[geeks] Whee! Lightning strikes, AGAIN!

[wa2egp at att.net]

-------------- Original message ----------------------
From: Phil Stracchino <alaric at metrocast.net>
>
> wa2egp at att.net wrote:
> > -------------- Original message ---------------------- From:
> > gsm at mendelson.com
> 
> >> 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.
> 
> Uh ... you just contradicted yourself.  You're saying, power station
> conductors have smooth rounded bends so that no arcs will be produced,
> but lightning rods have sharp points so that no arcs will be produced
> (i.e, no lightning attracted).

NO, I did not.  In both cases sharp points will cause charge to dissipate.  Power companies want to keep the charge, that's potential energy they sell.  They lose it, they lose money.  Lightning rod lose charge by dissipating it so the object does not build up potential and have a catastrophic breakdown of the air.  The "wires" in a power station are a whole lot closer than a cloud and the earth.
> 
> In this, you're completely missing the point of lightning rods.

No.  You did.

> 
> Van der Graaf generators have polished, rounded electrodes to keep the
> local field as flat as possible adjacent to them, thus minimizing charge
> leakage and ionization to prevent premature discharge and allow raising
> the potential as high as possible.  Lightning rods, on the other hand,
> have sharp points, and are set up as the highest point around, *because
> THEY ARE INTENDED* to create leakage currents and attract lightning.

You are correct until the last three words.  You contradicted yourself. If you build up a charge, you increase the electrical field until the insulator (air) breaks down.  A lightning rod prevents this buildup of charge. 

> The purpose of a lightning rod is not to prevent lightning strikes.  It
> is to prevent lightning strikes *from damaging buildings*, by attracting
> lightning strikes in the vicinity and inducing them to strike *the rod
> instead of the building*, and thus be harmlessly (in theory) conducted
> to ground.

Oh yes it is designed to prevent strikes.  And if it supposed to be struck by lightning then why to they put them on buildings with metal frames and no ground wires to the lightning rod.  If the frame takes the lightning strike, then why use lightning rods at all?

A good place to see where they are positioned is in Florida.  They have them all over the place at Sea World.  I've seen them in trees at Mt. Arlington.  Many buildings in Jersey City have them too and not one of them have been struck by lightning (buildings or lightning rods).

Bob

Bob