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

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
get a direct reading.

      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
- Suppose Wheeler is correct. Then Medhurst fails to give the
correct figure for Cself (if this were true, reading #1 suggests
Cself = 20.4pF, reading #4 suggests Cself = 78pF). This contradicts

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