# Re: Variable Capacitance

```Original poster: "davep by way of Terry Fritz <twftesla-at-qwest-dot-net>" <davep-at-quik-dot-com>

Hi Dave:

> It would appear to me to be both.  As the ball is raised,

> the physical object is changing due to distance between

> the "plates."

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.

> Due the large scale of distance added between the "plates"

> anything that can increase the strength of the dielectric

Time out.
Unique definition.
The rest of engineering uses a definition of capacitance
which DOES NOT DEPEND ON VOLTAGE.:
Area of plates, separation of plates,
dielectric constant (NOT WORKING VOLTAGE)
Are the only things that define capacitance.

I can have a 1uF cap at 1V, 10V, 100V, etc.
If i apply 1V to the 100V one it should act like the
1V one.  The unit is the Farad (or micro or...).

If a unique definition is to be used, it would help
to state it.

> must enter into the equation.

cf above.  The definition of capacitance does not
include voltage.  (the spec of a particular capacitor
may, there is a difference.)  (I'm over simplifying,
a little: there will be small (say under 1%) stray
differences.  But a 1 uF cap will be a 1 uF cap.
(and electrolytics, little used in Tesla work

> 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.

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
impossible.

> It has been pointed out by some that the earth has an electrostatic

> (due to the inverse square law of charge) there could be a

> substantial change in a 40 feet elevation difference.

Which has NO EFFECT on the theoretical value of a
capacitor, nor on its measured value (eg with a common
LCR meter or most other ways.).

> I'm looking for some detailed data on this electrostatic gradient.

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.

(I admit to NOT knowing when this was first documented.
May well have been AFTER Tesla was in Colorado Springs.
If he was using 'ac' techniques (resonating the suspended
sphere with a coil (1) I'd expect him not to have been
confounded by this.  If he was using DC techniques
(how long did it take the sphere to discharge), then,
Maybe....  Note that the _capacitance_ of the sphere was
not affected.  The interpretation of the measurement
technique may have been.)

==========================
Note:
Yes.  I have a copy of CSN.  Read it.  Years back.  Don't propose
to reread it for purposes of this discussion, as other confounders:
apparent redefining of capacitance, seem bigger barriers to
communication.)

best
dwp
(PS:
Just noticed i let a bad spelling of: capacitiance slip into
the spellchecker.  Hope this di not confound anyone.)

```