# Re: Isotropic secondary capacitance

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From: 	Malcolm Watts[SMTP:MALCOLM-at-directorate.wnp.ac.nz]
Sent: 	Sunday, September 07, 1997 5:05 PM
To: 	tesla-at-pupman-dot-com
Subject: 	Re: Isotropic secondary capacitance

Hello Jim,

> From:   bmack[SMTP:bmack-at-frontiernet-dot-net]
> Sent:   Saturday, September 06, 1997 10:07 PM
> To:     tesla list
> Subject:    Isotropic secondary capacitance
>
> Fr Tom,
>
> A very astute observation on the nature of distibuted
> capacitance animal!  That surface area capacitance could
> calculated(?) using the "free air capacitance" equation for
> the coil area and the standard  "plate equation" with curvature
> of the wire considered, to find the inter winding capacitance.
> Am I on track here?
>
> Now, are these two entities in parallel or series with respect to
> each other? Maybe a little of both? Ouch!
>
> When I run very low power tests to tweek the systems, placing ones
> hand near the coil body has as much effect on the resonance as getting
> near the top cap. This in my opinion clear evidence of isotropic activity.
>
> With a little help from our friends, there might even be a way to reliably
> predict the distributed capacitance mathematically!

There is indeed!! You'll be pleased to know that after a three month
search while researching a TC article for a magazine, I found it. It
is Medhurst's formula:

C = HD pF  where D is coil diameter in cm and H is a factor that
follows the law:
h/d = 2     H = 0.51
h/d = 5     H = 0.81
Interpolation is linear for this h/d range. H bottoms out at 0.46 for
an h/d = 1 (which as fate would have it was the final form Tesla's
extra coil took :)  The tabulated values for H remove the complex
mathematics from the formula.

Malcolm

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