Journey of Kilogram from Physical Constant to Universal Physical Constant ( h ) via Artefact: A Brief Review
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REVIEW PAPER
Journey of Kilogram from Physical Constant to Universal Physical Constant (h) via Artefact: A Brief Review B. Ehtesham1,2, T. John2, S. Yadav1,2, H. K. Singh1,2, G. Mandal2 and N. Singh1,2* 1
Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India
2
Physico-Mechanical Metrology Division, Electrical and Electronics Metrology Division, CSIR-National Physical Laboratory, Dr. K. S. Krishnan Road, New Delhi 110012, India Received: 16 May 2020 / Accepted: 13 August 2020 Metrology Society of India 2020
Abstract: The redefinition of mass adopted in November 2018 and implemented from 20 May 2019, i.e. World Metrology Day, eliminated the artefact-based approach dependent upon the International Prototype of the Kilogram (IPK), in favour of realizing the kilogram in terms of the Planck constant h by fixing its value as 6.62607015 9 10-34 J s. In this paper, the authors present a general outline of the circumstances and related developments that paved the way for the new definition that replaced the IPK after a period of 130 years since it was formally sanctioned to define the kilogram in 1889. The new definition opens up fascinating developments in mass metrology which include different realization techniques, realizing the unit at values other than 1 kg, numerous sources for traceability can be envisaged etc. Keywords: Redefinition of kilogram; Kibble balance; Metrology; Planck constant 1. Introduction The implementation of the new definitions of SI base units on 20 May 2019 has been very significant for the fact that the unit of mass, the kilogram, the last of the seven base units which was hitherto represented by an artefact, has also been defined by a universal physical constant like the other six base units [1]. The redefinition of the kilogram in terms of the Planck constant h is the successful culmination of a series of effort spread over a period of many years. Indeed, the inadequacy of the artefact definition of the mass had become evident long ago and the need for replacing its definition was recognized by the International Committee for Weights and Measures (CIPM) way back in 1960 [2, 3]. The most worrisome aspect about artefactbased primary standard of mass was its stability. Over the years, occurrences of significant drift in values were found between the International Prototype of the Kilogram (IPK) and its official copies kept at the International Bureau of Weights and Measures (BIPM) and the National Prototype of the Kilogram (NPK), maintained at various National Metrology Institutes (NMIs) worldwide [4]. The main issue
has been that it was very difficult to assign this drift either to the IPK and its official copies or to the NPKs, as there was no independent reference with respect to which the absolute drift could be determined. The only safe conclusion that could be drawn from this observation was that the primary standard of mass represented by the artefact was found to have drifted in value with the absolute value of this drift remaining unknown. Apart from that,
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