Re: mapping out equipotentials

[David Bowman <David_Bowman@GEORGETOWNCOLLEGE.EDU>]

Regarding Carl Mungan's question:
> ...
> The question is: WHY DO YOU FIND THIS? A couple of us have been
> arguing about this.
> ...
> Response: Well, that's what the experiment seems to say. Some people
> suggest the water somehow "confines" the field lines (like magnetic
> field lines through a transformer core?) to make this so. If this is
> true, please explain why water does this.

It's not so much that the water "confines" the field lines as it makes
them *rigid* against the effects of the intrusion of the voltmeter leads.
The field lines in the air can't be mapped with a voltmeter because the
impedence of the voltmeter is so low compared to any appreciable length
of air that both leads of the voltmeter act as a shorted conducting
equipotential region compared to the air.  The mere *presence* of the
voltmeter and its leads reconfigures that field pattern in a way that
makes both leads of the voltmeter have the same potential, so it
tends to read nearly zero for the potential difference even when one lead
is inserted into a region of (formerly before insertion) high potential.
Also the capacitance of the voltmeter's configuration of leads tends to
mess up the field pattern as well.  The most important rule in using a
voltmeter is to make sure that its internal impedence is *large* compared
to the characteristic impedences of the circuit for which it is to
measure.  In the case of air this is quite difficult with conventional
voltmeters.  The conducting water with its ions lowers the impedence of
the medium enough so that the voltmeter will not substantially perturb
the field pattern present in the water when the meter leads are
introduced.

David Bowman
David_Bowman@georgetowncollege.edu