An electroscope is an instrument used to detect charged particles. It has a long metal rod with a knob on top and two metal leaves attached to the bottom. The leaves hang straight down if the electroscope has no charge. If a charged object touches the knob and electrons are transferred, the metal leaves become charged. Because the leaves are the same charge, they repel each other and spread apart. The leaves will move in the presence of a negative or positive charge. An electroscope can tell us if an object is charged, but it cannot tell us if the charge is negative or positive.