Calibration of d.c. resistance standards and voltage-ratio boxes by an a.c. method

Some of the uncertainties and causes of error experienced in the traditional methods of calibrating d.c. resistance standards are discussed, and it is shown that they can be reduced or eliminated by the use of a.c. and inductively coupled ratio devices. Techniques are described whereby the zero-frequency (or d.c.) value of 4-terminal resistors can be readily determined to an accuracy of within 2 parts in 107 from a few measurements made at frequencies between 40c/s and 600c/s. A Kelvin-double-bridge network is used, the ratio arms consisting of adjustable inductively coupled dividers having an accuracy and discrimination of at least 1 part in 107. Similar techniques are used in a Wheatstone-bridge network to determine the values of d.c. voltage-ratio boxes. Any ratio between 1 : 1 and 1000 : 1 can be measured directly in one step to an accuracy of within about 1 part in 106. Experimental results and constructional details of the inductive dividers are given, together with the theoretical considerations underlying the principle of the method.