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# Re: [TCML] Not coil related, but a HV question

```On 4/11/16 1:27 PM, Paul B. Thompson wrote:
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```Recently I've gotten involved with a group of high altitude balloon
hobbyists. We're always looking for interesting experiments to try in
the stratosphere, and I thought of one with HV application. I'd like to
run it by everyone here as a thought-experiment first.

Air is a dielectric. Not a great one, but a sufficient air gap between
two electrodes prevents arcing. (I may not be expressing this the best
way, but you all know what I mean.) If you lower the air pressure
greatly, would a high voltage spark leap across the same distance? I
imagine something like a charged flash capacitor being lofted in a
balloon. The poles of the cap are connected to electrodes separated by a
gap sufficient to prevent discharge at normal air pressure. If the
balloon lifts the apparatus to, say, 100,000 feet, will we see the
apparatus arc over when the air is sufficiently attenuated? Or will we
get a kind of corona glow? Or nothing?
```
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Exactly, in fact, 100,000 ft is about where the breakdown voltage is lowest. The pressure is about 10-20 torr, which is the low point on the Paschen curve
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However, there's also the minimum sparking voltage, which is about 300 volts. Below that voltage, you won't get a breakdown, no matter how low the pressure is.
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One way to "measure" this might be to have a HV power supply of about 500-700V charging a capacitor through a resistor with a small spark gap across the capacitor. The rate at which the sparks occur will be related to the breakdown voltage: more frequently as the breakdown voltage goes down.
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A camera would be pointed at the gap to film the result.

Paul Thompson

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