# Re: spark gap voltages (Secondary capacitance)

```Original poster: "Antonio Carlos M. de Queiroz by way of Terry Fritz <twftesla-at-qwest-dot-net>" <acmq-at-compuland-dot-com.br>

Tesla list wrote:

> Original poster: "Dave Larkin by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<teslaman15-at-hotmail-dot-com>

> A small DC supply is used to charge the primary cap, until the spark gap
> breaks down.  The output spark length under these conditions _is_ (almost) a
> direct relation of voltage.  So if a ground terminal with a large ROC (to
> try and make the field a bit more uniform) is used and the single shot spark
> length measured, one can determine the approximate output voltage, using the
> fact that air breaks down at ~1MV/meter for large gaps.

The correct figure is 30 kV/cm for parallel planes, or terminals where
the radius of curvature (R) is much larger than the distance between the
terminals (D). When D>>R the voltage tends to be determinated by R only,
as V=60000/R (R in cm, assuming 2 identical balls). An approximate
expression for the voltage between 2 balls with radius R and distance D
is:
V=30000*R*D/(0.9*(R+D/2)), R and D in cm.
A spark with 20 cm of length between two balls with 2 cm of radius