Re: south in the north

["Donald E. Simanek" <dsimanek@eagle.lhup.edu>]

On Mon, 4 May 1998, Herbert H. Gottlieb wrote:

> > Joe Taylor is correct. The magnetic pole near the Earth's North pole 
> > is  South Magnetic pole.
>
> Joe IS N OT CORRECT. The magnetic pole near the Earth's North 
> Pole is a NORTH MAGNETIC POLE.  
>
> Herb Gottlieb from New York City
> (Where the Earth's north magnetic pole is close to the
> earth's north pole)

We always suspected there was something strange about New York City.

I side with Joe on this one. Of course, one could *define* it either way,
but the relation of magnetic field to current-carrying wires is defined in
one and only one way in all textbooks at all levels, just as is the
convention for currents (which many students wish had been done
differently). Therefore, we can check our compass' response to current
carrying wires, in coils, etc. and determine the direction of the magnetic
field "lines" from the magnet, using the standard textbook conventions. 
Thus we find which end of the compass is the magnetic North end. Then we
wee which way that end points.  It points to geographic North. This
indicates that geographic North of the earth is Magnetic South of the
Earth.

One of my favorite demos is to check this, starting from scratch. I use a
large lecture galvanometer. Of course we don't assume anything about the
galvanometer, so we check which deflection is positive and which negative
by use of a chemical 1.5v cell. It's quite difficult (impossible?) to
reverse the polarity of a chemical cell without drastically tampering with
it internally (remanufacturing it), or modifying the laws of
electrochemistry. So we can trust that if the cell has a terminal
potential of 1.5v, it's polarity is also what it should be. Now we know
which deflection corresponds to a particular direction of conventional
current. So then I put a loop of wire across the terminals and move that
wire through the poles of a strong (magnetron) magnet. The students must
predict which direction the galvanometer will deflect if the wire is
thrust in a certain way between the poles if pole X is N. Thus we get them
to think through the vector relations between v, B, and I, before doing
the experiment, and also we find out which pole of our magnet it N. Then I
take a small compass and see which end of it is attracted to the N pole of
the large magnet. Thus we have found out which pole of the compass is N.
Then we move away from all local sources of magnetic field and see which
geographic direction the N pole of the magnet points to. If no one has
tampered with the compass, one end of the needle has some marking to
indicate it is the "north geographic pole seeking" end, and that will be
its magnetic North pole.

Historically, that's why it was named the "North" end of the compass
needle. Only much later did we understand the physics behind it.

In all of this we have taken nothing for granted except the equations and
current and field conventions of the textbook, and the polarity of our
chemical cell. We did not assume we knew the polarity of the large magnet,
the compass, or the earth, nor the polarity of the terminals of the
galvanometer.

Of course the earth has no internal bar magnet, and no "pole" near the
arctic and antarctic regions, for the shape of the field is more like that
of a dipole. However, using the standard convention, the field lines
coming up and out of the earth in the Southern hemisphere have their
arrowheads outwardly directed.

When I took Freshman physics, Prof. John Eldridge used his own textbook. 
He told us that in the first printing someone at the printery looked at
his diagram of the Earth with a magnet inside and N and S labeled, and saw
that S was at the top. Thinking this was wrong, they turned it upside down
so N was at the top. Now it happens that N and S are letters which look
nearly the same inverted. No one noticed until the copies were printed. 
Careful examination of the Roman capitals' serifs showed that the N and S
were upside-down. Later printings corrected the error. I have copies of
both printings.

This leads to an interesting joke. Print a label for a package, using
Swiss font, saying "up (arrow up)  dn (arrow down)" Use lower case for
"up" and "dn", and use actual arrows where I've indicated. Put it on the
package, and whichever way they sit the package, it will read as if it is
correct. The abbreviation "up" inverts to "dn" and vice-versa with
non-serif fonts. 

	-- Donald

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Dr. Donald E. Simanek                            Office: 717-893-2079
Professor of Physics                                FAX: 717-893-2048
Lock Haven University of Pennsylvania, Lock Haven, PA. 17745
dsimanek@eagle.lhup.edu                 http://www.lhup.edu/~dsimanek
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