Re: [Phys-l] Lightning Rods

["Michael Edmiston" <edmiston@bluffton.edu>]

This message is mostly in response to John Denker.  Please note I am not 
necessarily disagreeing with him even though I am posting some 
ideas/data that are contrary to some of what he said.

(1) Is it fact or falsehood that lightning rods are sharp?

> From what I've read, Ben Franklin won the argument in this country. 
That doesn't make him correct; it just means more people in the USA 
followed his advice than those who didn't.

Typical physics books that bother to discuss lightning rods say they 
should be sharp.

All lightning rods I have observed (and I have observed a lot) are 
sharp.  This doesn't mean they are sharp as a needle and easily prick 
your finger, but it means they obviously taper to a point much narrower 
than the body of the shaft.  When working at Los Alamos I was living in 
the mountains.  Lightning rods were everywhere.  They were sharp.  Here 
in rural Ohio, where it is very flat and homes and buildings are 
generally 1 or 2 stories and are roughly the same height as trees, and 
there are a lot of barns... there are lightning rods everywhere, and 
they are sharp.

Charles Moore and the Langmuir Lab group wouldn't be advocating a switch 
to blunt rods if the past and current trend in the USA were not sharp 
rods.

(2) At the base of the clouds, is surface detail on earth all averaged 
out such that lightning rods have no electrostatic effect at cloud 
level?  That make sense.

(3) Can lightning rods achieve any degree of discharging the cloud and 
either prevent strikes or reduce their current and damage?

I suspect little affect from one building with lightning rods.  But what 
about a larger area with more rods?

Consider a van de Graaff accelerator.  How is the voltage of the HV 
terminal sphere regulated?  It is regulated with sharp corona points. 
The particle beam from the accelerator, after exiting the accelerator, 
makes a 90-degree bend in an analyzing magnet.  As it exits the magnet 
it passes through (and hits) a slit.  The slit plates are electrically 
isolated from each other and from ground so the slit current (the beam 
current hitting each side) can be measured.  If the far slit is getting 
more current the van de Graaff voltage is too high for the beam energy 
selected by the magnet settings, and so the control circuitry drives the 
corona points closer to the terminal to create more corona and reduce 
the voltage.  If there is more current on the near slit then the 
terminal voltage is too low and the corona points are withdrawn.  Of 
course this cannot react fast enough to hold the beam energy as constant 
as the nuclear physicists want, but that is done by the narrowness of 
the slits.

When the van de Graaff is operational, the belt is constantly charging 
the terminal, and the corona points are constantly discharging. 
Controlling the distance of the points from the terminal keeps the 
terminal near constant voltage.

Can we scale this up to thunderstorm size?  The wind and rain and cloud 
behavior are the van de Graaff charging belt.  An array of tall 
lightning-rod corona points on earth are like the control corona points 
in the van de Graaff.  If the lightning rods are high enough, and all 
the homes in a small town have them, are we approaching the same scale 
as the van de Graaff... a case where it clearly works.

Michael D. Edmiston, Ph.D.
Professor of Physics and Chemistry
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu