Heat and Glass Caps

From:  Thomas McGahee [SMTP:tom_mcgahee-at-sigmais-dot-com]
Sent:  Monday, January 26, 1998 1:51 PM
To:  Tesla List
Cc:  ghunter-at-mail.enterprise-dot-net
Subject:  Heat and Glass Caps

> From:  Gregory R. Hunter [SMTP:ghunter-at-mail.enterprise-dot-net]
> Sent:  Sunday, January 25, 1998 9:19 AM
> To:  Tesla List
> Subject:  Glass Caps
> Dear Readers,
> Anyone ever run a medium or large coil with a glass cap?  I'm trying 
> to figure out how bad the heating problem is at 1KVA and up.  I've 
> only built 3 small coils, all under 500VA.  All I've ever used is 
> glass caps, and I've only experienced one unexplained failure in a 
> foil & window glass cap on a 380VA coil.  A hasty autopsy revealed 
> glass panes laced with very fine, spider web-like cracks.  Might've 
> been due to thermal effects.  My current 450VA coil runs with salt 
> water bottle caps.  I think the water sinks so much heat, I'll never 
> be able to detect RF heating at such low power input levels.
> Greg

Heat fracturing of a glass cap can be due either to excessive heat
OR an excessive heat GRADIENT. When a saltwater cap is also immersed
in a saltwater bath, then the inside and outside temperatures are
kept pretty much equal. But if you are instead using foil on the
outside instead of saltwater immersion, then the outside of the
cap can get MUCH hotter than the inside, and this high GRADIENT
can cause fracturing. Also, with foil on the outside, pinpoint
corona effects can create localized hot spots. Note that in using a
saltwater bath, NO foil at all is used in the construction. Such
immersion caps have a higher capacitance (more than you would at
first think), much less corona problems, and much lower operating
temperatures. They tend to survive higher power levels, though
they are definitely NOT indestructible. 

Heating is much more pronunced in flat plate stacked glass caps, as
the temperature at the center can be many timns the temperature at
the outside. That is why many of the old-time flat glass plate designs
had each plate separated, so that cooling air could flow across each
plate and reduce the temperature.

Hope this helps.
Fr. Tom McGahee