Electroscope

thumb|Gilbert's versorium

The electroscope is an early scientific instrument used to detect the presence of electric charge on a body. It detects this by the movement of a test charge 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

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. 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 an insulator, 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. 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 to the object than the positive charges (C), their attraction is greater than the repulsion of the positive charges, resulting in a net attractive force. It consists of a vertical metal rod, usually brass, from the end of which hang two parallel strips of thin flexible gold leaf. A disk or ball terminal is attached to the top of the rod, where the charge to be tested is applied.]]The Bohnenberger electroscope was developed in the early 19th century by the German physicist Johann Gottlieb Friedrich von Bohnenberger as an improvement on earlier gold-leaf electroscopes. The instrument employed a single gold leaf suspended between two oppositely charge plates, increasing sensitivity and allowing clearer detection of both the presence and sign of an electric charge.

Bohnenberger electroscopes were widely used in 19th-century experimental physics and appear in university laboratories, teaching collections, and scientific manuals throughout Europe. The design influenced later high-sensitivity electroscopic instruments.

Eberbach & Son electroscope instruments were designed primarily for educational and laboratory use, following classical electroscope principles while emphasizing robustness and standardized construction for teaching environments.

While they did not introduce new electroscopic principles, they played a role in the standardization of electrostatics instruction in North America.

File:Gold-leaf electroscope-2.jpg|Condensing electroscope, Rome University physics dept.

File:Gold leaf electroscope with ground strips.png|Electroscope from about 1910 with grounding electrodes inside jar, as described above

File:Gold leaf electroscope homemade.jpg|Homemade electroscope, 1900

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