THEORY OF ABSOLUTE ELECTROMETER.

Referring to Fig. 152  g is the stationary disc, charged through the wire connection r; f is the movable disc, carried by a balance beam poised at i on a horizontal and transverse stretched platinum wire, acting as a torsional spring. The position of the end k of the balance beam shows when the disc f is in the plane of the guard ring h h. The end k is forked horizontally and a horizontal sighting wire or hair is fastened acro the opening of the fork. When the hair is midway between two dots on a vertical scale the lever is in the sighted position, as it is called, and the disc is in the plane of the guard ring. 224   STANDARD ELECTRICAL DICTIONARY. IMG:8956969607833025625_224F154.jpg: Fig. 154. PORTABLE ELECTROMETER. The general construction is seen in Fig. 154. There the fixed disc D is carried by insulating stem g1. The charging electrode is supported by an insulating stem g2, and without contact with the box pa es out of its cover through a guard tube E, with cover, sometimes called umbrella, V. The umbrella is to protect the apparatus from air currents. At m is the sighting lens. H is a lead box packed with pumice stone, moistened with oil of vitriol or concentrated sulphuric acid, to preserve the atmosphere dry. Before use the acid is boiled with some ammonium sulphate to expel any corrosive nitrogen oxides, which might corrode the bra . In use the upper disc is charged by its insulated electrode within the tube E; the movable disc is charged if desired directly through the case of the instrument. The upper disc is screwed up or down by the micrometer head M, until the sighted position is reached. The readings of the micrometer on the top of the case give the data for calculation. 225   STANDARD ELECTRICAL DICTIONARY. IMG:8956969607833025625_225F155.jpg: Fig. 155. LIPPMAN'S CAPILLARY ELECTROMETER.