A Compass Discussion

From: Richard Caldwell <rcaldwell@oklaosf.state.ok.us>

From: Bob Redding <bobredding@sprintmail.com>
> What happens at the equator, when it would seem that the
> north pole and south pole are equally distant?

The "N" end of the arrow... always points at the north magnetic pole of the Earth. Since the magnetic lines of flux are most parallel to the surface of the earth at the Equator, a magnetic compass works *best* there.

...Every magnet has a North magnetic pole and a South magnetic pole. They are so named because they match the polarity of the Earth's magnetic poles. The Earth can be though of as a huge bar magnet, with its ends at the 2 magnetic poles.

[Skip this if you didn't learn about magnetism in school! -Pat] Since like poles repel and unlike poles attract, the arrow in a compass is a small bar magnet with its South magnetic pole labeled "N", since that is the end that points at the Earth's North magnetic pole.

Except for localized effects, which I mentioned in my previous post, the little arrow will *always* align itself with the magnetic flux lines between the Earth's North magnetic pole and its South magnetic pole, and the "N" end of the arrow will *always* point at the North magnetic pole...
Richard C.

Bob <muskybob@sprint.ca>
...I have recently gone down to Austrailia and taken a hand compass with me just to see what would happen. I was kinda unsure what happened to the compass... Can anybody email me and explain why the compass doesn't read properly down there. I am in Canada and maybe it's a northern hemisphere compass. The needle just dug itself into the bottom and wouldn't read anything. Is the needle a magnetic strip with a north and south? and do the manufacture just replace the needle with a south and north? or am I way off... I am curious to find out what the difference is. Thanks in advance..

From: Richard Caldwell <rcaldwell@oklaosf.state.ok.us>
There is no difference in the needle for compasses used in the northern or southern hemisphere. Compasses work because the needle aligns itself with the magnetic lines of flux at the point in the world where you are standing. The magnetic north pole of the Earth attracts the "N" end of the needle, while the magnetic south pole attracts the "S" end, so there is no reversal effect.

However, if you are very close to one of the magnetic poles, the lines of flux are not parallel to the surface of the Earth. Instead, they angle downward toward the magnetic pole. This causes a large virtical dip in the needle. Very good compasses allow for this by allowing for a fairly large vertical play in the needle. But, the simple, flat compasses don't work well in this situation because the tip of the needle hits the surface of the compass.

Another thing that can cause a problem is local conditions. There are places on the surface of the Earth where fairly large iron deposits are under the ground. This will disturb the local shape of the Earth's magnetic field and cause the needle to be attacted to the underground deposits. Another cause of this effect can be man-made structures. I once tried to use a compass under a highway overpass, but the large amounts of steel in the I-beams that supported the overpass caused the needle to line up with them. I had to get away from the overpass before the compass would work correctly.

I hope this helps. Good Luck and Good Fishing! Richard

From: "Peter Moore" <mooreph@tig.com.au>
Hello Bob,
Correction.. you came UP to Australia :-)) Why compasses are different in the different hemispheres has to do with spherical geometry. In the Northern Hemisphere, a small counterweight is placed on the north seeking end [see below -Pat] of the compass needle such that the needle will be parallel to the earths surface and perpendicular to a line drawn from the centre of the earth to the position of the observer on the earths surface. In the Southern hemispere the converse applies and a counterweight is positioned on the south seeking end of the needle. That is why they only work in one hemisphere.
Peter...ex navy electonics engineer

From: "Steph"
If this is true, then the counterweight in the Northern hemisphere would have to be attached to the south seeking end of the compass, as the lines of flux show increased dip the nearer you are to a pole. And vice versa for a southern hemisphere compass.

From: "Peter Moore" <mooreph@tig.com.au>
Hello Steph,
Yes you are right I made an error, for the northern Hemispere the weight is placed on the south seeking end of the needle. The equilibriun of forces is such that the gravitational force on the needle is equal and opposite the magnetic force of attraction. It only becomes apparent in marine compasses as the whole compass dial is on a pendulum such that it always remains parallel to the earths surface while the boat moves about it in a seaway.

From: Richard Caldwell <rcaldwell@oklaosf.state.ok.us>
Thanks for that Info. I didn't know about the counter-weight. I do know that some better compasses use a floating ball, so that they don't have to worry about the "dip angle" of the Earth's magnetic field.

Another interesting tidbit is that these types of compasses actually measure the dip angle, because one can use that value to get a rough approximation of how far north/south you are. In other words, it gives a rough lattitude reading. Ship's captains used to compare this reading to their sextant sightings as a check.
Richard C.

From: Gary S. Colecchio<gsc@gate.net>
> the needle just dug itself into the bottom and wouldn't read anything.

That must be where the term "Down Under" came from.
or pehaps:
"What the flux?"
Capt. Gary S. Colecchio
West Palm Beach, Florida

"The beach? Only poor people go to the beach! The truly impoverished own boats." ? Capt. Gary to Ms. Dawn