# ReVoltage/Length (fwd)

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From:  John H. Couture [SMTP:couturejh-at-worldnet.att-dot-net]
Sent:  Wednesday, January 21, 1998 12:42 AM
To:  Tesla List
Subject:  Re: Voltage/Length (fwd)

Robert -

As you know the TC secondary voltage is a controversial subject and has
been for many years. From the response on the Tesla List it appears that
coilers believe one million volts equates to a spark distance of 4 feet to 9
feet. These distances are not with the identical conditions of a typical
Tesla coil.

Greg Legh's test was a spark of 2 feet for 275 KV. This could be
interpreted as
1000/275 x 2 = 7.27 feet   for one million volts? assuming the increase
is linear which it is not. This was at one PPS so again was not a typical TC
operation. A typical TC operation is with many PPS and the ionization of the
air with many sparks tends to increase spark length.

My 60 inch (5 feet) of spark for one million volts is based on typical
Tesla coils and the opinion of many coilers, not the opinion of one coiler.
This data was collected and converted into an equation by mathematical
regression methods and presented as a graph in one of my books. As no one
else has apparently done this to date it is not possible to make comparisons.

I do not claim the equation as a "hard reference" or as a "hard claim".
This would be ridiculous considering the fact that the TC sec voltage cannot
be accurately measured. I do not know of any coiler who uses this voltage
for determining other parameters of the TC design. The JHCTES TC computer
program uses this voltage to find the volts per turn. The volts per turn is
then used to estimate a safe insulation thickness for the secondary wiring.
It is obvious that great accuracy is not required and also that erring on
the high side is preferable. The insulation must be adequate to handle the
transients.

It should be noted that connecting a voltage divider to the secondary
terminal will reduce the sec voltage compared to a no load voltage. In order
not to have a voltage reduction the voltage divider would have to have an
infinite impedance. The reduction of the voltage can not be found indirectly
because the reduction is non linear. The TC sec voltage can only be
estimated and certainly cannot be a "hard reference or claim".

However, if a voltage divider is connected to the sec terminal, a voltage
can be obtained. My guess is that a coil with a 60 inch spark at no load
would give a voltage of about 300 KV with the proper divider. This could
vary considerably depending on the total impedance of the divider.

We will all be looking foward for your report of the results of your HV
capacitive divider column.

John Couture

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At 01:20 AM 1/20/98 +0000, you wrote:
>---------- Forwarded message ----------
>Date: Mon, 19 Jan 1998 19:21:53 -0500
>To: Tesla List <tesla-at-pupman-dot-com>
>Subject: Re: Voltage/Length (fwd)
>
>> Date:          Mon, 19 Jan 1998 12:26:49 -0700 (MST)
>> To:            tesla-at-pupman-dot-com
>> Subject:       Re: Voltage/Length (fwd)
>> From:          Tesla List <tesla-at-pupman-dot-com>

>> ---------- Forwarded message ----------
>> Date: Mon, 19 Jan 1998 19:17:09 +0000
>> From: "John H. Couture" <couturejh-at-worldnet.att-dot-net>
>> To: Tesla List <tesla-at-pupman-dot-com>
>> Subject: Re: Voltage/Length (fwd)
>
>John Couture wrote:
>
>>   Jeff, All -
>> These tables are based on spark conditions and
>> using apparatus that is very different compared to Tesla coils. For this
>> reason these tables cannot be used for Tesla coils.
><snip>
>> Also, the equation is based on benchmarks that have
>> been used by coilers in the past, such as 60 inches represents one million
>> volts for Tesla coils.
><snip>
>
>John,
>
>In my own experimental experience the distance benchmark you give of 60 inches
>for 1 Megavolt is not accurate for a Tesla coil discharge.  The actual terminal
>voltage required for such a distance feat is, in my own experience,
considerably
>less than 1 Megavolt.  Although there are at least two examples of so-called
>professional suppliers of commercial Tesla coils for sale out there who employ
>similar claims in their own advertising, actual laboratory measurements
>would surely more-often-than-not show such marketing claims as highly
>exagerated.
>
>Greg Leyh has, as I understand actually measured the output terminal
>voltage of his older 45 kW Tesla coil as no more than some 500kV
>while producing streamers between 20-30 feet in length (if memory and
>attention serve me correct).  His finding follows my own measurements
>albeit at somewhat reduced streamer lengths and input power levels to
>his own very impressive achievements.
>
>I am in the process of constructing a HV capacitive divider column
>probe to be able to measure both peak voltage and waveform shape on
>Tesla coil outputs up to either 500 kV or 1 MVolts (haven't decided
>how tall to make this sucker yet as one will be an inside capable
>tower, the other only outside capable given my present state of the
>lab).
>
>John if you err on this fundamental variability (and this particular
>parameter appears slippery in the present state-of-the-art), and employ it
as a
>"hard reference" in any of your software, your output data may not
>mimic reality at the level of accuracy implied.  You may wish to avoid making
>hard claims in "spark length VS voltage for Tesla coils" until more
>is known through actual physical measurement by researchers working
>in this field.
>
>Greg, would you like to share your own measurement experience again with
>this list?
>
>Always searching for the truth,
>
>Robert W. Stephens
>Director
>Lindsay Scientific Co.