Background and fundamental theory

The process of electrical measurement consists of comparing the unknown against a standard. Often the standard is in the form of a spring torque, and many elegant schemes have been developed for the electromechanical conversion. The accuracy of such direct methods however is limited, and for more precise measurements the comparison is made against an artifact of like kind, using bridge circuits for scaling and null techniques. Either the same current is passed through the unknown and the standard, and the voltage drops across them are compared, or the same voltage is applied to both and the current ratio is measured. Many fine bridges have been developed in the past but all have problems which limit their range of applicability and the accuracy attainable. The task in measurement development is to devise methods of reducing the impact of these problems.