Hi, when I was in high school, electric shop teacher had a Jacob's ladder made from a 15KV neon sign transformer. I've made some since, but his had a special feature I never figured out the secret to. It was strictly a one-shot device. Between the normal pair of electrodes, there was a short "starter" electrode. When you pressed the power button, the breakdown would start between the short electrode and the main electrodes: I forget if it had a preference for one vs. the other. I also forget how reliable it was, if you were guaranteed a start each time. But I do remember it never restarted after the first spark had extinguished itself, regerdless of the fact that the xformer was still under power.
Inside the wooden box was one, or perhaps 2 high voltage caps about half the size of a bar of soap: perhaps old mica transmitting caps. I don't remeber what their values or even their voltage ratings were, or how they were wired in. I failed to make a schematic and when I asked about it the teacher was evasive. The only thing I can figure is that it somehow took advantage of transients generated if and when it was started some distance from a zero crossing: and that it would never fire if controlled by a solid state relay with auto zero cross detection, for example.
It's a nice feature if only because Jacob's ladders have a strong tendency to overheat if they run continuosly. The electrodes get very hot too.
Has anyone seen a schematic of this trick? With two caps, three input terminals (arguably, if you conisder secondary midpoint ground) and three discharge terminals there are only so many combinations to try exhaustively, I suppose... but not knowing the values or the exact spacing between the electrodes adds too many variables.