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From: raoul@eplunix.UUCP (Nico Garcia)
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: <913@eplunix.UUCP>
Date: 23 Jul 90 23:04:05 GMT
References: <1990Jul18.111525.5749@ioe.lon.ac.uk> <2436@speedy.mcnc.org> <1047@tsdiag.ccur.com>
Organization: Eaton-Peabody Lab, Boston, MA
Lines: 31

[Various people discussing the effect of lightning rods drawing or repelling
lightning bolts]

Gentle folk, perhaps we could settle the question of repulsion/attraction by
examining a more basic question: what is a lightning bolt? Is it the massive
electrical current between thunderclouds and the earth carried by plasma,
accompanied by a lot of noise? Or is it any current carried between
thunderclouds and the earth large enough to create visible effects or carry
measurable current?

If I remember correctly from *my* old physics instruction, the lightning rod
acts to dissipate the electrical potential before the strike happens. Much
like doing your work on an anti-static pad, the slight conductive path it
provides dissipates local concentration of electrical potential energy and
prevents sparks between electrically isolated systems when brought into
contact (such as a thundercloud and the earth connected by a column of
rain-filled air.)  So if there is a strike nearby, since the damn things
tend to be at the top of buildings in the open and at ground potential,
it'll happen there. But the chance of a nearby strike is much less, so they
are effective protection.

The effect is much like putting a needle into a water balloon: left there,
it drains the pressure before the system ruptures. Yes, that is where the
system is likely to break down. But frankly, if I lived in a wooden building
(which I do), I'd much rather the lightning hit a big conductor tied to the
pipes and drain to ground than let it bust open my roof and light my attic
on fire. 
-- 
			Nico Garcia
			Designs by Geniuses for use by Idiots
			eplunix!cirl!raoul@eddie.mit.edu