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From: sjohnson@gumbee.mlb.semi.harris.com (Scott Johnson)
Newsgroups: rec.boats,sci.physics,sci.electronics
Subject: Re: St Elmo's Fire (was Re: Lightning)
Keywords: Induction, space charge, capacitance, and bulldrops ?
Message-ID: <1990Jul19.210925.165@mlb.semi.harris.com>
Date: 19 Jul 90 21:09:25 GMT
References: <1990Jul18.111525.5749@ioe.lon.ac.uk> <2436@speedy.mcnc.org>
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Organization: Harris Semiconductor, Melbourne FL
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In article <2436@speedy.mcnc.org> kgreer@mcnc.org.UUCP (Ken Greer) writes:
>In article <1990Jul18.111525.5749@ioe.lon.ac.uk> teexmmo@ioe.lon.ac.uk (Matthew Moore) writes:
>+++ [other stuff about a corona discharge (St. Elmo's Fire)]
>+++
>++I know this is the commonly accepted idea behind lightning rods, but as I
>++learned in my college physics class, lightning rods actually REPEL lightning,
>---------- stuff deleted -----------
>+Lightning conductors _tend_ to prevent lightning strikes. 
more stuff deleted, I think...
>Charge may be distributed irregurlarly on the surface of any charged
>conductor. In particular, it builds up and extends much further away
>from any pointed area. (This is known as point action). So the air
>above the pointy bits carries a high net +ve charge, and may become
>ionized, (causing the discharge seen). This ionized region is
>described as a space charge, and is what _prevents_ lightning
>discharge between ground and cloud.

OK,
The point will have a higher electric field and IF great enough it could
cause ionization of the air in the vicinity.
Ionization of gas creates a plasma (which will also be POSITIVELY 
charged, like the rod, since the electrons lost by the nitrogen
molecules will be
drained off by the conductor around which the field is concentrated)
If the field is only strong enough to ionize the air in the immediate
area, we have a cloud of glowing plasma around the conductor, (St. Elmo's
fire I would assume); if we have a strong enough field to break down
the air between the negative charge source in the atmosphere and
the rod, then plasma is created along the entire length and a lightning strike
results.

If you see a traveling bolt of lightning, then you are seeing a ionization
process begining first at the area of highest electric field.

In this discussion we are talking about charges and fields
of VERY LARGE proportions (look at the breakdown field for nitrogen)
and I would not believe that the leakage current 
in air could ever have an effect
on the eventual result (strike vs. no strike vs. other place struck).
Also, resistance of almost any object (lets leave out rubber, plastic
and such insulators, of course) is small when compared with the air, so whether we had an
excellent ground or a 10K ohm ground i think is also irrelevent
to the final result.

With all this handwaving, I think we all should hire out as
scarecrows! :)

Kim Greer writes:  

>  Ok, so "knowing" all this, as I understand it to be said, one should be
>safer standing on the roof next to a lightning rod, instead of somewhere
>else?  Or do I completely miss the point?
>-- 

Kim, I think they would tell you that this is a bad idea,unless you have
a well conductive pointy head (which I've yet to see at Duke) in which case
the lightning will avoid you also and strike a tree down the road.

I, however, have heard enough of this nonsense! I didn't study E-mag and
solid state physics this long to still be able to swallow these lines!
(by the way there is also a car down at the Ford place that 3 people 
committed suicide in. They can't sell it and its going cheap...)

I think maybe this 'lightning rods repel lightning' stuff was started as
a cruel textbook joke to baffle physics students by some screwed up prof.

CAN ANYONE COME UP WITH A REFERENCE?
SURELY IF THIS 'STRANGE BUT TRUE STORY' IS FACT, THEN IT'S DOCUMENTED.
i'D LIKE TO GET TO THE 'THEORY' BEHIND WHAT I CONSIDER VICIOUS
RUMOURS ABOUT MY FAVORITE 'FIELDS'. :)

	The earth is round.
	Rock n Roll is best loud.
	Sailing is expensive,
	and lightning STRIKES lightning rods!
        Thats how they protect a house, they deflect the current!
	Come on guys!

scott johnson:

sjohnson@gumbee.mlb.semi.harris.com
Melbourne, Fl

sjohnson@booboo.mcnc.org
Dept. of Electrical Engineering 
North Carolina State University 
Raleigh, NC