An electroscope is an early scientific tool used to detect the presence of a body's electric charge.
Due to the Coulomb electrostatic force on it, it senses load by the movement of a test piece. On an entity, the sum of charge is proportional to its voltage.
It takes hundreds or thousands of volts to generate enough charge to detect with an electroscope, so electroscopes are used with high-voltage sources such as static electricity and electrostatic machines.