Re: Inductance of a conical coil

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

Original poster: "Antonio Carlos M. de Queiroz by way of Terry Fritz <teslalist-at-qwest-dot-net>" <acmq-at-compuland-dot-com.br>

Tesla list wrote:

 > Original poster: "Barton B. Anderson by way of Terry Fritz 
<teslalist-at-qwest-dot-net>" <classictesla-at-netzero-dot-com>

 > Nature will know, but how accurate is your meter and can it read that low?

I don't have an inductance meter. To measure inductance, I resonate the
coil with a known capacitor. A problem may be the influence of
connection
wiring in these measurements of uH inductances.
I made a flat spiral coil today, to be used with the secondary that I
used in my transformerless coils in a conventional Tesla coil.
I planned it to have 58.4 uH, by Wheeler's formula. Fantc predicts a
bit less: 57.3 uH. The dimensions of the coil were: rmin=7 cm,
rmax=12.5 cm, 14.7 turns, #18 wire (more than enough for the case).
The design coupling coefficient with the secondary was 0.105, for
mode 9:10.
The measured coil measured as 60.20 uH, and the resulting coupling
coefficient for the design geometry ended as 0.125, lowering the
operating mode to close to 7:8.
Looking at what was wrong, I noticed that to obtain this inductance
I have just to increase the number of turns to 14.93. This is less
than 20 cm of wire, and in the measurements I certainly had more than
this. The increased coupling is more puzzling, but it was evidently
changing depending on how I was arranging the connections of the
coils. Theoretically, I could obtain this by rising the primary
coil by less than 1 cm.
The coil worked very well as primary in initial tests. I tuned the
coil by changing the length of an antenna in the secondary terminal,
as I prefer to do. 15-20 cm multiple streamers at the antenna tip,
with a 5 nF primary capacitor and a 5000 V, 30 mA NST. A bit better
that what I obtain with a directly coupled version (that operates in
a much slower mode). I will set up a page showing the coil soon.

 > I was thinking about the flat spiral case. Have you considered looking at a
 > mutlilayer formula? By "concept", the flat spiral is similar to a
 > multilayer coil, except the length of the coil doesn't really play a
 > significant role since the length becomes the wire size. I expect
 > multilayer formula's are derivitives of Wheeler, so maybe just stripping
 > down of what is not needed.

Maybe. I will take a look.

Antonio Carlos M. de Queiroz