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, V_c, applied to the vane must be large compared to the potentials V_A and V_B 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.

Quadrant electrometer