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Re: non-potential voltage



At 10:02 AM 4/14/00 -0500, Clarence Bennett wrote:
>How about considering Voltage to be whatever is indicated by a Voltmeter?

That's the right basic idea, but there are a few devilish details:

1) That depends on the definition of "voltmeter", bringing some danger of
becoming a circular definition. Suppose we have something that is
MIS-labelled a voltmeter. Do we consider voltage to be "whatever" it measures?

2) In the case of a non-potential voltage, the voltmeter reading depends
not only on where you put the voltmeter's probe-tips, but on the exact
layout of the leads. If (without moving the probe-tips) you put a few
loops in one of the leads, you can change the voltmeter reading.

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My recommendation is to define voltage to be energy per unit charge (in the
limit of a very small test charge, to exclude image-charge
interactions). In the case of a non-potential voltage, you can't speak of
the voltage difference between point A and point B without specifying the
path by which the test charge moves from point A to point B.

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Random tangential remark: Understanding non-potential voltages is one of
the keys to understanding the _ground loops_ that plague many laboratory
measurements.