Determination of an Absolute Capacitance by a Horizontal Cross Capacitor

A cross capacitor is a calculable one whose capacitance is ideally determinable from one length measurement of its electrodes. In the Electrotechnical Laboratory of Japan (ETL), a horizontal cross capacitor was constructed. In this model, ease of checking alignment of the horizontal electrode bars, and a simple driving mechanism to move the guard electrode, were emphasized. Because the movable guard electrode rests on, and is guided by, the horizontal electrode bars no special supporting mechanism is necessary. The displacement, which defines the length of the capacitor, is observed by interference fringes using a Hg198 source. The sensitivity obtained for electrical measurement is better than 0.01 aF and for length measurement better than 1/30 fringe. The errors of this capacitor are examined in detail and confirmed by experimental tests. The absolute capacitance was determined as 0.1 pF with an accuracy better than 1 ppm.