Uncertainty of the Ohm Using Cryogenic and Non-Cryogenic Bridges

We describe recent scaling measurements to decade resistance levels based on both cryogenic and non-cryogenic current comparator bridges. National measurement institutes and the International Bureau of Weights and Measures derive traceability for the SI ohm using the quantum Hall effect, with the primary comparisons made at the resistance value RK-90/2 = 12906.4035 Omega. Cryogenic current comparator scaling methods are preferred since large resistance ratios of 100 or more are achieved with unsurpassed uncertainty. The quantized Hall resistance standard based on graphene allow less complex and more cost-effective room-temperature resistance bridges to provide SI traceability.