UPDATE 12/21/04: Fixed timing capacitor error.
There seems to be some question as to what circuit to use for the interrupter. I have one that I have been using for some time that I like. It seems to be quite reliable and works for all of my coils. Below is the schematic:
The power comes from a 9V battery for convenience. The output signal is fed through an RCA cable to the input on the coil's end. Whatever you use to carry the signal, it must be shielded and that shielding must be grounded at the coils end (not at both ends!). You could also use fiber optic cable to send the signal, which would be optimal. This circuit was built in an aluminum project box. My older interrupters were not enclosed, and while they worked fine under most conditions, I found that they could become unstable depending on where I was operating the coil. So I strongly suggest a metal enclosure to completely shield the interrupter.