Skip to main content
U.S. flag

An official website of the United States government

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

A precise instrument to determine the Planck constant, and the future kilogram



Darine El Haddad, Frank C. Seifert, Leon S. Chao, David B. Newell, Jon R. Pratt, Carl J. Williams, Stephan Schlamminger, Shisong Li


A precise instrument, called a watt balance, compares mechanical power measured in terms of the meter, the second and the kilogram to electrical power measured in terms of the volt and the ohm. A direct link between mechanical action and the Planck constant is established by the practical realization of the electrical units derived from the Josephson and the quantum Hall effects. We describe in this paper the fourth generation watt balance at the National Institute of Standards and Technology (NIST), and report our initial determination of the Planck constant obtained from data taken in late 2015 and beginning of 2016. A comprehensive analysis of the data and the associated uncertainties led to the SI value of the Planck constant h = 6.626 069 85(23) 10-34 J s. The relative standard uncertainty associated with this result is 34 x 10-9.
Review of Scientific Instruments


El, D. , Seifert, F. , Chao, L. , Newell, D. , Pratt, J. , Williams, C. , Schlamminger, S. and Li, S. (2016), A precise instrument to determine the Planck constant, and the future kilogram, Review of Scientific Instruments, [online], (Accessed June 13, 2024)


If you have any questions about this publication or are having problems accessing it, please contact

Created June 21, 2016, Updated October 8, 2019