Electroscope

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

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

Electroscope. (P is a metal support terminating in knob K; B is the base; I is an insulator; and H is a cylindrical metal housing with flat ends and windows.) (a) Being charged by induction by negative charge on hard-rubber rod R. (b) Positive charge left on its leaf after induction process is complete. (c) Testing the sign of an unknown charge on test ball T.

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 also Voltmeter.