Re: south in the north

["Prof. John P. Ertel (wizard)" <jpe@nadn.navy.mil>]

On Sun, 3 May 1998, William Beaty wrote:
> Here is last week's ASK MARILYN column in the sunday paper Parade section. 
> I've not complained as yet. The email address is marilyn@parade.com
>
> > WHY DOES THE NORTH END OF A COMPASS NEEDLE ALWAYS POINT NORTH?  IF LIKE
> > POLES REPEL (AND UNLIKE POLES ATTRACT), SHOULDN'T THE NORTH END POINT
> > SOUTH INSTEAD?
> >
> > The north pole of a compass needle DOES point south.  It's the
> > "north-SEEKING" pole that points to the Earth's magnetic north.  So the
> > end of the compass needle marked N (for north) is actually the south
> > pole of the compass needle.  This is supposed to avoid confusion!

Sorry that I have been delayed in answering this one, but we are preparing
a final and I was up against the stops.  Bill Beaty is a much more
forgiving person than I am, I guess.  To me, this kind of "Bovine
Scatology" (as Stormin' Norman so elegantly put it during Desert Storm) is
exactly what makes all our jobs as teachers that much harder.  I find it
presumptuous, in the extreme, for someone to make such statements without
at least checking with someone in physics.  A little humility here would
help this lady immensely.

When it comes to science, I simple can't afford an ego.  I'd rather have
someone else's correct information than any of my best musings (no matter
how carefully I think I have thought them out).  Accordingly, just in case
it was me that had it all backwards, I checked with a number of
colleagues, including a very pleasant gentleman in the "Magnetic Effects"
branch of NIST (formerly NBS, and I still can't get used to the name
change).  The accepted standard is that the "north pole" of a compass
needle is defined to be at its North Magnetic Pole and marked as such;
i.e., this is "the pole from which lines of magnetic flux emanate" (boy,
those folks at NIST sure know how to talk).  From this, it must follow
that the North Geographic Pole of our earth is a South Magnetic Pole.*

Of course, Marilyn may have been trying to point out that the names
"north" and "south" are interchangeable as they are purely a convention.
However, the convention has already been well adopted!  The "north named
pole" or a magnetic body (such as a compass needle, a bar magnet, or a
current carrying solenoid) is the one which to an external observer
appears to be a "source" of lines of magnetic induction; while the "south
named pole" is the one which appears to be a "sink".  If we look also at
the internals of the magnetic body, we see that the field lines are
continuous.


* Actually, as every Boy Scout knows, "the best average location of the
maximum influx of magnetic field" on our earth (read: our North Magnetic
Pole) is not exactly at our North Geographic Pole but rather some distance
south.  This fact together with local variations is what leads to the
"declination" and "variation" corrections which may be necessary
(depending on the accuracy required) to properly run a navigation or
orienteering problem.

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