An International Comparison of Current-Ratio Standards at Audio Frequencies

The results and analysis of an intercomparison between the National Research Council (NRC), Canada, and the National Bureau of Standards (NBS), Washington, D. C., of two contrasting types of current ratio standards (current comparator and current transformer) are presented. The agreement achieved between the two laboratories when their respective designs and methods of measurement were quite different is emphasized. To this end, the basic theories underlying the design and operation of the current comparator and current transformer are contrasted; the origin and significance of their respective errors are summarized; the (N+1) method used at NBS to measure the errors of the multirange transformer standards up to 10 kHz is contrasted with the (N+1) method (one of several) used at NRC in calibrating the comparator standards. The comparison circuit used at NRC to compare the respective standards of the two laboratories is described. The errors of the transformers as measured at NBS and NRC are presented and contrasted in both tabular and graphical forms.