Re: Isotropic Capacitance

Tesla List wrote:
> >From nikki-at-fastlane-dot-netSun May 26 11:24:20 1996
> Date: Sat, 25 May 1996 18:40:28 -0500 (CDT)
> From: Bert Pool <nikki-at-fastlane-dot-net>
> To: tesla-at-pupman-dot-com
> Subject: Re: Isotropic Capacitance
> At 12:02 PM 5/25/96 -0600, you wrote:
> >>From brzozoww-at-rchland.vnet.ibm-dot-comSat May 25 11:56:06 1996
> >Date: Fri, 24 May 1996 15:54:18 -0400 (EST)
> >From: Wesley Brzozowski <brzozoww-at-rchland.vnet.ibm-dot-com>
> >To: tesla-at-pupman-dot-com
> >Subject: Isotropic Capacitance
> >
> > Richard Hull <hullr-at-whitlock-dot-com> wrote:
> >
> >> The charge figured relative to a point infinitely distant is a nice
> >> concept.  But, to aquire a charge, anywhere in the real world, work must
> >> be performed in a dielectric (space in this case) by sparating this
> >> charge relative to something (the point far off).  Metals can't have
> >
> >Ahhh... but the point that originally sparked this exchange was your quite
> >correct comment that the formula used for the capacitance of a sphere does
> >not match the measured values. And the reason for this is that the
> >assumptions made to derive the capacitance do not match anything that can
> >be done in the real world. Since most real-world problems don't have any
> >exact, closed-form mathematical solution, (at least that we've yet been
> >able to find) we are forced to either work with approximations or go with
> >computer-based numerical solutions. These are still approximations, but we
> >can usually make them very, very close to what's really measured.
> >
> [big snip]
> The capacitance of an isolated sphere in the real world is affected by close
> (and not so close) objects and the earth itself.  Tesla spent several pages
> (207 - 210) of his Colorado notes examining the capacitance of an elevated
> sphere at different heights above the ground.  You have undoubtedly seen the
> pictures of the Colorado Springs lab with the external tower with the
> hanging sphere - this was the raised capacitive test setup.
> It is interesting to note that tesla spent a lot of time and work making an
> exhaustive study of elevated capcitances, it therefore was of great
> importance to him.  You would think that the higher the sphere was above the
> earth, the lower the capacitance, but if you plot the values Tesla got, then
> you see this is not the case.  There is a trend toward lower capacitances,
> but only at the maximum altitudes.
> Bert  


There is a rule of thumb.  Don't over use it, though.  If a body is over 
30 of its maximum dimension distant from any other body, then its 
apparent isotropic capacity is stable and may be considered to be fully 

Richard Hull, TCBOR