The Cryo Current Comparator as a Calculable DC Ratio Standard

Cryo current comparators (CCC´s) are known as current ratio standards with small ratio errors. These small errors are easily proven for windings with 1:1 ratios only. For ratios other than 1:1, say 1: N, the errors must be determined by a step-up calibration procedure based on 1:1 intercomparisons of all windings. This procedure becomes tedious as N gets larger than 1. In addition the uncertainty in the overall error is enhanced by each calibration step. This paper shows that by using a toroidal CCC design the step-up calibrations become unnecessary. Instead of determining the actual error, the special winding configuration has a maximum error which, in principle, can be determined by two simple 1:1 measurements. Therefore, the tedious calibration procedure and the overall uncertainty are reduced. High dc current ratios can now be established with this technique with errors smaller than 1 part in 1010.