I'm doing an electrostatics lab and I'm getting a bit confused. The object of the lab is to establish an electrostatic series. First, I must choose a reference material, like plastic wrap. Then I charge this reference material by rubbing it in wool. Now, just by looking at any other triboelectric series I can see that wool is more electropositive than plastic wrap, therefore, the plastic wrap gains a negative charge. Then, I have to take another object, like a balloon, and rub it in wool so that it gains a charge. Then I have to move the balloon close to the reference material and notice a reaction. Both objects attract! Again, by looking at an electrostatic series, I can see that a balloon is more electronegative than wool, which means it too gains a negative charge. However, the negatively charged reference material and the negatively charged balloon attract. I know that with respect to each other, one must be more positive than the other. However, my question is, how do I know which one it is? (without having to a look at an already developed electrostatic series; since this defeats the whole purpose of the lab). Thanks a lot.