Demos: 6D-13 Jacob's Ladder

The secondary of a large transformer with a 15kV output is connected to a pair of heavy copper wires, mounted vertically. The wires are sufficiently close together at the bottom for the voltage to arc across. The wires diverge toward the top. Ionization around the discharge and convection of the heated air cause the spark to climb to the top and then start over again at the bottom.

Directions: Turn on the power to the primary. The spark should jump across at the bottom, where the wires are the closest to each other. (To aid in the upward movement, the spark is given a “jump start” by a flow of air directed through a hose near this point.)

Suggestions for Presentation: Say something about the old Frankenstein movies with the sparks climbing up conductors, etc. After the students have had time to enjoy the presentation, discuss the transformer effect and how this creates the spark across the gap. Discuss also why the wires diverge. (It is to insure that the spark begins at the bottom and not somewhere else.)

Applications: Horror movies!

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