Bulletin of the American Physical Society
APS April Meeting 2019
Volume 64, Number 3
Saturday–Tuesday, April 13–16, 2019; Denver, Colorado
Session Y06: The New SI System: Should 2 pi Have Units?Invited

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Sponsoring Units: FIP Chair: Roy Jerome Peterson, University of Colorado, Boulder Room: Sheraton Governor's Square 15 
Tuesday, April 16, 2019 1:30PM  2:06PM 
Y06.00001: The New SI system and the Definition of Frequency Invited Speaker: Judah Levine I will present the definitions of the fundamental constants in the new SI system, and I will discuss the differences between the new and old systems. I will also discuss the definitions of frequency, especially angular frequency, which is usually specified in radians/s and the Hz, which is commonly used to specify frequencies in engineering and physics. This will lead naturally to the question, should 2π have units? 
Tuesday, April 16, 2019 2:06PM  2:42PM 
Y06.00002: Reaching international consensus on fundamental changes to the International System of Units (SI) Invited Speaker: Richard S Davis After a decade of international effort, a historic change to the SI received final approval by a unanimous rollcall vote held on November 2018 in Versailles, France. The venue was the 26th General Conference on Weights and Measures (CGPM). The revisions to the SI come into force on May 20, 2019. The world was able to watch the voting in real time on the BIPM’s YouTube channel. What was approved by the CGPM were redefinitions of the kilogram, the ampere, the kelvin, and the mole, four of the seven base units of the SI. All seven base units are now defined by fixed (i.e. exact) numerical values of seven “defining constants.” Since all SI units can be written in terms of the seven base units, all SI units are now defined by these seven invariants. One of the four new constants with fixed values is h, the Planck constant, whose SI unit, the joulesecond, is J s = kg m^{2} s^{1}. Because the second and the meter were last redefined in 1967 and 1983, evidently a fixed value for h now redefines the kilogram. Previously, one kilogram was defined as the mass of a designated metal cylinder, which was put into service in 1889. Measuring h to parts in 10^{8} with respect to the mass of a 1 kg object, a prerequisite for the redefinition, required a heroic effort, but valiant attempts were also made to explain the significance of h to SI stakeholders unfamiliar with the rudiments of quantum physics. The revised SI has much to recommend it, but eliminating the last artifact definition of the system is what seized the public’s imagination even as they struggled to make sense of it all. So part of reaching a consensus was providing explanations by gifted scientists who are also gifted communicators. I will discuss these and other consensusbuilding challenges in my talk. 
Tuesday, April 16, 2019 2:42PM  3:18PM 
Y06.00003: Magnetic Flux Density, Magnetic Field Strength, and the Permeability of Vacuum Invited Speaker: Ronald B Goldfarb The International System of Units (SI) will be revised on 20 May 2019 such that all seven base units, including the kilogram, will be defined in terms of fixed numerical values of seven defining constants. The revised SI will include a redefinition of the ampere. One consequence is that the permeability of vacuum μ_{0}, equal to the ratio of magnetic flux density B to magnetic field strength H, will not have a fixed numerical value of 4π × 10^{−}^{7} N/A^{2}, but will become a measurable quantity. This prompts a review of historical arguments on the concepts of B and H, whether they are conceptually identical in vacuum, and whether their nature depends on the system of units used to express them. The argument can be made that, in vacuum, B is physically the same as H in all systems of units, but in the revised SI, it is helpful to recognize B as the primary magnetic field vector, μ_{0} as an experimental constant, and H as an arithmetically derived auxiliary vector. This does not affect the traditional Maxwellian definition of magnetic susceptibility, equal to M/H (not M/B), nor detract from the utility of M−H and B−H curves to characterize magnetic materials. R. B. Goldfarb, “The permeability of vacuum and the revised International System of Units” (editorial), IEEE Magn. Lett. 8, 1110003 (2017), doi: 10.1109/LMAG.2017.2777782. R. B. Goldfarb, “Electromagnetic units, the Giorgi system, and the revised International System of Units,” IEEE Magn. Lett. 9, 1205905 (2018), doi: 10.1109/LMAG.2018.2868654. 
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