Toroid
The toroid serves three purposes when used as the top terminal on a
Tesla coil. As mentioned in the theory section,
it forms a capacitor with respect to ground and thus helps to determine
the frequency at which the secondary oscillates. It also performs as an
electrostatic shield for the top turns of the secondary coil, preventing
potentially damaging arcing. Lastly, the large radius of curvature inherent
to the toroid shape helps to prevent arc breakout until the secondary potential
has risen very high. Breakout depends on the ability of the electric field
to ionize the gas around a conductor. The sharper the edges are on a conductor,
the higher the electric field is at the edge for a given potential. (Carbon
nano fibers, which, as the name suggests, are only a few nanometers in
diameter, can generate field emission with a potential of less than five
volts. This technology may show up in flat panel displays in the not too
distant future.) Bigger toroids give bigger sparks, until they give no
sparks at all. (The same principle is used for the output terminal of Van
de Graff generators.)
This was my first attempt at a toroid. Yes, that black thing at the
top inside is an air valve. Unfortunately, I made the mistake of wrapping
individual 2" X ~28" strips of aluminum tape around the inner tube. The
resulting seam (on the inside diameter) failed under slight changes in
pressure. I had to re-wrap it, but this time I used a continuous spiral
of tape. It has held pressure for several months now with no complaint.
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