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

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---------- Forwarded message ----------
Date: Sat, 18 Jul 1998 05:44:27 +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 -

The change in frequency is not exactly the same as the change in turns and
they soon part company. If Hull's extreme is correct there may be a
difference of several turns.

For example
With 1 turn change
Change in turns       =    7.14%     = 1/14
Change in freq        =    6.67%
Difference              =    0.47%

With 4 turn change
Change in turns      =    28.58%      = 4/14
Change in freq       =    22.22%
Difference             =      6.36%

With a 7 turn change
Change in turns       =    50.0%      = 7/14
Change in freq        =    33.3%
Difference            =    16.67%

I doubt the freq change is this great but is there any change at all due
to the capacitance of the streamers? How would the capacitance or freq
change vs spark length be determined? Apparently no one has ever tried to

John Couture

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At 12:08 PM 7/17/98 -0600, you wrote:
>
>
>---------- Forwarded message ----------
>Date: Fri, 17 Jul 1998 10:24:35 -0700
>From: Jim Lux <James.P.Lux-at-jpl.nasa.gov>
>To: Tesla List <tesla-at-pupman-dot-com>
>Subject: Re: Capacitance of a long thin rod (e.g. a spark) (fwd)
>
>Tesla List wrote:
>>
>> ---------- 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.
>
>Sure it would pull it out of tune. I suspect this is why you tap out a
>bit from the low voltage resonance. That would account for the reduced
>resonance. The coil doesn't need to be in as much tune to get the
>streamer started, and then as the capacity increases, the behavior
>changes...
>
>Just as a ballpark guesstimate, say you had 14 turns to get resonance
>w/o streamer and you need to go to 15 turns to get resonance with the
>streamer. In general inductance goes as N^2, and frequency goes as
>sqrt(1/LC), so the change in frequency will be the same as the change in
>turns, or 7% in my example, same ballpark, so it is consistent.
>
>I think that more precision in this sort of analysis would be spurious,
>unless you went to a lot of trouble to look at the dynamic behavior of
>the C.
>
>

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