Towards a fixed value of the Planck constant: Reproducibility and an updated NIST-3 watt balance

The scientific community is presently engaged in a historic attempt to fix the value of the Planck constant h so that it might ultimately serve as a base constant in a revised system of units. To assist this effort, NIST is examining its existing watt balance experiment, NIST-3, and will measure h once again, aiming at a final rigorous test of reproducibility. The main objectives are to modify and update the existing apparatus so that it can be used both to measure the Planck constant and to support other experiments related to the mise en pratique for mass, to attempt a determination of the Planck constant that is largely independent of previous NIST results, and to probe the discrepancy between the value of h measured in 2007 and other latest published results through a careful re-examination of the NIST-3 measurement procedures and uncertainty budget.