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. This coil was wound with 22ga. wire, 1398 turns and the Inductance is 84.8 mh. That is 2194 ft of wire!  The running frequency of this coil is 112.15 Khz. Tab on pictures to enlarge.