As is the case with all quadrant electrometers, this is an electrometer with an auxiliary field and is essentially composed of a very light metal strip in the form of an "8" (needle) suspended at its barycentre by a very thin torsion string which in the most sensitive instruments is made of quartz fibre made conducting either by silver plating or immersion in a solution of calcium chloride.
The needle is free to rotate inside a cylindrical container divided into four equal sectors (quadrants) by two diagonal cuts at right angles.
The quadrants and the needle are well insulated and screened by a metal case which is earthed. The opposing quadrants are connected to each other and in the way known as the needle connection, to create the auxiliary field. The two couples thus obtained are brought to equal and opposing potentials on connecting them to a battery, the central point of which is earthed.
Before making the connections, the needle is carefully centred with respect to the quadrants and its longitudinal axis is brought to lie on one of the diametrical planes of separation between them. ...continues