Path: utzoo!attcan!uunet!lll-winken!lll-tis!ames!ll-xn!husc6!uwvax!rutgers!bellcore!faline!thumper!karn
From: karn@thumper.bellcore.com (Phil R. Karn)
Newsgroups: sci.electronics
Subject: Re: Lightning rods. Was: Protecting computer equipment
Summary: strike prevention is controversial
Keywords: lightning, lightning rods, discharges, protection
Message-ID: <1228@thumper.bellcore.com>
Date: 14 Jul 88 22:08:47 GMT
References: <1988Jul8.045005.17867@utzoo.uucp> <6179@aw.sei.cmu.edu> <1323@kodak.UUCP>
Organization: Bell Communications Research, Inc
Lines: 30

> You have actually hit on the problem.  The high gradient at the point causes
> breakdown of the air near the tip.  The ion flow from the tip absorbs any
> increase in field strength above the ionization level.  The point thus protects
> itself from further increases in field strength.

If by this the author meant that a lightning rod can actually PREVENT
(or at least discourage) a lightning hit, then this is at least highly
controversial. While doing a little library research on lightning, I
found another textbook that attacks this belief as being false. The
author stated that a lightning protection system can only lessen the
damage caused by a lightning strike, not prevent it from happening.

Although I am not an expert on lightning, this latter view seems to make
much more sense.

Another interesting tidbit from the book: the "furrows" seen on the
outsides of trees that have been hit by lightning are not caused by the
vaporization of water under the tree's surface, as is commonly believed,
but rather by the explosive shock wave generated by an arc on the
*outside* of the tree. Trees are rather poor conductors. When they are
hit it is quite common for the lightning to find a better path to ground
along the outside of the tree than within it. This is why it's so
dangerous to stand under a tree during a lightning storm; if the tree
gets hit, the lightning may well decide to jump back out of the tree and
through you to ground.

BTW, the reason I'm interested in this stuff is because a pine tree only
about 10' from my tower (and about the same height as my antennas) has
the characteristic spiral furrow of a lightning hit...

Phil