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Re: [Phys-l] Lightning Rods

Michael Edmiston wrote:

* * * Typically-found Physics Description * * *

(1) Lightning rods are sharp

That assumes facts not in evidence ... facts that are not, in fact,
true. Some lightning rods are not sharp at all. I saw one once
that was the size and shape of a big round steering wheel, about
2 feet across. It worked just fine. It had obviously been hit
hundreds of times.

to induce corona discharge such that cloud to earth opposite charge build up can be substantially
reduced, thereby reducing the chance lightning will occur.

I find it highly implausible that the charge can be "substantially
reduced" by corona alone.

There are two different energy-scales here; let's not confuse them:
-- The "St. Elmo's Fire" energy scale, and
-- The "lightning bolt" energy scale.

This reminds me of proposals to prevent a magnitude-9 earthquake
by triggering "a few" magnitude-3 earthquakes. Hint: do the math.

(2) If sufficient reduction in charge cannot be obtained, such that a strike is going to occur,
then the lightning rod can essentially "attract" the lightning because an ionized stream of air
already exists (from the corona discharge) and this will be the preferred path for the lighting.

Yes, all rods attract lightning ... for reasons unrelated to item 1.

There is a so-called positive streamer that rises up from the object
that is about to be hit by lightning.

(3) With proper grounding technique, once lightning strikes the lightning rod the strike is
conducted over a prescribed conductive path that is less likely to cause a fire or electrical
damage in the building.

That's the whole point of the exercise. AFAICT there are many claims
but no serious evidence that lightning can be dissipated or repelled.

Also note there are various van de Graaff demonstrations that support (1).

I'm still very skeptical. To convince me would require a whole lot more
than a gee-whiz demonstration.

* * * Some not-so good explanations * * *

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.

Standard designs would be useless if they did not attract lightning.
Standard technique does not call for installing lightning rods
everywhere, and if there were no attraction, it would be common to
observe lightning strikes right beside the rod, bypassing the "safe
path".

The conventional rule of thumb is that a lighting rod protects
everything within a 45-degree cone. I suspect that two rods, one
at each end of the building, are even more protective than the
45-degree rule would suggest. You can estimate what's going on
using basic properties of the Laplace equation, but it might be
hard to be really precise because of the strong nonlinearities
in the problem.

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.

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.

The theoretical interpretation is that the electric field near the sharp point is higher than the
field on the radiused rod *if* you are observing near the end of the rod. However, as you go up
in the air above the radiused rod, the electric field becomes higher.

Higher than what?

Therefore, when the strike is about to occur, the ion path forms more easily over the blunt rod
because the electric field is higher there.

Again: higher than what?

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.

Also: From 100 feet up (let alone thousands of feet up) the cloud sees
only a washed-out average of what's going on near the ground. Laplace's
equation has powerful averaging properties. Feynman volume II section 7-5.

These and various other physics arguments lead me to believe that there
is no important difference between the two designs.

Also: note that this holy war between the little-endians and the big-endians
has been raging for over 200 years. As a matter of human nature, that
suggests that there is no strong evidence either way. Otherwise it would
have been settled by now.

I close with a quote from
http://www.fi.edu/pieces/hongell/

Franklin began to advocate lightning rods that had sharp points. His English colleagues favored
blunt-tipped lightning rods, reasoning that sharp ones attracted lightning and increased the
risk of strikes; they thought blunt rods were less likely to be struck. King George III had his
palace equipped with a blunt lightning rod. When it came time to equip the colonies' buildings
with lightning rods, the decision became a political statement. The favored pointed lightning
rod expressed support for Franklin's theories of protecting public buildings and the rejection
of theories supported by the King. The English thought this was just another way for the
flourishing colonies to be disobedient to them.