Re: very long sparks

To: tesla-at-pupman-dot-com
Subject: Re: very long sparks
From: Bert Hickman <bert.hickman-at-aquila-dot-com> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
Date: Mon, 10 Jan 2000 07:15:09 -0700
Approved: twftesla-at-uswest-dot-net
Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

Tesla List wrote:
<SNIP>
> 
> Couple of comments:
> 
> 1.  Having taken a look at the discharge, what amazes me is the path taken.
> 
> The discharge terminal at top of the building (you couldn't refer to
> something this size as a "coil") is of comparatively small dimension and
> the e-field should be such that the ground would have been a "better"
> target long before the transmission tower ? 100m away.
> 
> Seems odd.  I bet if we built a coil to a scale of this and placed a
> grounded tower equidistant as per the photo we would be waiting 'till hell
> freezes over before the tower was hit.
> 
> 2.  I think it is hard to ascertain the true distance of this
> discharge.  Granted it is large (and I'd be bloody pleased if it were my
> coil !) but the perspective of the photo is "optimized" to make it look
> larger.  Look at the fence leading to the towers, they are in fact well in
> the distance and we don't see the actual tower hit (which appears to be
> back towards the viewer).
> 
> 3.  The other thing which is strange is some of the shadowing in the
> photo.  Seems to be lit from above ???   I would be interested to get an
> expert photographic opinion !
> 
> Bert ...... I am just a little skeptical of this shot :-(
> 
> I think this comes from my training as an "Intensivist", our 3 rules:
> 
> 1.  Believe nothing
> 2.  Trust no-one
> 3.  Give oxygen !
> 
> What do you & others think ??
> 
> Cheers
> 
> Mark
<SNIP>

Mark and all,

A wee bit skeptical are we... ?  :^)

I suspect that the photo is indeed the real thing. Long spark discharges
seldom follow the shortest path, except when strongly overvolted. This
is fortunate, since it makes Tesla Coil and lightning discharges so very
interesting! There are some interesting aspects of the photo, and Stan
Darling has provided some possible explanations, and some confirmation
regarding the estimated spark length. Irrespective of the validity of
the photograph, there IS ample evidence of the phenomenon of long-spark
propagation under low average electric fields (compared to the often
quoted 30 kV/cm breakdown for air at STP).

While it is indeed hard to believe that a 5 MV impulse could generate a
100 meter spark, there is ample precident that very long sparks can
propagate under relatively low average applied fields - this includes
lightning! While the 5 MV discharge had an "average" field of only 500
volts/cm, it turns out that lightning strikes occur with an observed
average field stress that may be as low as 100 volts/cm when observed at
ground level ("Lightning" by Uman). 

Sometimes truth IS stranger than fiction...

-- Bert --