When the rod is first presented to the pith ball, the latter is neutral and does not possess an electrical charge. When the rod has touched the ball, however, some of the electricity from the rod passes to the ball, and after this they will repel each other. The reason is that the rod and the ball are _similarly_ charged and _similarly charged bodies will repel each other_. [Illustration: Fig. 23.—A Double Pith-Ball Electroscope.] If you are a good observer you might have noticed when experimenting with an electrified rod and the small bits of paper, that some of the little papers were first attracted and flew upwards to the rod, but having once touched it, were quickly repelled. The repulsion between two similarly electrified bodies may be shown by a double electroscope. A double electroscope is made by hanging two pith balls on two silk threads from the same support. Electrify a glass rod and touch it to the pith balls. They will immediately fly apart because they are electrified with the same kind of electricity. *The Gold-leaf Electroscope* is one of the most sensitive means which can be employed to detect small amounts of static electricity. [Illustration: Fig. 24.—A Gold-Leaf Electroscope.] It is a very simple instrument and is easily made in a short time. A couple of narrow strips of the thinnest tissue paper, or, better still, two strips of gold leaf, are hung from a support in a wide-mouthed glass bottle which serves at once to insulate and protect the strips from draughts of air. The mouth of the jar is closed by a plug of paraffin wax, through the center of which passes a small glass tube. A stiff copper wire passes through the tube. The lower end of the wire is bent at right angles to furnish support for the strips of gold leaf. A round sheet metal disk about the size of a quarter is soldered to the upper end of the rod. If an electrified stick of sealing-wax or a glass rod is presented to the disk of the electroscope, the strips will repel each other very strongly. If the instrument is sensitive, the strips should begin to diverge some time before the rod reaches the disk. It is possible to make an electroscope so sensitive that chips formed by sharpening a pencil will cause the strips to diverge. *There are two kinds of static electricity.* Rub a glass rod with a piece of silk and then suspend it in a wire stirrup as shown in Figure 25. Excite a second rod also with a piece of silk and bring it near one end of the suspended one. The suspended rod is _repelled_ and will swing away from the one held in the hand. [Illustration: Fig. 25.—Method of Suspending an Electrified Rod in a Wire Stirrup.] Now rub a stick of _sealing-wax_ with a piece of _flannel_ until the sealing-wax is electrified. Then bring the stick of sealing-wax near the end of the suspended rod. The rod will be _attracted_ to the sealing-wax. If you experiment further you will find that two sticks of sealing-wax will repel each other.