Re: Large flattened sphere output electrode

To: tesla-at-pupman-dot-com
Subject: Re: Large flattened sphere output electrode
From: Jim Lux <James.P.Lux-at-jpl.nasa.gov> (by way of terryf-at-verinet-dot-com)
Date: Fri, 28 Aug 1998 16:43:18 -0600
Approved: terryf-at-verinet-dot-com
Tesla List wrote:
> 
> From: Dan Kline <ntesla-at-ntesla.csd.sc.edu>
> 
> >X-Envelope-From: adder_black_the-at-yahoo-dot-com  Wed Aug 26 11:36:04 1998
> >
> >I just had an idea for a fairly easy large and hopefully cheap TC or
> >Van De Graff generator output electrode. I am sure that a lot of you
> >have seen those large "half moon" truck hub caps that a lot of the big
> >rigs use. If you took two of these hub caps and tack welded the inside
> >edges together this alone might make a decent output electrode. For an
> >even better electrode you could fill in the gap with Bondo and sand
> >the edges smooth. Then you could even cover the new rounded edge with
> >copper or aluminum tape to complete the electrode.
> 
> [snip]
> 
> >Any comments or story's on this sort of project are welcome.
> >Steven
> >black_adder_the-at-yahoo-dot-com
> >Dallas Texas
> 
> If you can find a company that manufactures air-compresser tanks, or
> propane tanks, or any kind of tank, cylindrical in form, with rounded ends,
> the rounded ends make great discharge terminals when welded together. Grind
> off the weld bead, and you're good to go. Cheap, and effective. Like a
> toroid with no doughnut hole, ;) I'm not sure how the shape affects the ion
> clound or field lines, but on my relatively small coils, these things
> actually worked better than dryer-duct toroids...maybe because of the
> smooth finish??

But, a bit heavy.  I think the smooth surface would definitely help,
though. I've now built about a dozen toroids in various sizes from Al
dryer duct and HVAC fittings, and in all cases, the little edges greatly
increase the corona losses (as measured by the charging current from a
500 kV Van de Graaf generator).  Actually, the HVAC ducting one (4
elbows in a ring) is better than the flex ducting one. I put Al tape on
the obvious sharp edges between the fittings, and it made a fair
difference.

In playing around with the Van deGraaf, I've noticed that even the edge
of a piece of copper foil tape burnished on the surface makes a big
difference in corona. Therefore, I think any scheme relying on covering
some larger thing with strips of metal tape is doomed (relative to some
scheme that produces a really smooth surface).

However, the secret to fun is "good enough". The aluminum dryer duct and
pie plates with aluminum tape to stick it together toroid is the most
fun. 10 minutes to make it and it works almost as well as anything you
can produce in less than a day or two.  When it gets dinged or dented,
you throw it way, and make another.