Essex magnet wire

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Gary Johnson by way of Terry Fritz <twftesla-at-uswest-dot-net>" <gjohnson-at-ksu.edu>

As I report to this list aperiodically, I am working on a solid state driver
to drive a Tesla coil secondary directly at the base (no primary).  A
knowledge of the input impedance is very important in this application, and
I have been side tracked for some months trying to make sense out of my
measurements.  I reported a couple of months ago that humidity was a factor,
and I now have some more information to share about humidity.

I bought a spool of 14 gauge Heavy Soderon magnet wire from Essex in 1994. I
recently wound a coil from this spool on a section of 8 inch PVC pipe, close
wound.  I put the coil in an oversize styrofoam box with an electric skillet
filled with water, and measured the input impedance as the relative humidity
rose from 30 percent to 100 percent.  The impedance rose by 17 percent, but
the dc resistance rose 13 percent because the temperature increased.  A 4
percent difference is close to my measurement uncertainty, so I cannot be
positive that humidity has even a small effect on this coil.  There was
condensate on the winding when I took the panel off.

When I was getting into this impedance measuring activity, I bought three
more spools of Heavy Soderon magnet wire from Essex, in 1997, (16, 18, and
20 ga).  Doing the same test with coils from these spools resulted in
changes in input impedance from 200 to 300 percent, as humidity rose from 30
percent to condensing.  It is my guess that the wire coatings on these
spools soak up water from the atmosphere, and that water in the coating
contributes a significant fraction of the total impedance (maybe 20 - 30
percent) even at ambient relative humidities below 50 percent.  It is also
my guess that Essex made some minor change in manufacturing process between
1994 and 1997 that caused this vast change in hydroscopic or hydrophillic
character in the Heavy Soderon coating. 

It is also my belief that input impedance is an important parameter in
disruptive Tesla coil operation.  I see reports on this list of some coils
that just do not work as well as it seems like they should, and I would not
be surprised if some of these were built with water absorbing magnet wire.
I don't know of any way to tell if this is a problem by just looking at the
wire.  One would need to measure the input impedance or the Q in
non-disruptive mode for two different humidities.

Lots of gotchas in this business!

Gary Johnson