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Re: [Phys-l] Lightning Rods






bc interleaves:

John Denker wrote:

Michael Edmiston wrote:


* * * Typically-found Physics Description * * *

(1) Lightning rods are sharp



cut

The foregoing were the "good" ones?


I started looking into this when a roofing firm was telling me about the subcontractor they use to install lightning protection on houses and buildings they roof. The lightning protection firm
says it is false that lightning rods prevent lightning, and it is also false that lightning rods
attract lightning. The only thing lightning rods do is route the lightning though a safe path.


Hmmm.


To me it seems inconsistent that the rods can route the lightning if they don't first attract the
lightning.


Yes, indeed: inconsistent.



No! one does not want a lighting rod to attract lightning as it might fail, i.e. burn out (open circuit) in a hidden spot, or heat up to the extent it burns the adjacent wood. What the builder meant is the number of adjacent area strikes is the same w/ or w/o the rod. Once a strike does occur, it is routed thru the conductor. (A little attraction., ok?)

It appears there has been some legitimate research that lightning rods should not be sharp, but also not be "too dull." A sharp point is bad, a large conductive ball end is bad, but a radiused
rod (perhaps a 2-cm diameter rod with rounded top) is best.


I doubt it.


bc agrees; see below.

The experimental evidence is that sharp rods and radiused rods have been put on a mountain, and the only the radiused rods get strikes. The sharp rods do not get strikes.


http://infohost.nmt.edu/mainpage/news/2000/30may03.html
http://www.usatoday.com/weather/resources/basics/2000-05-15-lightn-rod-tests.htm

IMHO there are a lot of poorly-controled variables in that study.
How nearby is "nearby"? What would have happened if other rods hadn't
been nearby? I'm not sure all the authors' conclusions are well supported
by the data.

I've seen sharp rods and very unsharp rods, all of which got strikes.



For the life of me I can't understand why the experimenters haven't used a single rod w/ a screw on end. Alternately using a blunt and a sharp end (resharpened after each strike, or have a bunch of them) and keep stats. over a long period of time to smooth out the other variable (different weather w/ time).

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It might be that the corona discharge from the sharp rod puts so many ions in the air around the end of the rod that the ion density is high enough over a large enough area that the sharp rod essentially appears as a rounded ball due to the space charge.


"That violates basic notions such as the intermediate value theorem.
At some point, the sharp point puts out enough space charge such that
the point+charge becomes just as blunt as the naturally-blunt rod.
From there on, both things behave the same. If the space charge gets
carried sideways or upward by advection or drift/diffusion, it effectively
makes the pointy rod wider or higher, which can't possibly hurt, I would
think."

It can't get blunter (the ion ball) than the blunt rod? How does it know how blunt the metal rod is??

I'm missing part of my brain, evidently, here.
JD's next claim, claims this is moot. If the builder's (and bc's) theory is correct, bluntness may make a difference.
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bc, who wishes he lived in lightning country instead of earthquake country, so he could be a Jr. Ben, easier to research too!