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

Abstract

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 the 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 83(22) × 10−34 J s. The relative standard uncertainty associated with this result is 34 × 10−9.

Measurement of the Planck constant at the National Institute of Standards and Technology from 2015 to 2017

“…The relative standard uncertainty of the result given here is less than half of that published previously [1]. This improvement is due to smaller uncertainties in three categories of the uncertainty budget (Table 1.1) labelled statistical, magnetic field, and electrical.…”

“…The main categories of the uncertainty budget are listed in Table 1.1. For comparison, the uncertainties that were assigned for 2016 publication [1] are also listed.…”

“…The biases caused by the magnetic fields are discussed in detail below. Comprehensive information on the alignment biases can be found in [1] and [21]. In brief, these alignment biases can be divided into three groups Virtual power contains the sum of five relative parasitic power terms.…”

“…For the result published in [1], a determination of β was made by measuring three different mass values: 0.5 kg, 1 kg, and 1.5 kg. The resulting relative bias on the final result due to ∆I 2 was estimated to be (17.5 ± 15.4) × 10 −9 for a 1 kg mass standard.…”

The performance of the KIBB-g1 tabletop Kibble balance at NIST

“…Since KIBB-g1 strives for relative uncertainties on the order of a few parts in 10 6 , the Planck constant only makes a subtle appearance as the means for absolutely calibrating the voltmeter and resistance standard used for the electrical measurements. For a detailed description of KB theory, see [4], [7].…”