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Re: Racing sparks



Jim Lux wrote:

> This is probably the same mechanism that makes the breakdown along an
> insulator's surface significantly less than that in free space (why those
> powerline insulators and "hockeypuck" HV rectifiers have ridges). It's also
> related to the really long sparks you can get on an insulated surface over a
> conductor or across the surface of a liquid. There is a  discussion of this
> in Bazelyan and Raizer, "Spark Discharge", pp256-260, but their analysis is
> somewhat qualitative, and is also oriented towards single impulses.  I don't
> recall if it was in Naidu or in Khalifa, but one of them said that the
> breakdown in a gap with an insulating column in the middle of the gap was
> 1/3 that of the same gap without the column.

Makes sense. The process may start with corona depositing mobile
charges on the surface of the insulator, maybe with some rectification
action too, progressively increasing the electric field at the edges
of the charged areas. Eventually breakdown occurs between charged
areas, possibly causing an arc across all the insulator.
Very probably this is a well studied phenomenon, but I don't have
recent references about this.
 
> The suggestion of semiconductive coatings is novel. I wonder though, if the
> dynamic behavior might screw you up (the time constant of the R(coating) and
> C(self) would be pretty long, so the charge would accumulate and the spark
> would form before it had a chance to dissipate). The spark propagation
> speeds are in the submicrosecond range.

The idea is to avoid the accumulation of DC charges over the surface
of the coil, that would be, if the theory is correct, the originators
of the "racing sparks". Certainly, if a single shot is enough to deposit
significant charges, the idea of a semiconductive coating would fail.
Does Tesla mention the phenomenon, with his cotton-covered wires and
wood forms, all rather semiconductive?
A simple test: spray some water over a coil that is showing racing
sparks, with the power off of course, turn it on and see what happens.

Antonio Carlos M. de Queiroz