可动屏蔽型计算电容复现电容单位的方法研究

Research for Reproducing SI Unit of Capacitance by the CaIcuIabIe Capacitor with a MovabIe Guard Rod

中国计量科学研究院建立了标准不确定度为2＆#215;10-8的可动屏蔽型计算电容装置。该装置复现电容单位量值只与屏蔽电极所移动的轴向长度有关。采用激光干涉仪测量这个长度时，一般无法提供干涉条纹数。经典方法采用替换方法，逐步从已知的小整数扩大到大整数。过程复杂，所需条件多。提出了一种新的复现电容单位的方法。对应复现电容量值的干涉条纹整数由一个准确度等级相对较低的仪器直接确定。其干涉条纹小数则通过电容电桥的两次平衡确定。所需结构及过程简单，操作方便。给出了该方法的详细内容及其优点，报告了检查装置线性的实验及其结果。

A new type calculable capacitor with a movable guard rod has been built in National Institute of Metrology of China at the standard uncertainty of 2 x 10 -8. The SI unit of capacitance reproduced by the calculable capacitor depends on the axial length which is measured by a laser interferometer. However the huge integral number of the fringe is in general not given by the laser interferometer. In a classical approach the substitution measurements were adopted to extend the integral number from a known smaller value to the bigger one. It needs some additive devices and complex operations. A novel approach is proposed. The required integral number is determined by the other capacitance standard with a lower accuracy, and the decimal number is determined by the capacitance bridge with a transfer capacitor. The details of this approach and its advantages are presented. An experiment to check the linear is reported.