Re: Electric Field Mapping

["John S. Denker" <jsd@MONMOUTH.COM>]

Tina Fanetti wrote:

> Okay so my students were given a point charge with a guard ring
> around it. It just wouldn't seem to work.

I don't think "guard ring" is the best
terminology.  For the usual elementary
experiment, all you've got is
 -- electrode "A" (point-like), and
 -- electrode "B" (ring-like).

Properly speaking, a guard ring is a third
electrode, used e.g. in delicate capacitance
measurements.

> One problem was that the potential difference on the guard ring
> wasn't constant.

What is the ring made of?
If it's metal, and not super-super thin,
it should be an equipotential.

The whole point of the elementary experiment
is to have both electrodes ("A" and "B") be
equipotentials.  So make them out of something
(e.g. metal) that will be an equipotential.