In these conditions, when the needle is earthed it is subjected to equal and opposing forces by the two couples of quadrants and remains immobile in that position. But when the difference in potential to be measured is applied between the needle and the case, the needle is repulsed by the quadrants charged with the same sign and attracted by the others. It thus rotates until the moment to which it is subjected is balanced by the moment of antagonist torsion of the suspending string.
Once the characteristic constants of the instrument have been determined, the potential that produced the rotation can be found by measuring the angle of rotation. As in galvanometers, the rotation angles are measured by means of a small mirror joined to the indicator and a graduated scale (Poggendorff's optical method ).
The symmetrical connection we have described is the one most commonly employed, but other connections of the needle and the quadrants can be used.