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# Re: sparks

A more readily accessible source may be the CRC Handbook - Spark Gap
Voltages. Tabulated there are peak voltages, different diameter spherical
electrodes, needle points, and the length of the spark gap, along with
corrections for temperature and pressure. These data are "based on the
results of the American Institute of Electrical Engineers."

For needle points, at a peak voltage of 20kV, the spark gap is 1.75 cm,
corresponding to a breakdown field of 11 kV/cm or 1.1MV/m.
For 5 cm diameter spherical electrodes, at a peak voltage of 20kV, the spark
gap is 0.6 cm, corresponding to a breakdown field of 33 kV/cm or 3.3 MV/m.

Dr. Lawrence D. Woolf; General Atomics, 3550 General Atomics Court, San
Diego, CA 92121; Ph: 858-455-447

At 17:45 2/18/01 -0800, Larry Woolf wrote:
According to the EB, the field strength for electrical breakdown of air at
atmospheric pressure is 10,000 V/cm. Within thunderclouds, fields of 1000
V/cm are exceeded only over limited areas. This information is also
available in the CRC Handbook in the spark gap section.

At 5:46 AM -0800 2/18/2001, John S. Denker wrote:
At 02:23 AM 2/18/01 -0800, Bernard G. Cleyet was skeptical of my
guestimate
of the breakdown field:
It's about 3 megavolts per meter.

Here are some values from a table of Sphere-gap Spark-over Crest Voltages
Table 4.92 Ref 1
Sphere diameter 6.25cm 12.5cm 25cm
spacing
1cm 31.3kv 31.7
2.5 68.8/69.3 70.8 72
5.0 105.5 127.0/132.3 136
10.0 *192.0 241/253

Error +- 3% except *

You can see that for small gaps and large electrode diameters
in Summer at sea-level,
the breakdown field strength exceeds 3 MV/m
and for larger gaps it is not quite 2 MV/m

Where two figures are given, the breakdown for a negative crest is
given first, then a postive crest value

Ref: sec 4-586 Standard Handbook for Electrical Eng.
Knowlton, McGraw-Hill (25degC, 760mmHg)
brian whatcott