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http://bogard.110mb.com/index.htm
http://bogard.110mb.com/TCTopload.html

Top load

  The top load is a chunk of conductor to which the top of the secondary is attached.  The top load is important because it adds capacitance to the coil changing the resonant frequency of the secondary ground system, so bigger is better, but only up to a point.  Think of it like a water tank, a bigger tank makes bigger ripples, but if it is two big, and a small stone is dropped inside, it will barely affect it at all.  Also, a smooth surface will encourage longer arcs, where as points will encourage corona (blue glow from the voltage breakdown of the air).  The toroid is favored by Tesla enthusiasts, It has high surface area for max capacitance and sheilding, but no sharp corners for corona breakout.  Another popular alternative is the sphere, because it is smooth.  Some coilers like to stack multiple toroids and spheres for higher top capacitance.  A simple toroid can be made by using a tube (like aluminum ducting) and covering it with foil or better yet foil tape, to smooth any rough edges, and provide a conductive surface for the top load.  I have also built a giant truck tire inner tube top load, it is a beautiful thing, but it is too big for my 6-in coil.

4.5in toroidTesla coil bolt onebig toroid on tesla coilsphere combo on tesla coilMOT coil top loadstove pipe top load
truck tube top load
These are some of the top loads I have tried.

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Scott Bogard. 2007