First Light

From:  Jim Lux [SMTP:jimlux-at-earthlink-dot-net]
Sent:  Thursday, April 16, 1998 12:29 AM
To:  tesla-at-pupman-dot-com
Subject:  First Light

In response to a challenge from a coworker to build a small demo coil, I
built a coil today from parts I had lying around. Not a very sophisticated
design, but it was fast to build. With the most cursory tuning and
adjustment, it produces 8-9" sparks.

Primary: Pancake of 1/4" copper tubing, 8.5 turns tapped at 6 turns, .5"
between turns (center to center), innermost turn at 3" radius. The primary
form is a piece of 1/4" plexiglass (acrylic) with holes drilled on the
diagonals every 1/2" starting at 3". Small nylon wire ties were used to
hold the copper tubing down. Calculated L is 11 uH (-at- 6 turns) to 20 uH The
primary capacitor is a pair of 2700 pF, 40kV doorknobs (I know they are
lossy, but, I had them lying around)

9000V, 30 mA NST. Gap across the NST output: a crude variant of a RQ gap,
made from three 2" sections of 1.5" diameter copper tubing, set up as 2
series gaps, about .0625 inches. The gap was set to just arc over with the
primary circuit open. 

Secondary is 500 turns of #24 enamelled wire on a 3" diameter paper mailing
tube, 14.5" long. Sprayed with 2 coats of clear acrylic. Toroid made from 4
duct elbows (4" diameter) put together, making it about 11" outside
diameter. The center of the toroid was 18" above the plane of the primary.
The bottom of the secondary was 1/4" above the primary.

Everything was wired with GTO-15 neon wire. It took about 8 hours to

Performance: The copper tubing in the gap gets pretty warm after running
for a while. The doorknobs get a bit warm (maybe 5-10 degree rise) after 15
minutes of running at full power. Changing the primary tap by 1/2 turn
kills the output.

Using a sheet metal screw taped to the "toroid" with the point out to force
breakout, thin violet streamers extend about 8-10 inches. It gets 8-9 inch
arcs to the end of a grounded piece of 1/4" tubing placed near the toroid. 
It lights up a fluorescent tube to easy visibility in normal room lighting
up to 6-7 feet away.  RF interference is quite noticeable.

The secondary mounting is quite rickety now, so I'll probably wind a new
one on some PVC pipe I found out in the scrap yard. I'll try to get the
resonant frequency down a bit, and build a better toroid.

It seems to work fairly well as a demo coil.

Easy/quick suggestions for improvement welcome, I don't want to spend too
much time on this particular unit.