Not Clear About Glass (fwd)

---------- Forwarded message ----------
Date: Fri, 15 May 1998 20:22:19 +0000
From: "Gregory R. Hunter" <ghunter-at-mail.enterprise-dot-net>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Not Clear About Glass

Dear List,

Ordinary glass (window glass, beer bottles, etc.) is widely credited
with a dielectric strength of 250V/mil.  However, my own limited
experience suggests that this figure is too conservative.  I think
glass is stronger than that--possibly much stronger.  I've only built
three rather modest-sized coils, and I used glass capacitors in all
of them.  In every case, the glass dielectric was supposedly too thin
to survive the peak charging voltage of my power supply, yet none of
the caps failed.  My present coil uses a 15KV NST to charge saltwater
bottle caps with a dielectric thickness of roughly 100 mils (more
than 1/16", less than 1/8").  By the formula, these should not be
able to withstand the 42KV peaks generated by the NST, let alone the
harsh conditions in the primary tank, yet they are holding up fine. 
Does anyone know where the 250V/mil figure came from?  Has anyone
else noticed that glass holds up to much higher voltages than it


East Anglia, UK