Positive? or Negative charge? How to tell on a Van de Graaff generator?

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Mike Johnson wrote:
It's awfully hard to get a picture of a spark. I've tried. If we get the science fair project closer to completion, I will see what I can do.
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Try this: use low ISO, low aperture, long exposure time. Fire the FLASH so you get a picture of the generator itself when not sparking, and eventually you're gonna capture a spark or two during the 2-3 exposure. Experiment and you will find a good combination of parameters