Addressing the Need for Wider Access to the SI Unit of Mass Following the Revision of the International System of Units

Abstract

The International System of Units (SI), which provides the basis for all physical measurements, is due to be revised in 2018. The current system, defined by seven base units, will be replaced by a "New SI" where all units are defined in terms of a set of seven reference constants, to be known as the "defining constants of the SI". The aim is to provide a simpler and more fundamental definition of the entire SI, which will also dispense with the last of the definitions based on a material artifact • the international prototype kilogram. In the new SI the kilogram will be defined in terms of a fixed value of the Planck constant, h. This definition theoretically gives universal access to the unit and facilitates a robust and egalitarian mass scale, but only if sufficient laboratories are able to realise mass from the new definition. Currently the projects able to realise the mass unit to the level of accuracy required are the Kibble (watt) balance and Avogadro experiments. The present implementations of these experiments are extremely expensive, difficult to duplicate and complicated and time consuming to operate. The Kibble balance experiment, which originated at NPL in 1975, equates virtual electrical and mechanical power. Once a numerical value of h has been fixed the Kibble balance can be used to determine mass in terms of quantum electrical phenomena (the Josephson and quantum Hall effects) and measurements of velocity and local gravity. NPL has proposed improvements to the Kibble balance which have the potential to significantly reduce the cost and complexity of both constructing and operating the balance. NPL is currently working on a technology demonstrator to test the viability of the proposed improvements. A second technology demonstrator will test the viability of a Kibble balance based on a "seismometer" mechanism using flexures for both weighing and moving, and incorporating a highly stable electromagnetic “tare” system making the apparatus much less sensitive to alignment issues. This paper outlines the proposed improvements in the Kibble balance design and examine more generally the likely effect of the revision of the SI on mass metrology in the future.