Re: Earth's Magnetic Field

[Maurice Barnhill <mvb@UDEL.EDU>]

Their explanation seems to be a long-winded way of saying that they have
deliberately labelled their magnetizer so that it makes mislabelled
magnets.  Sheesh!

The whole business is easy if you recognize that the magnetic pole of
the Earth which is located in the northern part of the planet is a
*south*-seeking pole. It therefore attracts the opposite poles of
compasses, the north-seeking poles.  This way the north poles of
compasses point roughly north and navigators go in the right direction.

The convention with respect to the direction of flux, out of
north-seeking poles and into south-seeking poles, also works with the
Earth's poles; *provided that you recognize that the south-seeking pole
is the one in Canada.*  In the northern (magnetic) hemisphere, the
Earth's magnetic field point northward and *down*.

Larry Smith wrote:
> Seems like this topic has come up before, but I just received a shipment of
> new equipment for our lab and read the following in the instructions for
> the Electro-Technic Magnetizer:
>
> -------begin quote------
> The convention for marking polarity for compass needles is to mark the pole
> seeking the Earth's north magnetic pole with a "N".  That way sailors are
> not confused when directed to steer a course.  The convention for marking
> bar magnets is such that a compass needle will point in the direction of
> the lines of flux as though they were traveling from the "N" pole to the
> "S" pole.  This places the "N" at the end of the bar that if suspended as a
> compass needle will point north.  Relative to the Earth's magnet field, a
> Nort seeking pole has to be the south pole of the bar.  Opposite poles
> attract.  Even so, the convention is to makr it "N".
>
> The Model 28000 Magnetizer is marked with a "N" on the right side and a "S"
> on the left.  When operated, the magnet produced causes a compass needle's
> north seeking pole to point to the end placed into the "N" cavity.  A
> magnet produced by the magnetizer if suspended by a string woudl point the
> end marked "N" to the south which is exactly opposite to the convention.
>
> The logic for doing it this way is that most customers are unaware of the
> convention, and in the beginning one of our distributors complained when a
> magnet thus produced caused a compass needle's "N" pole to be attracted to
> the "S" pole of the magnet.  We did some testing and discovered that most
> inexpensive magnets are made with the polarity reversed.  So
> Electro-Technic bowed to the popular perception even thought it is counter
> to the convention.  IN all the years and thousands of magnetizers sold, the
> number of complaints about polarity have been very few.
>
> For some, this may pose a problem.  If you have inadvertently magnetized
> your magnet backwards, and have been inconvenienced by this problem, we
> apologize.  If you want to reverse it, simply magnetize it correctly by
> placing the magnet pole marked "N" into the "S" cavity of the magnetizer
> and the magnet produced will be configured in accordance with the
> convention.
> ---------end quote-------
>
> If I'm even understanding this clearly, it seems to be backward to the way
> H&R and Serway have it.  True, a convention is just a convention, and we
> could have done it the other way; so I don't care which way we decide to do
> it, as long as we can all agree together.  What is the vote on this list
> for the convention?
>
> Thanks,
> Larry Smith

--
Maurice Barnhill, mvb@udel.edu
http://www.physics.udel.edu/~barnhill/
Physics Dept., University of Delaware, Newark, DE 19716