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Re: non-potential voltage



This very experiment, and other aspects of this thread, are treated in:

Am. J. Phys., Vol. 50, No. 12, December 1982 Pages 1089 - 1093
What do ``voltmeters'' measure?: Faraday's law in a multiply connected
region
Robert H. Romer

Bob

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor

----- Original Message -----
From: "Leigh Palmer" <palmer@SFU.CA>
To: <PHYS-L@lists.nau.edu>
Sent: Friday, April 14, 2000 11:43 AM
Subject: Re: non-potential voltage


How about considering Voltage to be whatever is indicated by a
Voltmeter?

No way, Clarence! Read on...

I perform a lovely demonstration which clearly demonstrates that
the reading on a voltmeter is *not* unique. This seems to be the
best way to free students of the misimpression that an induced
emf is a potential difference.

I take two resistors (say a 1 kohm and a 2.2 kohm) and solder their
pigtails together. (At this point I hesitate to say "in parallel"
or "in series" - that idea is undefined in this case.) I take the
combination, spread the resistors apart so I can slip the pair over
the yoke of a dissectable transformer, and close the transformer up
again. Now take two voltmeters. Connect one across the 1 kohm
resistor. Connect the other voltmeter across the 2.2 kohm resistor.
If you do this in the most straightforward manner you will find the
voltmeters read the same value, 0 AC volts. Now turn on the primary
of the transformer. A current will flow in the (now series) pair of
resistors, say 1 mA flows. One meter will read 1 VAC and the other
will read 2.2 VAC, as predicted by one of the sacred Laws of Ohm.
But hold on a minute. Those two voltmeters are connected to the
same places! How can they have different readings?

Of course the answer is simple, and the students seem to grasp it
well enough.

Leigh