Top Loads
Top loads form the capacitor for the secondary circuit. Along with the secondary coil they form a resonant circuit at the same frequency as the primary circuit.
As the secondary inductor is so large (500-2500 turns) and the primary inductor is so small (2-10 turns), the secondary capacitance can be small (8-100pf)
Any lump of metal will form a capaticance, but to allow as much energy to be stored in the topload with out the power "breaking out" the topload needs to be as smooth as possible. Any sharp point will accumulate electrical charge and discharge to the atmosphere.
The most common shape of topload is the torroid, but spheres and other shapes are also used.

Multiple  Torrids
Torroid made from aluminium ducting

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