New generation of low-frequency current comparators operated at room temperature

This paper details developments in the use of room temperature current comparators operating with low-frequency (<1 Hz) ac for precision resistance metrology. We consider the possible benefits of this technique over traditional reversed-dc operation. We describe the construction of high-sensitivity current comparators suitable for such low frequency. 100 ß to quantum Hall resistance (QHR) ratio measurements are demonstrated at frequencies below 0.2 Hz and with relative accuracies of 1 part in 10 or better. Resistance bridges based on this technology offer a lower cost, more robust alternative to those using cryogenic current comparators, and the results presented here indicate that the achievable uncertainties should be attractive for many applications. The combination of such a bridge with a compact next-generation QHR reference is our ultimate aim, and we include some first measurements on graphene samples with low-frequency ac.