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Re: Capacitance of a long thin rod (e.g. a spark) (fwd)



---------- Forwarded message ----------
Date: Fri, 17 Jul 1998 14:34:54 EDT
From: FutureT-at-aol-dot-com
To: tesla-at-pupman-dot-com
Subject: Re: Capacitance of a long thin rod (e.g. a spark) (fwd)

In a message dated 98-07-17 12:30:02 EDT, you write:

<< ---------- Forwarded message ----------
> Date: Fri, 17 Jul 1998 07:29:19 +0000
> From: "John H. Couture" <couturejh-at-worldnet.att-dot-net>
> To: Tesla List <tesla-at-pupman-dot-com>
> Subject: Re: Capacitance of a long thin rod (e.g. a spark) (fwd)
 
 >   Jim -
 
>  If the capacitance of the TC secondary is changed by the streamers
> wouldn't this put the system out of tune and stop the streamers?
 
>   Richard Hull said it was an extreme change implying much more than 5%. He
> did not say how he measured the frequencies with and without streamers under
> high voltage operating conditions.
 
>  John Couture
  >>

John C,

If I tune my S.G. coil for best results at low power, I have to
re-tune for higher power operation.  And when tuned for high power,
the sparks will be weaker when run at a lower variac setting, than
when tuned for the low power.  So it is clear from this, that the
coil must be tuned differently for different spark lengths for best
results.  The primary frequencies can be calculated from the pri L
and C at the different tune points.  The secondary frequencies should
follow.

So this suggests that the re-tuning for long sparks at high power
does make the coil *out of tune* as the sparks first
form, but the coil *goes into tune* as the sparks grow.  At that point
a positive feedback mechanism is at work bringing the coil into perfect
tune as the sparks grow, (it is wonderful or what?!). 

I generally have to move my tap point 1 turn outward (on a 20 turn pri)
at high powers compared with low power operation, for best spark
output.

The difference in tune is not enough to prevent the formation of the 
streamers (aren't we luckly).  Actually the delay in streamer formation
may be a plus, by helping to permit a higher voltage to develop before
streamer breakout.  The ROC voltage holdoff capabilities of toroids
may also be helped by this delay.  We may be getting a free lunch
here.  On the other side of the coin there's a lot of ionized air
floating around once the coil is running to help the streamer formation. 

John Freau