What device is used for producing high voltage traveling arcs and used as an electrical effect in classic movie portrayals of scientific laboratories?

A Jacob's ladder or more formally called a high voltage traveling arc is a device for producing a continuous train of large sparks that rise upwards. The spark gap is formed between two wires, approximately vertical but gradually diverging from each other towards the top in a narrow "V" shape.

The ionized air rises, carrying the current path with it. As the trail of ionization gets longer, it becomes more and more unstable, finally breaking. The voltage across the electrodes then rises and the spark re-forms at the bottom of the device.

This effect leads to an exotic-looking display of electric arcs, which is often seen in films about mad scientists. The devices built in the 1950s and 60s were typically constructed out of a Model T spark coil or any other source of high voltage in the 10,000–30,000-volt range, such as a television picture tube circuit and two coat hangers or rods arranged in a "V" shape.