First light

My 11 year old daughter had to do a science fair project for school, and was
asking about HV stuff in general so I told her about Tesla coils.  Since she
showed an interest, we did some surfing and came up with a taste for just
how much stuff there is on the net on this (including this mailing list).
We ended up building :

3.4" Black ABS pipe form				Free from the scrap
700 turns of #22 AGW wire salvaged from
	an old TV pwr xformer and re varnished 
	to cover the bare bits.  Several splices 
	to put the windings into one piece of wire		Free from a
scrap TV found by the road

8 salt water beer bottle caps				Free if you don't
count Mum's tinfoil and tape or
							the bottle deposit!

10 kv OBIT at 20 ma					Free from a generous

Primary of 16 turns of #12 Hard drawn copper wire		Free from
the scrap bin, but so hard to use, 
							I would definitely
look for soft drawn wire or pipe next time

Toroid of foil covered Styrofoam life ring			This was a
small life ring I found on the beach.  
							It is only about 14
inches across and 2 1/2 inches thick

Misc 10 AWG wire					Free from the scrap
Spark gap from 6 - 2.5" pieces of 1/2" copper 
	pipe epoxied on to a ceramic tile		Free from the scrap

Base of 3/4 " plywood varnished with PU			Free from the scrap

So far the cost has been pretty reasonable!

Yesterday we fired it up for the first time and got a good hot spark fairly
evenly across the spark gaps.  No arcing on the secondary (I was pretty
worried about the salvaged wire with all the bare spots and splices)  The
coil tunes very well with no spark at 8 turns and about a 5 1/2 inch steady
spark (actually looks like 4 or 5 sparks in parallel) at 14.5 turns.  Moving
1 turn either way drastically drops the spark so I am pretty sure I have
resonance and a moderate Q.  These values are fairly close to those
predicted by WinTesla (a few more turns on the primary than indicated, so I
suspect my toroid is a bit smaller than I told WinTesla)  I don't get a lot
of very visible corona unless it is completely dark and I don't get arc's to
anything other than a grounded target.

I realize that total output is the result of the weakest link in a chain,
and I suspect I am not going to get dramatically better results without
significant investment and upgrade of all parts. But... If I had to pick the
weakest link, based on discussions here, I am assuming I could use a better,
lower loss cap.  Unfortunately, MMC's look like they would require a
significantly bigger investment than already made (none of those in the
scrap bin!).  I should know better than to ask, but I assume that Microwave
Oven caps are inappropriate?  Any other type of cap like rolled poly look to
be well beyond what an 11 year old can do.  The beer bottle caps were a very
understandable and approachable technology for her.  (in fact, she wants to
build more)  I do have some 1 liter HDPE shampoo bottles with straight sides
and small necks.  They look to be about 30 mil thick.  Would there be any
benefit to trying these with parallel strings of two in series?  Is HDPE
similar to LD PE from an electrical point of view?  Am I correct in assuming
the lossyness of bottle caps are due to the glass dielectric?

In a somewhat unrelated question, I have several MOT xformers that have
minimal value to me.  I have seen several warnings about the potential
dangers of this kind of voltage and amperage.  I have seen lots of
discussion about running them in series and how after two, you start
exceeding the ratings pretty badly but the general consensus seems to be
that doubling the ratings has a chance of success at least for a while.
There were several implied warnings about doubling the input voltage but no
clear reasons why not.  Assuming some experimentation were to be done after
the 11 year olds bedtime, what are the risks / dangers (beyond the obvious)
of wiring two MOT's frame to frame and running the primaries off 240V in
series with an arc welder with a shorted secondary? (they are 120 V
primaries as designed)  If (when?) they fail, what are the likely results?
How much protection do I need from fire / shrapnel etc or will I just get a
popped breaker?  This seems like a much to easy way to get 8 - 10 KV at 700

Thanks again for all the help out on the net...

Brian Howden
Victoria B.C. Canada