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I was at a lecture yesterday and there was a demo of a van de graff generator. He held the smaller metal globe which is attached by a wire, about 4cm from the main globe. This created a spark between the two globes. At the same time the spark happened the speakers in the hall would click loudly, the projector (on the other side of the room) would turn yellow and the text moved down, then flashed black and back to normal. And the plasma globe that was next to the van de graff generator which was switched off (but on at the wall) would briefly turn on.

Is it possible for a spark to do that? Or is there something else happening at the same time?

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Ever heard about Hertz, Marconi? –  Georg Oct 21 '11 at 15:28

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The spark is part of a larger system that includes the two globes and the electrical system for generating a charge on the two globes, without which the spark wouldn't happen, but yes. The spark generates electromagnetic radiation at lots of frequencies, including light, but there will also be radio waves at lower frequencies, and there will be ultraviolet and even soft x-rays at higher frequencies. The intensity of the different frequencies from a given spark will depend on how long the spark is and the properties of the gas surrounding it.

The lower frequency electromagnetic waves that are generated by the spark can induce currents in any electrical circuits that are near enough to the Van de Graaff generator, which will cause different effects in different circuits, some of which you saw and heard. You could try repeating the experiment across different lengths to see how the different effects change, with which you can measure some of the electrical properties of the different circuits.

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