You have two identical looking metal bars. One of them is a magnet, with North and South ends. The other is simply an iron bar (which is attracted by magnets).

How can you tell which is which, using only these two bars?

First method: Hold one bar perfectly vertical, put the other on top, prefectly horizontal, if it starts moving and pointing north-south, the magnet is the horizontal one.
Second method: Rub one bar over another, this way heating it up.  If the temperature of one bar becomes above the Curie temperature of iron, test if there is still magnetic attraction, if not, the hot one is the magnet.  This method may take a while. 
Third method: Throw them high up in the air, perfectly vertical. The one that turns teh most to the horizontal direction is the magnet, being influenced by the Earths magnetic field 

This reminds me of how many ways there are to measure the height of a building using a barometer.

Posted by Hugo on 2005-01-26 20:42:39