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.

An Accurate Measurement of Planck's Constant



Edwin R. Williams, Richard L. Steiner, David B. Newell


Using a moving coil watt balance, electric power measured in terms of the Josephson and quantum Hall effects is compared with mechanical power measured in terms of the meter, kilogram and second, we find the Planck constant h = 6.62606891(58) x 10-34 Js. The quoted standard uncertainty ( 1 standard deviation estimate) corresponds to (8.7 x 10-8) h. Comparing this measurement places an upper limit of 2 x 10-8 / year on the drift rate of the SI unit of mass, the Kilogram.
Physical Review Letters


electron mass, fundamental constants, Josephson constants, kilogram, mass drift, Planck constant, Watt-SI


Williams, E. , Steiner, R. and Newell, D. (1998), An Accurate Measurement of Planck's Constant, Physical Review Letters (Accessed July 22, 2024)


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

Created June 30, 1998, Updated October 12, 2021