Re: Re-entrant portion of the sphere (VDG) (fwd)

[Steven Roys <sroys@xxxxxxxx>]

---------- Forwarded message ----------
Date: Thu, 16 Oct 2003 22:27:09 -0300
From: Antonio Carlos M. de Queiroz <acmq@xxxxxxxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: Re-entrant portion of the sphere (VDG) (fwd)

High Voltage list wrote:

> From: Finn Hammer <f-h@xxxx>

> This is interesting news to me.
> Should the distance between these rings be the same, and should the
> resistors equalizing them have the same value?

Yes, and yes. But this may be a difficult solution for a small machine
operating in plain air (professional machines run in compressed
gases), because very little current is available to bias the
resistor chain, and the resistors would have to be very large.

> I would like to build a nice VDG myself, and have been wondering how to
> design a ball terminal without creating too much leakage corona aat the
> entry point of the belt.

Note that the big double machine built by Van de Graaff in the 1930's
that is now at a museum in Boston used simple spheres as terminals,
and worked adequately, even probably producing some corona where the
spheres join the support tubes.

The usual solution is to make the lower part of the terminal as a
half toroid, or at least with the opening turned inwards. This is a
machine that I repaired recently, that has a terminal built in this way:
http://www.coe.ufrj.br/~acmq/ect/ectvdg.jpg

I have made a double machine with spheres, and with the belts
crossing the terminals through slits. There is some leak, but mainly
through the edges of the belts themselves.
http://www.coe.ufrj.br/~acmq/myvdg.html

> As for the belt, itself. I hear that this rubber belt from fysiotherapy
> (thera-something) is good. How do you glue it together.

Both machines use this material (Thera band). The joint in the belt
was made by cutting the belt at 45 degrees and gluing with cyanoacrylate
glue, with overlaps of about 5 mm, and care for correct alignment. The
joint is very strong, and I never had to replace a belt. The belt
stretches a bit over time, so it's a good idea to have adjustable
supports.

> I would like to use something a bit more substantial, any other ideas?

Nothing that I have seen and is easy to find.

Antonio Carlos M. de Queiroz