Re: Saltwater Caps

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Antonio Carlos M. de Queiroz by way of Terry Fritz <twftesla-at-uswest-dot-net>" <acmq-at-compuland-dot-com.br>

Tesla list wrote:
> 
> Original poster: "Dr. Duncan Cadd by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <dunckx-at-freeuk-dot-com>

> I can recall reading a Victorian amateur experimenting book which
> suggested that you should select glass for capacitors on the basis of
> how good a static charge you could get on the glass by rubbing it with
> e.g. a bit of silk or a dead dog or whatever.  Probably as good a
> guide as anything ;-)  It would be interesting to know how the
> different colours of glass stack up against each other on the basis of
> so simple a test.
> 
> Perhaps Antonio has some relevant experience here with all the work he
> has done on electrostatic machines, many of which are glass-based?

I have not tried to make an electrostatic machine using glass plates,
but I made some capacitors (Leyden jars) with glass. Comparing with
most plastics, they leak a lot, apparently through the surface. 
Varnishing the surfaces, or simply applying some wax, makes a good 
difference.
Classical machines with glass plates, capacitors, and other glass
insulators were always varnished with shellac varnish or covered with
sealing wax. The exception were the disks of friction machines, that
were bare glass. The guiding rule that you mention above is obviously
correct.
I don't know enough about glasses to translate the old recipes to 
modern glasses, but I imagine that too "alcaline" glasses
are bad, because they attract water. Borosilicate glass, as "Pyrex",
is better (I have made some good Leyden jars with Pyrex glass, waxed).
What kind of glass is used in those big insulators seen in high-
voltage power transmission lines?

For Tesla coils, the DC losses are not significant, because the
voltages at the primary are relatively low, and the drain will be
in the microamperes range, insignificant for power supplies in the
milliamperes range. AC losses are the determinant factor for
selection.

Antonio Carlos M. de Queiroz