Site Map
Q&A (subj)
For Teachers
Review Article
Central link

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)
Before you do, though, please read the


    69.     Magnetic Flux

        I am a second year physics A level student at a college in England.

        We are currently studying Magnetic flux and EMF as far as our syllabus demands, but what it doesn't do is actually tell us what magnets really are and why they work as they do. It is all just a case of "this is how it is and this is what you need to know, so learn it," which is great for the exam, but I and the rest of my class are really fascinated and would like to know more. On searching the internet I found your site and was wondering if you would be able to help us out at all.

        The information we have is that the reason iron makes such a good magnet is because of its unpaired electrons. This makes sense to me in that the negative charge orbiting the positive nucleus produces the same kind of effect as a when done with a wire wrapped around an iron core and the EMF produced. (I think this is right anyway, my understanding is only to A level standard)

        However I do not understand what magnetic flux is, how it 'prefers' to take the shortest route through the metal, but how, when it passes through air, it will curve and 'the flux lines will repel each other.' What is it though that is actually repelling? What is it that is carrying the flux and how is it that this attracts or repels? "What is it?" that I suppose is my real question. I was told that the atoms arrange themselves the same way around so that their electrons are all orbiting the same way, but why is it that that creates a magnetic effect?

        My class and I would be very grateful of any help you can offer us on this subject.


        It is hard to cover everything in a short letter, but luckily, similar questions have been asked in the past. Permanent magnetism ("ferromagnetism") is a complex subject, understandable only on the quantum level. You might think that iron magnets are magnetized by the electron charge orbiting the nucleus, but in fact the dominating cause is the magnetism of the electrons themselves. Think of the electron as a small charged sphere spinning around its axis, and this gives the right order of magnitude--electrons do have an angular momentum or "spin"--but the ratio between angular momentum and the strength of the source of magnetism ("magnetic moment") does not fit a spinning sphere in which mass and charge distributed the same way. If I recall it right, the electron is twice as magnetic as what you may think, a difference which has quantum reasons.

        Anyway, look up "What causes permanent magnetism" at
    and links from there. To round out your understanding, you may also look up "magnetic moment" at
            http;// "What is magnetic flux?" was also asked before: see

        I wish every science teacher could read the way you expressed your problem with the school syllabus! You wrote "what it doesn't do is actually tell us what magnets really are and why they work as they do. It is all just a case of this is how it is and this is what you need to know, so learn it, which is great for the exam but I and the rest of my class are really fascinated and would like to know more." Many students have a natural curiosity and would like to understand the way nature works, but that is too often ignored by textbooks and by teachers. Instead, they substitute dull exercises and book-learned facts, which the student is expected to memorize and to present at exam time. After that they are promptly forgotten. YOU are doing the right thing, and I wish you every success.

    70.     Why do moving electric charges create a magnetic field?

        Dear Dr Stern, I am currently studying year 12 physics, and we are covering magnetism. I know that moving charges, like a current in a wire, create magnetic fields but I was wondering why?


        The simple answer is, any electric current DOES consist of moving charges. If a current is generated in a wire connected to a battery, the charges move inside the wire. If they are electrons and protons in the Earth's magnetic field, they move on their own, but the effect is the same.

        On a deeper level... it gets more complicated and mathematical, like so much of physics. You have an electron moving in space, you expect it to generate a magnetic field and it does.

        But suppose you match velocities with the electron, so both of you move in the same direction with the same speed. In your frame of reference, the electron does not move. Does it create a magnetic field?

        No, it does not. It creates an electric field, due to its negative charge, but that's all.

        This is explained by recognizing that electric and magnetic fields are two facets of the same phenomenon, the electromagnetic field. By moving to a different frame of reference, the relative contributions of "electric" and "magnetic" may change, and in this case, the "magnetic" part is zero--but the electric part is also changed, so that the total force is the same (with some small corrections near the speed of light). This was recognized even in the second half of the 19th century, when the term "electromagnetic field" arose, but Einstein's theory of relativity modified it and found the proper relationship which holds up to the velocity of light.

        In Einstein's theory, electric and magnetic fields are parts of the same 4-dimensional entity (a tensor, if you need a label), and the way we observe it can be worked out for any velocity.

        I hope this is not too big an answer to a short and simple question!  

    71.     Weakening of the Earth's Field

    (1st of 2 questions)

        I just saw a science program on TV that said that the Uranium core of the earth is rapidly deteriorating. This is the nuclear engine that drives the magnetosphere, and keeps it alive and viable. It was estimated that the magnetosphere is deteriorating and that in 1200 years, it will be gone. This will expose the earth to direct solar winds, and the result will be the extermination of life of earth.

        I assume you have a PhD. in Physics. This is rather alarming. Any truth to this?


        I am sorry if you are alarmed by my physics degree! As for the Earth's core, you can rest easy. Still, I am curious to know what was that "science program on TV" and who prepared it, because it is rather misleading.

        One thing in it is correct: the main north-south magnetic field of the Earth is weakening, and the current trend goes to zero in about 1200 years. It is not clear if that trend will last that long--the field is changing all the time; if it does, the north-south polarity may indeed reverse, as has happened many times in the geological history of Earth. Even then, the field probably does not vanish. Other more complex magnetic components are currently growing stronger. All we would get is a smaller and different magnetosphere.

        Other claims are wrong. The core of the Earth is probably iron, certainly not uranium: there is just a little uranium around--also thorium and potassium, radioactive elements which provide heat in the interior of the Earth and should last billions of years. As for the solar wind--its particles can penetrate the atmosphere just a small bit (currently they only come down in two small patches near the poles). Maybe given billions of years it could erode the atmosphere--although it has not happened to Venus, which has a thick atmosphere but no magnetic field to protect it, and which faces a more intense solar wind, being closer to the Sun

        Much more can be learned from my site "The Great Magnet, the Earth," at
    Reversals of the Earth's field (4 queries)
    The movie "The Core" (1)
    The movie "The Core" (2)

    Weakening of Earth's magnetic field, #2:

    A monotonic advance towards reversal?

    So, if the Earth's magnetic poles reverse on a 0.5 - 1M year time frame, what happens in between? Does the magnetic field get weaker and weaker... etc. Is there a "dead" period where no magnetic field exists, then slowly builds back up in the opposite polarity, or direction, or whatever you call it?


        No, that would be far too simple. From all the records we have--fired ancient pottery, lava flows, and recently also lake deposits--the Earth's magnetic field varies irregularly.

        You should understand, of course, that it is very hard to reconstruct ancient GLOBAL magnetic fields from spotty observations at isolated locations--especially if you go back tens of thousands of years, to where dating becomes so uncertain that it is hard to compare two (say) sets of lava flow records, without precisely timing each.

        There also exist computer experiments (see end of and they also suggest irregular variations. This agrees with our ideas that the magnetic field is the result of a complex flow in the core of the Earth, of several linked eddies which slowly evolve, and the north-south polarity is just the dominant trend. Both the paleomagnetic record and computer simulations found "excursions" when the dipole field came close to reversing, then "changed its mind" and went back to the original polarity.

        The record of reversals from the ocean bottom is also irregular. I vaguely recall evidence of some very short intervals between reversals, and on the other hand of a "superchron" (in the Cretaceous era?) with no reversals for tens of millions of year. Mother Nature just doesn't always stick to simple schedules.  

    72.     Focusing magnetic fields

    Hello Dr. Stern.
        I have problem. Can we focus the magnetic field on a point? I am working on a project and I am from Iran. Thank you.    


        Yes, you can, though there is a limit to the amount of concentration. I suppose you have already read about magnetic fields in "Exploration of the Earth's Magnetosphere" and know how they can be described by magnetic field lines (lines of force).

        Where these lines are far apart, the field is weak, while where they are close together, it is strong. For instance, the field lines of the Earth are far apart where they cross the plane of the equator, then get close together as they approach the poles, where the field is strong.

        Iron and similar "ferromagnetic" substances channel magnetic field lines inside them, not letting them out. So on laboratory magnets you may see conical "pole pieces" of iron facing each other, to concentrate the magnetic field lines and the magnetic field, guiding them into a smaller volume, where the field is stronger.

        This is important, for instance, in magnetic recording devices--hard and floppy disks, also video and audio tapes. To record a signal--computer information, a sound etc--you need concentrate the magnetic field on a small area, because the more differently magnetized spots you can put on the disk or tape, the more information you can store there. Also, the field needs to be strong to magnetize the disk or tape permanently (even though it only needs a very short time to do so). To make this possible, all such recording device have the recording electromagnet end in a sharp cone, just above the disk or tape. If you look inside an audio recorder, you may see this tip. (In computer disks, the recording part is enclosed to keep out dust and you cannot look there.)
        I wish you success in your project!

    73.     Is gravity related to magnetism?

        I am an 8th grade student and recently we were learning about Newton's laws of motion and I was wondering what made gravity work. Then I asked myself what are all things made of and so I said atoms but something needs to be attracting them at the center of the earth. I was wondering if you knew if atoms magnetic fields are strengthened by heat (the center of the earth IM sure you know is very hot) and this is what might cause different weights.


    Gravity and magnetism are not connected (although you are by far not the first to feel that maybe they were).

    Magnetism is the effect of electric currents: see for instance
    and the historic section following it.

        Gravity is the property of massive matter. That was shown in the lab in 1796 by Henry Cavendish--look it up on
   and in more detail by Roland (or Lorand) Eotvös

        Gravity is proportional to mass, which may be defined as the resistance of heavy matter to motion. See above section for a description--it is a difficult concept for many students confronting Newton's laws (even for teachers--show this to yours!), and you should understand it clearly before applying it. Mass and gravitational pull seem to be proportional, and that was one of the foundations of Einstein's General relativity.  

    74.     Observing Magnetic Planets

        I live in India and came across your very interesting article on internet about the magnetic fields of planets. Although I am pursuing my Masters in Bioengineering and Medical Imaging, I love to explore the astronomy world. This world really fascinates me.

        After going through your article on Planetary Magnetism
    I just had a question as to how can it be known that a particular planet has a strong or mild magnetic field? Is there any specific device by which we can measure it or its just an inferential.


        Spacecraft that explore space around other planets usually carry magnetometers--instruments to measure the magnetic field. They are so sensitive that they are usually put at the end of long booms, to keep them away from the influence of electric instruments on the spacecraft itself. The one on the Voyager spacecraft was 13 meters long, looking like a folding tower--see
    By measuring the distribution of the strength of the magnetic field around the planet, one can calculate the total source strength ("magnetic moment") of the planetary magnetic field.

        The magnetometer generally used is of the "fluxgate" type, and you can read about it at
    You ought to find that page especially interesting, because it describes a biological experiment, performed with such an instrument.

    75.     How does magnetism spin aluminum disks in power meters?

        I am a retired lineman from a local power company and have wondered about how magnetic fields "retard" the movement of an aluminum disk in an electric power meter (kilowatt-hour meter).

        I was examining a kilowatt hour meter removed from a home for routine maintenance. On either side of the rotating aluminum disk are permanent magnets that can be adjusted to add drag to the disk. The disk is driven by a coil who's field strength is regulated by the current being drawn by the home.

            a) How is aluminum(non ferrous) driven by an electromagnet.

            b) how do the permanent magnets (which are arranged like caliper brakes on a bicycle wheel) retard the disk from moving?

        Thank you for your time, your website is most informative.


        Good for you--you want to know! The key phrase here is "eddy currents"

        Actually, I do not have a precise answer to your question, but I hope you know that magnetism is not always the force between iron magnets, or associated with iron at all. It is fundamentally the force between electric currents, and the involvement of iron ("ferromagnetism") is an interesting sideshow of nature. In the technology of our society, of course, it is enormously important to applications of magnetism, but out in space (which I used to study) currents are almost always carried by free electrons and ions, with no iron in sight.

        For more about magnetism and its history, see my web collection "The Great Magnet, the Earth" with home page
    The story of how the effect of electric currents was discovered by a Dane named Oersted and a Frenchman named Ampére, that is there too.

        Now the thing to note is that if a conductor moves in a magnetic field, it can have electric currents induced in it. That is the "dynamo effect" described in one of the sections there, including a dynamo which Faraday devised using a rotating disk between the poles of a magnet. Aluminum (not yet discovered in Faraday's time, I think) makes a very good disk for this, because it conducts electricity very well. (I think his disk may also be used as a motor, if you inject a current, but not a very good one.)

        Your electric meter is not like this because (1) it has no external contacts and (2) it works on AC. An AC magnetic coil, however, will also induce electric currents in the disk, and I would guess those are repelled by the magnetic field of the coil and produce the motion.

        Permanent magnets also produce eddy currents in a moving disk. I guess in a meter they can retard motion because the disk then absorbs energy by its ohmic resistance to those currents.

        My son in California lives across from the "Great America" amusement park, and a big attraction there is "The Drop Zone", a 220' tower for experiencing free fall. People are strapped into seats in groups of 6, are hauled to the top, and after a short interval, allowed to drop. You hear the screams a long way.

        How are they stopped? Well, I am told the seats have magnets, and about 1/3 of the distance from the ground these magnets begin straddling thick copper vanes. The magnetic field (with the motion) induces currents, the currents cause energy loss, and the seats lose speed until the slowly glide to a rest. (At least that's what I heard unofficially.) Eddy currents do it all.

        Have a good retirement and a happy new year!  

    76.     Magnetic Poles in Druid times?

    Im sorry to bother you but I have been searching forever for an answer of any sort.. Is there documentation anywhere that might show the past path of magnetic North Pole? or even an estimated guess? I have a theory, that magnetism played a very large role in druid/celtic times. If you know of such documentation, would you consider sharing the knowledge?


        The best I can offer is the motion of the pole back to 1600, and of that, the positions before 1831 are somewhat uncertain. In 1831 of course the north magnetic pole was visited and marked--a real stroke of luck, by a poorly equipped expedition at a time when the pole was unusually far south (read section 9 in "A Millennium of Geomagnetism" at )

        You will find a map with the estimated positions of the pole at

        I don't think the Druids had compasses. I looked at Gilbert's book--and he collected all the ancient European references he could locate--and could not find a reference to Druids or Celts. A German woman once wrote to me, wondering how Stonehenge got aligned properly without a magnetic compass--but of course, you do not use a compass there, you use the stars.  

    77.     Magnetism linked to Global Warming?

        Having been at sea and after circumnavigating the world a few times I have always been a skeptic of the current theories about global warming. Our activities seem such a small part of this planet .

        I wondered if you see or have conceived a link between geomagnetism and the changes in the earths climate.


        The energy involved in the climate is much, much greater than what creates the Earth's magnetic field. Plus, since the atmosphere is an electric insulator, it's hard to conceive a way for it to interact with climate.

        The amount of matter contributed by humanity to the atmosphere is also small. However, molecular gases (like methane, carbon dioxide and water vapor) have a great effect on the opacity of air to infra-red radiation, out of proportion to their actual amount. So it is possible for them to have an appreciable effect.

        Computers have simulated global warming. Also, there is no question that the amount of carbon dioxide, which is constantly being monitored, is rising. Arctic sea ice in the northern polar cap is breaking up earlier and in general climate is warmer--not just this winter, which is unusually warm where I live, but also statistics of the distribution of the 10 warmest winters on record, etc.

        The situation is complicated by the fact that climate itself is subject to fluctuations, which are not sufficiently understood--say, the warming of the western Pacific, which triggers of "El Niño." When the warming trends were first noticed, some people attributed it to a fluctuation and predicted it would fade again. But it seems less and less so now. And even without the evidence of the last decade, it always pays to err on the side of caution.

        Magnetism plays no significant role in all that.  

    78.     Uses of Magnetic Energy

        I am supposed to write a report on magnetic energy in my physical science class. One of the requirements of the report is to tell the advantages and disadvantages of the energy. I have researched it everywhere but cannot find any. Please help me!


        I cannot teach you a whole course here,. Magnetic energy is contained in electric currents or in electromagnets, and is generally very diffuse. The quantity of energy is usually very small, but it can respond very quickly, and that can be important: look up the section on the fluorescent lamp.

        The important thing is that it can very rapidly exchange energy with (say) electric energy. So if you need store magnetic energy--just a little, but 120 times each second, as in a fluorescent ballast coil--magnetism helps. And when you tune your radio or TV to a station, you use the above interchange of magnetic-electric energy, which you can arrange to be most effective at just one frequency. That way you make your receiver sensitive to that frequency--just one station--and ignore all the other electromagnetic signals floating through space.

        Sorry, but that is all I have space for. You can also look at the design of electric transformers, which use oscillating magnetic energy.  

    79.     Can sparks generate magnetic fields?

        Can anyone tell me if a spark generates a magnetic field and if so where can I find the relevant results?


    Your answer depends very much on what you mean by "magnetic field."

    A steady field like that of a magnet--no. A spark does not generate one.

        However, it does generate a radio wave, which is an oscillating interplay of electric and magnetic fields, at high frequencies. See http"//

        Lightning is a giant spark, and you can hear its emission on your radio, especially on an AM receiver, during a thunderstorm (the main frequency is closer to that of AM). Early radio communications used spark generators to create short and long signals, dashes and dots on the Morse code, which is why in German, the word for transmitting a radio signal was "Funken" ("sparking") and why the radio operator on a ship had the nickname (in English) "sparks".

        The reason for the radio transmission is that a spark is actually a discharge which oscillates at a high frequency. Its current therefore also oscillates, and it emits radio waves.

    80.     Can a magnetometer detect cracks in an oil well?

        Hello, I came across a webpage of your information while researching about fluxgate magnetometers. I am hoping you may be able to provide some insight into this project.

        I am involved in the oil and gas industry in Western Canada particularly in the fracture stimulation of oil and gas wells. I am looking for an alternate method to detect the induced fractures in the formation. As a background if you are not familiar with the technique, high pressure pumpers pump a slurry down the wellbore through holes in the steel casing over the zone of interest. They pump at high enough pressures to fracture the rock and fill the fractures (1 to 10mm wide) with the sand laden slurry. We currently add a radioactive tracer to this slurry and run in the wellbore after the fracture treatment is done to "see" where the fracture has gone. Typically these fractures can extent 1 to 500 meters away from the wellbore and grow vertically 1 to 100 meters in height. The current method of fracture detection only identifies the radioactive material up to 20 inches away from the wellbore. It would be advantageous to be able to map the direction, length and width of the fractures for their full length.

        Based on this limited explanation do you see potential for some type of magnetometer to be useful in this application. Keep in mind there is steel casing in the wellbore. As well we could add magnetic particles to the slurry which could in effect make the fracture a magnetic conduit?


    2 February 2006

        I tried hard to figure out a way magnetometers could help you, but could not find any. Even with highly magnetized material, the field gets rapidly weaker with distance, and quickly gets undetectable..

        If the slurry were highly conducting electrically, you could perhaps put a high voltage pulse on the pipe and see how "echoes" of the pulse is received at various locations on the ground--a bit like an EKG. However... high voltage electricity near oil and gas is not a good idea, and electrical properties of the slurry are probably not much different from those of the surrounding rock.

        I believe that magnetometers were used for a long time in prospecting for gas an oil in Canada, and one of the pioneers was Lawrence Morley, who made a great name in science: he was the first to suggest that the magnetic stripes on ocean-floor rock came from gradual spreading of such rocks, which led to the current view of plate tectonics. I wrote about that in
    and in more detail in

    The journals refused to publish Morley's article, considering it too far out. A little while later Vine and Matthews arrived at the same conclusion, had it published, and for a long time they alone were given credit, before the world learned about Larry Morley.

        I hope he is still with us today: we met once and I found him charming. More about him at
   (that's the spelling there!)
    also at

    February 19th is his birthday--go ahead, raise a toast to him on that day!  

    81.   Telling about magnetism    

    I am a Junior at Gabrielino High School in San Gabriel, California and am currently working on an expository speech about magnetism for our nationally renowned speech team. I will then memorize the speech, create presentation boards for it, and enter with it a state-wide competition. Last year, I placed 9th best expository speaker in the state of California.

        An expository speech is basically an educational speech, and I've found it difficult to thoroughly explain the workings of magnets and magnetism in simple words, without sounding confusing. I am specifically looking for current breakthroughs, inventions, and/or other interesting facts to keep my audience engaged. That's what I hope you can help me with.

        I currently have three points: History, Science (which explains how magnets work), and Society (which explains why magnets are important in our world: Maglevs, MRIs, etc.)

        If you had 10 minutes to tell an audience everything there is to know about magnets, what would you say?


    I cannot write your speech for you, but you may perhaps get ideas from the questions and answers on my web site, all actual correspondence (selected from many more as being the most interesting). Some thoughts:

      --Most people do not know much about magnetism, and feel it is a mysterious force. It is actually well understood.

          --Many people have heard that the Earth's magnetic field may reverse polarity, and are afraid of it happening soon, and causing great danger. Neither is true.

          --Magnetism has a long history of usefulness to humanity, from the magnetic compass (would Columbus have reached America without it, or even dared sail west?) to computer disks and electric machinery. All generators of electric current use magnetism, as do transformers which make it possible to transport electric power across the country at high voltage and small loss, later (with other transformers) reducing the voltage for safe home use.

          --We once believed that magnetism was just the property of special iron and minerals. It is actually intimately connected to electricity. The world as we know is held by 4 fundamental forces--gravity, electricity and magnetism (two sides of the same force, responsible among other things for light and radio phenomena), and two kinds of forces holding together atomic nuclei. Thus it is one of the foundations of the natural world.

          --We once believed the Earth was special, being magnetic. Now we know most planets are magnets (Jupiter's is 20000 times stronger than Earth), sunspots are magnetic, pulsars and other distant objects, and magnetism is widespread in the universe. We still don't know all the processes responsible--but that's why physics is a frontier science.

    Take it from there. Make number 1!

    82.   Does North-South orientation slow down iron corrosion?  

        I was told today something apparently known to a few contractors. Namely, metal placed north and south is a lot less likely to be eroded than when placed east and west. Any comments?


    I know of no magnetic effects which affect corrosion, but if the effect is indeed observed, maybe there exists a different explanation.

        Suppose you build a bridge, using I-beam girders. If the girders point north-south, the sun shines on one side in the morning and on the opposite side in the afternoon, keeping them dry.

        On the other hand, a girder placed in the east-west direction will have its northern side in the shade all day. With rain, dew or snow, that side may stay wet much more frequently, and water promotes its corrosion.

    83.   Why two magnetic poles and not more?

        I am a student at a Turkish university. For one of my lessons I need an information about earth's magnetism. My question is:

        "Why does the earth has two magnetic poles, north and south? What is the reason for this? If possible there will be another poles?"


    There may also exist more magnetic poles--it all depends on the sources of magnetic field. The only requirement is that the amount of magnetic flux (see below) leaving the Earth equals the one re-entering it. For the definition of magnetic flux, see


    As for defining the magnetic pole, several versions exist--here let's take the simplest, the point where the magnetic force is perpendicular to the surface of Earth.

        The Earth has two poles, because among all the magnetic sources inside the Earth, the strongest ones are those associated with a two-pole structure: see


    and in more detail


      However, some people speculate that during magnetic reversals, when the two-pole (dipole) source gets weak (and finally reverses direction), for a while the other sources dominate, and more poles can exist. You might say this situation exists on the Sun, which has a weak global dipole field, but in addition also has concentrated fields in sunspots, creating additional local "magnetic poles" by the above definition.

Back to the Index of Questions

Back to the Master List

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

Last updated 23 February 2008