RE: Variable Capacitance and Inductance

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

Original poster: "Jan Wagner by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jwagner-at-cc.hut.fi>


> Wow! And until a couple weeks ago nobody seemed to care that Wheeler's
> formula was outputting 1000 inches (and many probably still don't care.)
> It was a point clearly made by some that we just don't have any formulas
> that give direct capacitance or direct inductance.  

Well, we do have, but it gets to iterative solving of rather ugly
formulas. Not anything one would do by hand. For example microstrip
antenna design for slightly more complicated antennas... no-one would
calculate that by hand.

But, all accuracy won't help for real-life situations where the object
under measurement is not really in free space. Different surroundings
affect the result differently. How much the error will be depends on a lot
of things...

> Why can't our "better instruments since then" give us exact inductance
> and capacitance measurements? 
> Could it be because inductance and capacitance are variable;
> just as Tesla noted?

Yes, L and C do vary, if the surroundings of the place of measurement are
changed. If the surroundings remain absolutely fixed, exactly, L and C
won't change.

Here's an experiment for your (maybe for others too!):
take a small metal box that can be closed completely and tightly, mount a
plate capacitor inside (firmly!), maybe  even fill up the box with some
for of resin or other hardening material so that nothing will move/rattle
around, and have a connector on the box wall (so that you can connect your
measurement equipment from the outside).

Then make a second plate capacitor similar to the first one, but not
enclosed. Maybe mount it on top of the box? There's no need for
capacitances to be exactly equal...

Then do a long simultaneous measurement series with both capacitors,
at least over a few days, and at various locations / "measurement sites".
Does the capacitance of the enclosed cap change? And how about the
non-enclosed cap?

I'll probably have to try that out myself too, once I get the time.

cheers,

 - Jan

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