william@pyr1.cs.ucl.ac.uk (07/22/88)
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) ***********************************************
adrian@cs.hw.ac.uk (Adrian Hurt) (07/25/88)
In article <44000015@pyr1.cs.ucl.ac.uk>, william@pyr1.cs.ucl.ac.uk writes: > > I think it works like this: We have a pretty enormous charge on the > 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. Ever seen a church with a copper roof? When I was on holiday in Germany, I noticed that the church tower clock had stopped. I found out from the people I was staying with what had happened. The church had recently had its roof repaired, and covered with copper. Within a few days, there was a thunderstorm, and of course it got hit, clobbering the clock. The same storm also wiped out the local power supply for a short while; I was told someone opened their fridge door and nearly got fried. The locals blamed the power failure on the new copper roof. -- "Keyboard? How quaint!" - M. Scott Adrian Hurt | JANET: adrian@uk.ac.hw.cs UUCP: ..!ukc!cs.hw.ac.uk!adrian | ARPA: adrian@cs.hw.ac.uk
henry@utzoo.uucp (Henry Spencer) (07/25/88)
In article <44000015@pyr1.cs.ucl.ac.uk> william@pyr1.cs.ucl.ac.uk writes: >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. Ah, but if you do get a local discharge, you want to be very sure it goes into the rod, not the building frame. So there is still a reason for trying to make the rod conspicuous to the lightning. -- MSDOS is not dead, it just | Henry Spencer at U of Toronto Zoology smells that way. | uunet!mnetor!utzoo!henry henry@zoo.toronto.edu