An electrometer that consists of a flat cylindrical metal box formed from individually isolated quadrantal segments and containing a light foil-covered vane that is supported by a quartz fibre and is free to move (see diagram). Opposite quadrants are connected together. The instrument is usually arranged so that the vane hangs symmetrically within the quadrants when it and both pairs of quadrants are at zero potential. In addition the potential, Vc, applied to the vane must be large compared to the potentials VA and VB applied to the quadrants. Under these circumstances the deflection, θ, of the vane is given by
or if one pair of quadrants is at earth potential by
where k1 and k2 are constants and characteristic of the instrument. The deflection of the vane is observed by using a small mirror that reflects a spot of light attached to the torsion thread.