June 7, 1998: In Final Problem 5, the planes containing the sheets of charge and current should be at y = +d and y=-d (not z!)

Recap of email message of May 26:

 
One person asks:
@
@ I'm going over the final, problem number 2, and I have to admit, I'm doing
@ algebra but I don't really understand what a duality transformation is, or
@ why one would wish to do it.  Any clues?     

   The duality transformation transforms magnetic into electric and vice
versa.  It illustrates something about the existence or not of magnetic
charge - perhaps all electrically charged particles are also magnetically
charged?  That is the general idea.  Take it and run with it!

                        Roger B.

Recap of email message of May 21:

Hi, Everybody,

   I wonder how you are all coming along on the midterm.  Are you making
progress on NP 5.04?  On that problem, once you set up the program and run
it, you see that the field near the center is not constant enough.  In
general in this situation with a magnet, the field tends to get weaker
near the edges.  You need to correct this by shaping the pole pieces.
This is generally referred to as shimming the magnet.

   To correct the inhomogeneity you get with square pole faces, you need
to add some material near the outside of the pole pieces.  This tends to
make the field stronger there, by reducing the gap.  This compensates for
the field initially being stronger at the center.

   Here is an example:

			Square pole pieces

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   And here is an example of shimmed pole pieces
	

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To accomplish this in the program, you need to change the array which
defines which points on the x-y grid are re-calculated in the iteration
and which are held constant.  You should try this first with the 11x11
array of the starting program.  But it will be necessary to increase the
size of the arrays to get the fine control necessary to get a very uniform
field.

   If you need any further discussion, call me or email me.  

   I will be in today (Friday) from about 3:00 for a couple of hours.
Look me up then if you weant to discuss any part of the midterm.

			Good Luck!

				Roger B.