1. electrometer, quadrant (a) Sir William Thomson's electrometer, a simpleform of which is shown in the cut, consists of four quadrants of metal placed horizontally; above these a broadflataluminumneedle hangs by a very fine wire, acting as torsional suspension. The quadrants are insulated from each other, but the opposite ones connect with each other by wires. The apparatus is adjusted so that, when the quadrants are in an unexcited condition the needle is at restover one of the diametrical divisions between quadrants. The needle by its suspensionwire is in communication with the interior of a Leyden jar which is charged. The whole is covered with a glass shade, and the air within is kept dry by a dish of concentrated sulphuric acid so that the jar retains its charge for a long time and keeps the needle at approximately a constantpotential. If now two pairs of quadrants are excited with opposite electricities, as when connected with the opposite poles of an insulated galvanic cell, the needle is repelled by one pair and attracted by the other, and therefore rotates through an arc of greater or less extent. A small concave mirror is attached above the needle and its image is reflected on a graduated screen. This makes the smallest movement visible. Sometimes the quadrants are double, forming almost a complete box, within which the needle moves.
When the "quadrant electrometer" is spoken of it may always be assumed that Sir William Thomson's instrument is alluded to. Henley's instrument is properly termed a quadrantelectroscope. (See Electroscope.)
Definitions of electrometer, quadrant
