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# Re: Capacitance of a coil

```Tesla List wrote:
>
> Original Poster: Hollmike-at-aol-dot-com
>
> Ok, for all you diehards who want to try to best the Medhurst formula, I
found
> this equation in an ooooold electrical engineer's handbook(copyright 1914,
> 1922, and 1936 John Wiley and Sons).  I have the 1936 edition.
>    This equation was tested on inductors ranging from 1 - 200uH and matched
> the measurements at 30MHz with good agreement(whatever that meant in those
> days).
>     I have NOT used this or even tried it out, so someone can tell me if
it is
> worth anything.  Here goes:
>
> Cself = pi*D/(3.6cosh^-1(s/d))    in pF
>
> where D = the coil diamter(in cm), s = pitch of widing(in cm) and d = bare
> wire diam (in cm)
>
> In excel, that trig function would be ACOSH (or the inverse hyperbolic
> cosine).
>
> A further note in this book states that the important parameters in the coil
> capacitance are the diameters of the winding and the ratio of the pitch
of the
> winding to the diameter of the bare wire.  The capacitance is practically
> independent of the number of turns.  Of course, the wire insulation
thickness
> and dielectric constant  and the form on which the coil is wound will alter
> the capacitance of the coil.
> If you try this, Let me know how it works.
> Mike

Mike,

The winding length of the coil is not a parameter in this equation??
When I use Medhurst's formula on my 10" coil, I come very close (15.88
pF) to the measured value (based on reverse calculating from Ls and Fo).
However, if I use the above formula, I only get 3.88 pF. Is there an
error in the algebraic representation above?

BTW, is the book Pender and Delmar's, "Electrical Engineers' Handbook :
Electric Communication and Electronics"? I've got volume IV (Power) but
not volume V. Is volume V worth getting, and how does it compare with
Terman's Radio Engineers Handbook?

-- Bert --

```