Re: Voltage/Length (fwd)

---------- Forwarded message ----------
Date: Sat, 17 Jan 1998 11:20:49 +0000
From: wysock-at-ttr-dot-com
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Voltage/Length (fwd)

> Date:          Sat, 17 Jan 1998 10:29:34 -0700 (MST)
> To:            tesla-at-pupman-dot-com
> Subject:       Voltage/Length (fwd)
> From:          Tesla List <tesla-at-pupman-dot-com>

> ---------- Forwarded message ----------
> Date: Sat, 17 Jan 1998 11:05:59 -0600
> From: Jeff Larson <jflarson-at-starnetinc-dot-com>
> To: Tesla List <tesla-at-pupman-dot-com>
> Subject: Voltage/Length
> Does any one have the voltage per length approximation formula?  This
> would be used to approximate the voltage of a coil output based on the
> measured spark length.  I imagine the spark length would be dependent on
> the humidity regardless of the voltage?
> -- 
> 	Jeff Larson		jflarson-at-starnetinc-dot-com     
> 	Design Engineer		Home phone 847-934-1877      
> 	Propheteer INT		Work phone 847-359-8988       
> 	Palatine IL

Jeff, all;

The question of voltage per unit of measurement i.e., "x"
kv per inch length, is a subject of long-term debate and
considerable research.  Humidity and barometric pressure
notwithstanding, it turns out that when the operational
frequency of the discharge rises above about 10 Khz, the
voltage required to ionize across a "standard spark gap"
with 1" air spacing, falls dramatically.  At d.c. with
the "standard spark gap" being defined as a pair of
polished copper spheres of 1" diameter, the required
breakdown point may be > 20 kv.  At 60 Hz a.c., this
voltage will be less then the d.c. point, but not by much.
At, say for example, 30 Khz, this voltage may be less
then 10 Kv.  An extensive study of air breakdown at
ELF (r.f.) was published in the transcripts of the
AIEE (forerunner of the IEEE.)  This was in the PAS
Journal (Power, Aparatus, Systems) and this article is
referenced in my paper "The stoy behind 13M" which
you can down load from my web site: www.ttr-dot-com.

This reference explores various forms of spark gap
geometry including sphere/sphere, sphere/plane, rod/rod
and rod/plane configurations.  Careful notation was given
to ambient air conditions of the published results i.e.,
barometric pressure, humidity and tempreture.  The
mean average of this study showed that for the test
frequency investigated (~ 28 Khz.,) the required voltage
per inch of discharge length dropped to about 8.4 Kv.
It must be borne in mind that the source generating the
r.f. voltages for this study was a high power ELF 
transmitter (used for Navel Research work,) and produced
strictly speaking, a sinusoidal continuous waveform.

Further investigations have since been published, using
very fast rise time pulsed wave sources.  Some of these
studies support the notion that depending on wave-front
rise time and duration, the actual breakdown voltage for a
given lenght of spark gap becomes even more non-linear.

Like many folks, I always used to believe that 20 Kv/inch
was a "good" benchmark.  In Tesla coil work, this turns out
to be far from the truth.

Bill Wysock
Tesla Technology Research