The Climbing Arc or Jacob's Ladder uses a high-voltage, high-frequency power supply to create a stationary arc at the bottom of a pair of diverging electrodes where the gap is smallest.  In a few minutes heat will build up between the arc electrodes and cause a rising air current.  The arc will then begin to rise up between the electrodes, following the rising heated, ionized air.  It will extinguish itself near the top where the arc length becomes longer than can be supported by the applied voltage.  As soon as the arc is interrupted at the top, a new one strikes at the bottom, and the cycle repeats.