I came across this - 'Jacob's Ladder' while doing some research and think it's amazing!!! If you google the term check out the images, some are fantastic!
Borrowed from - Visual Entertainment - http://www.answers.com/topic/spark-gap?cat=technology
A Jacob's ladder (more formally, a high voltage traveling arc) is a device for producing a continuous train of large sparks which rise upwards. The spark gap is formed by two wires, approximately vertical but gradually diverging away from each other towards the top.
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 1MV/m (24kV/in), 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.
The heated, ionized air rises, carrying the current path with it. As the trail of ionization gets longer, it becomes more unstable, finally breaking. The voltage across the electrodes then rises and the spark re-forms at the bottom of the device.
This cycle leads to an exotic-looking display of electric white, yellow, blue or purple arcs which is often seen in movies about mad scientists. The device was a staple in schools and science fairs of the 1950s and 1960s, typically constructed out of a Model T spark coil, or any other source of high voltage in the 10,000 volt - 30,000 volt range, like a neon sign transformer or circuit (10-30 kV) or a television picture tube circuit (flyback transformer) (10-28 kV), and two coat hangers or rods built into a "V" shape. The sparks can burn through thin paper and plastic and start fires; contact with the exposed high voltage can be lethal.
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