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Re: Flat conductors (was I need help).

Brian W.hatcott wrote:

... but if you use a three and a half digit DVM as I suppose
many experimeters would, there seems to be no great
advantage in heating the resistance paper, and the possibility
that heating would tend to reduce the repeatability of such
experiments via ageing effects.

I agree on "no advantage." Why did I use 300 V? Because just
before this I tried to see if something like Hall effect can be
observed. It was silly, but being an experimentallist I tried.
A strip carbon paper with two silver leads was made and two
floating silver lines were painted above and below. I hooked
the milivoltmeter to these floating lines and immediately
noticed a clear non-zero reading. Excited that "even terrestial
magnetism does it" I brought a strong magnet but this had
no evvect on the mV, even at 300 V. My floating electroded
were not perfectly painted and they adjusted to different
potentials. That is what I was probably reading. A silly idea
but it was only a matter of 20 minutes or so. Then I started
fishing for equipotential points at the same 300 V.

Responding to this:

2) There is one question I keep asking but nobody answers.

In air, or on an acetate sheet, two small silver dots separated
by a distance much larger than their diameters, would produce
a field of a 3D dipole (except very close to circles). The electric
field lines would not be confined to a single 2D plane because
all meridian planes are identical. An electric current, of the
order of pA would be flowing, more or less along the E lines
in the 3D space.

If the resistance of the acetate sheet were progressively lowered
then more and more current would flow through the sheet and
less and less above and below it. Eventually we would have a
flat conductor carrying practically all the electric current. The
distribution of E is no longer like for a 3D dipole, it is like
that of a 2D dipole (two infinitely long cylinders perpendicular
to the 2D sheet). It looks like if the electric current, in process
of changing its path, also changed the configuration of E. The
current is saying to the field "I become 2D and you must do
the same." Why is it so?

Brian wrote:

Where the current is planar the electric voltage distribution in
the plane due to that current is predictable.
But in the surrounding insulator, there is still a field
("field" = force available to remotely move a charged object)

In this space, the field is not like the bipolar field due to two
isolated charged spots, and not like the field due to two charged
cylinders in 3 space. That's because in this air space, a charged
particle would see a planar potential gradient spreading from
one spot and converging on another which goes with a different
3D field distribution than any of the ones already depicted.

The only way I can rationalize this is to assume that static
surface charges associated with the steady state flow are
responsible for what really happens. How else can this be
explained? Static charges, by the way, were discussed here
recently in a thread whose name was "Chabay/Sherwood."
Ludwik Kowalskil

Static charges associated with a charge flow seems to me to
be an unhelpful or misleading mind picture in this case.
You can measure point voltages, both on the paper and
(at least conceptually) in the air to derive a picture of electric fields.

It is not immediately obvious. If static charges play a role in
circuits made from wires and compreesed resistors why would
they not play a role in flat resistors?
Ludwik Kowalski