# Re: [TCML] Copper tube spark gap

```On 6/4/15 7:25 AM, Doug wrote:
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```Thank you all for your replies. Another question, should I assume that
the distance between a given number of tubes is, cumulative, so if my
preferred, single spark gap distance is .125 I should split that
distance between what ever number of tubes there are in my assembly?
Doug J.

```
```
Yes, but...

```
The gap breakdown voltage is a (nonlinear) function of the spacing and radius of curvature. if the gap is very much smaller than the radius of curvature, the field will be fairly uniform and the gap is small, so you can use this simplified relationship:
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```
For air, and gaps on the order of a millimeter, the breakdown is roughly a linear function of the gap length: V = 30pd + 1.35 kV, where d is in centimeters, and p is in atmospheres.
```

Note the 1.35kV breakdown for a "zero length" gap.

```
Most people set the gap(s) empirically: build your spark gap, hook up just the HV transformer, and adjust them until the gap just fires (with the fan on).
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```
Note that if you get hot spots in the gap, the firing voltage goes down: if you have parallel tubes, the sparks should be occurring along the length of the tube, not all clumped at one end or another (which would indicate non-parallel: or at least, non uniform field).
```

```
More gaps = more loss: when the gap has fired, there's a voltage drop of 100-300 hundred volts across it. So there's a trade on multi-gaps: more gaps is better quenching (because the heat is distributed across more surfaces) but more loss, too.
```

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