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.