Re: How can I tell which end of a magnet is positive and which is negative?

	To explain why some things are magnetic and others aren't requires 
understanding of two major concepts:  electrons in atoms, and domains.  

Electrons of one atom may have a net magnetic spin, or moment, which depends 
on the angular momentum.  Between atoms with moments, the spins interact in 
two ways in a magnetic field:

1)  the spins can align parallel to each other; this is called a 
    ferromagnetic interaction

2)  the spins can align antiparallel to each other; this is call an 
    antiferromagnetic interaction.

Below a temperature called the Curie temperature, the magnetic moments in a 
ferromagnetic solid align with each other.  Magnets that we use every day have 
very high Curie temperatures, like 1000 Celsius, so at room temperature are in 
their ferromagnetic state.

Ferromagnetic substances, like iron, in addition to having ferromagnetic 
interactions between atoms in a magnetic field, form domains.  These are 
collections of atoms (like millions) which spontaneously align even in the 
absence of a magnetic field.  When a magnet loses its magnetism, these domains 
still exist; they are all just randomly oriented, so that the overall effect 
is nonmagnetic.  The domains, when heated above its Curie temperature in a 
magnetic field, will again realign.  When a magnet is working correctly, the 
direction of alignment determines the two poles, which can be north and south 
(analogy to the earth's magnetic field) or, as you suggest, plus and minus.

So now for your pole determination.  Take two of your "known" magnets, and 
first bring together the two + poles.  Observe how it behaves (they should 
repel, if they are labeled correctly).  Then bring together + and -.  These 
should attract.  By using a "known" magnet, you can then correctly label your 
"unknown" magnets.

A fun and related experiment is to make your own compass using a bowl, water, 
and a magnet.  As mentioned, the earth has its own magnetic field which is 
utilized greatly with navigational tools.  Fill the bowl partially with water, 
then float the magnet on top.  The south pole should be facing north (it seems 
strange to me that this is the case, but this is what I read).  Enjoy!