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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>
To: tesla@xxxxxxxxxx
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 
spark too.

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 
point-plane rectifier.


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 
their appearance.

Jeff Behary, c/o
The Turn Of The Century Electrotherapy Museum