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.
