The above pictures are of a double-coned coil I acquired on eBay. The seller purchased it from the estate of a deceased FBI agent. One of the bidders, who owns a similar coil without wires, informed me that it was designed by Nikola Tesla himself. The wooden frame he owns was given to his father by Nikola Tesla as payment (Tesla had no money at the time).
Each cone is 6" in diameter and 6" in height. Each cone has approx—127 ft of wire. The end-to-end inductance of the secondary is 2.36mH. The quarter wavelength frequency calculates to 1.937MHz for a straight solenoid, but this coil resonates at about 1.12MHz as determined by a frequency counter and oscilloscope.
The two cones are connected in the middle such that the entire length is one wire. There is no tapping point between the two coils. Due to age and wear, I had to replace the copper primary, but I am keeping the old copper tubing to verify the coil's age. This coil was likely built in the early 1900s and is identical in layout to Tesla's patent schematics from the late 1890s.
Rumor has it that this coil exhibits antigravity properties. I have not seen antigravity effects when applying a standard spark gap discharge to the primary.