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[TCML] Fixing up an old coil -- update and more questions



Last month, I wrote:

Hi, all, I'm new to this list. I'm a physics professor at a small liberal arts college. About 5 years ago, before I was hired, one of our physics students built a tesla coil for his senior project. It's been gathering dust in our machine shop ever since. Some of my current Electricity and Magnetism students and I are working on putting it back into working order.

Original coil specs (got more info since last time)

12 KV, 30mA neon sign transformer
6 brine-filled glass bottle capacitors made from root beer bottles
Spark gap: single gap made from two trailer hitch balls mounted on a wooden frame Primary circuit wiring: 6-gauge primary wire, alligator clips to primary coil Primary coil: Inverted conical helix made from 1/4" copper pipe, about 7-8 turns, smallest diameter 14 inches, largest diameter 30 inches, height about 20 inches, mounted on triangular plywood supports. Secondary coil: 14 gauge wire on white PVC pipe, 5"ish diameter, 36" length Top load: 24" toroid made from 4" diameter metal ventilation ducting, mounted on foil-covered plywood disk.
Secondary resonant frequency measured to be 389 khz.
Bottle cap capacitance measured to be 5 nF (that was a nice homework problem for the students.)


Following the advice of folks on this list, we built a second six-pack of beer bottle capacitors. I also cleaned up a lot of the connections and built a little wooden box to hold the bottle caps, and maybe provide some containment on the off chance they explode.

I retuned the thing (I'm now only using about 5 turns of the primary), and am getting maximum 14-16 inch sparks to a grounded pole, with a snarling lightning storm at 8-10 inches. With the pole removed, there's an "octopus"-like pattern of dozens of 5-inch streamers into the air, coming off the sharp lower edge of the foil-covered plywood disk supporting the toroid.

Not as awesome as some of the things I've seen on this list, but definitely enough to impress the students on their last day of class. One of the beautiful things about a tesla coil is that to explain how it works, you need to touch on almost every aspect of E&M, so it rounds out the semester really nicely.

Anyway, I'm still hoping for more. Can you give me some suggestions on the following?

1) Get rid of the beer bottles and get some better capacitors. Right now, they hiss and seethe with corona discharges and small streamers, and they occasionally give a firecracker-loud "pop". Definitely some serious energy losses, and I worry that they'll blow up and put a student's eye out. I can spend a little money on this (department chair is enthusiastic), so what's the best route? Solder together a dozen kilovolt-scale polypropylene capacitors, or get something like http://www.amazing1.com/capacitors.htm (0.005 uF, 10 KV, "tesla coil capacitor", four of these hooked up in series/parallel)? I can't find any single-unit capacitors with the right properties, with the exception of "glassmike" caps, which I hear like to blow up.

2) Smooth off the sharp edges on the topload. The topload is a 24" disk of 3/4" plywood, covered with aluminum foil, with a torus of 4" flexible ductwork sitting on top. The bottom edge of the disk is a sharp right angle, with creased aluminum foil over it, so all discharges start from this edge. I'm thinking of disassembling the top load and using a router to round out that corner. But at what point do I have to worry about going "too far", and building a top load that's so smooth that it won't arc or corona into air at all?

3) Replace the trailer-hitch spark gap with something better, or at least easier to adjust. Would a multiple-gap setup made with short lengths of copper pipe be appropriate for this size coil? These air- blown, motorized monstrosities you guys are building seem like overkill for this setup.

4) Something I'm still stumped by: grounding. Our workshop is on the second floor of the building. I can't move the TC outside. I figure grounding it to the third pin on a wall socket is a good way to destroy every computer in the building, so I'm grounding to the building compressed air supply pipe, which runs around the room mounted to the wall by standoff brackets, and then disappears into the floor to parts unknown. I hook both the grounding rod and the ground end of the secondary to this pipe, close together. The problem is that when I put the grounding rod close to the topload, so I get a huge lightning storm, every one of the pipe's mounting brackets lights up with a little spark. I gotta figure that ain't good. Any suggestions?
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