Re: HV Arc Resistance

• To: tesla-at-pupman-dot-com
• Subject: Re: HV Arc Resistance
• From: "Jim Lux" <jimlux-at-jpl.nasa.gov> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
• Date: Sun, 11 Jul 1999 15:40:24 -0600
• Approved: twftesla-at-uswest-dot-net
• Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

> 
> 	I "think" if you double the distance, the resistance basically doubles. 
I
> think a good measure of this is when one draws an arc such as off a
Jacob's
> ladder.  If you double the distance, the arc just seems to get longer
> without the intensity of the arc changing greatly.  So if it is twice as
> long, the arc is twice as big and the resistance would seem to be
proportional.

Arc column resistance/unit length is determined by the current... So, if
the current is the same, the resistance of a spark twice as long is 2 times
the shorter gap's resistance. (Only for arcs, not sparks, and only for arcs
long enough that you don't have the effects right next to the electrode.). 
 You have to watch this,though, because if you are driving from a voltage
source (i.e. a capacitor,etc.) doubling the arc length doubles the
resistance, which drops the current by a factor of 2, which in turn
increases the resistance, etc.etc.etc.  (Why a Jacob's ladder or drawn arc
doesn't stretch forever). 

The actual value is determined mostly by the heating from the current...
Only that part of the arc column which is more than about 5000-7000K
actually conducts (the conductivity of a plasma vs temperature has a pretty
sharp cutoff).  Essentially, the arc column diameter grows until it is big
enough so that the heat lost through the sides of the arc balances the heat
dissipated in the IR losses of the arc.  It is a volume vs surface area
thing.

> 
> 	I think many electrode gaps probably have the same resistance when they
> are conducting but have much higher losses at the zero current crossing
> where the arc goes out.  A single gap will stay hot and it is easy for
the
> arc to restart.  However, a multi gap cools quickly and the arc has a
much
> more difficult time restarting.  This appears effectively as higher
losses
> but more effective quenching.  A gap with say a hundred small gaps would
> have super good quenching but the losses would be extreme.  A single gap
> has very low losses but practically no quenching.

This is taken to an extreme with things like Fruengels multi disk argon or
hydrogen quenching gap.
> 

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