ELECTROSCOPE (from Gk. TIXeKroop, •lek iron, amber -- o-Kotrae, skopei». to look). An instrument for the detection of the presence of electricity. it depends for its action on the prin ciple that bodies charged with like electricity repel, while those charged with unlike electricity attract. each other. The ordinary pith-ball sus pended on a silk thread is the simplest form of the instrument. The most common typo of elec troscope is that devised by Bennet in IT57, and known as the gold-leaf electroscope. It consists of two strips of gold leaf, or thin aluminium toil, suspended from the lower extremity of a conduetor within a glass bottle or jar. The upper coil of the conductor terminates in a ball, or a plate in ease the instrument is to be used as a condensing electroscope. (See CONDENSER.) If a body charged with positive electricity is brought near to the knob of the electroscope, the negative electricity will be attracted to the knob and the positive repelled to the leaves, which then diverge. If now the finger is touched to
the knob. the positive electricity is drawn off and the leaves collapse. while the negative electricity is held hound. Removing- the charged body. the leaves will then diverge again, charged with nega tive electricity. In this ease the instrument can be used to determine the nature of the charge of a body brought near it. as with a positive charge the leaves will collapse, and with a nega tive charge spread farther apart. in the Bohnen berger electroscope a strip of gold leaf is sus pended between the poles of a dry pile (q.v.). If a charge is eommunicated to the gold leaf it will be deflected toward one or the other of the two poles.