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The secondary structure of a tesla coil should be made of a substance of low loss. Materials such as polycarbonate (Lexan) or acrylic (Plexiglass) are appropriate. The coil form should be as thin as feasible, but must have enough strength to support the winding procedure and to sustain the Toroid terminal. A Tesla coil secondary has its own particular electrical makeup determined in part by the length of secondary. Like a tuning fork, it wants to vibrate at a particular frequency. The secondary is inductively energized by the primary coil. The secondary is four by 20 inch plexy form wound with AWG26 magnet wire on  sealed top and bottom  with about 19 inches of winding. There is 15 coats of varnish on this  four inch coil . Tab on pictures to enlarge!