# [TCML] measure VDG voltage

Barton B. Anderson bartb at classictesla.com
Sat Dec 22 18:53:31 MST 2007

```Hi Bert,

I measured a 1" ball gap back when doing TSSP measurements for Paul. I'm
at a very low altitude of about 100 ft above sea level here in a CA. I
measured 26kv/cm at that time. I think the 30kv/cm is a nice roundabout
number, but I think 26kv/cm is a more accurate number and not just from
my own measurements. I know where the 30kv/cm is from, but from my own
measurements and others who have measured (including papers on the
task), I've seen values between 26 and 27 and "never" higher. That
4kv/cm does make the terminal voltage little lower.

When corona inception voltage is reached, I have to assume breakout at
that point in this application. Although the corona itself "grows" the
ROC a tad bit, it's not enough to change the voltage significantly.

The ROC is the best method of predicting top volts. Far lower than
energy calcs and certainly more real.

Take care,
Bart

> You can estimate the corona-limited maximum voltage for the VDG based
> on the radius of curvature of the top terminal. However, this only
> provides the maximum that the VDG generator could achieve, not
> necessarily what your generator is achieving. The following
> relationship assumes an air breakdown voltage of 30 kV/cm at sea level
> and a polished sphere. The maximum voltage will be reduced for
> locations with higher elevations or if the sphere's surface was not
> polished. The Radius of Curvature (ROC) is in centimeters. So, a
> spherical topload 20 cm in diameter would have a ROC of 10 cm, and an
> estimated maximum VDG voltage of 300,000 volts.
>
> Vmax = 30*ROC
>
> You can also approximate the actual maximum voltage by using a spark
> gap and a suitable table that converts gap distance to voltage. This
> has the advantage of not loading down the VDG until actual spark-over
> occurs. You can use the VDG top terminal as one gap electrode and a
> similarly curved grounded spherical terminal (a gazing globe or a
> metal "float" ball or even a suitably round metal bowl) as the other
> gap electrode. Jim Lux's HV Handbook contains sphere gap construction
> hints and a gap-voltage table: