In this diagram, we find that B attracts C, A attracts D, A repels
C, and B repels D. This is not enough information to deduce our law. B and C
might be like poles, or they might be opposite. We need a third magnet: Return to my Physics pages
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© Copyright 1999, Jim Loy
Let's pretend that we do not know about the law that opposite poles attract and like poles repel. We experiment with two magnets. And we find that sometimes we can make the magnets repel, and sometimes we can make them attract.
In this diagram, we find that B attracts C, A attracts D, A repels
C, and B repels D. This is not enough information to deduce our law. B and C
might be like poles, or they might be opposite. We need a third magnet:
Now we see that B and D act identically. They are like poles, and
they repel each other. A little more experimentation clarifies the law:
opposite poles attract and like poles repel.
Our
third magnet could have been the earth's magnetic field. We could have
experimented by using our magnets as compasses and seeing which end points
north. We can deduce the same law in that way. Traditionally, the north end of
a magnet is the end that points north. Since it is attracted to the earth's
"north magnetic pole," we can deduce that the earth's "north
magnetic pole" is the SOUTH end of a huge magnet. Of course, there
is no metal bar running through the earth. The earth itself is the magnet. And
its north magnetic pole is a south pole. Does that sound confusing?
The "north magnetic pole" is in Greenland, and moves around somewhat, from day to day. The magnetic poles swap, about every 250,000 years. The last reversal was about 700,000 years ago. So we are overdue for one. Such a reversal takes about a thousand years.