First I am going to describe what many physics books and physicists say
about lightning rods, and then I am going to present some more recent
things I have read, and ask if anybody on this list knows more about the
subject.
* * * Typically-found Physics Description * * *
(1) Lightning rods are sharp to induce corona discharge such that cloud
to earth opposite charge build up can be substantially reduced, thereby
reducing the chance lightning will occur.
(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.
(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.
Also note there are various van de Graaff demonstrations that support
(1).
* * * Some not-so good explanations * * *
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.
To me it seems inconsistent that the rods can route the lightning if
they don't first attract the lightning.
* * * Another idea * * *
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.
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.
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. 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. 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.
Comments?
Here a couple references. You can find many more with a Google search.
The first is a lightning protection company used in the area. The
second two are references to the blunt-rod experiments.
Not mentioned in the Langmuir experiments is anything about whether the
sharpened rods, by earlier corona discharge, might actually prevent some
strikes from occurring.
Michael D. Edmiston, Ph.D.
Professor of Physics and Chemistry
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu