We are doing an experiment to verify coulomb's law, and we are currently using a stable voltage source. However, we were assigned to do it using static electricity, and we were wondering how to establish inverse r^2 dependence if the amount of the charge on the ball isn't the same every time we do the experiment. When we knew the charge was the same every time (assuming same voltage begets same charge), we were able to cancel the q1q2 term and be left with only 1/ r^2 dependence. This is apart from the issue that we aren't actually able to create the forces required to measure significant deflection. Any help would be much appreciate.