billc@prism.UUCP (07/02/87)
All this talk about ground resistivity and lightning reminds me of a
question I came across in freshman physics. Wait! Don't skip to the
next note yet!
To put it in historical perpective, when Benjamin Franklin invented the
lightning rod, he advocated that the rods should be pointy on the end
in order to provide the greatest protection. On the other side of the
Atlantic, however, the British got the idea that lightning rods with
a ball the end were better, and in fact a law weas passed that all
lightning rods in England should have balled ends.
Of course, the freshman physics problem was to compute the electrical
properties of both rods, which we dutifully did.
Now the clincher: No one has any arguments about the different properties
the two types of rods have, since no one argues about the physics. But
the story I heard from my professor (whom I very much respect as a physicist
and teacher) is that opinions were changing as to which rod was really more
effective at protecting a house in an electrical storm. The traditional
answer is that pointy rods are better, but some people re-examining the
problem are (were) beginning to think that perhaps the 18th century
House of Commons may have actually had a point (though probably for the
wrong reasons, since they didn't know much about modern EE.)
I would appreciate respones on this, or posted replies if you have something
on this of general interest.
Bill C. billc@mirror.TMC.COM
{mit-eddie, ihnp4, wjh12, cca, cbosgd, seismo}!mirror!billcwtm@neoucom.UUCP (Bill Mayhew) (07/04/87)
Several books I have on antenna design recommend using a small metal or even plastic ball on the tip of whip or pole type antennas. The two books I looked at don't show mathmatical proof, but claim that the increased surface area of the ball makes it easier for static to dissipate. The claimed result is that generation of small sparks is reduced, thus cutting down on the amount of crackle emitted from the speaker of a receive attached to said antenna. Supossedly, this is especially helpful on car radio antennas. I once had a car where to little ball on the tip fell off during the time I owned the car. I didn't notice any difference in the performance of the radio after the tip fell off. Since the purpose of a lightning rod is to dissipate static that could attract a lightning bolt. It seems that the increased surface area of a ball-tipped lightning rod would be helpful. Does anyone out there have any information on dealing with lightning strikes in *very* tall buildings. On a building several hundred feet tall, lightning hits would essentially be assured. I've seen some commercail radio transmitters with quite ealborate spark bussbars and gas filled tubes, etc. I've never had the chance to take to the engineer at such a facility about lightning, but I'd imagine that radio and TV towers are also routinely hit. --Bill (wtm@neoucom.UUCP)
ken@rochester.arpa (Ken Yap) (07/05/87)
|Since the purpose of a lightning rod is to dissipate static that |could attract a lightning bolt. It seems that the increased |surface area of a ball-tipped lightning rod would be helpful. No. The purpose of a lightning rod is to preferentially attract lightning. A sharp point does much better at breaking down the dielectric (air) than a smooth surfaceveyic R
ornitz@kodak.UUCP (barry ornitz) (07/06/87)
About three years ago, the Journal of the Franklin Institute published an
article on the current state of lightning research. Based on mathematical
models and experimental evidence, a pointed lightning rod is not optimum.
In general, most lightning rods are actually hit several feet below the tip.
The best shape from mathematical simulations is an ellipsoidal tip. This
article dispels much of the popular "scientific" notions about lightning
rods preventing strikes by draining the charges with ionization from a sharp
point. I recommend this article highly to anyone seriously interested in
lightning protection. The Journal normally prints papers on advanced control
theory and mathematics, but from a historical background, they always give
space for papers on lightning research.
Barry
-----------------
| ___ ________ |
| | / / | | Dr. Barry L. Ornitz UUCP:...!rochester!kodak!ornitz
| | / / | | Eastman Kodak Company
| |< < K O D A K| | Eastman Chemicals Division Research Laboratories
| | \ \ | | P. O. Box 1972
| |__\ \________| | Kingsport, TN 37662 615/229-4904
| |
-----------------wtm@neoucom.UUCP (Bill Mayhew) (07/07/87)
In article <455@sol.ARPA>, ken@rochester.arpa (Ken Yap) writes: <<stuff about ball on tip vs. pointy tip>> > No. The purpose of a lightning rod is to preferentially attract > lightning. A sharp point does much better at breaking down the > dielectric (air) than a smooth surface. Aside from plain or sphere on tip, it is my understanding that the function of a lightning rod it to allow static to bleed from the air around a structure into the ground. By neutralizing the charge, the likelyhood of a bolt landing there should be reducded. The wording in the previous article gives the impression that the lightning rod is designed to attract lightning rather than deter it. Bill
ron@topaz.rutgers.edu.UUCP (07/08/87)
> |Since the purpose of a lightning rod is to dissipate static that > |could attract a lightning bolt. It seems that the increased > |surface area of a ball-tipped lightning rod would be helpful. > No. The purpose of a lightning rod is to preferentially attract > lightning. A sharp point does much better at breaking down the > dielectric (air) than a smooth surface. Guess again, Chucko. The lightening rod is designed to disipate static to avoid arcs (bolts). This is why you'll find lightening rods around to avoid lightening in places you are explicitly trying to avoid arcs (like in refeuling areas). There's a wonderful Bureau of Mines saftey film on this subject. -Ron
ken@rochester.arpa (Ken Yap) (07/09/87)
|> No. The purpose of a lightning rod is to preferentially attract |> lightning. A sharp point does much better at breaking down the |> dielectric (air) than a smooth surface. I should have worded it to mean that the lightning rod preferentially takes the damage in the event a bolt hits because the conduction path is easier via the rod than the structure itself. Ken
dje@datacube.UUCP (07/09/87)
I understand that this rule applies: A direct lightening strike will be drawn to a thing only if the lightening was going to hit within a cone defined by the thing's top and a circle on the ground whose radius was that thing's height. That only counts for direct strikes. The Electric fields near the lightening get very high. I was unfortunate to be out on Mass Bay in an INTENSE lightening storm in a sailboat (40' mast) with ungrounded rigging. No direct hit, but I saw giant bolts hit 100 feet away. I recieved shocks from wet ropes and saw a 2" discharge from my hand to boat hardware. New underware was in order. I agree that lightening rods dissipate the field from nearby lightening, and I think they prevent damage from direct strikes. All masts should be grounded at *least* to a metal keel. If you get caught out in the water and you wonder if you're grounded, shackle a length of chain from a stay or shroud and let it drag in the water. Then stay away from everything. Dave Erickson ---------------------------------------------- ------ Datacube Inc. / /| 4 Dearborn Rd. ------ | Peabody, Ma 01960 || \| | ihnp4!datacube!dje || /|/ Human:(617)535-6644 ------ Fax: (617)535-5643
lee@minnow.UUCP (Gene Lee ) (07/10/87)
>I should have worded it to mean that the lightning rod preferentially >takes the damage in the event a bolt hits because the conduction path >is easier via the rod than the structure itself. > If you stopped to think about what you are saying you'd realize how ridiculus it sounds. If a bolt of lightning (MEGA volts at MEGA amps) struck the lightning rod sitting on a wooden barn or house, you would really be talking FLAMES. Even if the wire cable that would be carring the current to ground was low resistance, at that current there would be a LARGE voltage drop across the cable. The power disapated would heat the cable red hot; not exactly something you want attached to your wooden structure. -- Gene Lee UUCP: ...ihnp4!{meccts,dayton,rosevax}!ems!minnow!lee UNISYS Corporation ATT: (612) 635-6334 If not for the courage of the fearless crew, the minnow would be lost.
ron@topaz.rutgers.edu (Ron Natalie) (07/10/87)
If the purpose of a lightning rod is to take hits of lightning (which it isn't but let's just assume it is at the present), then even though a point is better for causing the bolt to form, doesn't make it ideal for the end of the rod. If you've ever seen equipment that is designed to draw sparks, you will notice that they use round ends on the electrodes. This is to provide more surface area exposed so that they will not wear out as fast. You're nice pointy end would probably be gone after the first stroke. -Ron
ken@nsc.nsc.com (Ken Trant) (07/13/87)
in article <915@minnow.UUCP>, lee@minnow.UUCP (Gene Lee ) says: > Posted: Fri Jul 10 08:50:29 1987 >>I should have worded it to mean that the lightning rod preferentially >>takes the damage in the event a bolt hits because the conduction path >>is easier via the rod than the structure itself. > across the cable. The power disapated would heat the cable red hot; not > exactly something you want attached to your wooden structure. > Gene Lee UUCP: ...ihnp4!{meccts,dayton,rosevax}!ems!minnow!lee Just as an aside, when I worked for 'Western Electric' we used #8 awg cable as the 'single point ground' for the #4ESS machine. This was a 6' to 8' (approx) fusable link in case of a lighting strike, in such a case this #8 cable would melt like the fuse in your car. Ken Trant {JOAT} Ollie North for President in '88'
ken@rochester.arpa (Ken Yap) (07/14/87)
| If you stopped to think about what you are saying you'd realize how |ridiculus it sounds. If a bolt of lightning (MEGA volts at MEGA amps) struck |the lightning rod sitting on a wooden barn or house, you would really be |talking FLAMES. Even if the wire cable that would be carring the current to |ground was low resistance, at that current there would be a LARGE voltage drop |across the cable. The power disapated would heat the cable red hot; not |exactly something you want attached to your wooden structure. How do you account for those real pictures of lightning rods on tall skyscrapers taking hits? Ken
agn@unh.cs.cmu.edu (Andreas Nowatzyk) (07/14/87)
I feel that excessive flaming is inappropriate for technical discussions, especially if you didn't look up the facts. Lightning protection systems are quizz material for ham-licenses (at least in Germany). The basic purpose of such systems (reqired for most out-door antenna installations) is damage control in case of a lightning hit by providing a safe, well defined path to ground. You can't do much about being hit, but you can make sure that your equipment and you body is out of the current path. A 10 mm^2 copper cable (solid core, you need more if it is stranded) can carry the current of a lightning discharge safely (no red-hot BS). About 80 mm^2 of iron cable is needed to do the same job (higher resistance, but cheaper). Layout, grounding systems, contact point to water-pipes, electrical installation, test-points etc. are not trivial because the high current transient will induce high voltages on nearby conductors. -- Andreas