A type of cylindrical capacitor is described for which the capacitance per unit length may be computed with very great precision. It consists of a hollow conducting cylinder divided into four insulated sections by gaps parallel to its axis. The defined capacitance is the direct capacitance between pairs of internal opposing faces of the cylinder. If the two capacitances so obtained are equal, both are equal to (log_ε 2)/4π² e.s.u./cm whatever the shape of the cross-section of the cylinder. If the two cross-capacitances are not quite equal, the mean capacitance per unit length is very nearly equal to the above value. A measurement of the effective length of the capacitor is the only mechanical measurement required for the computation of the mean cross-capacitance.The length may be defined by using guard electrodes, and techniques are described for eliminating uncertainties in the position of the guard gaps. An alternative is to measure the change of capacitance for a known movement of a terminating shield.An experimental capacitor constructed from round bars is described. The computed capacitance (1pF) agreed to 1 part in 10⁵ with the value measured in terms of standards of resistance and frequency.