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Re: Formula for self C of a Coil (not Medhurst)



Hi Jim,

Thanks for your reply.

I believe your just the man I need for the formula of the mutual inductance
of two helical coils. If so can you send it to me or a link. Thanks in
advance.

So I can assume there are  analytically solutions for  cylinder above a
ground plain.

The Medhurst  formula generates, for a square cylinder with either the same
volume or area of sphere, approximately half the C of the sphere. A cylinder
with the same diameter of a sphere must have a H/D ration of 2.5 to have the
same C as the sphere. I find this  difficult to believe even if the cylinder
is a tube. But I know intuition is a poor substitute for field equations and
integration.

I have seen formula for the C of solid cylinders in round and square
cavities. They may have been derived empirically.

Regards Bob.



-----Original Message-----
From: Tesla List <tesla-at-pupman-dot-com>
To: tesla-at-pupman-dot-com <tesla-at-pupman-dot-com>
Date: 25 April 2000 18:23
Subject: Re: Formula for self C of a Coil (not Medhurst)


>Original Poster: "Jim Lux" <jimlux-at-jpl.nasa.gov>
>
>Medhurst is an empirical formula determined by a tabular fit to measured
>data, and as such, is entirely composed of fiddle factors.
>
>I note that if you try to analytically calculate the capacitance of a
>cylinder above a ground plane, you get results very similar to Medhurst.
>There are several ways to look at the integration problem, for instance,
>consider it as concentric cylinders where the space between the cylinders
>varies as a linear function of distance along the cylinder. However, the
>integration gets real evil real fast.  Numerical methods will come up with
>results similar to Medhurst.
>
>The field around a cylinder is sufficiently different from that of a sphere
>that a discrepancy between the two isn't surprising.  For instance, if the
>cylinder is long and skinny (a sort of limiting case), the field drops off
>as 1/log(r) (I think) as you move away.  The field drops off as 1/r for a
>sphere.
>
>Check the archives.. there was some discussion about this about 2 years
>ago.
>
>----------
>> From: Tesla List <tesla-at-pupman-dot-com>
>> To: tesla-at-pupman-dot-com
>> Subject: Formula for self C of a Coil  (not Medhurst)
>> Date: Tuesday, April 25, 2000 1:25 AM
>>
>> Original Poster: "Robert Jones" <alwynj48-at-earthlink-dot-net>
>>
>> Hi all,
>>
>> Does anybody have the formula or a link for the capacitance of an
>isolated
>> cylinder that is not due to Medhurst?
>>
>> My suspicion is that the Medhurst formula is incorrect or it contains a
>> fiddle
>> factor. If you compare the values it generates to the C of spheres it
>> appears
>> to underestimate  C by at least a factor of 2.
>>
>> I believe the formula actual produces a value that when used with the
>> inductance of a coil  correctly  calculates the resonance frequency but
>it
>> is not the self C of the coil.  Its just an imperical relation to
>calculate
>> Fr.
>> I have assumed the formula in wintesla is an accurate Medhurst formula.
>>
>> Thanks in advance.
>>
>> Regards Bob
>>
>>
>>
>>
>>
>>
>>
>
>
>