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There is an area where I live that prevents my compass from giving a solid direction; instead it just spins.

Does this mean that this area lacks a magnetic field, or has to many of them for my compass to get a reading? (Yes. My compass otherwise works correctly everywhere else.)

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    $\begingroup$ It means that you have an AC magnetic field somewhere from a power line, transformer, machine etc.. $\endgroup$ – CuriousOne May 16 '16 at 1:51
  • $\begingroup$ Yeah, I agree with Curious one, presence of any source that creates electromagnetic radiation will disturb the compass. $\endgroup$ – UKH May 16 '16 at 3:47
  • $\begingroup$ Long-term effects under a heavy magnetic field like that can be harmful. Is it only your building or is it a larger area? $\endgroup$ – Neil May 16 '16 at 7:55
  • $\begingroup$ The explanation of @CuriousOne is possible, but it's not conclusively the correct explanation. When you say it "just spins" do you mean that it spins constantly in one direction, or that it wanders first one way, then the other ... $\endgroup$ – garyp Sep 3 '16 at 15:44
  • $\begingroup$ Spins? Indefinitely? How fast? Any compass worth carrying will be damped enough that it won't spin even one whole revolution unless something is spinning it. $\endgroup$ – Solomon Slow May 16 '17 at 15:09
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There are at least two spots on Earth where the magnetic field of the planet is not horizontal, but is vertical. Other than those (the magnetic poles), one expects a compass to take on a stable directional reading, unless affected by local (not planetary) magnetic field sources.

Large direct currents, such as the Pacific Direct Current Intertie see map might have such an effect over significant distances.

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  • $\begingroup$ While this is true for a sphere centered on a mathematical dipole, it may not be true for the Earth's field. Actually I'm not sure, and so I've asked this follow-up question. $\endgroup$ – uhoh Jul 20 '17 at 6:09
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A compass needle will align itself with external magnetic fields. If the external field is static then the compass needle will remain stationary. If the external field is in motion, changing direction, the compass needle will follow accordingly.

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One thing's for sure: your compass reading showed an unusual magnetic activity. To know which are the causes you should take your compass (or even better, a set of them) on walks or rides around your place and map the magnetic field lines. Maybe there is some current in the earth which is close by in comparison to other places, maybe it's a persistent solar wind (which I can't really imagine), or maybe there are man-made structures which cause this strange behavior.

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