electroscope, instrument for detecting the presence of an electric charge or of ionizing radiation, usually consisting of a pair of thin gold leaves suspended from an electrical conductor that leads to the outside of an insulating container. An electric charge brought near the conductor or in contact with it causes the leaves to stand apart at an angle because, according to Coulomb’s law, the like electric charge either induced in or transferred to each leaf causes them to repel each other. Radiation from radioactive materials introduced into a charged electroscope ionizes the gas within, permitting the charge on the leaves to leak off gradually. The rate at which the leaves of a charged electroscope converge to their parallel uncharged position is proportional to the intensity of radiation present.