Electroscope, an instrument used for detecting electric charges or for measuring small electric voltages or currents. It is used in the laboratory—for experiments and demonstrations—and in industry, where it is connected to meters and other sensing devices. A simple electroscope consists of a glass jar in which two strips of gold leaf are suspended from a metal rod that conducts electricity. The rod, which enters the jar through a stopper made of a material that does not conduct electricity, has a metal knob on the end outside the jar.

The strips of gold leaf hang straight down when they are not charged. When a charged body is brought near the metal knob, both strips acquire a like charge (that is, they both become negative or both become positive). As a result, they repel each other and spread apart to form an inverted V. The electroscope is then charged. If an oppositely charged body is brought close to the knob, the charge on the strips is neutralized, and they again hang straight down. The electroscope is discharged.

By using a specially calibrated microscope to observe the movement of the strips, a scientist can measure the voltage of the charge in microvolts (millionths of a volt). When the electrical capacities of both the electroscope and the body producing the charge are known, electric currents moving through ionized air can be measured Even when the capacities are not known, these currents can be detected. Therefore, the electroscope is used for detecting X rays, cosmic rays, and radiation from radioactive material. These rays ionize the air and pass through it as a kind of electric current. The current either charges or discharges an electroscope.