Re: Corona and Sphere - Puzzle

>          The question:  Why does a single rooted streamer or just a 
> few streamers appear when the breakdown voltage of the sphere 
> has clearly been reached?  Why not all around?

The appearance of a streamer (and its location) are determined by the
electric field distribution. The E field is influenced not only by the
sphere, but also by the other electrode (i.e. the surroundings). Unless you
are operating a perfect sphere within a perfect sphere, or two very large
flat plates, the field isn't uniform. The "breakout" will always start
where the field is strongest.

Even a small dust particle on the electrode is sufficient to create a
localized field concentration.  A somewhat more practical example is
sticking a thumbtack or screw (with the point facing out)on the toroid to
force the streamers to come from that point. 

Another example is if you have a smooth top electrode, and you put a
grounded electrode near it (even a few feet away), the sparks will emanate
from the location on the top electrode nearest the grounded electrode.