ELECTROSCOPE, an instrument for detecting differences of electric potential and hence electrification. The earliest form of scientific electroscope was the versorium or electrical needle of William Gilbert (1544– 1603), the celebrated author of the treatise De magnete (see ELECTRICITY). It consisted simply of a light metallic needle balanced on a pivot like a compass needle. Gilbert em- ployed it to prove that numerous other bodies besides amber are susceptible of being electrified by friction.' In this case the visible indication consisted in the attraction exerted between the electrified body and the light pivoted needle which was acted upon C and electrified by induction. The next im- provement was the invention of simple forms of repulsion electroscope. Two similarly electrified bodies repel each other. Benjamin Franklin employed the repulsion of two linen threads, C. F. de C. du Fay, J. Canton, W. Henley and others devised the pith ball, or double straw electroscope (fig. 1). T. Cavallo about 1770 employed two fine silver wires terminating in pith balls suspended in a glass vessel having strips of tin-foil pasted down the sides (fig. 2). The object of the thimble- shaped dome was to keep moisture from the the pith balls were supported, so that the apparatus could be used in the open air even in the rainy weather. Abraham Bennet (Phil. Trans., 1787, 77, p. 26) invented the modern form of gold-leaf electroscope. Inside a glass shade he fixed to an insulated wire a pair of strips of gold-leaf (fig. 3). The wire terminated in a plate or knob outside the vessel. When an electrified body was held near or in contact with the knob, repulsion of the gold leaves ensued. Volta added the condenser (Phil. Trans., 1782), which greatly increased the power of the instrument. M. Faraday, however, showed long subsequently that to bestow upon the indications of such an electroscope definite meaning 1 See the English translation by the Gilbert Club of Gilbert's De magnete, p. 49 (London, 1900).it was necessary to place -a cylinder of metallic gauze connected to the earth inside the vessel, or better still, to line the glass shade with tin-foil connected to the earth and observe through a hole the indications of the gold leaves (fig. 4). Leaves of aluminium foil may with advantage be substituted for gold-leaf, and a scale is sometimes added to indicate the angular divergence of the leaves. The uses of an electroscope are, first, to ascertain if any body is in a state of electrification, and secondly, to indicate the sign of that charge. In connexion with the modern study of radio-activity, the electroscope has become an instrument of great usefulness, far outrivalling the spectroscope in sensibility.