An electroscope is a device that indicates the presence of charge. An electroscope is made from conducting material (generally metal). Charge is free to flow on a conductor - if you put charge at a particular point it will distribute itself over the surface of the conductor.

An electroscope generally has a rotating arm to indicate the presence of a net charge. When our electroscope is uncharged the arm is usually vertical, and when the electroscope is charged the arm moves away from vertical. This is true as long as there are no charged objects nearby - in a few minutes we'll discuss what happens when there are charged objects nearby.