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Re: [TCML] voltage

The measurement of high voltage without expensive equipment is really difficult, as the many letters in this thread attest. (It's often not that easy even with expensive equipment.) Spark length is subject to so many variables that it is essentially useless. Besides pressure and humidity, it depends greatly on electrode radius. The breakdown strength of air at sea level pressure is about 30 kV per cm. However, this is a local phenomenon--an arc can start if any part of the apparatus has a field strength greater than this, even in a tiny area. The electric field strength at the surface of a sphere is equal to the voltage on the sphere divided by the radius. But this is only at the surface. As you go out, the field decreases. This assumes that the sphere is perfectly smooth and clean. Any dust, grime, lint, etc. will cause local field enhancements that can start discharges at lower voltages.
If the breakdown field is exceeded, a discharge will form at the surface. So for a sphere of 1 mm radius, this field is reached at 3kV. But the discharge is a conductor with a tip of very small radius and corresponding high field, so it will propagate into the reduced-field region, forming a streamer. The length of this streamer depends on many factors. With a Tesla coil, the initial breakdown will happen when the voltage on the toroid first reaches a value sufficient to create a field of 30kV/cm at its surface. (The field at the surface of a toroid is a little less than double that of a sphere.) Then the voltage goes to zero, but the next half cycle comes along before the streamer can deionize, so the renewed high voltage creates a high field at the tip of the streamer and it propagates further. This happens for several cycles until the voltage drops enough to allow the streamer to cool down and extinguish. This process is why a TC with a 30 inch high secondary can throw arcs of 13 feet or more, even though the voltage on the toroid may not be more than 300kV to 500kV.
The best way to meaure the voltage with sparks is probably to put spheres of different sizes on a toroid large enough not to break down itself, and see what radius sphere will or will not break down. With careful work, this should be within 20% or so.
One crude way to estimate voltage would be to use the transformer turns ratio and mutiply by the primary voltage, and assume that the real voltage is a significant fraction of this, like 1/2 or 2/3. That seems to sorta work for my coil. 

---Carl





Bob,
I agree will all you have said, apart from one item. I come from the UK where inches and mm are interchanged regularly. (and with more success than NASA) So I have always used the rule of thumb of 25Kv per inch as this equates to a rule of thumb as 1Kv per mm.
This Rule of thumb satisfies both measurement systems more easily I think... 

    Derek

On 25/01/2012 18:04, Bob Svangren wrote:

Hi guys,
I notice that there is a lot of conversation on measuring the terminal voltages on these fantastic coils. I have read a lot of books on the subject over the years and found that it takes very complicated and costly equipment to measure these very high voltages. It seems that everything has a bearing on the accurate measurements such as the electrostatic field, barometric pressure, humidity, AC- DC. frequency and many other factors. Of course we all like boasting rights of our coil output and it's great to say that we are putting out a million volts but in reality, no one is sure of this number The best experts cant agree on what an inch of arc is equal too but it ranges from 10,000 volts per inch to 50,000 volts per inch of arc. The makers of stun guns claim anywhere from 100,000 volts to a million volts for less than a three inch arc as one example. Who would believe any of this to be fact. We do need our boasting rights and I for one would be in favor of adopting our own voltage standard which we could all understand and would do away with all this guess work. I have determined that 20,000 volts would be a good and close to accurate standard for us to use and we would all be on the same page. Be it right or be it wrong the 20KV per inch should be close for our use. If we should adopt the 20KV volts per inch standard, I would be in favor of naming our new standard after the late and great Mr. Harry Goldman who gave much of his life to making coiling fun and interesting for all of us to enjoy. We could simply call this the Goldman standard. A 50 inch arc would equal a million volts. Doesn't that have a nice ring to it? 
   So, the big question is, what do you fellows think of this plan.
Bob _______________________________________________ 
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