Josephson Voltage Metrology for Watt Balance Experiments at NIST

The Planck constant, h, is one of the seven fundamental constants proposed to redefine the SI. It has been measured by the watt balance experiments since the 1980s. In its early days, the watt balance experiment played a vital role in determining the Josephson constant KJ-90 which was adopted internationally on January 1, 1990. The development of Josephson technology, especially the implementation of the Programmable Josephson Voltage Standard (PJVS) in voltage metrology has improved the uncertainty of voltage measurement in the watt balance experiment by more than two orders of magnitude. The Josephson voltage measurement in the watt balance experiment plays an important role in the proposed SI redefinition based on a set of fundamental constants. This paper will describe a brief history from the original NIST-1 watt balance in 1980s up to the most recent development of the NIST-4 watt balance. We will also discuss the role of voltage metrology in the watt balance experiments and the impact of the SI redefinition to the voltage metrology in the future.