### Re: Why are each of my compass different in magnetic north.

Date: Mon Jan 6 13:38:12 2003
Posted By: John Moulder, Faculty, Radiation Biology, Medical College of Wisconsin
Area of science: Earth Sciences
ID: 1041602179.Es
Message:

Compasses

"Why are each of my compass different in magnetic north? I put them a part away from all metal yet each point to a different north pole, why is this?"

If the compasses are in the same place they should point the same direction. They should point to magnetic north, which is not quite the same as true north.

On the earth's surface, a compass points to the magnetic North Pole rather than the geographic North Pole. The angular difference between these two directions is called magnetic declination. Where I live the difference is only a few degrees, but in Seattle Washington it is 20 degrees.

I can only think of a few things that could make a compass point the wrong way:

• A bad pivot/bearing so that the needle has trouble adjusting itself.
• Some part of the compass contains iron or steel (but a good compass does not).
• Some part of the compass has become magnetized.
• The compasses is tilted (that is, not horizontal with the ground).

I would do some experiments:

• Find out which, if any, of the compasses is correct. And for that you will need to look up the magnetic declination. The source I use is: National Geophysical Data Center (http://www.ngdc.noaa.gov/seg/potfld/geomag.shtml) .
• Find out what the errors are on the other compasses; then see if they are wrong by the same amount when they are in different places or when they are rotated (in a horizontal plane).
• If the problem is a faulty bearing or a tilt, then the errors should be variable.
• If the compass contains iron or steel, then the error may be more-or- less constant.
• If the compass is magnetized or the pivot is frozen, then the needle will probably point to the same part of the compass no matter how you rotate it.

John Moulder
Medical College of Wisconsin

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