rsd@sei.cmu.edu (Richard S D'Ippolito) (07/27/88)
In article <1988Jul25.163404.18143@utzoo.uucp> Henry Spencer writes: >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. Without getting into the arcane subject of tip design, here is the basic theory of how lightning rods function: Air moving across the earth's surface picks up charges, usually negative, resulting in a difference in potential, i.e., a voltage difference. Thus, there is generally an electric field present between the air (and clouds) and the ground. When the field exceeds the breakdown of the air (variable depending on humidity, etc.), the air will ionize (charges will be stripped off of the atoms) and a current will flow (cloud to cloud or cloud to ground). (Please, one can have a visible arc or current in a vacuum.) The tip of the lightning rod is at ground potential and held there by means of the cable. It is designed to cause the charged air to relinquish its charges to the ground in the nature of a steady and safe current before they build up to a dangerous level. You can measure it if you insert an ammeter into the ground wire. Please, the mechanisms for safely discharging the air are not important, and can be obtained from references. Yes, lightning can strike the rod if other nearby conditions cause it to ignite in the vicinity, or if its capacity is exceeded. The best protection comes from an array of them. But, the important point is that the rods are intended to _prevent_ lightning, not to draw it or to initiate it. Rich