Path: utzoo!utgpu!water!watmath!clyde!att!pacbell!lll-tis!lll-winken!uunet!mcvax!ukc!dcl-cs!nott-cs!pyr1.cs.ucl.ac.uk!william
From: william@pyr1.cs.ucl.ac.uk
Newsgroups: sci.electronics
Subject: Re: Lightning rods. Was: Protecting com
Message-ID: <44000015@pyr1.cs.ucl.ac.uk>
Date: 22 Jul 88 16:22:00 GMT
References: <6179@aw.sei.cmu.edu>
Lines: 31
Nf-ID: #R:aw.sei.cmu.edu:-617900:pyr1.cs.ucl.ac.uk:44000015:000:1664
Nf-From: pyr1.cs.ucl.ac.uk!william    Jul 22 16:22:00 1988


The lightning conductor should prevent a strike.  If it doesn't, then
the conduction path is a last-ditch protection.  

I think it works like this:  We have a pretty enormous charge on the
ground/building relative to the cloud.  The conductor is pointed at the
top so the electric field projects WAY up into the cloud.  As the charges
in the cloud are generated (displaced) by the movement of fast air currents,
they are immediately discharged to the pole.  This means that you get a
constant current flowing which is enough to keep the area within striking
distance below the breakdown potential.  

Calculations:  Suppose the area of cloud over a building generates enough
charge to cause a strike every 30s, and a strike lasts, dunno, say 0.1s,
then the current flowing would be 1/300 of a lightning strike, and this 
assumes a TOTAL discharge.  I suspect that you also get a kind of insulation
effect because there will be an accumulation of cloud charge near the 
pole, so other cloud charges may be repelled from that locale.  (That bit's
all guessing!)

If this were not the case, then lightning conductors would not stick into
the air, because this positively encourages a local discharge. A church,
forinstance, would have a metal plate, as unpointed as possible, on its
spire, so as to discourage discharge but still provide a path to ground.

			... Bill
	
************************************************************************
Bill Witts, CS Dept.     *
UCL, London, Errrp       *       Don't believe everything you hear,
william@uk.ac.ucl.cs(UK) *       or anything you say.
william@cs.ucl.ac.uk(US) ***********************************************