Design principles for gas-dielectric capacitors of high stability are discussed, with particular reference to parallel plate capacitors. A form of construction is proposed in which each of the two sets of plates is supported by a single mounting column and the mounting columns are secured to a single flat mounting plate. It is shown that for units of the highest precision it is desirable to have a 3-terminal construction in which the solid insulation is intercepted by the guard and shielded so that it serves a mechanical purpose only, so far as the direct capacitance is concerned.A marked improvement in stability is gained by hermetically sealing the capacitor in order to exclude changes in humidity and pressure.Constructional details and test results are given for two 1 000-μμF nitrogen-filled capacitors embodying these principles. The capacitance values have remained constant relative to one another within 1 part in 106during the period of approximately two years following their construction. Close agreement between the temperature coefficients of capacitance and of linear expansion of the metal electrodes is taken as evidence that the capacitors are free from deleterious mechanical constraints.