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

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.

Jes sayin'.
(new bigger site has taken 2 years and should be ready in 6 months)

-----Original Message----- From: Jeff Behary
Sent: Monday, September 02, 2013 11:34 AM
To: tesla@xxxxxxxxxx
Subject: [TCML] Low power Tesla Coils producing DC spark phenomena

Most know the story of how I came across the glass plate negatives of TB Kinraide, and the mysteries discussed over the years concerning why his spark photos made with high frequency coils showed distinct positive and negative polarities. In replicating his work, the output my low power coils was exactly the same as his, faint brush or corona discharges of unusually distinct shapes, 1/2" thick forked positive discharges and unusual feather shaped plume-like negative discharge - often appearing with little logic as to why, united back-to-back, and even changing polarity with even minute (1/10,000th of an inch) adjustments of the spark gap.

The following video shows a 3.5kV 8mA NST connected to a .01 mfd condenser. A small micrometer spark gap was inserted in series with an Oscilloscope/Ondoscope tube used to show polarity for induction coils. These tubes were little more than geissler tubes with two very closely spaced wires that almost meet in the middle. When operated from oscillatory currents both wires glow evenly (as in the start of the video). When operated with pure DC the negative wire lights brightly. When operated with "dirty" DC (as in an induction coil with a lot of "inverse currents") the negative lights as well as part of the positive wire. In the early X-Ray days spark gaps were often inserted in series with the X-Rays tubes to correct the inverse currents, and when an oscilloscope is used to show polarity the condition will change from an impure DC to a pure DC once the gap was opened beyond the voltage of the induced currents in the "make" of the circuit. This filtered them out to a certain
extent and left only the desirable "break" currents...


In the video you can see that the polarity is rather distinct positive or negative at some moments and partially DC with oscillations present at other times, and still purely oscillatory at other times. The only thing changing is the length of the spark gap.

So at low powers, there is a sort of rectification taking place somehow with the inductance of the transformer and capacity of the cap in combination with a spark gap. By replacing the tube with a Pancake Coil of relatively few turns (2 1/2 turns on the primary, 200 on the secondary) these unusual discharges take place, forked branches 1/2" thick in all directions and over a foot long under certain conditions of the gap when favorable. At other instances shorter 2-3" negative plumes can be seen interspaced, often with a positive oscillation occurring from the surrounding air. And still at other instances it operates like a normal oscillatory Tesla Coil. It explains the unusual nature of Kinraide's spark photos. The sound of the spark gap is unusual as well, some of these effects can be heard or distinguished by the sound of the gap without even looking at the coil/tube. It's strange, but consistent.

I've shared this in person with some of the top lightning experts in the country. The distinct polarity of little more than a normal TC circuit operated with low powers was not expected, but definitely is the case. A proper explanation might be given by others, Bert I'm sure will comment better. Funny to think a low powered Tesla coil actually behaves differently (and drastically so) from a normal powered coil.


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