Xref: utzoo rec.boats:4186 sci.physics:13771 sci.electronics:13118
Path: utzoo!attcan!uunet!spock!grayt
From: grayt@spock (Tom Gray)
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: <3984@healey>
Date: 25 Jul 90 12:41:11 GMT
References: <1990Jul18.111525.5749@ioe.lon.ac.uk> <2436@speedy.mcnc.org> <1990Jul19.210925.165@mlb.semi.harris.com> <1047@tsdiag.ccur.com>
Reply-To: grayt@healey (Tom Gray)
Organization: Mitel. Kanata (Ontario). Canada.
Lines: 43

In article <1047@tsdiag.ccur.com> davet@tsdiag.ccur.com (Dave Tiller N2KAU) writes:
>In article <1990Jul19.210925.165@mlb.semi.harris.com> sjohnson@gumbee.mlb.semi.harris.com (Scott Johnson) writes:
>-
>-
>-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!
>-
>-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'. :)
>
>Popular Mechanics had an article on lightning protection for mountaintop
>antenna installations.  It used giant mushroom shaped wire mesh structures
>to avoid the point charge buildup so the lightning would hit the mushroom,
>_not_ be deflected toward the antenna tower.  Unfortunately I can't provide
>the chapter and verse, since it's been a few years, and my Pop Mechs are in
>Va.  Anyone else read this article?
>-- 

The July 1990 issue of TE&M magazine contains an article describing the latest
thinking in lightning protection.

It confirms that a space charge does build up around a lighning rod which
reduces its efficency. A lightning leader must make a closr approach to 
the rod to trigger the upward intercepting leader to complete the ground
cloud path for the bolt.

Another contributer mentioned the use of heavy guage wire to conduct
the down current from the rod. As you might expect for currents
with such fast rise times and magnitude things are not that
simple. There is massive amounts of capacitive coupling from the
down conductor to the buildoing structure. This will cause the
structure to rise in potential with the down conductor. This may
not be bad since it causes electrical equipment to rise in in 
potenetial with their surrounding and cabling which can prevent
destructive breakdowns.

Lightning protection is a matter for specialists and I am certainly
not one.