Re: "Gas burner" corona from STSG driver

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-qwest-dot-net>" <evp-at-pacbell-dot-net>

Tesla list wrote:
> 
> Original poster: "Kurt Schraner by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <k.schraner-at-datacomm.ch>
> 
> John, Gary -
> 
> To my knowledge, what has been used for X-Ray were induction coils, not
> Tesla coils. Probably John meant this (please correct me if I'm wrong)?
> - The design of those inductors took into consideration, to make the
> secondary sparks voltage-waveform with as low "inverse" as possible,
> meaning a waveform with high DC part. This is elucidated i.e. in
> M.A.Codd, Induction Coil Design, E.& F.Spon, London 1922, reprint
> obtainable by PV-Scientific.
> 
> Cheers,
>          Kurt
> 
> Tesla list wrote:
> >
> > Original poster: "Lau, Gary by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> <Gary.Lau-at-compaq-dot-com>
> >
> > Be careful how you state this.  The terminal voltage of a Tesla coil is AC.
> >  The secondary oscillations are centered about zero volts.  Insulated
> > objects near the secondary will develop a DC or static charge deposited
> > upon them, due to the asymmetric conduction (a.k.a. rectification) of
> > corona.  This is what an electroscope will indicate.
> >
> > Gary Lau
> > MA, USA

	These comments are correct.  The output (voltage between the two ends
of the secondary winding) of ANY Tesla coil is a sinusoidal RF signal
with zero mean value (in other words, "pure AC").  The presence of a DC
component would require a DC current to flow in the secondary, which
just isn't possible with a resonance-transformer setup.  [Actually, if
an asymmetric load such as a diode were connected across the secondary a
small DC current would flow in it, but the Q would be so spoiled that
there wouldn't be much coupling and power transfer.)  Consider a
secondary winding with a DC resistance of perhaps 100 ohms and a nominal
output voltage of 100 kV.  In order for the output voltage to be have
even a slight DC component, say 10 kV, a DC current of 100 amps would
have to be present.  No way!

	The "old-time" X-Ray equipment did indeed use a large induction coil
(Rhumkorff or "spark" coil), whose output voltage consists of a large
short spike of one polarity followed by a longer and much lower-voltage
output of the opposite polarity.  As a kid (about 1934, I think) I broke
a wrist and the doctor reset it using an X-Ray setup with a fluoroscope
to watch his progress.  I remember seeing the coil very well, with a
safety gap across it showing some corona discharge.  Seems like pretty
antique equipment for the teaching hospital of the University of
Missouri, but that's the way it was.  In retrospect I remember that the
tube was out in the open, connected to the coil by long helical coils of
wire such as were commonly used in such equipment.  This type of
connection may have been used because the large radius of curvature
reduced corona, because it was flexible, or perhaps just because it
"looked good".

	Having said all of the above, I should mention that I do have an old
magazine article on X-Ray tubes (circa 1918, I'd have to dig it out)
which does mention tubes designed to be excited by a Tesla coil, which I
presume in this case meant the conventional type with which we are all
familiar.  Wonder if anyone has any description of such an installation
- I haven't been able to find one.

Ed