Re: [TCML] Low power Tesla Coils producing DC spark phenomena

[Jeff Behary <jeff_behary@xxxxxxxxxxx>]

Pete - In the large induction coils, the voltage used is in the "break" of the circuit, not the "make"...it's actually
the collapsing field when the points are open that does the work.  You are correct in that its more DC, the difference is that when the contacts
"break" there is a very high voltage that is useable and when the points "make" there is a lesser voltage pulse in the opposite
direction that tends to interfere with some experiments (like X-Rays).  In the "make" the voltage is less because it 
is only the turns to turns ratio of the input voltage (12V etc) whereas on the break the back EMF might be a few hundred
volts (and therefore a much higher voltage induced in the secondary).  For a large coil it may be like a pulse of 20kV on the "make"in one direction and 100kV on the "break" in the other direction...a spark gap in series with the output set to 20kV
would eliminate most of the unwanted 20kV in the wrong direction...  and give a really clean DC output.
In the case of an old X-Ray tube, the difference is seeing the bones clearly in a fluoroscope vs. seeing very little contrast between
bones and flesh.  It's amazing that a small gap is all it takes to make a world of difference in how the tube operates...and visually,
the tube fluoresces in the same way and looks no different.  Yet the X-Rays produced are like night and day.  Worthless vs. useable.

Agree on the low power coils, the sparks alone in low power coils tend to have DC spark leaders at the tips, where with large powers
these leaders are consumed by the sparks themselves finishing the path (if that makes sense...?).  I'm sure Bert can comment more.
Also agree in slightly unturned coils these DC effects are more present as the spark discharges are more erratic at the terminal.

But...specifically for this post, the TC has been removed from the circuit.  The DC polarity is actually coming from the tank
circuit itself.  Much like the induction coil running from DC, the small NST run from AC is (at times) having an output of DC,
how pure seems to be a function of the length of the spark gap used in the circuit.  At times it is DC in one direction, 
at other times in the other direction, sometimes fairly pure, sometimes impure with a predominant pole in one direction.
The polarity indicating tube is showing this well.  It goes in line with past observances that micrometer adjustments of
these gaps (thousands of an inch or less) have a drastic change on the actual output of the streamers in the air.  

If you have seen the Kinraide spark photos on my site, you can see things like 1/2" thick forked branches in some photos,
very well defined patterns.  In the case of operating these flat coils, whether these branches are 1/8" thick or
1/2" thick depend on nothing more than these minute changes of the gap.  What has been so strange in deciphering the reasons
of this have been that the spark gap as you open it over its full functional range will allow these thick branches at
various settings of the gap with little reason.  Let's say for conversation if the gap is set to .070", .100", or .130"
they appear beautifully.  And if at .069" or .071" or .099" or .101" they appear thinner and more diffused.  Change the
capacity of the condenser by a small amount, .001 mfd, and the same thing happens but at different gap settings,
a few thousandths of an inch or less being all that's required to change the condition from one to the next.
The reason is now clear that its the DC being fed to the coil causing this...the question for me is the reasoning
as to why the output is the way it is.  I'm feeding the 3.5kV 8mA transformer with regular 120V from the wall.
If I were to feed a regular 30mA NST with a variac and series reactance coil to limit voltage and current the exact
same thing happens at various settings on low power...

At low powers, say 30W or less, NSTs can give DC output, and fairly pure at some critical settings. It isn't usual
to have a spark gap adjustable in such small increments, especially less than .001", but when you do, you can have nearly a pure
rectified output at some settings.  At what's unusual is that at some settings the polarity of this actually reverses.  Which is
why, in discussions with Bert and others off-list, I have seen pure positive discharges in one direction and then after
a minute adjustment negative discharges with positive forming at their tips.  The same phenomena can be seen in the old
glass negatives.  

If anyone has a proper scope, a good experiment, find one these baby NSTs or use a 30mA and connect a ca. .01-.015 mfd cap to the xfmr,
limit the voltage and current and monitor the wave form of the output with a small spark gap in series.  
You will see the changes I am mentioning.  It's kind of an interesting surprise really.  
If anyone can really analyze things professionally, I can offer to lend a micrometer gap with a dividing head capable of
adjustments less than 1/10,000th of an inch.  Its an interesting thing...and explains why some really strange
observances have been witnessed by Kinraide, myself, and those who have seen the stuff up close here.  Jeff


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I could be completely wrong here but a few thoughts on DC in Tesla coils.

Firstly, an induction coil is fed by a sharp spike in one direction as  contacts meet and DC is fed to the coil primary. The back EMF when the  contacts open is dampened by a "condensor" across the points.  The induction  coil should give an asymmetrical output and be more DC in nature. One commercial induction coil advertising 100kv had a positive and negative  polarity switch, presumably on this principle.  Note there is no resonance  to give an AC ringing to counteract the DC. Secondly I believe that small TC's tend to be a bit polarised and I base  this on the performance with an ion spinner. These work great on my small  briefcase TC which is not tuned.  They do not work at all on my medium TC  with 4 foot sparks.  Even with a 14 foot spinner just 60 kV DC will send it  happily spinning but the TC will not budge it in the slightest. The  difference perhaps is that a properly resonant system will be AC rather than  a pulse of DC.  		 	   		  _______________________________________________
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