Re: stepped leaders
>>From MALCOLM-at-directorate.wnp.ac.nzTue Aug 20 22:08:59 1996
>Date: Tue, 20 Aug 1996 18:17:24 +1200
>From: Malcolm Watts <MALCOLM-at-directorate.wnp.ac.nz>
>Subject: Re: stepped leaders
> Firstly, I don't even begin to pretend to know all of what is
>going on here. I'm as interested as anyone in finding out more. I
>simply offer a few thoughts for what it's worth.
> Drawing on the previous two posts, I'd like to offer a couple of
>observations. The first is about the Q of the secondary (from Skip's
>post). The Q is high ONLY when no load is placed on the coil.
>Measurements made by others show that it drops by around half when
>corona ONLY is produced. I also have noticed this drop but have not
>quantified it. Prolonged rings still result but not as prolonged as
>when a minimal amount of energy is lost. But an attached spark is an
>entirely different matter. I have scoped the rings as dropping so far
>in number that the multiple beats simply disappear. Since Q is by
>definition the inverse of losses (internal to the coil or external to
>it), you can see that it is not going to ring for very long at all
>when energy is sucked out. Under these conditions, output current is
>maximized, and in the extreme, for each half cycle of primary ring-
>down, the energy that transfers to the secondary is lost in the arc.
> I'd just like to illustrate what I mean. A typical system has
>a coupling constant around 0.1 or so. The condition for critical
>coupling (max power transfer) is given by : kc = 1/SQRT(Qp.Qs).
>Taking the primary Q with gap of around 10, and an unloaded secondary
>Q of around 100 (err on the worst side), this gives an unloaded value
>for kc of 0.031. By definition, critical coupling occurs when k = kc.
>k is set, kc is load/loss/Q dependent. Assuming Qp is constant (spark
>gap), the secondary Q has to drop to 10 to meet this condition. The
>system can do no better than lose energy at the rate at which value of
>k allows it to be transferred from primary to secondary. From this
>you can see where the oomph of a magnifier comes from - a much
>higher value of k (greater rate of energy transfer) with a consequent
>reduction in gap conductions and hence losses. Also, a far greater
>rate of charging the toroid to max potential. I'd suggest that this
>improvement in performance shows just how bad the gap really is.
> The multiple beat envelope one gets shows that the system is
>vastly overcoupled before spark production.
> To something slightly less tomey (phew) - I think using the
>camera is a must if we are to make progress in analysing spark
>behaviour further. The eyes lie - there's no doubt in my mind about
>that. When my coil strikes an attached arc, to the eye the discharge
>goes from a purplish streamer to a bright thick blue white channel
>that slowly rises. The camera tells quite a different story. It shows
>the Richard's banjo effect in detail, and also shows that the
>individual streamers are not not as bright as the eye would suggest.
>This brilliance may be a cumulative time-lapse trick by the brain.
>Ready for the bricks to arrive,
With a very good post like that you shouldn't expect the delivery of
bricks,...unless you are planning to build the foundation for another
Yeah, you seem to agree with me, we need 'high speed eyes' to qualify
very much further the streamer behaviour which we have been
discussing. Too bad household video cameras don't offer adequate temporal
resolution. On the other hand, consider how miraculous they are in the first
place, technologically speaking. They aren't quite as good as we
coilers would like, but 'wow' they sure are helpful and useful
I have to tell you, and remind the audience, that you have had a distinct advantage
over most of the rest of us in your ability to actually 'scope' waveforms
pertaining to coil operation. I have gotten to where I am now
mainly, by mental visualization and then tuning the design for the biggest arcs.
I am ashamed to admit that I have a fairly good test equipment
laboratory which has been in storage for the past 4 years, and due to lack of
facilities or space where I have done all my coiling over the past two years,
I have been prevented me from digging out the old storage scope and getting an
enlightened inside view.
You said: "From this you can see where the oomph of a magnifier comes from
- a much higher value of K (greater rate of energy transfer)
with a consequent reduction in gap conductions and hence
(increasing?) losses. <snip> I'd suggest that this improvement in performance
shows just how bad the gap really is."
Are you refering to the fact that with a higher system K, because the
gap dwell time is reduced, assuming the gap never gets any better per
unit of time than C as a conductor, shorter dwell means reduced net C?
You then conclude: "The multiple beat envelope one gets shows that
the system is vastly overcoupled before spark production." Yes I
agree with you, this seems intuitively correct.
DING, light going on! This is the scientific explanation why on a sizeable
system, when big power is going in, and no streamers are yet coming out, the
rotary break is beating itself up! (with greatly increased light
output). I used to give this the casual explanation that the input energy was
merely 'piling up' at the break contacts, but you've just cleared it up for me!
The much higher K (no streamer condition) DEMANDS that the interplay
between primary and secondary in the gap last for many more RF cycles, than
if a streamer is pulling power from the system at the output end. In
this situation the mechanics of the break are overcome and a form of
'power arcing' (extended interplay) is apparently innevitable by this explanation.
To not power arc in this situation would be to destroy energy rather than
convert it to matter, which is as we all know, quite illegal.
Thanks for helping me learn another chapter from Tesla Coils 101.