The Tesla coil Top load
This is the top load, it is formed in the shape of a torous or toroid. Although other shapes can be used the toroid has many advantages. The top load adds capacitance to the resonant system and lowers its operating frequency. The bigger the top load the bigger the spark BUT input power must be increased to allow break out to occur. 
The toroid also acts as a corona shield for the top of the secondary coil. 
Our toroids are made from ally heating duct which is then covered with ally tape to give a smooth finish.

The next step from making your own toroid is to buy a comercialy avalibe spun alluminium toroid, these are fantastic with regard to electrical properties and they good great BUT they cost the earth but well worth the expencive
As was stated above the top load does more than just protect the top turns of the secondary coil, it all so adds capasitance to the secondary system, by adding "Top C" we can lower the oparating frequency of the Tesla coil, below are the forulars needed to calculate the capasitance of a toroid top load shape.
C  = Capacitance in pF
D1= Major diameter
D2= Minor diameter
PI = 3.14160 

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