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Copper vs. Brass in SG electrodes



I'm slowly and carefully planning for my next undertaking, a sync RSG.
I plan to use cylindrical electrodes, both stationary and rotating,
either 1/4" or 3/8" diameter, so that the arc forms on the sides of the
cylinders, rather than the ends, as appears to be more typical.  I see
several advantages to this:

1) With the larger radius of curvature, premature breakdown will be
   reduced, so the distance spanned by the arc will be less, with
   correspondingly lower losses.

2) The arcing surface will be spread out over a larger area.  This should:
   a) reduce electrode wear
   b) reduce localized electrode heating

3) Motor bearing play along the axis of the shaft won't matter.

Brass seems to be a much more common material than copper and I'm
wondering if anyone can tell me why most references suggest that copper
is the better electrode material?

I'm also planning on using 1/2" lexan for the disk.  I've heard many
times that plastics should not be used for the disk as electrode heating
will loosen them and send them flying.  With an earlier attempt at a sync
RSG I used 1/4" copper tubing cylinders, and the extreme turbulance on
the rotating electrodes left them quite cool.  I'd be happy to hear from
any others who have used lexan for disks.  FWIW, I'm using a 15KV/60mA NST.

Regards, Gary Lau
Waltham, MA USA