Bringing A Charge Near An Electroscope

An electroscope is a device used to detect the presence of charge and it's relative amount. The electroscope is usually constructed with a metal plate or sphere at the top of a metal post with thin foil leaves hanging from the bottom of the post. This setup is typically supported in a flask or some other transparent enclosure. If a charged object is brought near the top of the electroscope, the leaves at the bottom spread apart. The greater the charge, the farther apart they move. The animations below show what happens if charged objects are brought near the electroscope. Try to focus on only one animation at a time, cover one with your hand if you have to.


Electrons pushed by negative object toward the bottom of the electroscope. The foil leaves at the bottom have a negative charge so they repel each other.	Electrons attracted by the positive object toward the top of the electroscope. The foil leaves at the bottom have a positive charge so they repel each other.
When the charged object moves away, the electrons in the electroscope redistribute evenly so the leaves fall back down. The observable behavior is the same, so we can not use this test to determine what kind of charge we have.