Re: wimshurst (fwd)

[Steven Roys <sroys@xxxxxxxx>]

---------- Forwarded message ----------
Date: Mon, 28 Jul 2003 21:37:49 -0300
From: Antonio Carlos M. de Queiroz <acmq@xxxxxxxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: wimshurst

High Voltage list wrote:

> From: Erik Von Halle <erikvonhalle@xxxxxxxxxxx>

> I'm thinking that the William Welch Wimshurst are high Voltage and extremely
> low current. Is that Correct? Are these similar to Van Der Graft generators
> in that the charge is limited by the size of the sphere? At a certain point
> the Van Der Graft generatrs ust arc there electricty away.
>
> My question is what is the limiting factor on the William Welch Wimshurst
> machines? Do they arc and dscharge when they hit full voltage?

Welch and Wimshurst are two different persons. Welch was an instrument
maker. Wimshurst was the inventor (or at least popularizer) of that
machine.

All electrostatic machines have the output current limited by the
maximum
electric field that air can sustain, that limits the charge density in
the charge transport structures (disks, belts).
A typical Wimshurst machine with 30 cm disks can produce 20 to 40 uA.
A Van de Graaff generator with this same capability would be a quite
big machine, as it's more difficult to move a belt very fast. 10 uA
is ok for a tabletop machine.
See: http://www.coe.ufrj.br/~acmq/efield.html
A Van de Graaff can produce higher voltage, however, due to the simple
and well insulated structure. The maximum voltage in a VDG is limited
by the size of the sphere, in about 30 kV per cm of radius (more
probably one half of this in a typical machine).
See http://www.coe.ufrj.br/~acmq/myvdg.html
A Wimshurst machine eventually sparks across the disks if you try to
obtain a too high output voltage. This practically limits the
spark length to about one half of the disk diameter, and large
terminal balls will only reduce the maximum spark length between
the terminals. The actual maximum voltage is difficult to estimate.
A sectorless Wimshurst machine produces more current than a regular
machine (~2x), and greater voltage too. Still limited by sparking across
the disks, but without the sectors making bridges, it's not difficult to
reach 3/4 of the diameter of the disks, maybe more, in spark length.
See: http://www.coe.ufrj.br/~acmq/wimshurst.html

Antonio Carlos M. de Queiroz