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

[dave pierson <dave_p@xxxxxxxxxxx>]

> 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.
     I would expect any coil to have a 'resonance', with its own
     stray capacitance, with any condenser which might be present,
     etc.  These resonances may be 'weak' weak, but are expected.
     There may be more than one.  (Playing with spectrum
     analyzer, sweep generator etc will demonstrate.)

     Once one gets to sparks and coils 'pure' AC and pure
     'DC' are not common.

> 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.

     Perhaps the higher voltage, forcing/providing corona 
     more edges of the spinner accounts for the differences?
     At least as much as the 'ac-ness' or dc-ness?

> The difference perhaps is that a properly resonant system will be AC
> rather than a pulse of DC.
----------------
> 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...
> 
> https://www.youtube.com/watch?v=j0A0-izBdnI
> 
> 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.
     My recollection is the voltage at which corona formation
     takes place is measurably different depending n polarity.

> 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.
    I would, rather, expect differences.  For one thing the
    lower powered coil will not get so far into corona,
    or breakout?   
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