Re: Conical and Pancake Coil Arguments... (fwd)

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Date: Tue, 19 Jun 2007 21:10:57 +0000
From: Jeff Behary <jeff_behary@xxxxxxxxxxx>
To: tesla@xxxxxxxxxx
Subject: Re: Conical and Pancake Coil Arguments... (fwd)

Bart,

Its great to see some interest in Pancakes and Flat Spirals.  Probably you 
know about this, but there
are a few things I forgot to mention that I probably should.

"Pancake Coils" are used as a generic term, and include both multilayered 
coils and flat spirals.  In the 1890s portable coils that Tesla formed the 
basis of most of his circuit controller patents for, he always used the 
multilayered variety.  Of course the beautiful flat coil from NY represents 
the other style, which is basically "one turn per layer".  (The half scale 
reproduction posted a while back is beautiful beyond words!)

Oil insullation for these coils becomes tricky.  I made one flat spiral 
Pancake this way:
http://www.electrotherapymuseum.com/2005/KinraideAcrylic/index.htm

I put a funky support on the acrylic forms to allow the oil to penetrate 
everywhere.  I stole this idea from Kinraide.  It worked well for oil, but 
when I switched over to wax it was a disaster.  (details below).

Kinraide's original design:
http://www.electrotherapymuseum.com/2006/KinraideSeeThrough/index.htm

Originally Kinraide used glass rods as supports that were heated and smashed 
on the end so that silk thread may tie them together and "sandwich the 
pancake" as it were:
This formed a patent for the first non-Tesla Pancake Coil.  Of course it was 
still a Tesla Coil, and hundreds of other people patented them since then 
just by getting creative too.

http://www.electrotherapymuseum.com/_PatentLibrary/_KinraideInductionXRayCoil/index.htm

The original coils made like this are here amongst some of the other 
artifacts:
http://www.electrotherapymuseum.com/2007/KinraideRecovered0207/index.htm

My first attempts at making Pancakes & flat spirals were completely 
worthless for the most part.  I attempted to make them by sandwiching the 
wire between acrylic, as many people are doing today:

http://www.electrotherapymuseum.com/2005/KinraideAcrylic/index.htm

I can tell you now, that this method is not the way to go at all.  Once the 
coils are tuned to resonance, they can produce an arc equal to their 
diameter or greater.  Any chance of breakdown with these that can occur, 
will occur.  Materials like plastic and rubber that may be good insulators 
at low frequencies are lousy at high frequencies, and eventually a carbon 
track will prevail and the output of the coil will dwindle to nothing.  I 
have a movie of this for proof, a 100% goof fully documented:
http://www.electrotherapymuseum.com/Films/675.wmv

What you'll see is low power (30 watts, actually a Violet Ray circuit) with 
about 2" sparks.  Then I close a knife to allow another coil in parallel 
with the kicking coil to get 100 watts.  The sparks jump to around 6", but 
immediately you see breakdowns in the coil.  After a short while one of 
these occurs between two wires and shorts them, and the output of the coil 
dies to less than when the 30W circuit was used.  Ouch!  That was a few 
hours time lost in a few seconds!

With any insulator, even wax, the wire itself must be held and positioned in 
such a way to have the wax or oil be as close as possible to the only 
insulation used, or a discharge will try to occur on the second best 
insulation...if that makes sense?  For example, I tried to use that flat 
acrylic star shape to emulate Kinraide's glass rods.  Despite the 1/4" width 
of the acrylic, it was enough to eventually cause a short.  Kinraide's rods 
look primitive, but the wire is touching only on the tangent so the actual 
contact with the rod is reduced to a minimum.  This means the difference 
between "life and death".

Even with paper interleaves, you still must be careful if you raise the coil 
above the ground surface .  I've placed 2" tall paper-interleaved 
multi-layered and flat spiral coils on rubber insulators to get some extra 
height to allow the wax to fully flow underneath the coil (good practice) 
but used pieces too wide before and had an arc form through the wax and 
around the rubber straight up through the wax again to another turn of wire 
and short it out!

The key is to have the thinnest supports possible, or better still (and wax 
is about the only thing you can cast like this in two shots) pour the coil 
once with wax in a form and then position it on supports, add your primary 
coil, and then pour a second time to allow complete immersion in and around 
both coils.  Impregnating the primary coils is also a good idea.

Now, the surprise:  Where do Pancakes normally fail?  You might think (and 
its logical to think this way) that the centre-most windings will fail 
because of the high stresses and voltages raising higher and higher in this 
area to its maximum:  In reality, most arc-overs and short circuits occur in 
the outer 10% or so of the winding.  Its actually the lowest potential part 
of the coil, but the windings are longer and have the most resistance.  In 
coils I've made that "burned out", most 9" coils will burn out around 1/2" 
or 1" from the OD, only a short distance from the ground.  Logical would 
seem like the opposite way, but logic looses in this case.  It doesn't make 
any sense at all until you consider the resistance.  Even one of Kinraide's 
coils is burned out in about this very spot.
If you check out that video again, you'll see what I mean!
http://www.electrotherapymuseum.com/Films/675.wmv
(of course this coil has no insulation at all, its just sandwiched between 
1" acrylic discs)

If you choose to wind a coil without interleaves, the easiest way is to pass 
cotton or silk-covered wire through a trough of wax.  This automatically 
glues it together as your winding.  I use acetate sheets in the form so I 
can see through and also remove the coils easily afterward.  They are still 
fragile, but try to do this without the wax and you'll have an exploding 
"slinky" of wire everywhere.
http://www.electrotherapymuseum.com/2007/5inKinraideCoils/index.htm

A last minute thought, for interleaves, cash register paper works really 
well and so does dry-wall seam paper.  Both are cheap and can be tossed in 
some hot oil and let to sit overnight.  You end up with oiled paper that 
sticks to itself nicely.  The oil seals nicely with hot wax too, and the 
fact that its already soaked avoids many problems with air pockets.

A final note is for small Pancakes - silicone baking pans make the best 
molds imaginable.  You can simply turn them inside out and pop out the coil. 
  Non-stick pans and other types of forms can get brutal when it comes time 
to remove the coils from them.

Jeff Behary, c/o
The Turn Of The Century Electrotherapy Museum
http://www.electrotherapymuseum.com

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