Re: What happens if you are standing ON the magnetic north pole with a compass?

The earth's magnetic field can be viewed as if the earth were a very big 
bar magnet (this is a simplification, but one that works fine for most 
purposes).  Magnets produce forces on other magnets and on magnetic 
materials.  Magnets also have directions and every magnet has a north 
pole and a south pole.  You can see this in action if you get a couple of 
refrigerator magnets and play with them.  The forces produced will tend to 
attract magnetic materials to the magnet and will tend to both attract and 
repel other magnets in such a way that the magnets end up stuck together 
north pole touching south pole
   ___________  ___________
  |_S_______N_||_S_______N_|

So far, so simple, however magnets have three dimensional effects.  The 
forces are most often described by a "field".  This field is typically 
shown by lines that loop outward from the poles.  Again, to oversimplify a 
little, these lines lie along the directions in which the magnetic force is 
acting.  On the earth, the field loops run from south magnetic pole to 
north magnetic pole (more about poles in a bit).  You can see a nice 
diagram of the field around a bar magnet and the earth at:
 http://liftoff.msfc.nasa.gov/academy/space/mag_field.html

Near the equator (which has no significance for the magnetic field, it's 
just a marker halfway between the geographic poles) the field lines run 
north-south and are essentially parallel to the surface.  By the time you 
reach the mid lattitudes, the lines still run north-south, but they are now 
at a significant angle to the surface.  This angle is the magnetic 
inclination. Approaching the poles, the field lines get even steeper, until 
they are vertical at the pole itself.  Looking from the side:


  -- --     \  \          | |
 _______    _\__\__     __|_|___
 equator  mid-latitude   at pole

The compass needle works because it's a small magnet in a big field and it 
is pushed by the forces between the earth and itself until it lines up with 
the earth's field lines.  However, as you point out, the way we use a 
compass only uses two of the three dimensions of the field.  If your 
compass needle were free to move in any direction, it would point north end 
down toward the ground as well as toward north.  In fact, you can 
soemtimes see this downward deflection and some sophisticated compasses 
have adjustable weights on the south end of the needle to compensate for 
it and keep the needle level so that it swings freely.  There are also 
compasses designed to swing only in a vertical plane that are specifically 
used to measure the inclination.  These are called "dip needles" since they 
measure the dip, or downward angle, of the field.  If you take a standard 
compass to the magnetic pole, then the needle, which swings in a horizontal 
plane is trying to align itself with a vertical field.  The result is that 
the alignment of the needle is meaningless.  In practical terms, the needle 
will probably swing wildly in response to any slight movements of the 
compass.  If you want to know when you have found the magnetic pole, a 
compass is the worng instrument, what you need is a dip needle.  When it 
goes completely vertical, you know you are there.

A compass normally responds to earth's field, which is not very strong (if 
it were strong, you would have a hard time moving your fridge magnets 
around).  Any magnets or magnetic materials will cause changes in that 
field or will completely overwhelm it, which is why compasses do not work 
well when held too near to steel belt buckles, Boy Scout kerchief holders, 
car bodies, or even rocks with magnetic minerals in them.  If you wanted to 
see how a compass would behave at the north pole, you could get a good 
strong bar magnet and hold it vertically, then place your compass directly 
over the north pole end of the magnet.

You also asked about the magnetic and geographic poles.  They are 
different.  The geographic pole is defined by the spinning of the earth and 
it is in the same spot all the time.  This pole is the one that sits 
directly below Polaris, the north star.  This is usually what people mean 
when they talk about the earth's poles.  The magnetic north pole is defined 
by the earth's magnetic field, which is in turn defined by swirling 
currents within the liguid iron outer core.  The magnetic pole is roughly 
aligned with the geographic pole, but wobbles around it, getting up to 
about 10 degrees away.  This variation is what produces the magnetic 
declination, the difference between true north (geographic) and magnetic 
north.  Declination varies from place to place and year to year as the 
magnetic pole wobbles.  If you look at the positions of the pole over tens 
to hundreds of thousands of years, then the _average_ magnetic pole will be 
the same as the geographic pole.

All of what I have said about the north pole is true also of the south 
pole, except there the north end of your compass needle would want to point 
straight up instead of straight down.

A google.com search on the words magnetic field turned up the following, 
where you can find more information:
 http://lep694.gsfc.nasa.gov/lepedu/lepedu_hp.html
 http://csep10.phys.utk.edu/astr161/lect/earth/magnetic.html

I hope this helps.

David Smith, Ph.D.
Geology, Environmental Science, and Physics, La Salle University, 
Philadelphia