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Basics
Any lightweight material that can pick up and hold an electric charge can be fashioned into the pith ball. Originally, scientists used pith, the spongy material that holds water inside vascular plants. Nowadays, balls made from plastic, or from aluminum foil, or even from aluminum foil wrapped around plastic balls, will work nicely. Rub a glass rod with silk to remove some electrons from the rod, giving the rod a positive charge. Touch the rod to the pith ball, which is suspended from a string. The ball acquires the rod's positive charge, because the rod has drawn off some of the pith ball's electrons. Now rub the rod again, restoring its positive charge, and try to touch it to the pith ball. The ball moves away from the rod, because objects with like charges repel each other.
Coulomb's Law
Try that experiment again with an electroscope equipped with two pith balls, each hanging down from its own string. Both balls will become positively charged and will repel each other. The balls swing apart, forming an inverted "V." How far apart the pith balls spread depends on how many electrons each one has lost to the glass rod; the more electrons lost, the greater the positive charge on each ball, and the greater the repulsion. That's Coulomb's law. The balls are just the right distance apart for their repulsion to balance the tendency of the pith balls to hang down at the ends of their strings -- also known as gravity.
Opposites Attract
You also can use the pith ball electroscope to show how objects with unlike charges will be drawn to each other. First, get those pith balls positively charged with the glass rod you rubbed with silk. Next, rub an ebonite rod with fur. Ebonite is a hard rubber, which draws off electrons from the fur, making the ebonite rod negatively charged. Move it toward the two pith balls, which will be drawn toward the rod, even if the balls still try to stay apart from each other. Positive charges are drawn to negative charges, and vice versa. That's the electromagnetic force. It keeps negatively charged electrons circling positively charged protons in an atom's nucleus, until some outside force -- say, a good rubbing -- changes the balance.
Induction and Contact
Charges can be imparted by contact, two objects touching, or by induction -- two objects nearly touching. A balloon rubbed with fur becomes negatively charged, and when moved near two hanging pith balls will make both pith balls negatively charged, so the balls spread apart. Move the balloon away, and the balls draw together again. Touch each pith ball separately to the charged balloon (carefully, to avoid discharging the balls), and the balls move apart and stay apart. The difference is that the induced charge isn't permanent.