Try this ...

http://farside.ph.utexas.edu/teachin...es/node94.html

This is one of a series of lectures by a prof at Texas Uni.

In fact, if you go right back to the home page of
http://farside.ph.utexas.edu/teaching,
this leads to a most excellent revision of the necessary EM theories, and,
briefly
glancing thereto, the post grad stuff even exceeds my current interest and
knowledge.

I'm fairly sure now that this area is where I came across the governing
formula that I
alluded to recently in this NG when doing my own revision previously in
2005, although
the URLs and lecture node numbers have changed since then.