We put together a makeshift jacob’s ladder tonight for fun at Radio Mondays, using a 7.5 kv neon transformer.
This is two brass tubes (one bent to put the arc point above the temporary frame), carrying high voltage arc just to the point of being capable of jumping. (the rule of thumb is 1mm per 3kv). The convection of the superheated air pulls the arc upwards, until it can’t sustain the arc, and it breaks.
Things were a little fiddly, as you can see in the above video, so we pulled out at 12kv transformer, to see if more power improved things.
The larger transformer seemed to help: