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Q & A: Small magnetic field and compasses

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Most recent answer: 10/22/2007
Q:
if the earths magnetic field is very weak at the surface, how come a compass can face north?
- josh (age 12)
MD
A:
The earth's field is about half a Gauss or so at the surface. This is enough to pull a compass needle around. Of course the actual torque on a compass needle depends on how magnetized the needle is too. Most compass needles are very light and are mounted on bearings or nails with very little friction so they can respond to even weak magnetic fields.

The weakness of the Earth's field makes it easy to disrupt a compass. Put a magnet near the compass and your compass will easily point in a direction that's not north. Compasses are not reliable when there is a large amount of iron-containing metal around. This is an important detail, as many instances where people would like to know which way is north are instances where lots of iron or steel is around -- people in ships or cars or trucks, for example.

For this reason, and also for the reason that the magnetic north pole isn't located quite at the geographic north pole (and moves around!), modern navigation systems use gyroscopes which always point in the same direction in space (they even continue to do this as the earth turns underneath).

Tom

(published on 10/22/2007)

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