The NIST watt balance experiment is one of several such system that will provide a value for the Planck constant h and the elementary charge e to be used when the CGPM approves a SI based primarily on scaling the SI so that c, e, h, k, and N_A have fixed values. The present relative standard uncertainty on h from the NIST experiment is 3.6 x 10⁸. Work is continuing with the target of improving about a factor of two. Consistency with other determinations of h will likely be the key factor on deciding to proceed with the SI redefinitions in 2011.

Benoît GRÉMAUD

Last modified: Mon Feb 19 15:30:25 CET 2007