The practical uses of AC-DC transfer instruments

Alternating currents and voltages are measured most accurately when they are compared with nominally equal and known dc currents and voltages. The comparisons are usually made with thermal transfer instruments which respond nearly equally to ac and dc quantities. Practical information and recommended procedures are given for using these instruments along with diagrams of apparatus and examples of typical data and calculations. Methods for minimizing difficulties caused by dc reversal differences, thermal drift, energy picked up from local electromagnetic fields, and the deviation from square law response of these instruments are considered. Causes of ac-dc differences are discussed, and methods for measuring them and applying corrections are also described.