English: Diagram showing how a pith-ball electroscope works. The molecules (yellow ovals) that make up the pith ball (A) consist of positive charges (atomic nuclei) and negative charges (electrons) close together. Bringing a charged object (B) near the pith ball causes these charges to separate slightly. If the object has a positive charge, the electrons ( – ) in the molecules will be attracted to it, and will move to the side of the molecule near the object. The nuclei ( + ) will be repelled and will move to the side of the molecule away from it. This is called polarization. Since the electrons are now closer to the external charge than the nuclei (C) their attraction is stronger than the repulsion of the nuclei, and therefore the result is a weak attractive force. The separation of charges is microscopic, but since there are so many atoms in the pith ball the total force is strong enough to pull the pith ball toward the external charge.