Jacobs Ladder

A Jacobs Ladder is a novelty device designed to produce high-voltage arcs. It consists of a source of high voltage (usually a step-up "transformer" which takes the 120 volt AC utility power voltage and increases it to several thousand volts AC) and a pair of stiff wires or metal rods with a small air gap near the bottom and a large air gap near the top:

                                                    

When powered, an arc develops at the point where the two rods are closest, then travels up the length of the rods, becoming wider and wider, until it "breaks" off the top of the rods. Once the arc is extinguished, a new arc forms at the bottom of the rods again.
The arc begins at the shortest gap because that is where the electric field strength (volts per inch) is greatest. Once the air ionizes, its resistance decreases dramatically, and the voltage between the two rods likewise decreases due to the "loading" effect of the arc.