Re: wimshurst (fwd)

[Steven Roys <sroys@xxxxxxxx>]

---------- Forwarded message ----------
Date: Wed, 30 Jul 2003 21:54:54 -0300
From: Antonio Carlos M. de Queiroz <acmq@xxxxxxxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: wimshurst (fwd)

High Voltage list wrote:

> From: Ralph Zekelman <hyperion@xxxxxxxxxxxxxx>

> Antonio, thanks for reminding me of the 30 cm disc size for the
> old William Welch Wimshurst generator. William Welch (now VWR)
> was a Chicago supply house and maker of scientific equipment.

So, the company still exists?

> On page 244 of the 1990 Central Scientifics catalog is a Wimshurst with 31
> cm plates. The price is $479. The description does not state the material
> used for the plates but the large photo shows a lot of Lucite parts,
> including the housing for the two Leyden jars. The specs call for 12 cm
> sparks and a short circuit
> current of 30 uA.

The disks and insulators are almost certainly acrylic. Leyden jars are
sometimes still made in glass. At least, the resulting capacitance is
larger. The specifications are reasonable.

> I would suppose that the use of Lucite rods and
> fittings improves the corona control of the older William Welch
> model of the 1930s and 1940s era.

The  construction of the modern machines is practically identical to
what was the practice in the early 1900, maybe with simplifications.
Modern plastics are not much better insulators than the ebonite
(when new) used in those days. I have restored some old machines
with ebonite disks and insulators. When the ebonite is polished to
eliminate the decayed surface, the machines work very well. But the
procedure has to be repeated after not much time. Acrylic lasts longer.
Disk surfaces show a different aspect after some time, but this
doesn't appear to be a problem.

> A not too bright experiment we did with the Welch Wimhurst was to hold a
> piece of
> thin glass between the discharge balls. With the balls touching the glass on
> opposite sides, we had a high dielectric capacitor in parallel with the
> Leyden jars and cranking
> away on the generator built up a voltage high enuff to arc around the edge
> of the glass at least twenty inches. QUESTION: Why didn't the generator arc
> between the discs?

Probably because the electric field adjacent to the terminals, at
the surface of the glass plate, was much more intense than at the
machine's disks. The plate may have been dirty, and so it was easy
to make long sparks track along its surface. But 20 inches?

Antonio Carlos M. de Queiroz