Re: Strange shock (fwd)

["John Williams" <jwilliams-at-edm-dot-net> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

	I worked with a friend developing a large Van DeGraff generator
for several years.  One of the things I had a difficult time impressing on
him was that it doesn't take much surface contamination to render
a material incapable of holding a static charge.
	He would wash down the collumn on his generator with
denatured alcohol assuming that it was clean.  I asked him to shut off
the lights and turn on the generator.  The collumn lit up with a pattern
that looked like a town at night seen from a high flying jet aircraft.
	The material used to denature the alcohol was enough to
provide leakage paths across an otherwise insulating material.
	He once built a prototype generator and counter to my arguments
used a brush type a.c. motor to drive it.  The generator is the self exciting
type and very dependent on the cleanliness of the materials used
in construction and for the rollers and belts.
	On initial start up the machine, which stands nearly seven
feet tall, tossed off a six foot arc.  But, then stopped performing so well.
Now if he can pull a foot and a half off of it just after washing it down
he's lucky.
	I believe the reason for the persistant degradation of his machines
is that the generator draws the minute carbon particulates from the
brushes, blown out into the surrounding air in the normal course of
operation, back to the surfaces and allows the static charge to bleed off.  It
contaminates all the surfaces of the machine, collumn, belt, rollers etc.
	It is likely that the coils that don't display this surface charging
during operation have some contamination in the coating or on its'
surface that allows the charge to bleed off, and, as I stated above, it
only takes a very minute contamination for this to happen.

	John


>I read this after replying to David's post. Since you observed the same
>phenomenon with a cardboard form, then I would think that the coating applied
>is probably the cause. I still think PVC and similar forms may cause this
>problem, but also the coating type. The charge properties of your shellack may
>be very different than the Marine Spar Varnish I used. I've ran my coil in
>relative humidity levels down to 20, but never got a shock. I have a habbit of
>touching the secondary after runs checking for a static charge and have never
>had one on the coil.
>