The Bailey–Borwein–Plouffe formula allows calculation of an arbitrary digit of pi without calculating the preceding digits, as required in a series expansion calculation. Although this formula calculates a pi hexadecimal digit (base-16), there's an easy way to convert the result to a decimal digit. (Equation rendered using Inkscape.) |

Old school metrology. This is a replica of the international prototype kilogram (left), and an illustration of the former International Prototype Metre (right). The meter bar, made from a 90% platinum - 10% iridium alloy, was the standard of length until 1960, when the meter was redefined in terms of the wavelength of light from a krypton-86 lamp. (left image and right image via Wikimedia Commons.) |

"It is ordained that 3 grains of barley dry and round do make an inch, 12 inches make 1 foot, 3 feet make 1 yard, 5 yards and a half make a perch, and 40 perches in length and 4 in breadth make an acre."[6]There are many simultaneous experiments conducted for precision measurement of various physical constants, and these generate values that disagree slightly. The February workshop revealed that the measurements of the Boltzmann constant, which converts particle energy to temperature, are converging on the same value. In the future, the kelvin temperature unit will be defined by the Boltzmann constant.[3] Also converging are measurements of Planck's Constant, which will eventually help to define a new kilogram standard.[3] Says NIST's Peter Mohr, coauthor of the summary paper about the workshop,[2]

"The Planck constant was problematic in the past, as there were disagreeing values obtained by different experiments. However, the values seem to be converging to a sufficiently reliable value for the redefinition of the SI to move forward... The new definitions will make many of the physical constants that are measured now exact in the future. Others, although not exact, will be more accurate.. This will stabilize the values of the constants and provide accurate measurement standards."[3]The metrologists' goal is to define all SI units in terms of fundamental constants by 2018, thereby replacing the artifact standards.[3] A major hurdle in this is the kilogram definition, and there are efforts underway to define the kilogram in terms of fundamental constants.[3-5] One method for this is the "watt balance" that relates mass to electric current and voltage (see figure). It derives its name from the fact that the unit of electrical power, the watt, is the product of voltage and current.

A watt balance at NISTIn a watt balance, the magnetic attraction between current-carrying coils balances the weight of a kilogram. Knowledge of the local gravitational acceleration allows conversion of the weight to a mass. (NIST image.) |

A silicon kilogram.The number of atoms in this nearly perfect sphere of silicon-28 is known to about 20 per billion. (Photograph by Enrico Massa and Carlo Sasso.) |

"Prior to redefining the kilogram, we must demonstrate that the new realization is indistinguishable from the present one, to within the accuracy of the world's best balances... Otherwise, when changing from the present definition to the new one, all users in science, industry, and commerce must change the mass value of all the existing artefacts."[5]

- CODATA Internationally recommended 2014 values of the Fundamental Physical Constants at the NIST web site.
- Savely G. Karshenboim, Peter J. Mohr, and David B. Newell, "Advances in Determination of Fundamental Constants," Journal of Physical and Chemical Reference Data, vol. 44, no. 3 (September, 2015), article no. 031101, DOI:http://dx.doi.org/10.1063/1.4926575.
- Constant change - Advances in determination of fundamental constants to guide redefinition of scientific units to rely on constants of nature instead of physical standards, American Institute of Physics Press Release, July 14, 2015.
- G. Mana, E. Massa, C. P. Sasso, M. Stock, K. Fujii, N. Kuramoto, S. Mizushima, T. Narukawa, M. Borys, I. Busch, A. Nicolaus, and A. Pramann, "The Correlation of the NA Measurements by Counting 28Si Atoms," Journal of Physical and Chemical Reference Data, vol. 44, no. 3 (September, 2015), article no. 031209, DOI:http://dx.doi.org/10.1063/1.4921240.
- More precise estimate of Avogadro's number to help redefine kilogram, American Institute of Physics Press Release, July 14, 2015.
- Supposedly in volume nine of Owen Ruffhead, "The statutes at large: from Magna Carta to the end of the last parliament," (M. Baskett, 1765), but there was too much there to scan for verification.