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Re: Flat conductors (was I need help).



I forgot to mention that one of Pasco's CSRs (customer service representative)
suggested that one should paint the periphery and ground it. Ground it? Earth
ground. My question about the relation ship of the power supply to any ground
confused him -- I thought it maybe should be the mid point of the P/S. Now I
realize it should be earth ground and the P/S is isolated. They, and I, use 9
V Le Clanché.

My measurements indicate that the paper is uniformly impregnated:

The resistance of a strip is the same either side (very ~ 74 X 6.5 mm). ~ 90
k ohm between Ag. spots. Painting duplicate spots on the other side: no change
and the same either side.

Scraping the paper with a sharp knife increases the resistance (duhhh) --
difficult to do uniformly, but when the resistance had doubled its thickness was
~ half. The paper is very fibrous (The name of the mfgng. co.) The stuff
scraped is rather long fibers and black powder. Perhaps it's a fiber mat (like
felt) then impregnated with carbon in a glue. Remember its resistivity is
several orders of magnitude > "pure" graphite.

bc

P.s. The resistance between slightly < 3 X 3 cm Ag squares (on a 3 X 3 square)
is ~ seven ohms. In this case the resistance of the Ag. complicates, as it is ~
2 ohms (probes over 5 mm separation.)

kowalskil wrote:

Larry Woolf wrote:

I do a number of experiments with graphite lines on a piece of paper and
have looked at the lines using a scanning electron microscope. They are
certainly of non-uniform thickness, consisting of many touching flakes. I
agree with David that there is likely significant junction resistance
between carbon coated material, as well as between the metallic electrode
and the carbon material. So the measured resistance would be expected to be
much higher than expected from the carbon material resistivity, cross
sectional area, and length.
To complicate issues further, don't forget that graphites and carbons are
anisotropic, with conductivities much higher in the graphitic planes
compared to perpendicular to the planes.

Interesting. But all this should not prevent us from trying to
explain the observed patterns of equipotential lines quantitatively.
It differs from what was expected for a vary large sheet of paper.

1) Is it a "pure paper size" effect?
2) Is it a "pure static surface charges" effect?
3) Is it something else?
4) Are #1 and #2 unseparable?

My guess is that #4 is correct. Even an infinite sheet must have
surface charges to bend electric field lines in a flat conductor.
Ludwik Kowalski.