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Compass needles

  1. Feb 4, 2009 #1
    The topic of magnetic compass needles came up on another forum. This led me to get out some cheap compasses and start messing around with them.

    I lined up three compasses in a north-south row. I expected them to show magnetic north and they pretty much did. I also thought they would do about the same thing in an east-west row.

    When the compasses were an inch or so apart they didn't affect each other so much, but when I placed them almost touching the needles turned to the northwest. They vibrated about for a bit and when they settled down the outer compasses ended up pointing northwest, but the center compass needle was pointing due west. Why west?

    Any ideas on this?
  2. jcsd
  3. Feb 4, 2009 #2
    The needles want to line up co-linearly, to bring each one's north nearer to another one's south (since the Earth's field is weak in comparison). So in an east-west row, it is more stable for them to align either east or west rather than north. Which part specifically do you want to understand?
  4. Feb 5, 2009 #3
    Thanks for the reply. After about two hours all three needles pointed in the same direction WNW. I tried the experiment in four rooms in the house and the needles ended up pointing in a westerly direction in each case.

    I am curious about the "handiness" or preference of the needles to "choose" the west.
  5. Feb 5, 2009 #4
    please forgive me if i have not understood your question. As cesiumfrog said the earth magnet is very weak in this case actually what happens in your case is the compass (magnet) tries to align with the strangest magnetic field next to it(Another compass). you can make sure this by placing another compass close it. Now the compass direction will differ.
  6. Feb 5, 2009 #5
    Why does it always choose west rather than east? I thought it should have been random, but you might be forcing it to pick west if your east-west row is actually aligned just very slightly southeast-northwest or if there is a particular handedness in the way you bring the compasses together (as you slide each successive compass into position to the east of the preceding compasses, try making the new compass approach from the north-east rather than south-east)?
  7. Feb 5, 2009 #6


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    Gold Member

    I'm trying to figure out whether or not the direction of Magnetic North would bias the needles. Assuming you live in N.America, Magnetic North is well west of true north.

    How do you know you're pointing the compasses (compi?) Magnetic North to start?
  8. Feb 6, 2009 #7
    change your compass position alignment it will show different direction, if the magnetic compass has more field strength then the earth magnet. the effect on the each compass is sum of the compass and earth field strength.
    1. You can try this with strong magnet by hanging them in thread (actually it work as a compass when you tie them in a thread allow them to rotate freely).
    2.Or try with different compasses that is each compass should have different field strength.
  9. Apr 17, 2009 #8
    Thanks for the interest and replies;

    I finally got around to putting the demonstration on youtube.

    It seems to be more coincidental "accidental " than having any underlying significance attached to the change from magnetic east-west to geographic east west. It sent me on a mental "wild goose chase " for awhile, but I'm satisfied with your answers.

  10. Apr 17, 2009 #9


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    Staff: Mentor

    I'm not sure how to say this without sounding too condescending, but it would actually require some serious effort to not know which direction magnetic north was when setting the experiment up. Such as setting it up with your eyes closed and after spinning in place 10 times.

    The experiment is being done with magnetic compasses so the first one you place on the ground will tell you what direction magnetic north is, so you can use it as a guide to place the others.
  11. Apr 17, 2009 #10


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    Staff: Mentor

    No one addressed this part:
    You didn't ask so you may already understand it, but the reason they are not all pointing the same direction is the outer compasses have one magnet deflecting them away from north while the center compass has two.
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