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device for detecting electric chargecharge,
property of matter that gives rise to all electrical phenomena (see electricity). The basic unit of charge, usually denoted by e, is that on the proton or the electron; that on the proton is designated as positive (+e
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 invented by NolletNollet, Jean Antoine
, 1700–1770, French clergyman, experimental physicist, and leading member of the Paris Academy of Science. He constructed one of the first electrometers and developed a theory of electrical attraction and repulsion that supposed the existence of a
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 in 1748. There are various types of electroscopes. The most common has a cylindrical metal case closed by two round, flat, glass faces. A charge sensor is mounted within the case and electrically insulated from it and is joined to an external terminal by a conductor, e.g., a metal rod. The sensor consists of two leaves of metal foil (usually gold) or a metal vane mounted so that it can freely rotate about a metal rod. If a negatively charged body is brought near the terminal of the electroscope, it will cause electrons to be repelled into the sensor; a positively charged body attracts electrons out of the sensor. In either case a net charge is induced on each part of the sensor, and the two leaves will fly apart or the vane will swing away from the rod. It the electroscope is given a known charge by conduction, e.g., by touching its terminal with a negatively charged rod, it can then be used to identify an unknown charge. If the unknown charge is like that on the electroscope, when it is brought near the terminal the leaves or vane will move even farther; while if it is opposite that on the electroscope, the leaves or vane will fall toward the uncharged, neutral position. The charged electroscope can also be used to detect ionizing radiation. The charge on the sensor will be neutralized by oppositely charged ions formed by the radiation from the surrounding air molecules; the rate of discharge provides an indication of the intensity of the radiation.


An instrument for detecting the presence and sign of an electric charge. It is the simplest type of ionization chamber. See Ionization chamber

The illustration shows a common type of simple gold-leaf electroscope. Gold leaf (L) is used because it is an extremely thin conducting foil which has low mass per unit area and is very flexible. Hence, it responds quickly and vigorously to small electrostatic forces. In the illustration H serves as a grounded electrostatic shield, as well as a shield against air currents. The hard-rubber rod R (illus. a) with its negative charge has set up the charge distribution by the process of electrostatic induction. The response shown is a test for the fact that R has a charge. See Electrostatics

Electroscopeenlarge picture

To leave the electroscope with a net charge, a grounded conductor is touched to K so that the surplus electrons on P and L go off to ground, leaving the bound positive charge on K. The ground connection is then broken and R is removed. At this stage (illus. b) the electroscope is said to have a positive charge because there is a positive charge on its leaf system.

If an electroscope has a charge of known sign, as in illus. b, it can be used to test the sign of an unknown charge, as in illus. c, where the metal test ball (T), with its insulating handle (J), has the unknown charge. In the situation pictured, L moves farther away from P as T is brought slowly up toward K, showing that T has a positive charge. If T had a negative charge, L would move toward P, as T slowly approaches K. The converse situation, if the leaf system in illustration c had a negative charge initially, can be readily visualized.

Although electroscopes have been built with a wide variety of geometries, the principle of operation is essentially the same for all. If an electroscope has a scale, permitting quantitative measurements, it is called an electrometer or electrostatic voltmeter. For information on electrometers See Voltmeter



a simple device for the detection of electric charges and the approximate determination of their values. It consists of a metal rod with one or two small light metal leaves attached at the bottom and usually with a small sphere on the other end. The rod is placed in a glass vessel and secured with a plug of insulating material. When the sphere is touched by a charged body, a portion of the charge is transferred from the body to the leaves, and they repel one another (in electroscopes with a single leaf, the leaf is repelled from the rod). From the divergence angle of the leaves it is possible to estimate the value of the charge of the leaves and, consequently, the charge on the body.


Kalashnikov, S. G. Elektrichestvo, 3rd ed. Moscow, 1970. (Obshchii kurs fiziki, vol. 2.)


An instrument for detecting an electric charge by means of the mechanical forces exerted between electrically charged bodies.