or IGRF predictions. Ferromagnetic ore deposits; geological features, particularly of volcanic origin, such as faults and lava beds; topographical features such as ridges, trenches, seamounts, and mountains; ground that has been hit by lightning and possibly harboring fulgurites; cultural features such as power lines, pipes, rails and buildings; personal items such as crampons, ice axe, stove, steel watch, hematite ring or even your belt buckle, frequently induce an error of three to four degrees."
Could you explain me how faults, trenches, power lines, pipes or even a steel watch can cause magnetic anomalies?
    I assume that it is because of their ferromagnetic origin, but then i wonder how it is possible that a trench or a ridge can influence a magnetic field?

Reply

Dear Sébastien          I looked up the site you mentioned.  The author seems to define as "anomaly" anything that can make your compass deviate from the global magnetic field. Some are large scale--like lavas (see section on sea-floor spreading in "The Great Magnet"), and maybe trenches and ridges too--meaning features like the long ocean trench along the eastern coast of South America, or west of Japan, and volcanic features like the mid-Atlantic ridge  (same section). Power lines are more local, and I think the main offender is DC current. Buried iron can affect magnetic observations, and lightning is mentioned because it can in principle create lodestones (though only rarely; see section about those).  Even bricks are a bit magnetic, which is why magnetic observatory buildings avoid them.  

127.    Magnetic pole reversals

    I got this email address from http://www-spof.gsfc.nasa.gov/Education/Intro.html as I was looking for information regarding moving earth's magnetic pole and changes in earth's magnetic field strength. I was led to my research after hearing of rumors of magnetic pole shift acceleration and magnetic field weakening. I ask, that, if possible, you point me to information regarding those topics please.

Reply

Dear Arnie
              The web-course whose home page you cite deals mainly with the Earth's magnetic field in space.  A similar (shorter) course dealing with the field inside Earth, its study and variations, is "The Great Magnet, the Earth," home page http://www.phy6.org/earthmag/demagint.htm .  You may  look up section 15 there on magnetic reversals and at its end answered questions about it, but I would also encourage you to read through it, it's not that long.

          The magnetic field from the Earth's core is gradually weakening by 5-7% per century, but the continuation of this trend is uncertain.


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Author and Curator:   Dr. David P. Stern
     Mail to Dr.Stern:   earthmag("at" symbol)phy6.org

Last updated 30 April 2012