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Corona and Sphere - Puzzle -addition
> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com <tesla-at-pupman-dot-com>
> Date: Saturday, October 23, 1999 4:39 AM
> Subject: Corona and Sphere - Puzzle
>
>
> >Original Poster: "Malcolm Watts"
> <malcolm.watts-at-wnp.ac.nz>
> >
> >Greetings all,
> > Following some very
> interesting discussions offlist
> >with Boris Petkovic, I and he have a problem we
> would like to throw
> >into the arena for comment and possibly some
> answers. I can
> >formulate the problem like this:
> >
> > I have a sphere of a chosen diameter. It is
> connected to the top
> >of a TC. At some voltage, the sphere will issue a
> visible streamer.
> >We know that air streamers do not lower the Q of a
> system to a high
> >degree. The relatively high Q of system doing this
> has been
> >measured by myself and others and manifests itself
> as a continuing
> >series of beats which are visible on an e-field
> oscilloscope trace of
> >the secondary.
> >
> > The question: Why does a single rooted
> streamer or just a
> >few streamers appear when the breakdown voltage of
> the sphere
> >has clearly been reached? Why not all around?
> >
> > Something to consider: I have operated a coil
> such that the
> >voltage it reached was not sufficient to promote a
> visible streamer
> >but if the sphere was viewed in *very* dark
> conditions, the terminal
> >was indeed surrounded by a ball of wispy streamers
> issuing in all
> >directions.
> >
> > Does anyone have any comment on this? Is there
> any *good*
> >reason why, when terminal voltage has reached such
> a level as to
> >allow the issuing of a single bright streamer more
> are not issued?
> >
> > The problem appears simple on the surface but
> perhaps not so
> >simple if one delves deeply enough into what is
> happening.
> >
> > I've tried to formulate the problem as
> concisely as I can but
> >perhaps Boris might like to elaborate if he feels I
> have missed
> >something. This is written in some haste as I am
> about to go away. I
> >look forward to reading the answers when I return.
> >
> >Regards,
> >Malcolm
> >
----------
Hi List,
Firstly ,thanks to Malcolm for formulating the
problem.The formulation is a good one.I would add the
same formulation of mine as I had formulated it to
Malcolm (2 months ago).
Think that question may be put the way is the strenght
of local electrical field ,at the surface of smooth
sphere,the only thing which determines breakdown of
air at the other point parts of sphere after first
streamer somewhere already issued (standard
atmospheric conditions assumed) ?I'm affraid that
there are cases in praxis offering negative answer to
this question.
Now ,a one more formulation of the problem :
Let us consider behavior of smooth sphere excited by
TC h.f wave form.Let sharp point be added at one side
of sphere to allow issuing of spark.Let be considered
only single pulse mode.Let diameter of sphere be
10".When voltage on sphere reaches 300-350 KV sphere
should discharge in all directions in air,not only
through already formed sharp point path judging by
experiments of the same sphere without sharp point
added.However,it proceeds to discharge through sharp
point path despite voltages of 400 KV or more.At last
when voltage in the order of 500 KV is reached ,sphere
would start to form another streamer paths from its
surface but even then those new streamers are not
reaching so far distances as very first one ..In all
the books can be found data the most responsible
factor for breaking down of air is strenght of local
electrical field (which is ~30 KV/cm for DC ,low
frequency range or even somewhat lower for TC h.f
range).But before forming new streamers el.field
reaches 40 KV/cm in described case!
I got as the same explanations like some of yours as
I got from some physicist and HV engineers:Well ,you
see the sphere electrode already discharges through
adjanced sharp point path..or formed streamer offers
less impedance than rest of surrounding air.."But when
I formulated the problem like follows:
Half of sphere with adjanced wire point is situated in
black box (so you cannot seen if discharging occurs
there or not),another half of smooth sphere is exposed
and can be seen .When powered by TC you can measure
strenght of el.field >40 KV/cm and nothing
(caracheristic visible corona doesn't appear).The
question :How do charges at exposed side "know" that
discharging already occurs in the black box and follow
that path instead of forming new streamers despite
more than sufficient el.fields reached?What mechanism
determines and allow such behavior (I know it is
powerful streamer in the black box but what
exactly?).After that some of them start scraping their
heads...The problem is not as simply as someone may
think.Anyway thanks for thoughts/tries.
Regards,
Boris
=====
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