This project is a favorite of mine, and the one shown here was built for the “laboratory of the Scientists from Krypton that saved Superman from the exploding planet room thingy” of a past event.
The Jacob’s Ladder is a pair of vertical (or in extreme cases, horizontal) electrodes that are parallel to each other and connected to a source of High Voltage. The air between the bottom of the electrodes has a Dielectric Breakdown due to the high voltage potential and creates an Electrical Arc.