Re: 600kV voltmeter

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Bert Hickman <bert.hickman@xxxxxxxxxx>

Tesla list wrote:

[snip]
> It is a bit disappointing to discover your 4 ft. long continuous spark
> shrinks to only 7 inches in the pulsed mode and gives a true output of
> approx 160 kV.  It's hard on the ego after you have told all your friends
> your coil is running at 1/2 a million volts!!
>     FWIW, I had this epiphany a few months back. What happens every 10mS 
> that makes the spark channel dissipate?
>     Or as Richard Hull posed, why does the streamer *go out* at all?
>     Why don't all streamers follow the same channel? I suppose the same 
> goes for non-TC sparks...
> -Phil LaBudde

Hi Phil,

Interesting questions. I took a shot at answering some of these on 
Wikipedia - go to http://en.wikipedia.org/wiki/Tesla_coil and read the "Why 
do Tesla Coil Sparks jump to open air?" section. Feel free to 
edit/change/add as appropriate... it IS a Wiki... :^)

The 10 mS thing mostly has to do with the spark channel cooling down 
between shots. The thinner the channel, the more rapidly it cools down. 
Cool air is denser and harder to "break down". This (plus space charge 
effects at the streamer tips) is why the streamer root seems to change only 
slowly, while the far ends constantly dart and change location. Space 
charge effects are also likely the reason why a well established discharge 
path may suddenly become replaced by a new path on the opposite side of the 
toroid.

This effect is also what limits high current spark gaps to repetition rates 
of ~100 Hz without external air flow/cooling. At faster rates, the gap 
doesn't have a chance to fully recover its dielectric strength between shots.

Best regards,

-- Bert --
--