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Re: Fr. Tom Strikes Again

Subject:  Re: Fr. Tom Strikes Again
  Date:   Tue, 13 May 1997 08:35:06 -0400
  From:   "Thomas McGahee" <tom_mcgahee-at-sigmais-dot-com>
    To:   "Tesla List" <tesla-at-pupman-dot-com>

>         <a beautifully commonsensical dissertation deleted for
> bandwidth's sake>
> 
> >Fr. Tom McGahee
> 
> Perhaps I'm missing a point or going off on a tangent here, but is
a
> single
> cycle sinusoidal pulse really a pure sine wave?  The voltage is
constant
> at
> zero up to the start of the pulse and returns to stay at zero at
its
> termination.  There's (ideally, of course) a discontinuity at each
> point.
> These should generate an infinite number of
harmonics--uh--shouldn't
> they?
> 
> Your students are lucky to have you as a teacher.
> 
> Norm

Norm,
You get a cigar! As long as a sine wave is sinewaving it is a pure
sine wave. Once it STOPS sinewaving (and before it starts
sinewaving), it ain't a sine wave no more. You are absolutely correct
in your surmise that the start and stop *points* introduce
discontinuities.  Such points will always introduce the same sort of
things that we see in square waves. If we look at the waveform at
these *points* we notice sharp "edges" if you will. These "edges"
have MANY frequency components, as you stated.

One of the problems involved in explaining any given point is knowing
where to begin and where to end. And every explanation (hopefully)
raises new questions. That's where the real fun is. No explanation of
anything is really ever complete, because to do so would mean
encompassing EVERYTHING. My greatest praise is reserved for those
moments when one of my students asks a REALLY GOOD QUESTION. I
definitely don't have all the answers. But I have a lot of fun
playing with the questions and seeing where that takes me. Knowledge
always begins with asking the right questions.

To give this post a Tesla slant (since this IS the Tesla list, after
all...) if we dump energy into a resonant circuit, SOME of the dumped
energy will have the proper frequency components to "excite" the
circuit. In a tube type Tesla coil you can PULSE the tank circuit as
well as run it in pure CW mode. In CW mode you get greater *average*
POWER, and in pulse mode you get greater *peak* power. If you pulse
it with a square wave you will get LESS peak power than if you pulsed
it with a SHAPED pulse that closely resembles the resonant frequency.
Note that the square wave pulse has more TOTAL ENERGY, but spread
across MANY frequencies. The shaped pulse will have MORE of its
energy MATCHED to the operating characteristics of the circuit to
which it is being delivered.

This shaping of the delivered pulse is more difficult to accomplish
in a classic Tesla coil, but I believe that one reason that "synergy"
is sometimes reached is that the primary circuit itself is
efficiently *shaping* the delivered energy. For example, more turns
in the primary helps to SHAPE the initial pulse more readily. 

The careful adjustment of a rotary spark gap also helps to INCREASE
the frequency component of *interest*, and the phasing is subtley
affecting the SHAPE of the delivered energy. 

Someone asked me about the cigars. No, I don't smoke, nor do I
actually give my students cigars. It is just an expression I use when
someone gets something right. "Give that guy a cigar!"

Fr, Tom McGahee