Coil notes

["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 All,
 
I've been doing some experimenting with different coils tonight.  In particular
I've been experimenting with flat spiral and solenoid combination secondaries. 
I'll give the coil particulars if anybody is interested, but the general
effects were worth passing on for others who want to experiment with flat
spiral / solenoid combination secondaries.
 
So far I have gotten my best results from making the solenoid coil three times
the wire length of the flat spiral coil.  Using a 250 watt NST and magnetically
quenched single static spark gap, I managed to get about 8 simultaneous and
continuous streamers about 12" long from the terminal.  This is straight
radiation without a ground lead to draw from the terminal.  All the streamers
were equal distances from each other and emanated from the top hemisphere of
the copper ball.  In this coil combination the flat spiral was 25" diameter
wound with 12 gage wire and the solenoid was 48" x 3.5" wound with 21 gage
wire.  
 
I had tried a shorter solenoid on the coil and the results were streamers about
5" long and in the same pattern.  To compare with this I built a 14" diameter
flat spiral with about a 28" x 1.75" solenoid.  The wire was 21 gage all
through.  I intentionally chose lengths that were not proportional to see if it
made a difference.  It wasn't possible to tune this coil at its resonant
frequency but I could tune it to a much higher frequency using a smaller
primary capacitor.  In this resonant case, there were no sparks coming off of
the terminal, but it would light a fluorescent bulb brightly about 18" away.  
 
I could also tune the larger combination coil at the resonant frequency and it
would start by emitting streamers but then I could hear the coil come into
resonance and the sparks would disappear.  This coil would light a fluorescent
tube several feet away with no sparks of any kind coming from the terminal.  In
order to get the larger coil to put out streamers I had to tune just above or
just below resonance.  This fact was verified with my oscilloscope and
frequency meter.  When testing the coil I would connect both positive leads
from the frequency generator and oscilloscope to the outer lead of the flat
spiral and connect the ground leads of both pieces of equipment to ground. 
When testing all the flat spiral / solenoid coil combinations there was just
one resonant point characterized as a dip in voltage between the sharp voltage
peaks.  It is when the coil is tuned for either of these voltage peaks that the
streamers emanate from the terminal.  When the coil is tuned for the sharp dip
between the two voltage peaks the terminal does not emit streamers when
resonance is achieved, but it radiates a strong EMF.  
 
I still don't have enough data to conclude whether the longer wire length
contributed to the longer sparks, or whether it was the proportion of the flat
spiral length to the solenoid length, or both.  But so far my suspicion that
proper proportions and long wire length both contribute to combination coil
efficiency.  On all these experiments I used the same 250 watt NST and spark
gap and adjusted the primary capacitance and primary turns as needed.
 
Dave