Re: Radioactive SparkPlugs

["Daniel L. MacIsaac" <macisaac@bohr.phy.nau.edu>]

> LUDWIK KOWALSKI wrote:
> > I do remember reading somewhere about using alpha-radioactive sources
> > at the top ends of lightning rods. Can somebody confirm this? The
> > purpose of using them this way is less obvious than in spark plugs.
Since nobody has overtly answered the obvious question
"Why use radioactive elements to initiate/enhance electric spark discharges?"
and I recently (18 mo ago) learned why from a careful reading of that
outstanding 1st yr calc-based E&M text by Chabay & Sherwood, I'm going
to gratuitously inflict an unsolicited explanation on you-all...  :^)

Electric sparks in air are caused by accelerating a single free electron 
through an E field.  The e- picks up KE which in subsequent collisions with
neutral gas molecules in air knocks more e- free via an avalanche or cascade
mechanism (think of a G-M tube or a photomultiplier tube dynode chain).  
When enough charged particles are produced, a considerable current can 
flow through the air.  (A nice calculation of this is featured in the latest
Physics Teacher, btw.)

The problem is: free electrons to start the cascade are RARE, and are not 
typically produced from neutral gas molecules even in strong E fields.  You
need an outside mechanism to give you these initial charged particles.  C&S
claim that such free electrons are usually the result of cosmic ray 
interactions with the atmosphere (p73; p89) knocking e- loose continuously.
Cosmic rays set up conditions necessary to start sparks.  

An alternative method is to use charge carriers produced by radioactive 
decay processes, like positively charged alphas or betas (fast electrons).  As
an aside, I had the opportunity once to attend a wine and cheese party at the
SF Exploratorium, where I spend two hours imbibing white wine over their
cloud chamber, marvelling over the sea of energetic particles passing through
the air around me (and me, and my wine).  I decided there were lots of e-
being continuously freed from molecular tyranny, that these chamber gismos
were very much too cool for words, and that free wine was a terrific idea also.
It was an epiphany of sorts, although my wife wasn't as enthused :^).

Well, back to writing quizzes and preparing meaningless activity reports,

Dan M

PS -- C&S is described on the web at http://cil.andrew.cmu.edu/emi.html
and if you teach E&M you need a copy.

Dan MacIsaac, Assistant Professor of Physics and Astronomy, Northern AZ Univ
danmac@nau.edu  http://www.phy.nau.edu/~danmac/homepage.html