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B Magnetic compass and Earth's magnetic field

  1. Jun 11, 2017 #1
    Hello ,

    I am having a slight confusion while reading the chapter on Earth's magnetism . If we place a magnetic compass on the floor I.e on Earth's surface , the North (red) of the compass needle points towards magnetic north of earth .

    But would it be correct to say that "the compass needle aligns itself such that it becomes parallel to the horizontal component of Earth's magnetic field " ??

  2. jcsd
  3. Jun 11, 2017 #2
    That's true, but it's of course simply because compasses are constrained to horizontal movement. If you had say a magnetic needle suspended in a solution, it would orient itself parallel to the actual Earth's field lines. At th the North Pole the needle would be pointing upward.
  4. Jun 11, 2017 #3

    And the compass needle always lies in the magnetic meridian ??

    In other words it is always possible to draw a plane that passes through the compass needle and the magnetic axis of earth .This plane is called the magnetic meridian at the point of location of compass .

    Is it correct ?
  5. Jun 12, 2017 #4


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    I think the N pole of the magnet seeks the N magnetic pole of Earth, which is underground, so the compass needle you describe would point down when placed over the North Magnetic Pole.
  6. Jun 12, 2017 #5
    @rumborak , please reply to post#3 .
  7. Jun 12, 2017 #6
    A .... qualified yes. That is, "meridian" is the term for the circles that go through Earth's *rotational* poles, not the magnetic ones. Those differ when you get close enough to the poles.
    In a less strict sense your statement is correct though.

    Same qualified yes. It's a simple geometric consequence that the two will share the same plane. After all, that's what "aligning" means.
  8. Jun 28, 2017 #7


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    I remember we had at school a special compass mounted in a vertical plane, which was used to measure "the angle of dip" , I suppose the vertical component of the earth's field.
    I can't remember any use for knowing this! So I looked in WikiP, but they don't seem to know any reason to measure this angle either. My guess is it's mainly of interest to geologists - either interested in the Earth's magnetism, movement of the poles and what have you, or looking for magnetic anomalies to indicate mineral deposits etc.
  9. Jun 28, 2017 #8


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    A use for knowing this?
    Breaking down the earth's magnetic field into its x, y, and z components was the only way I was able to complete an experiment a few years back.

    This post shows the basic setup, but the results were all wrong.

    From a later post; "I'm still trying to figure out why, when I'm told that the earth's flux is ≈0.54 gauss where I live, I come up with a smaller number [0.33 gauss]."

    Here, knowing I only needed one of the components, I concluded the experiment with a reading of 0.43 gauss, which was close enough to the 0.49 gauss it was supposed to be.

    Anyways, that's one use.

    ps. I would link to the thread in general, but we went so far off topic, it was embarrassing. And we never did figure out how a 'clip on' ammeter works! Though, it was fun trying.
  10. Jun 28, 2017 #9


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    other way around, opposites attract, similar repels

    but it is a bit more complicated than that
    what we call the magnetic north pole on the earth is actually magnetic south pole polarised
    and the magnetic south pole ( off Antarctica) is magnetic north pole polarised

    why you ask ?

    The magnetic poles are defined by which end of the magnet that the field lines emerge and re-enter the magnet. Field lines emerge from the north pole and re-enter at the south pole


    the earth's magnetic field is thus ......


    so as can be seen the magnetic polarities are opposite to what is expected

    A bar magnet or compass needle 's N pole end seeks out the magnetic south pole
    (opposites attract) which currently happens to be up in the geographic north of the northern hemisphere

    Another magnetic reversal of the earth's field at some time in the future ( as has happened often in the past) will restore the normal. but then all the red end of compass needles will indicate the other way :wink: :rolleyes:

    Last edited: Jun 28, 2017
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