Original poster: Steve Conner <steve@xxxxxxxxxxxx>
Nowadays, everyone uses a "toroid" as a "topload" on their coil(s). I believe the term "topload" is credited to the good folks on this Tesla List.
I don't know "who" first called it a "topload" but I believe I know the reason for using that name. In radio engineering you often see reference to "top-loaded" antennas. These have a plate or network of wires attached to the top of the mast to add capacitance and allow a shorter antenna mast to be used. The top-loading capacitor is sometimes referred to as a topload. Some antennas also have a loading inductor at the bottom or in the middle.
Now, the analogy to a Tesla coil seems pretty obvious to me. A Tesla coil is just a short vertical antenna with a loading inductor at the bottom and a loading capacitance on top, and the antenna mast in between is then made shorter and shorter until it's completely gone. It is now near useless as an antenna and radiates practically nothing. Instead the input power piles up in the resonator before bursting out in the form of enormous voltages and colossal discharges.
If you think of the coil as a modified radio antenna, "topload" is the natural name for whatever you place on top.
Steve Conner http://www.scopeboy.com/