Brian, A 1.25 inch needle gap will breakdown around 30KV in dry dust free air.
For clean spherical surfaces at least 1.5 inches indiameter the volatage is
around 60KV for the 1.25 inch gap distance. "An approximate rule for uniform
fields at all frequencies up to at least 300 mhz is that the breakdown gradient
of air is 30 peak kilovolts/centimeters or 75 peak kilovolts/inch at sea level
(760 millimeters of mercury) and normal temperature (25-degress centegrade.)
The breakdown voltage is approximately proportional to pressure and inversely
proportional to absolute (degrees-Kelvin) temperature.
Hope this will help you.
Kingshill, VI 00851
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