Their operation is based on the principle of like sign charge repulsion. Two sheets or leaves, cut longer than they are wide and made of very thin, electrically conductive material, are hung adjacent and virtually in contact with each other. The leaves are ordinarily made of material sufficiently thin so that they have no rigidity and hang down limply. When the leaves, which are electrically connected, become electrically charged, they push apart from each other. The angle they form correlates on the amount of electric charge on the leaves. See the figure below. If the instrument is shielded so that the capacitance is fixed, then the angle can be with some precision to static voltage. It should be noted that the electroscope actually indicates potential, not charge.
The leaf electroscope does not distinguish positive from negative charge, though there are some simple methods for doing so.
In a calibrated electroscope, the two leaves are made of fine hammered gold leaf and the voltage is determined by viewing and measuring the separation angle with a low-power microscope.