RE: How to Tune a Flat Spiral Coil

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

Original poster: "David Thomson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <dave-at-volantis-dot-org>

Hi Paul,

I inspected the coil much closer today.  You are most likely correct about
the number of turns.

The construction technique I used is 1/4" Plexiglas over 3/4" plywood.
Before winding I coated the surface with freshly mixed Envirotex Lite.  The
center leads are fastened by the center 1/4" brass bolt.  The Plexiglas is
bolted with the center bolt and large weights were placed over the
Plexiglas.  This made a blind condition for winding the coil.  Without the
weight, the wires would cross.  Apparently the weight is not good enough,
for as the wires don't cross, they still bunch up.  This trifilar is
essentially a three layer, space-wound coil, wound with three individual
wires.  I had to use a magnifying glass to notice this because it looked
quite uniform otherwise.  I can't confirm how many turns are in the coil
without cutting the coil with a saw and counting the layers.  I'll leave
this as an option, though.

Using the Ohms per foot of the wire and a more accurate average radius of
6.8125", I came up with 227 turns per wire.  You can't see anywhere near
that many turns in this coil, but I agree with you that it is likely there.

>Two things could cause this: Either or both:
>a) The effective inductance is very high, due to the action of internal (in
this case radial) capacitance.  We see this effect is short H/D solenoids,
and I rather think it may be much more pronounced with the spiral,
especially a narrow one!!!
>b) The total external capacitance of the secondary is much greater, either
because this is a characteristic of the spiral itself, or because Dave's
setup is adding extra C, eg by the coil being sat on a table, proximity of
primary, or something like that.

My coil is mounted on a 4" PVC pole 24" off the ground.  The capacitors and
spark gap are 24" below the lowest point of the flat spiral secondary.

>A very good point. Take Dave's 250kV over 220 turns, thats rather higher
than in a conventional TC.

I'm using a 15KV 60mA NST.  I'm sure I'm well over 300,000V between the
primary and secondary.

Dave