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Re: Isotropic Capacity

Subject:  Re: Isotropic Capacity
  Date:   Tue, 20 May 1997 00:26:51 -0400 (EDT)
  From:   richard hull <rhull-at-richmond.infi-dot-net>
    To:   Tesla List <tesla-at-pupman-dot-com>


At 09:46 AM 5/19/97 -0500, you wrote:
>Subject: Isotropic Capacity
>  Date:  Mon, 19 May 1997 06:25:24 +0000
>  From: "John H. Couture" <couturejh-at-worldnet.att-dot-net>
>    To: tesla-at-poodle.pupman-dot-com
>
>
>  To All -
>
>  The dictionary defines "Isotropic" as "equal physical properties along
>all
>axes". As an example, an isolated charged sphere in space would have an
>isotropic capacity because the charge on the sphere would extend equally
>in
>all directions. 
>
>  Isotropic capacity can only be a theoretical possibility. However, if
>the
>earth, moon, etc. are charged they would have isotropic capacity. Does
>anyone know if the earth, etc. has a charge? 

The earth does indeed have a charge.  Most planets have net charges. 
Most
of the charge accumulated relates to solar wind activity and varies a
good
deal with time.  

R.hull, TCBOR
>
>
>  It is obvious from the above that the secondary terminal of a Tesla
>coil
>cannot have an isotropic capacity because the charge is interrupted by
>the
>coil and other objects.

Absolute isotropic capaciy occurs at a range of about 10 times the
largest
dimension of the object in question from other objects. Near isotropic
capacity for spheres can occur at much closer ranges.  The toroid or any
other terminal has a mixed bag of isotropic and regular capacity with
the
coil form, wire sheet capacitance, etc.  Most normal toroids, at normal
altitudes, have little or no capacity relative to the ground or earth as
it
is too far away.  Thus most of the terminal capacitance is isotropic and
that shared with the resonator.

The superb article on Isotropic capacity in TCBA news by Schoessow of
some
years ago was reprinted two months ago in the Electric Sparce Craft
journal
with permission.  The equations are close to telling the tale
accurately,
but can't be relied upon for tuning and the like.  The key and salient
point
to be had was that the size of the largest object has almost nothing to
do
with any smaller object's isotropic capacity.  In short, it is the size
of
the smaller object which will determine not only its isotropic capacity
but
the range from any giant body many orders of magnitude larger as to
where it
will effectively exhibit full isotropic capacity.  The large body just
doesn't enter into the capacitive figuring more than that 10X the large
dimension of the smaller body distant.

We did a bunch of real world measurements with real terminals and found
that
a toroid in space hung over the ground really kept its isotropic
capacity to
within 3% when within 5 large dimension diameters of the ground.  Thus a
12X3" inch toroid is isotropic about 5 feet off the ground.  We put most
of
the tests on a report tape.  We also tested various wound resonators for
frequency variations, with elevation too.  This also related to altitude
but
related more to the diameter and l/d ratio of the coil.  A 1"X10 coil
was
the same frequency at 18" off the grounded plane as 60" up.  A 10"X36"
resonator needed to be 3-4 feet off the floor before stabilizing.

R. Hull, TCBOR