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

[mrbigstuff]

if you are ever in Boston, it's worth a trip to the Museum of Science to see the world's largest Van de Graaf generator in action.

the kids - and some adults - jump out of their skin when the fireworks start.

(and a quaint web page about it from the MoS circa 1995!)