Richard S. Davis

The values of Newton’s constant of gravitation, G, reported by determinations at the International Bureau of Weights and Measures (BIPM) in Quinn et al (2001 Phys. Rev. Lett. 87 111101/1–111101/4) and in Quinn et al (2013 Phil. Trans. R. Soc. A 372 2–28) are some 200 parts per million (ppm) larger than the value of 6.67430 × 10 − 11 m³ kg⁻¹ s⁻² which was recommended by CODATA in Tiesinga et al (2018 J. Phys. Chem. Ref. Data 50 033105). The CODATA value has an assigned uncertainty of 22 ppm and the reported uncertainties in the BIPM values were 41 ppm and 25 ppm, respectively. The discrepancy therefore amounts to about seven times the combined uncertainty of the latest BIPM and CODATA values. We examine experimentally the hypothesis that the difference is due to a bias produced by stray alternating (AC) magnetic fields in the vicinity of the experiment. Whilst the BIPM torsion balance was located at the University of Birmingham a coil, having a similar scale to the balance, was used to generate AC magnetic fields of magnitude approximately 1 μT which in turn produced torques of magnitude approximately 0.2 % of the gravity torque. The magnitudes of these torques were compared with simple analytical models and finite element analyses. Measurements in the laboratory in Birmingham and, more recently, at BIPM give a very conservative upper limit on the rms magnitude of ambient AC magnetic fields of 100 nT. As the torque was demonstrated to vary with the square of the ambient field, these observations and our analyses suggest an upper limit to the possible bias in the BIPM determinations of the order of 20 ppm. It is therefore unlikely that torques due to ambient magnetic fields could have significantly biased values of G determined at BIPM.

In a useful introduction to “natural units” [1], the author contrasted natural units to the SI as it existed at the time of writing (2016). The SI was characterized in [1] as a system that “relies heavily on precision measurements of standard prototypes, objects or systems that define a physical unit.” In particular, the SI indeed relied on one standard prototype, called the International Prototype of the Kilogram. The mass of this unique object defined 1 kg. By contrast, “natural units,” as the name implies, are completely defined in terms of selected constants used to describe the natural world. Different systems of natural units are based on different selections of these constants.

2019 was a big year for metrology. The international system of units was revised on World Metrology Day, May 20 ^th , that year [1]. What will 2020 bring? In this article we discuss five promising advances that we have on a watchlist for 2020. First, we describe the measurement of volume and gas pressure using electromagnetic waves. These measurements rely on the fixed value of the speed of light in vacuum c ₀ . We then pivot to the Planck constant h. SI traceable measurements of mass and force can be obtained from h. Interesting developments are coming in mass metrology since the definition changed from the mass of the international prototype of the kilogram to the value of the Planck constant. Adding the elementary charge e to h gives access to resistance and impedance measurements via the quantum Hall effect. This has been a very interesting field for some time, since the discovery of graphene in 2004. The last section explains how the noise across a resistor can be used to measure thermodynamic temperature. As will be shown, the temperature can be linked to the quotient of the Boltzmann constant k _B and the Planck constant.

We are now well into 2019, the International Year of the Periodic Table (IYPT), celebrating the 150th anniversary of pioneering work of Dmitri I. Mendeleev (1834-1907). He not only made sense of the zoo of chemical elements that were known in 1869 but also predicted the existence of several elements that were unknown at the time, as well as their expected properties. Mendeleev took advantage of advances that had been made in the measurement of atomic weights (relative atomic masses is the preferred term in our century) of the elements to guide his thinking, which was indisputably bold and prescient.

Following the revision of the International System of Units (SI), that takes effect on 20 May 2019, the unit mole is defined by using a fixed number of elementary entities. This number is the fixed numerical value of the Avogadro constant, which is the defining constant of the unit mole. This definition was made possible because the determination of the Avogadro constant had reached a level of relative uncertainty that allowed its value to be fixed and, at the same time, safeguard continuity of measurement results before and after the definition. The motivation for the revision of the SI and the mole in particular will be explained and the experimental work that allowed it is summarized.

In November 2018, the General Conference on Weights and Measures (CGPM) gave its unanimous approval to Resolution 1, a major revision of the SI to take effect on May 20, 2019 [1]. The intentions of the CGPM had already been set out in a resolution adopted in 2011. All of the work needed to meet the requirements of the 2011 resolution was monitored by the International Committee for Weights and Measures (CIPM), which drafted both resolutions. The CIPM relied heavily on its technical Consultative Committees, composed primarily of experts from National Metrology Institutes and international science bodies with a focus on metrology.