# Re: Spaced Coil measurements.

```Hi Malcolm, All,
Using your measurements I get 18.25 pF from Medhurst's formula.  I
plotted your data in Excel as 1/(f^2) VS C according to Howe.  A
linear trend line fit to the data crosses the axis at about -50 pF.
50 pF seems a bit high.  You gave Ns = 38 turns for the whole coil?

----- Original Message -----
From: Tesla List <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Monday, May 31, 1999 8:27 PM
Subject: Spaced Coil measurements.

Original Poster: "Malcolm Watts" <MALCOLM-at-directorate.wnp.ac.nz>

Hi all,
As promised, here are the measurements (and they do contain a
surprise):

The Coil:   Hs = 31.5",  Ds = 12.3",  Ns = 38 turns  Dwire = 0.8mm
Winding pitch = 1.2 TPI,  former = thinwall PVC tubing

According to Wheeler, Ls = 147.5uH
"      " Medhurst, Cs = 17.8pF (or thereabouts - I estimated "H")

Measurement of L was rendered impossible by the stray electrical
signals flying round, mostly from local radio transmitters :(  I was
bitterly disappointed by this and will figure out some other way to

However, if one takes Medhurst's Cself as gospel and adds that
to the shunt capacitances listed below, one gets a remarkable result:

Bare Coil,    w. 100pF shunt    w. 228pF shunt    w. 336pF shunt

Fr    2.9MHz          1.113MHz         773kHz           644kHz
Ls    169.2uH         173.6uH          172.5uH          172.6uH

Now that I hadn't expected. Possible interpretations:

- Medhurst works and lumped rules hold good for this coil BUT Wheeler
fails
- Suppose Wheeler is correct. Then Medhurst fails to give the
correct figure for Cself (if this were true, reading #1 suggests

Until Ls can be measured directly and definitiively, no firm result
but something clearly doesn't work properly for this case.

Malcolm

```