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Re: [TCML] voltage

On 1/26/12 9:56 AM, Bob Svangren wrote:


Hi again guys, Thanks to you fellows who have responded to my letter
of adopting a voltage standard for our Tesla coils. As usual, there
are many ideas on how to measure the voltage in an inch of arc. No
one seems to agree on this subject.
I think that if you look at peer reviewed literature from the last 100 years, everyone agrees within 1%. Where there is disagreement, it's because you're not comparing apples to apples.
Ever since Friedrich Paschen wrote his dissertation (published in 1889), there's not been substantial disagreement. His experimental work was very high quality. Later, workers such as Meek, Craggs, etc, extended it, and offered alternate measurement methods that are more convenient, but ultimately, there's no significant disagreement on what the breakdown voltage is over a given distance in a specified gap. Modern measurement tools reduce the uncertainty and allow tests in more unusual enviroments (at work, we did a bunch of work on RF breakdown in simulated Mars atmospheres).
The exception is if you've got something which is inherently inconsistent... a highly non uniform field gap, perhaps with dust or being irradiated by UV or charged particles. But that's more a reflection of the uncertainty of the environment, not the underlying breakdown behavior of the gas in the gap.
Those little transient voltage suppressors that use gas gaps are very consistent, as are things as mundane as neon bulbs, fluorescent lights, etc. 

Again, there is substantial agreement on breakdown vs voltage.



 Down through the years, I have

read many statements from some very smart men and none of them could
agree either. The facts are that measuring high voltage is not an
exact science.


Actually it *is* an exact science.  Who is it who is disagreeing?



 I now find myself having to defend my statements on a

voltage standard for all of us. If you reread my letter you will find
that my intent wasn't to state that 25KV was an exact science but
just a voltage standard.
No, what you wanted to do was claim "a coil makes sparks X long, so that means it has Y voltage", which is demonstrably and provably incorrect.
it is no more appropriate than saying "I measured the sound level of the car going by as X dBA, and that means it's going Y mi/hr".
In both cases, the two are correlated, but do not have a direct relationship (as might be expressed in an equation or table). 


Like it or not, those of us who have built

Tesla coils are showmen to some degree. What point is there to
building these fine machines if one couldn't show them off to family
, friends, schools, ETC. In the process of displaying our fine
contrivances, someone will invariably ask, how many volts the machine
is producing. How do I answer this question?  Do i say, i'm not
really sure?
That is EXACTLY what you say, and what anybody should and does say, in the absence of a measurement. Unless you want to lie or dissemble.
You can say, the voltage cannot be higher than X, because the radius of curvature of the top terminal is Y, and there's a fundamental physical principal that limits the voltage.
You can say, mine is similar to Joe Bloggs's coil and he's measured(!) his as being X kV, so mine is probably about the same. 



 Do i say, it depends who's book you read.
No.. because which book you read doesn't really change the story. Spark length in a tesla coil is not very well correlated with voltage. It's well correlated with input power.
All the books (all the reputable science books, anyway) give pretty much exactly the same numbers. Sure, they differ by a few percent because of differing underlying assumptions (what's "standard air temperature" for instance). 

Do i say, it

depends on the math and formulas i use.


Not really, because the math and formulas are always the same.. that is:
There is NO equation that relates spark length from a tesla coil to topload voltage. 



 None of this would have any

meaning to them and i'm sure they would be disappointed in my
answer. The 20 KV per inch of arc could be dead on or completely
wrong but this is not the point.


That *is* the point.
There is no "kV/inch" for tesla coil sparks.



 The point of my letter has been

missed by most. I would much prefer to tell my audience that my coil
is producing a million volts. How could they dispute this.
They could dispute it by simple physics, which has been explained at length. And if they did, you will look like a fool in front of your audience. Why subject yourself to such abuse. Just tell them the truth.. 

What you could tell your audience is
"Some people claim that tesla coils put out millions of volts, but that's wrong. The actual peak voltage is typically several hundred kV, but there's no way to relate spark length to voltage, because sparks *grow*, just like lightning."
There is a picture that's widely distributed of a spark that is well over 100 meters long, and that is from a (I think) 5 Megavolt source. That's about 1kV/inch.
Lightning is another example. The E-field under a thunderstorm might be 10kV/meter, but you can get a several km long spark to bridge that.
In fact, one of the most fascinating things about long sparks is that they require remarkably low voltage. This was a source of great consternation to HV equipment designers as they moved above 500kV, because they discovered that simple linear scaling for safety distances did not work. Equipment was destroyed and people injured and killed because they were trying to use something like you advocate: a simple distance/kilovolt linear relationship.
And you can also tell people that this is what is fascinating about tesla coils. They can make very much longer sparks than you would expect from the voltages available. A single pulse at the TC voltage will NOT generally make a spark as long as the repetitive pulses characteristic of most tesla coils 

(this is the "single shot" distance)
There has been a lot of empirical research, reported on this list (and in other places) to try and figure this out. There are pulse rates (break rates) that seem to be better at making long sparks than others.
The advent of modern SSTCs has helped, because they're a lot more controllable, if somewhat lower peak power, than spark gap coils. 




So, when they ask what the voltage is you can say:
"I don't know, but what's cool is that I know it's not the million volts that some people claim, but it can make sparks much, much longer than a simple scaling of 3MV/meter (breakdown for air) would imply" 





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