On Tue, 23 Mar 2004 09:58:40 GMT, loopfan wrote:
Ah, ok, so a round mass (pipe,tubing etc) is more efficient than a flat
strip even though they seem to share the same area? (ie a 6-inch wide
strip compared to a 3-inch diameter pipe) Is it kind of a shape vs. area
thing, assuming things like loss resistance are the same?

Hi Jack,

Basic physics of electrostatics teaches us that potential, in the form
of charge accumulates on a conductor at its smallest radius. Hence we
have lightning rods with sharp points, not flat metal plates occupying
the roof as a tile or shingle. The classic Van de Graff generator has
a dome construction whereby charge is moved up inside the dome, and it
immediately transfers itself to the outside surface (self shielding,
the interior has a negative radius in this sense). The dome serves
as a storage for the charge and presents the economy of a large radius
(the charge pump of the rubber belt presents a small radius inside
it). On the other hand, the Jacob's ladder consists of narrow wires
that emit continuous streams of arcing with much less voltage
(although at impressively high enough potential it is generally 1/10th
to 1/100th that of the Van de Graff).

A wire has an obvious radius (in cross section), and the charge is
distributed equally over its surface. However, if you hammer this
wire flat, the charge then seeks the edges (the smallest radius) and
abandons the flat area, starving it of conduction (resistance climbs).

The same phenomenon can be observed in variable capacitors that arc
further from their separated edges than from between their more
closely situated, meshed flat surfaces. Even with arcs between these
flat surfaces, it is always initiated by a site dislocality in the
form of a metal whisker or spur (small radius). Hence comes the
caution to finely polish the plates of high voltage capacitors.

It is a mistake think surface area alone as the geometry of a circular
cross section is more important.

73's
Richard Clark, KB7QHC