Electroscope

An electroscope is an early scientific instrument used to detect the presence of electric charge on a body. It detects charge by the movement of a test object due to the Coulomb electrostatic force on it. The amount of charge on an object is proportional to its voltage. The accumulation of enough charge to detect with an electroscope requires hundreds or thousands of volts, so electroscopes are used with high voltage sources such as static electricity and electrostatic machines. An electroscope can only give a rough indication of the quantity of charge; an instrument that measures electric charge quantitatively is called an electrometer. 'Condensing' electroscope, Rome University physics dept.Electroscope from about 1910 with grounding electrodes inside jar, as described aboveKolbe electrometer, precision form of gold-leaf instrument. This has a light pivoted aluminum vane hanging next to a vertical metal plate. When charged the vane is repelled by the plate and hangs at an angle.Homemade electroscope, 1900 An electroscope is an early scientific instrument used to detect the presence of electric charge on a body. It detects charge by the movement of a test object due to the Coulomb electrostatic force on it. The amount of charge on an object is proportional to its voltage. The accumulation of enough charge to detect with an electroscope requires hundreds or thousands of volts, so electroscopes are used with high voltage sources such as static electricity and electrostatic machines. An electroscope can only give a rough indication of the quantity of charge; an instrument that measures electric charge quantitatively is called an electrometer. The electroscope was the first electrical measuring instrument. The first electroscope was a pivoted needle (called the versorium), invented by British physician William Gilbert around 1600. The pith-ball electroscope and the gold-leaf electroscope are two classical types of electroscope that are still used in physics education to demonstrate the principles of electrostatics. A type of electroscope is also used in the quartz fiber radiation dosimeter. Electroscopes were used by the Austrian scientist Victor Hess in the discovery of cosmic rays. In 1731, Stephen Gray used a simple hanging thread, which would be attracted to any nearby charged object. This was the first improvement on Gilbert's versorium from 1600. The pith-ball electroscope, invented by British schoolmaster and physicist John Canton in 1754, consists of one or two small balls of a lightweight nonconductive substance, originally a spongy plant material called pith, suspended by silk or linen thread from the hook of an insulated stand. Tiberius Cavallo made an electroscope in 1770 with pith balls at the end of silver wires. Modern electroscopes usually use balls made of plastic. In order to test the presence of a charge on an object, the object is brought near to the uncharged pith ball. If the object is charged, the ball will be attracted to it and move toward it. The attraction occurs because of induced polarization of the atoms inside the pith ball. All matter consists of electrically charged particles located close together; each atom consists of a positively charged nucleus with a cloud of negatively charged electrons surrounding it. The pith is a nonconductor, so the electrons in the ball are bound to atoms of the pith and are not free to leave the atoms and move about in the ball, but they can move a little within the atoms. See diagram at right. If, for example, a positively charged object (B) is brought near the pith ball (A), the negative electrons (blue minus signs) in each atom (yellow ovals) will be attracted and move slightly toward the side of the atom nearer the object. The positively charged nuclei (red plus signs) will be repelled and will move slightly away. Since the negative charges in the pith ball are now nearer the object than the positive charges (C), their attraction is greater than the repulsion of the positive charges, resulting in a net attractive force. This separation of charge is microscopic, but since there are so many atoms, the tiny forces add up to a large enough force to move a light pith ball.