James Watt (1736-1736).During Watt's time, conventional steam engines, such as the Newcomen steam engine, condensed the steam inside the work cylinder by a spray of cold water. Watt's engine had an external condenser, which increased efficiency dramatically. (From Popular Science Monthly, 1877, via Wikimedia Commons.) |

NIST-4 Watt Balance.This Watt balance has measured the Planck constant to within 34 parts per billion. According to theory, mass can be expressed as a function of this constant and other fundamental constants. (NIST image 16PML013 by J. L. Lee .) |

h = 6.626 069 83 x 10The Watt balance work at NIST and other standards agencies has been done towards the aim of redefining the kilogram in 2018 in a way that separates this standard from a physical object.[4] As the graph below shows, there's a continuing problem in transferring the standard from the international prototype kilogram, maintained at the International Bureau of Weights and Measures near Paris, France.^{-34}joule⋅sec ± 22 x 10^{-42}Joule⋅sec

Left, a replica of the prototype kilogram at the Cité des Sciences et de l'Industrie, Paris, France. The prototype kilogram is made from an alloy of 90% platinum and 10% iridium, so it resists oxidation. The problem with this artifact standard is that it is only useful through use of transfer standards calibrated to itself. As the graph on the right shows, these transfer standards have been changing over the course of time, probably through accumulation of airborne contamination. The average change has been 50 micrograms over the course of a century. (Left image, via Wikimedia Commons. Right image, from NIST.)[5] |

- CODATA Internationally recommended 2014 values of the Fundamental Physical Constants at the NIST web site..
- Jonas Bylander, Tim Duty, and Per Delsing, "Current measurement by real-time counting of single electrons,", Nature, vol. 434, no. 7031 (March 17, 2005), pp. 361-364, doi:10.1038/nature03375. Also at arXiv.
- D. Haddad, F. Seifert, L. S. Chao, S. Li1, D. B. Newell, J. R. Pratt, C. Williams, and S. Schlamminger, "A precise instrument to determine the Planck constant, and the future kilogram," Rev. Sci. Instrum., vol. 87, no. 6 (June 21, 2016), Article no. 061301, DOI: 10.1063/1.4953825. This is an open access article with a PDF file available at the same URL.
- In Its First Measurement of Planck’s Constant, NIST’s Newest Watt Balance Brings World One Step Closer to New Kilogram, NIST Press Release, June 21, 2016.
- Redefining the Kilogram: the Present, NIST Web Site.
- Redefining the Kilogram: Planck's Constant, NIST Web Site.