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A one magnet method...



Those ultimately cheap compasses are easily remagnetized in the opposite
direction with two magnets of very modest strength. Bring either pole of
one magnet up to the case so that one end of the compass needle is
strongly attracted to it. Now bring *the same pole* of *another* magnet up
to the other end of the compass needle. (You should find that the needle
stays in its original orientation.) Remove the first magnet and, finally,
remove the second magnet. The compass needle will now be magnetized in
the opposite direction.

You can even do this with one magnet if you kind of sneak up on the
compass needle from below, trapping it frictionally in the case while
maneuvering the magnet so that its south (or north) pole is next to the
needle's south (or north) pole. In fact, this is so easy to do
accidentally that I suspect it is the primary reason for the commonly
observed lack of any statistically significant correlation between the
needle markings and the directions in which they point.

John
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A. John Mallinckrodt http://www.intranet.csupomona.edu/~ajm

I know that the normal compasses the we get for lab and even most you buy
in the sporting goods stores are easily reversed. My student with bar and
ceramic magnets get them reverse magnetized all the time. The are easily
corrected. One can even do it with one magnet; a slight simplification
(maybe) over John's method above. Just bring a magnet up to the edge of
the reversed compass and let whatever end that is attracted point to it.
Now all you have to do is make one slow swipe of that same end of the
magnet across the compass body directly along the length of the needle and
then follow through with that end of the magnet off the trailing end of the
needle. When you are finished the needle's magnetization will be reversed.

Dewey

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Dewey I. Dykstra, Jr. Phone: (208)385-3105
Professor of Physics Dept: (208)385-3775
Department of Physics/SN318 Fax: (208)385-4330
Boise State University dykstrad@varney.idbsu.edu
1910 University Drive Boise Highlanders
Boise, ID 83725-1570 novice piper

"Physical concepts are the free creations of the human mind and
are not, however it may seem, uniquely determined by the external
world."--A. Einstein in The Evolution of Physics with L. Infeld,
1938

"Don't mistake your watermelon for the universe." --K. Amdahl in
There Are No Electrons, 1991.
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