🚀 go-pugleaf

On Fri, 2 Jun 1995, Steve Luzader wrote:

[snip]
>    So you're right--the field inside the dome is essentially
> zero, but the potential isn't zero unless you have defined the
> dome itself to be at zero potential.  If you adopt the zero
> of potential to be at infinity, then the region inside the
> dome is at a large potential.  (Incidentally, I sort of recall
> reading somewhere that van de Graafs usually end up putting
> excess electrons on the dome, so the potential is actually
> negative.)

Have any of you tried running the device with the dome grounded to a
nearby water pipe? Predict what would happen before doing it.

Someone asked privately what's the sign of the charge on the dome, saying
their departmement members were having some disupte on the matter.
Reminds me of the alleged medieval argument about how many teeth has a
horse, without bothering to go and look.

Charge an electroscope to use as a test. Test it with a glass rod which
had been rubbed with silk (the glass is positive). Test it with a plastic
comb which has been run through your hair (the plastic is negative). Test
with the other standard objects: cat's fur, sealing wax, etc...

Now bring it near the running van deGraff, and see what sign of charge is
on its dome.

Now *why* should it be that polarity? Look at its construction. What
carries the charge into the dome? Usually a rubber or plastic belt. Now
where is rubber and plastic in the 'electron-affinity' series? (See an
older textbook for the relative electron affinity of common objects). As I
recall, almost anything rubbed against rubber makes the rubber negative.
If my recollection is right about that, then we predict the van de Graaf
dome will acquire negative charge, as the rubber physically transports
negative charge (against the field) into the dome where it passes near the
needle comb (electrically connected to the dome) and is yanked from the
rubber by the strong field between the needle point and the rubber..

Can anyone here explain, *in detail* the operation of a Wimshurst
machine? This is a rhetorical question, for hardly anyone, teacher or
student, can. Then why use it, if we can't explain it? A complete
explanation would have to answer the question "What physical
considerations determine (1) the optimal spacing of the metal strips, (2)
the optimal speed of roation for a given size, (3) the optimal angle of
the two arms which position the brass brushes, relative to the position
of the needle points. Now does anyone care to try?

Old textbooks are rich sources of information and pictures of various
electrostatic machines. The chapter XXXVIII "Electrical Machines" in
"Deschanel's Natural Philosophy" edited by Everett (Appleton, 1873) is
marvelous. It (and its clear engraved pictures) inspire me to try to build
some of these generators from scratch. I envision an electrostatic
generator made from concentric counter-rotating plastic Skippy peanut
butter jars, one huge, one small inside it, with Erector of Meccano strips
bolted to it, and supports and driving mechanism built from such parts. It
would be useful, and could be visually interesting (like a work of art) as
well. Anyone tried something like this? There's a challenging student
project here, folks.

        -- Donald