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RE: One-shot coil? (fwd)
---------- Forwarded message ----------
Date: Fri, 13 Jul 2007 21:35:16 +0000
From: Jeff Behary <jeff_behary@xxxxxxxxxxx>
Subject: RE: One-shot coil? (fwd)
I've done experimenting along this line before.
The "single flash" method was used at one time to produce very rapid
radiographs using both TCs and induction coils. It was used to photograph
the heart early on when film (glass plate negatives) exposures were slow and
a normal x-ray would make a blurry image because it would capture the heart
beating several times.
I tried this method to make electric autographs on film using Tesla Coils.
I took a 1KW coil and placed the smallest fuses I could find in series with
it (fractions of an amp). By throwing the knife switch a single "bang"
occured and allowed to capture onto the film the minute details of the
discharge that would have been lost by surrounding sparks with even a 1
second exposure, and weren't bright enough to show up by a normal
The fuses would blow instantly but not too quick for a nice photo and a nice
For reference, the same photo produced by closing the opening the switch as
fast as possible produced this:
Its difficult to discern the sparks on this one! Still a neat photo though.
To try the same thing with a second exposure or more resulted in the film
catching on fire from the discharge.
The coil was a bipolar Pancake. The cap was huge - .6 mfd! The transformer
was rated for 1,000V at around 1500 mA. The gap was 1/4" tungsten, with
copper finnned heatsinks.
In experimenting with Tesla Coil tuning, I was noticing that with very small
input currents and large capacities , slow spark gaps, the spark output
wouldn't necessarily grow longer but would branch differently. The corona
effects would also change gradually. By changing the capacity, the shapes
of the discharges would change too. The sparks began to look like static
electricity, but not exactly - like a merge between a static spark and a TC
One experiment was with very low currents (4000V 15mA or so stating out...)
but high capacities (.15 mfd+), with a spark gap in the form of a
Depending on the distance of the gap compared to the input power, you could
get it fire similar to a normal spark gap or as slow as 1 "bang per second"
or even one every 2 seconds. These slow "bangs" produced some interesting
sparks similar to those in the film exposures above, but shorter in length
from the lack of current.
(When the gap was operated in a normal fashion, it produced a longer and
steadier discharge - this is seen in the movie below). The gap is clearly
seen in action, although at one point the discharges were in synch with the
shutter speed of the camera (30 frames/second) and for some reason the flash
of the discharge is seen but the spark is not! On another movie with the
gap adjusted a bit this isn't the case and the sparks of the gap can be seen
Photograph of the spark produced with the small bangs:
My experiences were that with full power and a quick bang you get a spark
very similar to what the machine would produce normally. With low input
powers and a big bang you get smaller sparks with a greater variation in
Jeff Behary, c/o
The Turn Of The Century Electrotherapy Museum