Eventually, there is not enough negative charge left on the 1st bar to draw back the ball. And so the cycle is broken.
The ball"s strange behavior is explained by two phenomena:
1) Different types of atoms can transfer electrons to one another.
2) Once electrons are concentrated in one area, "electrostatic force" will drive them to spread farther apart. Positive ions left behind may then be attracted (by the same force) to the remaining negative electrons.
