A Jacob’s ladder (more formally, a high voltage traveling arc) is a device for producing a continuous train of large sparks that rise upwards. The spark gap is formed by two wires, approximately vertical but gradually diverging from each other towards the top in a narrow V shape. It was named for the “ladder to heaven” described in the Bible.
When high voltage is applied to the gap, a spark forms across the bottom of the wires where they are nearest each other, rapidly changing to an electric arc. Air breaks down at about 30 kV/cm, depending on humidity, temperature, etc. Apart from the anode and cathode voltage drops, the arc behaves almost as a short circuit, drawing as much current as the electrical power supply can deliver, and the heavy load dramatically reduces the voltage across the gap.