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From: teexmmo@ioe.lon.ac.uk (Matthew Moore)
Newsgroups: rec.boats,sci.physics,sci.electronics
Subject: Re: St Elmo's Fire (was Re: Lightning)
Summary: lightning conductors prevent strikes
Keywords: Induction, space charge, capacitance, point action
Message-ID: <1990Jul18.111525.5749@ioe.lon.ac.uk>
Date: 18 Jul 90 11:15:25 GMT
Reply-To: teexmmo@ioe.lon.ac.uk (Matthew Moore)
Organization: Institute of Education University of London
Lines: 63

>> [other stuff about a corona discharge (St. Elmo's Fire)]
>>
>>Maybe one could argue that by bleeding off the charge like that, one is
>>less likely to take a hit (reducing the potential difference); but on the
> ^^^^^^^^^^^^^^^^^^^^^^^^^
>This part is probably true.
>
>>other hand, pointy vertical rods are a much better target than other
>             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>>things (lightning rods, while generally saving a structure, do tend to
>>increase its liklihood of being hit), and the fact that there was all this
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>I know this is the commonly accepted idea behind lightning rods, but as I
>learned in my college physics class, lightning rods actually REPEL lightning,
>rather than ATTRACTING it.  The physics behind it goes something like this:
>  The lightning rod MUST be grounded (electrically connected to the earth),
>  otherwise it's useless.  The other important feature of a lightning rod is
>  that it is very pointy, not merely the highest point on a structure. 
>  A few basic facts need to be introduced before getting to the actual 
>  explanation.  
>    - Lightning is a discharge of excess negative (-) charge toward a source 
>      of excess positive (+) charge.
>    - Unlike charges (+ & -) attract; like charges (- & - , or + & +) repel.
>    - The earth is a natural source of negative charge.
>  Okay, with those things in mind, the lightning rod is therefore (by being
>  connected to ground) a source of negative charge.  Due to the shape of the
>  tip of the lightning rod being pointy (more precisely, a VERY small radius
>  curve), the excess negative charge is radiated out from the tip of the rod
>  in all directions.  (an explanation of that gets rather technical)
>  Therefore, with this field of negative charge radiating
>  out from the rod, a bolt of lightning (also negative charge) is REPELLED
>  from the rod, NOT attracted.
>

Lightning conductors _tend_ to prevent lightning strikes. 

A LC is an earthed rod, attached to the top of the protected
structure. The base of any nearby thundercloud will be negatively
charged wrt the earth. This charge induces a positive charge at the
top of the rod (ie like charges repel, so the free electrons at the
top of the rod are repelled down it, leaving net +ve charge at the
top). 

Charge may be distributed irregurlarly on the surface of any charged
conductor. In particular, it builds up and extends much further away
from any pointed area. (This is known as point action). So the air
above the pointy bits carries a high net +ve charge, and may become
ionized, (causing the discharge seen). This ionized region is
described as a space charge, and is what _prevents_ lightning
discharge between ground and cloud.

The mechanism seems to be similar to capacitance, in that potential
between the two plates of a capacitor is reduced because one plate is
earthed. Similarly, earthing the LC allows the flow of charge between
earth and the pointy tips, causing a reduction in potential difference
between cloud and ground.

Cross-posted to physics and electronics, corrections very welcome via
posting or email, particularly on the inferred similarity with
capacitance. The convention wrt to current flow has been ignored as
confusing, and the mechanism of lightning strike (leader spark from
earth to cloud, followed by return discharge from cloud to ground)
omitted for brevity.