Model Tests of Electrical Compensation Method for the New Calculable Cross-Capacitor at NIM

In a cross-capacitor with guard electrodes, the equivalent electrical length is sensitive to any variation in the cross section of the main electrodes unless measures are taken to compensate for this. The problem arises particularly in the case of a movable guard electrode, which is conventionally fitted with a nosepiece of such geometry as to compensate for variations in the geometry of the main electrodes from one guard position to another. In the new National Institute of Metrology cross-capacitor, an alternative approach, in which a fraction of the measuring voltage is applied to an isolated electrode in the face of the guard electrode, has been investigated. Results indicate that this modified active compensatory electrode performs as well as or better than an equivalent geometrical compensating nosepiece.