If the pith ball is permitted to touch the glass rod, the latter will transfer some of its electricity and charge the ball. Almost immediately the pith ball will fly away from the glass rod, and no matter how near the rod is brought, it will refuse to be touched again.
This action is much the same as that of the magnetized needle suspended from a thread when the similar pole of the magnet is presented to it.
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.
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.