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Q & A: Forces on a magnetic compass

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Most recent answer: 11/14/2012
Q:
science question: for a compass to work accurately the arrow needs to be able to spin freely (so friction needs to be small) and also protected from air currents. Explain why this is so. i am so confused
- alexia Salem (age 11)
london
A:
Hi Alexia
This is a nice question and it shows how difficult it can be to accurately measure very small forces and their effects.   A magnetic compass is really a magnetized needle mounted on a low friction suspension.  The needle wants to align itself with any external magnetic field, such as that of the earth.   Now this alignment force is very very small so any extra addition force will tend to make the needle jump around.   Air currents are but one of these unwanted forces.   That's why a good compass will be enclosed in a air tight container with a glass top in order to prevent this effect.

LeeH

(published on 11/14/2012)

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