This chapter serves as an introduction to the book “Coaxial Electrical Circuits for Interference-Free Measurements”. It provides a discussion on interactions between circuits and eliminating electrical interference.

This chapter presents the application of coaxial network for interference free measurements. Interference-free balanced-current circuitry evolved from the need to develop bridge measurements of standard impedances to obtain ever higher accuracies to satisfy demands for better characterisation of passive electronic and electrical circuit components. In this book, we are concerned with conductor-pair networks where one network of the pair has a low impedance compared to the other, and where the current in any one conductor is matched by an equal and opposite current in the other conductor of the pair. The potential differences between corresponding nodes of the pair of networks are quantities of interest because, for example, detectors or sources are to be connected between a pair of corresponding nodes.

We now begin our discussion of balanced current coaxial techniques as applied to impedance measurement with AC bridges. As mentioned in the introduction, this provides a good example of how these techniques can be applied in general to sensitive electrical measurements of other parameters -voltage, current, power and so forth. Before moving on, it is as well to note the advantage of a substitution measurement in comparing two nearly equal impedances, and indeed in metrology in general. We can make a formal statement of the philosophy of a substitution measurement as follows.

In this chapter, we are concerned with the other category of transformer that may deliver some power, but the design objective is rather to approach an ideal transformer. In an ideal transformer the EMF induced in each and every turn is equal and the voltage developed by a winding is very close to the sum of these induced EMFs, i.e. to one part in a million or better. Isolation may also be required. In the case of current transformers, the ratio of the currents in the windings is the important quantity, and the accompanying voltages are only of incidental interest.

This chapter discussed all the concepts and devices described so far to show how they may be combined to make practical high-accuracy bridges. The number of networks that may be devised is endless, and therefore, we have chosen just those examples that either are of considerable practical importance to national standards laboratories or illustrate the use of some device or concept. In principle, we show how to relate like impedances of different values and then how to generate the impedance standards of R, L and M from the unit of capacitance. The reader should feel encouraged to devise other networks to serve other special needs. We will often present a network as an admittance bridge; of course the distinction between this and an impedance bridge is purely one of algebraic convenience.