Toroids
Toroids are the circular tube that sits above the secondary coil. This nifty item "collects" the charge from the secondary coil and when the voltage from the secondary is high enough, the charge stored in (on) the toroid is eventually released to the closest grounded object or the air. Toroids also help "shape" the electrical field produced by the primary/secondary combination. What exactly it does is beyond me, but maybe one day I'll figure it out. What I do know is that the height above the secondary is important and the size of the toroid also helps in producing longer streamers. The general rule of thumb about toroids is that the bigger it gets, the more voltage is needed for the streamer to "break out".
There are many different ideas as to what voltages are coming off of the toroid, some say it can be as high as 1,000,000 volts. The general concensius believes it is more like 750,000 volts. But hell... 3/4's of a million volts is nothing to sneeze at :) b.t.w. this is for streamers at about 4 - 5 feet long.
Most people use aluminum dryer ducting for the toroid.
There are other options, but they are a bit more difficult to build or
they are expensive. The real toroids ( spun aluminum ) can cost well over
100 dollars for a unit that has a 5" diameter height and a 20" overall
diameter width.