Xref: utzoo comp.misc:2804 sci.electronics:3321
Path: utzoo!attcan!uunet!lll-winken!lll-tis!mordor!joyce!ames!mailrus!tut.cis.ohio-state.edu!rutgers!att!ihlpa!travis
From: travis@ihlpa.ATT.COM (Marlatte)
Newsgroups: comp.misc,sci.electronics
Subject: Re: Lightning rods. Was: Protecting computer equipment
Message-ID: <8523@ihlpa.ATT.COM>
Date: 15 Jul 88 15:22:27 GMT
References: <1988Jul8.045005.17867@utzoo.uucp> <6179@aw.sei.cmu.edu> <1041@gethen.UUCP>
Organization: AT&T Bell Laboratories - Naperville, Illinois
Lines: 51

Rich D'Ippolito correctly wrote:

	"Ligthning[sic] rods work by _preventing_ the buildup of static
	charges in the vicinity of the pointed end(*)."


In article <1041@gethen.UUCP>, Alan Bostick writes:

	"Because of the sharp tip of the lightning rod, the electric
	field strength there is much greater than on the (more or less)
	flat ground. It becomes much easier for the insulating air to
	break down in the vicinity of the rod.  Hence, the preferred
	path for the lightning is for it to strike the rod, and travel
	down the wire to the ground.

	"Remember that those thousands of amperes are flowing for a small
	fraction of a second.  If the wire is stout enough it will be
	able to sustain the flow for the needed time without melting."


While it may be true that a stout wire could sustain a lightening
strike, that is not the intent. Rich is correct. A lightening rod will
prevent, or more accurately, minimize the build up of charge in the area
around the rod.

Alan's description is basically correct. But the point on the rod does
not attract or induce a strike, but prevents it by causing a slower
discharge. If lightening actually strikes a lightening rod on a building,
it is as good as striking the building. The wire is attached to the
building, the building is attached to the ground. The wire provides a
very good path, but the building offers a close second.

This is not a perfect solution. In severe thunderstorms, the
build up of charge can be very sudden. The presence of moisture, like
rain, can alter the affects of a lightening rod. Also, charge build up
can be quite isolated. This is why a tree within a close distance of the
lightening rod can get hit.

All of this can be seen in an experiment at home. Ready boys and girls,
here we go. You know how you can scuff your shoes on the carpet and get
a shock when you touch a door knob? Well this is SIMILAR to
lightening. If you approach the door knob with your hand or finger, the
static build up is sufficient to arc several centimeters. Now ask your
Mommies for a straight pin. You know, the kind she uses when she sews.
If you hold the straight pin, and approach the door knob holding the
point of the pin out, you will not get a shock.

-- 

Travis
*****************************************************************
Travis Marlatte    ihlpa!travis    312-416-4479    AT&T Bell Labs