Spaced Coil measurements.

• To: tesla-at-pupman-dot-com
• Subject: Spaced Coil measurements.
• From: "Malcolm Watts" <MALCOLM-at-directorate.wnp.ac.nz> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
• Date: Mon, 31 May 1999 18:27:55 -0600
• Approved: twftesla-at-uswest-dot-net
• Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

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 
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 
  fails
- 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 
badly.

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

Malcolm

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