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 passes 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
brass.
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.