Designing Demo Coil

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Original poster: Michael Johnson <mjjohnson.geo@xxxxxxxxx>

I've been reading and searching this amazing site, and
finally decided to ask a question.

I'm designing a coil to be used in demonstrations here
at ASU (Arkansas State), as well as at various local
public schools.  I've discussed the general design
with one of the professors, and he suggested something
that would put out around 36" streamers would be good.
 We don't want it to be too big, as this has to be
reasonably portable, plus we don't want to have to
worry too much about sensitive equipment (like
computers) nearby.  It seemed like it would be easier
to not do damage if we restricted the size a little
bit.  However, we DO still want it to be somewhat
impressive, and three feet seemed good.  And by the
way, I have a budget of $200.

I found the equation for maximum spark length (1.7 *
Sqrt(input power in watts)), and that suggested a 450
watt system should do the job (a 15/30 NST).  How
optimistic is that?  Should I go for something larger?
 I may be able to do a 12/60 system, depending on what
I can scrounge from the local sign shops.  (I already
have one 12/30 that seems to work fine, albeit with a
slight buzz--I don't know why it was discarded.)  I've
read a fair amount here, but have no practical
experience yet.  I'm not sure just how efficient my
first coil will be, so I don't know how much power I
want.

I was also planning on using a 4" secondary since I
have the pipe, but I haven't settled on the other
parameters yet, except in general terms (TCBOR SG, LTR
MMC).  Does 4" sound reasonable for something in the
450-900 watt range?

Thanks to everyone for your help!

-Michael Johnson