RE: newbie question - spark gap and toroid

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Lau, Gary" <gary.lau-at-hp-dot-com> 

Hi Shu:

My comments are interspersed below:

 > Original poster: "ho \[shu\] huang" <shuhuang-at-singnet-dot-com.sg>
 >
 > Hey!
 >
 > I've got my tesla coil to work but I've got a couple of quick
questions for
 > you pros out there: -
 >
 > 1. My SG spark gap's made from aluminium curtain rail, not copper
pipe. The
 > rail is flat, so it's easier to mount. Does it matter? My coil works,
but
 > I'm not sure if aluminium hinders performance in any way.

I've not personally tried it, but I suspect that as electrode material,
aluminum is inferior to copper for several reasons.  It has a much lower
melting point than copper, so it will melt and vaporize (ablate) more
rapidly than copper.  It is also oxidized more readily than copper.

 > 2. Does it matter that air isn't blown directly at the sparks? Do the
 > sparks require a lot of air to be "quenched", or will a bit of moving
air
 > be enough? In other words, is more wind better?
 >
 > My spark gap (pic here:
http://web.singnet-dot-com.sg/~shuhuang/spark_gap3.jpg
 > ) sparks at the ends of the rails, which is sort of shielded by the
10mm
 > thick plastic support. I can easily reconfigure it so that the sparks
get
 > most of the breeze from the fan, but I'm wondering if it's worth the
 > effort. The fans blow, rather than suck, BTW.

If I'm interpreting your photo correctly, it looks like your fan is
blowing into a chamber below the gap segments, and the air exits the
chamber between the aluminum segments.  This in itself is a fine plan.

I've found that air flow always improves performance over no air flow,
but there is a point above which more air does not help.  It's difficult
to indicate or predict how much is enough, but I don't think heroic
measures, i.e. shop compressors and leaf blowers, are needed.

I think the reason your gap tends to arc preferentially at the ends is
because there is less (or no) air flow there.  Air that has just had an
arc through it is hotter and has ions and gasses that are more conducive
to arcing than is fresh, cool air.  This means the gap breakdown voltage
is less at the ends, so that's where it will break down.  A coil's
performance diminishes with lower gap breakdown voltage, so this is why
good airflow is important.  I'm not sure that "quenching" is a factor in
the airflow equation at all.

 > 3. I used 2 aluminium wok covers joined together
 > (http://web.singnet-dot-com.sg/~shuhuang/tesla_coil_lab.jpg ), ~210mm wide
and
 > 320mm in diameter, as a toroid but I suspect a traditioanl toroid
might
 > work better. Believe it or not, I'm actually having a hard time
flexible
 > drainage pipe. It's apparently not used very often any more...where I
live
 > (SIngapore) anyway. I came across the pool float toroid design on a
 > webpage, and I've found one on sale cheap. It's 23" in diameter and 8"
 > thick. Is that too big a toroid for my 15kV 30mA, 90mm 900 turns AWG
24
 > wire secondary coil, set up?

The 2 woks actually look quite good!  But the problem with using a
spherical top load is that it doesn't shield the top of the secondary
from corona as well as a toroid would.  You may find that you may need
to bring the woks closer to the top of the secondary to prevent corona
there.  If flexible drainage pipe is difficult to find, perhaps you can
go to a store that sells cloths dryers; they should be able to provide
4" corrugated aluminum dryer duct.

It's been several years since I ran my coil with a 15/30 power supply,
but I think I was using a 4" x 15" toroid.  One could probably go
somewhat larger, but my gut says that an 8"x23" may be too large.

 > thanks!
 >
 > Shu

Regards, Gary Lau
MA, USA