1. A 6 inch outside diameter, 24 inch long 1/8 inch thick acrylic tube.
(I got rid of the previously mentioned tubes 2, 3 and 4 in 2007. And I am aware that I would get greater Q with the coil's diameter greater than 1/4 of its length.)
Tube 1 got wound with 27 AWG magnet wire (approx. 1400 turns) in 2005, and not quite as neatly as I hoped for. Recently, I learned that AWG 24, 25 or 26 will work better, so I plan to get AWG 26 wire and redo this more neatly.
CONSTRUCTION STATUS: UPDATE 9/22/2008 - 2/27/2013: I intend to sometime get some MP108 power opamps for this coil, to drive it by the low impedance primary feed method, in a self-oscillating scheme. I still need to get a top electrode for this coil. At this moment, the coil still has its top termination being the end of the AWG 27 magnet wire. If this wire is tucked into the top end of the secondary and bent upward so that its end is pointing upward, centered on the coil axis, and level with the top of the winding or a little lower, there is no corona or "brush discharge" at a peak voltage of 40 maybe 50 KV.
UPDATE 12/14/2022: I have this in a low power self-oscillating circuit using the low impedance primary feed method. I still plan to replace its AWG 27 winding with a neater one using AWG 26 wire. Even with low input power with a simple circuit made with a few cheap parts and non-neat winding and lack of a proper top electrode, and resulting low output voltage less than 10KV RMS 14 KV peak, this Tesla coil lights neon lamps and fluorescent bulbs up to 4 - 5 feet away. This spectacular performance is from the large size of this coil. The continuous oscillation that it has contributes to good brightness of lamps lit up at impressive distances.
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