Any economical quantitative electrostatics experiment?

[Dorea]

Do you know any economical quantitative electrostatics experiments for the university level?
I'm asking for some experiments for undergraduates in the electrostatics context. We have a Van-de-Graaff generator in our fundamental physics lab but this kind of devices are useful for demonstration, Can they be used for quantitative experiments, too?!

 

[Bystander]

Semi-quantitative good enough? Two (or more) Leyden jars with different interior electrode areas charged from same source attracting/repelling an unknown test charge (ping-pong ball) suspended by an insulating filament? Humidity and drafts could be a killer.