Heh. It didn't take me long to realise that a lamp flasher module (complimentry BJT astable) could drive the coil, and for some reason, the frequency was drastically increased. I know now that it was due to the back-EMF. Some experimentation to find a transistor that could stand up to the voltage spikes, and heatsink it, and I had something that could drive most small transformer secondaries from a supply between 6V and 12V and sustain a continuous discharge from the former primary. T then got into Cockcroft-Walton voltage multipliers . . . . . .
I packaged the whole circuit up with a 9V battery in a lantern battery shell and took it to school. Oddly enough charged HV capacitors (small enough to be reasonably safe) didn't make the teachers very fond of me once they'd made the mistake of confiscating what I was fiddling with and getting a 'bite' off them.
However the Physics master and the rest of the science department were vastly amused, and my detention was IIRC a lecture on practical electrical safety, followed by the return of the offending device with strict instructions not to bring it to school again without written permission from the Physics master, or he wouldn't be so lenient the next time.