I’m confused. One of those books I mentioned has a picture of a panel placed under a model house hit by simulated lightning bolts. There are no hits immediately under the lightning rod, and they don’t start to appear until you get out to the edge of the “cone of protection”. If that doesn’t constitute proof that lightning rods work, what does? G.E. had a laboratory devoted to this. I’m sure they produced tons of papers, if only to justify their own existence. I admit that I never looked this stuff up myself, but I find it impossible to believe that there is “no scientific proof” that lightning rods work.
Certainly there are questions about how they work. Look up the Am. J. Phys. article I cite above – it deserves to be a classic – but that’s not the same as empirical demonstrations that the rods work as advertised.
Folks, there’s just no way to scientifically test this question. Lightning is a mostly-random event. To do a “real” experiment, we’d need identical houses, in identical locations, one with lightning rods, the other without; then have both of them struck by identical size lightning bolts, probably 10 times each to minimize random variations. No can do.
We CAN do 1/10th scale experiments with artificial lightning generators. Many, many, many have been done with things that really matter, like airplane gas tanks. No surprise, the tanks that were well grounded didnt explode. In those where there were ungrounded items, like poorly-fitting gas caps, there was sparking and ignition.
No surprises here either.
One could surmise that the reason why Cecil couldnt find any actual experiments, is that it’s hard to come up with any way for lightning rods to NOT work. In general, it’s better to short a few megawatts to ground thru a lightning rod and grounding wire, rather than have the megawatts heat up and explode your roof and walls.
grg,
Actually, science can be applied even when experiments are impossible, such as with studies of human cancer.
Also, medicine has shown us that scale models such as mice (in which much cancer is curable) don’t always apply to the real thing. Note the following text from the pdf document that I cited above:
Despite these findings, it is generally agreed that there is little correlation between the
performance of air terminals in the high voltage laboratory and field test results under natural
lightning conditions. Bazelyan and Raizer detail Russian results from laboratory experiments
beginning in the 1940’s. They detail the conclusion of long experimentation in the following:
“ Laboratory investigations of lightning attraction were initiated in the 1940s…… At that time, experimental
researchers expected to derive information necessary for a numerical evaluation of lightning rod
effectiveness. The naïve optimism has long since vanished. The measurements showed that the attraction
process did not obey similarity laws.” 39
Umm… SOME Faraday Cages will protect you against lighting strikes. I worked at a laboratory doing extremely sensitive experiments inside a cage of a very fine, thin copper mesh. It worked perfectly at screening radio emissions as is required of a true Faraday Cage, but would not have given any protection against a huge lightning bolt, it would have vaporized instantly and opened up the interior to any random grounding path.
If you saw a demonstration of a van Degraff generator with a demonstrator inside a cage, I assure you it was made of thick copper cables or thick mesh, it would have had sufficient capacity to carry the large charge to ground, but it would not have necessarily had the properties necessary to screen out general radio frequencies. It wouldn’t really have qualified as a Faraday Cage, the same effect could have been achieved by a grid of grounded cables between the source and the demonstrator, in essence, lightning rods. Or perhaps it could have been a long tunnel-like cage with one open end. This would not have offered shielding from radio waves, but would have offered protection from the van Degraff (if the open end was pointed away from the source).
CK:
It must be obvious that we don’t go for your explanation. It seems to hinge on what your definition of “to work” is. This is semantic quibbling. As grg says, there is no way for a lightning rod to fail.
The cage at the Science Museum is a pretty large open-work grid cage. It has to be, for the audience to see and hear the speaker. It probably wouldn’t have been very good at screening radio waves, but it worked very well at blocking the “lightning bolts”.
Yeah, I’ve worked in places with the copper mesh walls, too. To tell the truth, it’s not clear to me that a true lightning strike would have vaporized it – there’s a lot of conducting capacity and huge cross-sectional area, when you consider all the wires.
That said, arguing that one is a “true” Faraday cage while the other isn’t seems like quibbling to me. My understanding is that a “Fraday Cage” is a complerte or nearly complete metal enclosure that is intended to block em effects inside the enclosed space. Whether or not it works perfectly for all wavelengths isn’t part of the definition, it’s a qualifier. The MOS cage wouldn’t be approved by our lab, but definitely falls within the usage of the term.
>grg,
>Actually, science can be applied even when experiments are >impossible, such as with studies of human cancer.
I never said science couldnt be applied, just not well-controlled experiments.
>Also, medicine has shown us that scale models such as mice >(in which much cancer is curable) don’t always apply to the real thing.
Er, mice are not scaled-down humans! On The Other Hand,
the scaling rules for electrical surfaces are well known and tractable.
>Despite these findings, it is generally agreed that there >is little correlation between the
performance of air terminals in the high voltage laboratory >and field test results under natural
>lightning conditions.
I didnt say boo about air terminals. I was referring to the obvious advantage of grounded lightning rods over your basic unprotected building.
It seems to be generally agreed that lightning rods attract lightning. As Cecil (and blain3) also pointed out, lightning rods do not always work perfectly. If the danger posed by the additional strikes outweighs the protection, then the rods would in fact be a hazzard. This seems unlikely, but there’s no way to know for sure without scientific study.