[geeks] Whee! Lightning strikes, AGAIN!

[wa2egp at att.net]

-------------- Original message ----------------------
From: gsm at mendelson.com
>
> On Mon, Jul 27, 2009 at 10:17:16PM +0000, wa2egp at att.net wrote:
> >
> >Enlighten me.
> 
> Zap!!!!!   :-)
> 
> 
> http://en.wikipedia.org/wiki/Lightning
> 
> "When a stepped leader approaches the ground, the presence of opposite
> charges on the ground enhances the strength of the electric field. The
> electric field is strongest on ground-connected objects whose tops are
> closest to the base of the thundercloud, such as trees and tall
> buildings. If the electric field is strong enough, a conductive
> discharge (called a positive streamer) can develop from these points.
> This was first theorized by Heinz Kasemir. As the field increases, the
> positive streamer may evolve into a hotter, higher current leader which
> eventually connects to the descending stepped leader from the cloud. It
> is also possible for many streamers to develop from many different
> objects simultaneously, with only one connecting with the leader and
> forming the main discharge path. Photographs have been taken on which
> non-connected streamers are clearly visible.[21] When the down and up
> leaders meet, the flow of electric current greatly increases."
> 
> "Once a channel of ionized air is established between the cloud and
> ground this becomes a path of least resistance and allows for a much
> greater current to propagate from the Earth back up the leader into the
> cloud. This is the return stroke and it is the most luminous and
> noticeable part of the lightning discharge."
> 
> What it does not say, is that if you disipate the charges from the ground
> side, there are no up leaders, and therefore no connections. 

OK.

> 
> A Franklin designed lightening rod (grounded point source) focuses the
> ground charges and creates up leaders, forcing connections to them. 

No, he did not.  The sharp point of the lightning rod causes the immediate field around the tip to ionize the air and dissipate the charge so the field doesn't build up enough to create the leader.  the air needs to ionize the full length of the potential path.  This can be demonstrated with any van der Graff machine or other high voltage generating device.  See St. Elmo's fire.  Perfect example.  Look at any power station.  Any turns in their conductors have rounded corners, no sharp points, so no charge will be dissipated.
 
> With no up leaders, you either get a discharge after a negative leader
> reaches the ground, or more likely, an up leader from a different source
> forms and reaches a down leader. Remember we are talking about cloud
> sized objects in the sky and relatively small ones on the ground.

Lightning rods also work like an anti-theft device sold here called "The Club".  It doesn't keep your car from being stolen, it just makes other things look like an easier target.

Bob