I have been experimenting with joule thief's and decided to put the winding around a fly back transformer core. And it works! Using only one transistor you can get super high voltage. The transistor that works the best is a 2N3055 it will put out 1 1/4 inch arcs. In the video I am using two 2SC5047. Because I burned out my 2N3055 transistor. For some reason the 2S part is not stamped on it. They are hooked in parallel to get double the power. But is not really necessary because they can handle 25 amps each.