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This is my very first tesla coil. I wanted to make a very small "proof of concept" version. I am very pleased with the results. The primary circuit is a 12 volt 555 timer running at about 20 Hz with a 20% "on" duty cycle. This drives (through a 2n3904) a TIP120 power Darlington transistor. You can see the heatsink in one of the shots. The Darlington switches an automotive ignition coil to provide the HV supply. I have a 470 pF 25KV capacitor, a 2.2 mm spark gap and 10 turns of #18 on the primary. The secondary is 1200 turns of #28. I wrapped electical tape, adhesive side out, on the cores and wound the coils by hand. The primary core is a Fuji 35mm film canister. the secondary core is from a bulk cotton thread spool.

I am able to get good "gas glow" on a 48" fluorescent tube by holding one end of the tube in my hand and bringing the other within about 4 inches of the top of the coil. In the dark, I can get arcs of about 2 inches. The corona in open air is a deep violet. While I have a plastic UV filter over the spark gap, I do wonder if there are short wavelength UV or X-ray emissions coming from the device when I connect it to a large heat sink with many fins. It lights up like a deep purple Christmas tree with little glowing balls at the corners. I suspect there is not enough power in the entire apparatus to produce a lot of X-radiation or high-energy UV.