Jim Lux wrote:
I have the formula implemented in the Inca program, in a version that accepts balls of different diameters. It agrees quite well with the tables. There is an approximate formula too, for identical spheres, that works well.For how long a gap? If the gap is much more than 1/10th the size of the spheres, it's not a uniform gap anymore (for which that nominal 30 kV/cm number applies). The actual calculation for spheres is a pain (no exact analytical solution that doesn't involve infinite series), and, there's a lot of stuff that can affect the exact voltage (surface finish, dust, etc.)
http://www.coe.ufrj.br/~acmq/programsOf course, the ideal case of highly polished spheres and air breakdown at precisely 30 kV/cm of electric field are approximations, but quite good ones. A curious fact about spheres is that if they are too far apart, breakdown continues to occur due to excessive electric field at the ball surfaces (at 30 kV of potential per cm of radius of the spheres), but there is no guarantee of a spark between the spheres. Another is that the breakdown voltage for a ball-plane gap is about one half of the breakdown voltage for a pair of identical spheres, for the same spacing. An useful trick to obtain long sparks.
Antonio Carlos M. de Queiroz _______________________________________________ Tesla mailing list Tesla@xxxxxxxxxxxxxx http://www.pupman.com/mailman/listinfo/tesla