Neil's News

Jacob's Ladder

15 December 2012

A friend visiting my apartment for the first time looked around and commented, "I was expecting more 'mad scientist'." While my apartment does occasionally feature cannon balls swinging from the ceiling^[?] or lava lamps moving at high velocities^[?], most of the time it looks disturbingly normal. Challenge accepted.

Nothing looks more 'mad scientist' than a Jacob's Ladder. These are the high voltage traveling arcs seen in the background of really bad sci-fi movies from the 1950s. To build one, I acquired the transformers out of four microwave ovens, and daisy-chained twelve light bulbs and a hair drier as ballast. This frightening assembly actually worked pretty well. As this was my first time playing with multi-thousand volt circuits, I invited a coworker, David, over for safety.

But then I stumbled upon a neon sign that had been thrown out. Neon signs contain compact, high efficiency transformers that have built-in ballast. This route seemed a lot safer than the microwave oven transformers. The resulting Jacob's Ladder was very temperamental, but when adjusted perfectly would run reasonably well.

The problem was tuning it. If the bars are too far apart, no arc forms. If the bars are too close, the arc doesn't heat enough air to cause it to rise. It took just the right distance, temperature and humidity to get the arc both formed and rising. The solution was to add a small electrode between the bars that's attached to one side with a 2 megohm resistor. This allows the gap width to be doubled, since the arc will form one half at a time. I also added a glass tube for safety.

With the center electrode added, the Jacob's ladder runs perfectly. It looks especially stunning in the dark. I think this helps address the earlier deficit of 'mad scientist'.

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