I acquired a double-coned coil from eBay that a deceased FBI agent previously owned. According to one of the bidders, who owns a similar coil frame without wires, this coil was designed by Nikola Tesla himself. The wooden frame of his father's coil was given to him by Tesla as payment when Tesla was out of money.
The coil has two cones, each with a diameter and height of 6 inches and approximately 127 feet of wire. The secondary inductance from end-to-end is 2.36mH. The quarter wavelength frequency for a straight solenoid is calculated to be 1.937MHz, but this coil resonates at about 1.12MHz as determined by a frequency counter and oscilloscope.
The two cones are connected in the middle with one wire, and there's no tapping point between the two coils. Due to age and wear, I had to replace the copper primary, but I'm keeping the old copper tubing to verify the coil's age. This coil was most likely built in the early 1900s and has the same layout as Tesla's patent schematics from the late 1890s.
There's a rumor that this coil demonstrates antigravity properties. However, I haven't seen any antigravity effects when applying a standard spark gap discharge to the primary.