One criterion which can be applied to the evaluation of the electrostatic design of high-voltage equipment is the ratio of the average to the maximum electric field. This may be termed the ‘utilisation factor’ η, and shows how inferior the arrangement is when compared with two infinite parallel-plate electrodes where η = 1.0. Tables of utilisation factors, calculated by means of a digital computer, are given for several standard electrode systems. η takes no account of the space necessary to accommodate the conductors; this can be done by deriving a related quantity λ, termed the normalised effective electrode separation. Values of λ and conditions for it to be a maximum are also given.