The authors have between them more than 60 years of experience in making electrical measurements in National Measurement Laboratories. These laboratories are the source of measurement standards and techniques for science and engineering and are dedicated to maintaining the international system of units (SI) by establishing and disseminating the values of measurement standards with the lowest possible uncertainty. Careful attention to detail is required in designing measurement systems that eliminate electrical interference and are as simple and as close to first principles as possible. This book draws on their experience by offering guidance and best practice for designing sensitive electrical measurement circuits.In particular the book describes examples that demonstrate the elegance, flexibility and utility of balanced-current coaxial networks in obtaining the ultimate in noise-matching and interference elimination for precise and accurate voltage, current and power measurements. It also updates an earlier book on coaxial AC bridges by including recent AC measurements of quantum Hall resistance to establish a primary quantum standard of impedance and by extending impedance measurements in general to higher frequencies.