Re: Variable capacitor

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Antonio Carlos M. de Queiroz by way of Terry Fritz <twftesla-at-qwest-dot-net>" <acmq-at-compuland-dot-com.br>

Tesla list wrote:
> 
> Original poster: "Harold Weiss by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <hweiss-at-new.rr-dot-com>

> I have built a Cu/PVC tubing variable cap somewhat similar to what you
> describe.  It did not work!  PVC has too low a puncture voltage.  My 6/30 NST
> could breakdown the 3/4" PVC tubing dielectric.  You need to go with
Acyrlic or
> polyethelene in oil.  The cap I made used a 3/4" Cu patching pipe over
3/4" PVC
> pipe.  The repair pipe was 12" long.  For the other plate, I used 1/2" Cu
> pipe.  I measured this for capacitance, and was able to get 100pF.  This cap
> failed as soon as I applied power.

Without some special insulation at the edges of the metal plates the
electric field there is too intense. Corona forms, heats the dielectric,
and it soon fails. Oil may be a solution.
This is a variable tubular capacitor that I made recently. Could reach 
400 pF. Didn't last much.
Open as in the picture below, there was corona everywhere and the
dielectric surfaces started to become conductive.
http://www.coe.ufrj.br/~acmq/tesla/567c2.jpg
When I tried to eliminate the corona at the foil borders with a layer
of wax it failed in seconds with 20 kV at 300 kHz applied.

Antonio Carlos M. de Queiroz