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Re: Variable Capacitance

Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-qwest-dot-net>" <evp-at-pacbell-dot-net>

Tesla list wrote:

>         This touches on something that would help, is keeping
>         clear in each posting whether conventional caps or
>         capacitance of isolated objects is under discussion.
>         The usual capacitor is a discrete, defined object,
>         while a ball over ground is a deal vaguer.

	The ball over perfectly conducting ground  is exactly analagous to one
half of a system of two isolated balls.  There is an "image" ball on the
"other side" of the ground plane.  

> > Due the large scale of distance added between the "plates"
> > anything that can increase the strength of the dielectric

> > One has to question if we fully understand all the "stuff",
> > if any, that is getting between the ball and ground as well
> > as the effect of surrounding charge changes.

DOUBTERS, those with opposing view  must provide convincing and
reproducible evidence to support their claims.  Nobody has to prove
anything to anybody, of course..........
>         IF the standard terms (capacitance, as measured
>         (approximately) by a common LCR meter) are used in the
>         standard way, then they are understood.  If  unique
>         definitions are adopted, communication becomes
>         impossible.

	Standard "LCR meters" don't really measure capacitance or inductance
directly, but "infer" them from some phenomenon (such as a resonant
frequency) which they produce. 

> > It has been pointed out by some that the earth has an electrostatic
> > gradient.  If the electrostatic gradient is logarithmic
> > (due to the inverse square law of charge) there could be a
> > substantial change in a 40 feet elevation difference.

>         Usual values (as someone posted) of 100 to 200V/m.
>         Varies with weather, humidity, gets LARGE in vicinity
>         of thunderstorm.  cf any good book on lightning or
>         atmospheric electricity.  Tricky to measure, due to VERY
>         high impedance.

	Easily measured with a simple field mill.

	For the record, the "customary" definition of capacitance is the stored
charge per unit voltage (coulombs per volt, for example).  It is not
voltage dependent.


End of this subject.