Cryogenic current comparator bridge for quantum Hall resistance ratio measurements

A cryogenic current comparator (CCC) bridge for quantum Hall resistance ratio measurement has been constructed. Compared to previous CCC systems at the National Physical Laboratory, UK, the new system differs in two aspects. Firstly, it uses a type-II CCC, i.e. the ratio windings are on the outside and the detection winding is on the inside. This has the advantage that the CCC can be made very compact and fits into a standard storage Dewar. Secondly, the bridge electronics uses a digital servo and the SQUID feedback loop is disabled during current reversals. The overall relative measurement uncertainty of the system is of the order of 10^-9.