Round vs flattened primary tubing

From:  Jim Lux [SMTP:jimlux-at-earthlink-dot-net]
Sent:  Monday, June 08, 1998 10:16 AM
To:  Tesla List
Subject:  Re: Round vs flattened primary tubing

> Suppose the tubing was run through some rollers to flatten it out.  It
> would still have rounded edges for corona control, and it would have the
> same skin effect surface area. 
Except that the radius of curvature (which is what is important for corona
control) is now smaller on the edges of the tube. For instance, if you had
3/8" OD tubing with 1/16" walls, and you smashed (smoothly) it flat, you
would get a tube with the cross section being a rounded rectangle 1/8"
thick and about 1/2" wide. The radius of curvature of the ends would now be
1/16" as opposed the previous 3/16". This radius is what is the determinant
of corona.

 It could now be wound more compactly which
> would increase the inductance per length slightly.  Or, the same spacing
> could be maintained, which would increase the turn to turn voltage
> breakdown.

Except that the smaller radius of curvature reduces the breakdown voltage.
Granted, it is not as bad as if the two small radii were facing each other,
but you still haven't gained much.

  Would there be any real practical advantage to such a primary,
> other than it could be secured easily by winding it through saw slots in
> the supports?
> I suspect the effort to flatten the tubing would not be worth whatever
> advantage there might be.  Comments?

OTOH, if you have some 20 kV across an coil with 6 turns, you are only
looking at 3 kV/turn which is probably not enough to get corona, even from
a piece of 14 ga wire. The big advantage is a compact coil with low skin
effect. (Although, the magnetic field from the adjacent turns will affect
the skin depth.)