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