An International Direct Comparison of Two Josephson-Effect Voltage Standards

A direct comparison of the Bureau International des Poids et Mesures (BIPM) and the National Physical Laboratory (NPL) Josephson-effect voltage standards has been accomplished by transporting the BIPM apparatus to the NPL during April 1978. The voltage of a saturated standard cell as measured by the BIPM system, V(BIPM), was found to differ from that measured by the NPL system, V(NPL), such that [V(BIPM) - V(NPL)]/V(BIPM) = 7 ± 2 × 10-8. This difference is 1.6 times the total expected one-standard deviation uncertainty for the combined systems. Direct comparison of the two 2e/h systems without an intermediate standard cell was also accomplished and is reported for the first time.