# Re: spark gap voltages (Secondary capacitance)

```Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

One could use a rod gap, which is a deliberately nonuniform field, and one
for which tables have been published.

Of course, the rod itself will greatly reduce the voltage the TC reaches..

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
>
> I am well aware that the textbook breakdown for air is 3MV/meter.
> 1MV/meter is a crude approximation to account for the fact that the field is
> seriously non-uniform.  The equation you posted below seems a much better
> way of doing things!  Is there another equation which accounts for inequal
>
> The really accurate method for the test I describe is not to rely on a
> calculated voltage, but to hook an EHT power supply up to the (formerly)
> grounded electrode, and actually measure the voltage taken to break down the
> gap.  However the requirement for a multi hundred kilovolt test supply means
> that for most people it'd probably be easier to simply build the fiber optic
> probes!
>
> -Dave-
>
> >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
> >corresponds to about 111 kV.
> >Of course, this is for single sparks.
> >
> >Antonio Carlos M. de Queiroz
> >
> >
> >

```