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URL: https://pubmed.ncbi.nlm.nih.gov/35023879/

⇱ The watt or Kibble balance: a technique for implementing the new SI definition of the unit of mass - PubMed

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Abstract

The redefinition of the SI unit of mass in terms of a fixed value of the Planck constant has been made possible by the Kibble balance, previously known as the watt balance. Once the new definition has been adopted, the Kibble balance technique will permit the realisation of the mass unit over a range from milligrams to kilograms. We describe the theory underlying the Kibble balance and practical techniques required to construct such an instrument to relate a macroscopic physical mass to the Planck constant with an uncertainty, which is achievable at present, in the region of 2 parts in 108. A number of Kibble balances have either been built or are under construction and we compare the principal features of these balances.

Keywords: Kibble balance; Planck constant; kilogram; mass measurement; redefinition; watt balance.

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Figures

👁 Figure 1.
Figure 1.
The Kibble balance in weighing mode.
👁 Figure 2.
Figure 2.
The Kibble balance in moving mode.
👁 Figure 3.
Figure 3.
Schematic drawings of the four permanent magnet designs used in Kibble balances. All magnets exhibit rotational symmetry around the dashed line in the centre. The grey shaded parts concentrate the flux and are typically manufactured of mild steel. The hatched parts represent the active magnetic material. The arrows indicate a possible direction of the magnetic polarization of the material. (a) NPL-type. (b) BIPM-type. (c) LNE-type. (d) MSL-type.
👁 Figure 4.
Figure 4.
Typical circuit topology during force mode. The current is passed through the coil, drawn as an inductive and a resistive element and a measurement resistor. The voltage drop across the measurement resistor is compensated with a programmable Josephson voltage system (PJVS) and the residual voltage is measured with a digital voltmeter (DVM).
👁 Figure 5.
Figure 5.
Results of published results obtained by Kibble and joule balances. The abbreviations on the left give the institution where the experiments were performed. The numbers on the right indicate the years the results were published. The dark vertical line is the value of h as recommended by CODATA in its 2014 adjustment [142]. The error bars denote the one sigma uncertainties given by the researchers in their respective publications. See table 4 for references.
👁 Figure 6.
Figure 6.
Measurements of h with relative standard uncertainties below 2 × 10−7. The dark vertical line is the value of h as recommended by CODATA in its 2014 adjustment [142]. The uncertainty bars show the standard uncertainty of each result. See table 4 for references.
👁 Figure 7.
Figure 7.
The required relative standard uncertainties for E1 and E2 weights as a function of the nominal mass value according to [157].

References

    1. Kibble BP 1976. A measurement of the gyromagnetic ratio of the proton by the strong field method Atomic Masses and Fundamental Constants vol 5, ed Sanders JH and Wapstra AH (New York: Plenum; ) pp 545–51
    1. Vigoureux P and Dupuy N 1978. Determination of the ampere and of the gyromagnetic ratio of the proton Technical Report DES 44, NPL
    1. Kibble BP and Robinson IA 1977. Feasibility study for a moving coil apparatus to relate the electrical and mechanical SI units Technical Report DES 40, NPL
    1. Thompson AM and Lampard DG 1956. A new theorem in electrostatics and its application to calculable standards of capacitance Nature 177 888
    1. Kibble BP, Robinson IA and Belliss JH 1990. A realisation of the SI watt by the NPL moving-coil balance Metrologia 27 173–92

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