Re: Flat conductors (was I need help).

[kowalskil <kowalskil@MAIL.MONTCLAIR.EDU>]

"John S. Denker" wrote:

> First of all, to keep things
> from getting unreasonably complicated, I assumed that the
> resistor paper was lying a distance L above a conducting
> "ground plane".  This gives me a definite capacitance per
> unit area:
>
>         rrrrrrrrrrrrrrrrrrrrrrrrr   (resistor paper)
>          |
>         (L)
>          |
>         ggggggggggggggggggggggggg   (conducting ground plane)

Yes, the metallic plate was 48 mm from my paper. Can this
influence locations of equipotential points? I do not think so.
But who knows, devil has his own rules. So I put the Pasco
stand on a wooden table and checked it. What do you think
happened? The C line (at 187 V) did shift a little near the
margin. How can it be? I put the Pasco setup back to the
metallic table (48 mm above it) and traced the line again. It
was exactly the same as it was on the wooden table. I also
did it with 30 volts (instead of 300 volts) and found no
effect of voltage. The 18.7 V line was exactly the same as
the 187 V line.

So why did the line shift with respect to what it was
yesterday? This brings me to the question JohnM asked
last night: what are the diameters of circles. The diameters
are small; about 5 mm. But this does not count, I think.
What counts is the 8 mm diameter "washer" sitting
on top of each silver dot. The "washer" is actually the
eye lead (1 mm thick) of the wire supplied by Pasco; wires
are used to apply the DOP to the silver circles. The entire
structure consists of the horizontal cork plate, the carbon
paper and isolated wires connected to silver dots as
described  above. It is not an ideal setup but that it is.

It turns out that the exact location of the 187 V
equipotential line near the margins (but not near the axis)
is quite sensitive to the location of the eye lead. Moving it
in the direction of another circle by as little as 1 mm moved
the line endings in the same direction by about 1.5 cm. I was
not aware of this. In order to measure the diameters of silver
spots a I had to remove the washer and put it back again,
apparently not exactly in the same position. That is why the
line shifted. I moved the washer by 1 mm and the line
became as it was (very nearly so). Yes, the devil is always
present but we can often avoid his traps.
Ludwik Kowalski