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Re: Variable Capacitance and Inductance

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Saturday, May 25, 2002 6:43 PM
Subject: RE: Variable Capacitance and Inductance

> Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>
> Hi Dave,
> At 08:10 PM 5/25/2002 -0500, you wrote:
> ....
> >
> >The modern Wheeler's equations already account for the earth's effects.
> >What I'm doing now, as a result of this discussion, is searching NASA
> >for any information about air core coils in space.  If I don't find
> >this weekend, I'll write NASA and ask for information.  If I'm right
> >Wheeler's formula already taking the earth effect into account, then an
> >core coil in space will have a slightly higher inductance reading.  If
> >wrong, it won't.
> Jim Lux may know such things.

Speaking for myself, and not for NASA (or JPL), I don't think you'll find
anything there.  You might find some early technical report discussing
parameter changes of components in vacuum or air, and you'll certainly find
some stuff discussing the impact of being in a zero-g environment
(mechanical and thermal changes).  Not having air to cool things does make a
big difference.

Wheeler's formula is an approximation good to a few percent based on a more
precise analytical formulation of the inductance of the spiral.  (for one
thing it does not take into account the diameter of the wire, which DOES
have an effect, particularly at higher frequencies, where the "current
filament down the center" approximation doesn't hold.  It's also been
empirically validated against real inductors over the past century. Any
possible change that you propose for an "earth effect" would be smaller than
the precision of the Wheeler formula.

Let us also be clear about the difference between the actual inductance and
the measured inductance.  If you hook some inductor up to a LCR meter, or an
impedance bridge, or whatever tool, such things as stray capacitance can
affect the number that shows up
on the display.  This will amost certainly result in a a change as it is
moved from near the earth's surface to far away, although, once you get,
say, 100 coil radii away, I suspect it doesn't make a huge difference.
Also, if there is anything conductive or magnetic in the vicinity of the
coil, it will affect the MEASURED inductance.

If you really think parameters change in space, you can buy a "Get Away
Special" canister for the space shuttle and test it out.  The manifests are
a bit full these days, what with ISS, but you never know.  There's some
website at www.nasa.gov that will give you all the details. ($50K, as I

There are also some low-cost launch opportunities for small cubical
satellites, about 8" on a side.  I believe they are coordinated through
Stanford. I've worked with several student groups on designs for these, and
they are really really inexpensive (mostly because they don't impose all the
reliability requirements...)

And, the Russians may still have a "throw it out the door" program like they
did for Mir ($10K to carry your small package up and literally throw it out
the door from Mir)