Re: Second Commandment

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

Original poster: "Kennan C Herrick by way of Terry Fritz <twftesla-at-uswest-dot-net>" <kcha1-at-juno-dot-com>

[snipped]
 
Herrick comments interspersed...
 
> Freau:  Ken, all,
> 
> I do think that CW coils tend to create a lot of corona on the
> surface of the coils, which makes this type of breakdown more
> likely, compared to spark gap coils.  On my 36" spark VTTC,
> I had a lot of problems with corona, that I never had on a spark
> gap TC.  CW coils seem to be plagued with corona and flashover
> problems, if there's not enough spacing, etc.  And I used a 
> breakout
> point.  Letting the energy build up before breakout will make the
> corona even more intense I suppose. 
> 
> I often used no coating on my coils, but sometimes
> I did, and I didn't see any difference either way.  I'm not sure 
> about
> the effect of horizontal sparks in a tube coil, my guess it 
> wouldn't
> make any difference, but in spark gap coil, the sparks are 
> generally
> horizontal anyway.
> >  
> > Herrick:  Have small height:spark-length coils been sucessfully made where 
> spark
> >  breakout is allowed from the periphery of the toroid?  If so, I 
> wonder
> >  what it is--if not coil-turns protection--that keeps the spark 
> from just
> >  going to ground via the shortest way, i.e., down the coil?
> 
> Freu:  Yes, many coils of that type have been built.  It is especially at
> high breakrates that the sparks can reach 4 or 5 times as long
> as the secondary winding length.  This is because the bang size
> is relatively small in this case.  A 120 bps coil cannot give a
> spark that is 4 or 5 times the secondary length as easily.  My
> 23" tall secondary begins to break down when the sparks reach
> about 55" to 60".
> 
> I think it's because of the growth of sparks phenomonen, and the
> field shaping effects.  Once a spark emits from the toroid.... and 
> this
> spark is usually shorter than the coil's length.  It has determined 
> 
> the direction of the streamer.  Then as the spark length grows over
> successive bangs, it adds to the existing streamer length.  So in
> effect, the streamer is being shepparded outwards, rather than
> downward.  Also the field around the coil, helps to keep the sparks
> away from the coil's surface.  This applies to spark gap TC's.  But
> in the tube coils, the same thing occurs.  The spark starts small,
> then grows, and is guided to continue in the direction that it 
> starts,
> at least when fed with raw AC with a breakout point.  In the case
> of your coil running on DC, without a breakout point, things may
> be a little different.
 
Herrick:  Perhaps what happens is this:  
 
1.  Breakout occurs at a low enough voltage so that arcing along the coil does
not occur.
 
2.  The spark's path becomes very hot, of course, and some of the heat remains
after the spark stops.
 
3.  In higher rep-rate devices, the next spark comes along and that spark goes
in the same direction due to the residual heat of the previous spark's path; it
takes less energy to re-heat that path so that is where the next spark tends to
go.  The presence of ozone there might also be a contributor.
 
4.  If initial-breakout voltage is allowed to be too high in relation to the
coil height, e.g. from a larger-radius toroid, then the spark may initially go
along the coil instead of thru the air--and then it will keep going that way.
 
5.  I'd think that the finite decay-time of spark-ionization, after the
initiating energy has gone away, would not be much of a contributor to this
phenomenon since I understand that to be only on the order of tens of
microseconds--short compared to the usual intra-spark interval in Tesla coils. 
I have first-hand experience with that particular phenomenon as regards my
late-lamented commercial product, the Neon Bubbles neon-sign "transformer",
which produced segmented luminosity (the "bubbles") in neon signage.  There, in
order to capitalize on the decay-time phenomenon for that effect, I needed
rep-rates, i.e., excitation frequency, in the low KHz.
 
In my system, I've been keeping the rep-rate relatively low (up to ~10/sec,
variable at will) because I have wanted to maximize the breakout voltage & thus
the power put into each spark (by allowing the storage capacitors to fully
recharge between sparks)--thinking that that's the way to get longer sparks. 
But perhaps I need to concentrate on spark breakout at lower voltages while
trying at the same time for a greater rep-rate, so as to take advantage of the
successive-bang phenomenon that you describe.  Since I utilize current-source
mains supplies, I can readily crank up that rate--at least up to 120 Hz which
would be the limit due to my present timing scheme--without overload.  I
briefly tried that this a.m., going up perhaps to 50/second or so, and noted no
particular difference in the appearance or length of the spark from the 3"-dia
breakout ball or the toroid (but I did enjoy that buzz!).  In fact, the
greatest length seemed to occur at around 10/s but other factors were probably
at work; I haven't spend much time at it so far.
 
> >  
> >  Herrick:  I don't understand "a point must be used on the coil" in your 
> comment...
 
Herrick:  Could you clarify that?
 
 
[snipped]
   
> Freau:  Did you look at my 36" VTTC spark photo at my webpage to
> see if they look something like your sparks?
> 
>    
<http://hometown.aol-dot-com/futuret/page2.html>http://hometown.aol-dot-com/futuret/
page2.html
 
Herrick:  Yes--very nice site.  Several observations:
 
1.  I note that all sparks/breakout points are located upward, away from the
coils.  Have you had success, with those coils, in having sparks break out
essentially horizontally--from a toroid, say?
 
2.  The sword-like spark is especially nice.  The particular sword is, I find,
the Maylay creese, or kris.  Very wicked.
 
3.  The other discharge you show is, I suppose, the product of a
time-exposure.  Since my eyeballs don't work that way, it would be hard for me
to make a comparison with my sparks, at this juncture.  Each of my sparks is
branched and what I would characterize as "bushy"--at least, when I make the
spark duration ~5 ms.  With a ~300 us duration, the spark is thin and more
blue.
 
4.  I see an apparent division in one of the coils.  What is that?
 
5.  Just for my edification, did you create the site using some kind of service
from AOL?  I use AOL occasionally & have never yet looked into making a Web
site myself.
[snipped]
 
Ken Herrick