Probably the beginning of many a hobbyists' high voltage dreams, the Jacob's Ladder is a very simple, yet dramatic, demonstration. First, a high voltage arc is struck at the bottom, where the electrodes are nearest. Since the neon sign transformer is current limited, via an air-gapped core, all hell doesn't break loose at this point and blow a fuse. Next, as the air surrounding the arc quickly heats up and moves upward, the resistance of the arc itself increases, until it finds a new lower resistance path just above the original arc. The arc continues to step up the gap until the low resistance region exceeds the height of the electrodes (or the sustaining voltage limitation) and it is extinguished. Finally, since the closest point now regains its voltage potential after the collapse of the arc, the process may begin again.
In the pictures above, the shutter speed was a long 1/8 of a second. This allowed the arc to strike several times in that window. It made for a cool wavy-flag effect.
To the left you can see my little 5-amp Variac driving a 9kV, 120mA neon sign transformer. I overdrove the Variac by around 16%, so I'm thinking that the striking voltage was around 14.7kVpeak (That's 9kV*1.16*sqrt(2)). That 120mA made for some nice fat arcs.
As I understand it, neon sign transformers (NSTs) do not maintain 9kVrms and 120mA simultaneously. It will maintain nearly 120mA, but the voltage falls off somewhat. There is some sufficiently complex interplay between voltage and current in that gapped core that I'm not qualified to explain.
I took a video that I'd like to post, but first I need to upgrade to Weebly Pro, but there's no extra money in the budget this month. Maybe next time!