The paper describes the design and construction of a three-terminal capacitor covering the range 0.01-10 μμF, and capable of adjustment to any value within this range correct to 1%. Part of the wide range of variation is obtained by retracting one of the electrodes into a hole; this device gives an exponential relation between the penetration of the electrode into the hole and the capacitance, the latter decreasing by a factor of about 100 for a retraction of the electrode equal to the diameter of the hole. The capacitor, used in conjunction with a pair of silicon rectifiers and a d.c. measuring instrument, can be used as a radio frequency peak voltmeter covering the range 1-20 kV, at frequencies between 50 kc's and 50 Mc's.