Re: whimshurst--RESEND (fwd)

[Steven Roys <sroys@xxxxxxxx>]

---------- Forwarded message ----------
Date: Mon, 28 Jul 2003 15:46:00 -0600
From: Gomez <gomez@xxxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: whimshurst--RESEND (fwd)

On Monday, July 28, 2003, at 03:10  PM, High Voltage list wrote:

> Original poster: Steven Roys <sroys@xxxxxxxx>
>
>
>
> ---------- Forwarded message ----------
> Date: Thu, 24 Jul 2003 17:37:43 -0600
> From: Erik Von Halle <erikvonhalle@xxxxxxxxxxx>
> Reply-To: Erik_Von_Halle@xxxxxxxxxxxxxx
> To: hvlist@xxxxxxxxxx
> Subject: Re: whimshurst--RESEND (fwd)
>
> Hi Gang,
>
> I'm thinking that the William Welch Wimshurst are high Voltage and
> extremely
> low current. Is that Correct?

  Yes.  Of course "high voltage" and "low current" vary somewhat between
the different
types of static machine.

>  Are these similar to Van Der Graft generators
> in that the charge is limited by the size of the sphere?

  Partly.  All terminals will leak charge off, depending on how smooth
they are, how dry the air is, altitude, cosmic rays, and the phase of
the moon.  (kidding on the last two).   All other things being equal,
the radius of curvature of the top terminal will determine the maximum
charge voltage and the quantity (coulombs) of stored charge. (ie; the
size of the virtual capacitor)

>  At a certain point the Van Der Graft generatrs ust arc there
> electricty away.

  Van de Graaf generators also are limited by their charging rate.  At
some voltage, the top terminal will probably start to leak as fast as
the belt can replace it.  You can either:
A) add a "spray" supply, to spray charge onto the belt,
B) make your top terminal bigger and smoother
C) run the belt faster.
D) add "down charging", which runs charge on the belt in both
directions, so that you are (for example) "taking" negative charge from
the sphere and also adding positive charge to it.

  Professional machines do all of the above, run the whole machine in
vacuum or sulfur hexafluoride (insulating gas), and use "chains"
instead of belts, which consist of little round metal slugs on an
insulating cord, operating at really high speeds.

> My question is what is the limiting factor on the William Welch
> Wimshurst
> machines?  Do they arc and dscharge when they hit full voltage?

I'm not sure, but I would bet that this guy knows:
http://www.sparkmuseum.com/FRICTION.HTM