Fundamental Constants Data Center

Technical Highlights

• New ANSI Standard: U.S. GUM. The ISO Guide to the Expression of Uncertainty in Measurement, commonly called the GUM, has been officially adopted by ANSI (American National Standards Institute) as an American National Standard. This adoption culminates an effort begun over two years ago by writing group Z540-2 of the NCSL (National Conference of Standards Laboratories) Accredited Standards Committee on General Requirements for Calibration Laboratories and Measuring and Test Equipment, or Z540. The official designation of the new standard is ANSI/NCSL Z540-2-1997, and its full title is American National Standard for Expressing Uncertainty - —U.S. Guide to the Expression of Uncertainty in Measurement. B.N. Taylor of the FCDC and C.E. Kuyatt of the Director’s Office, contributed significantly to the work of the Z540 Writing Group that steered the GUM through the rather intricate ANSI standards-writing process. The Writing Group was chaired by J. Wehrmeyer of the Eastman Kodak Co. and consisted of a number of other representatives from industry.
  
  As a consequence of the adoption of the GUM as an ANSI standard and the resulting availability of the GUM at a readily affordable price through the NCSL, it is expected that the method of expressing measurement uncertainty advocated by the GUM (and adopted by NIST in 1992) will find ever increasing acceptance and use. This acceptance has in fact been quite phenomenal –- it exceeds by far the initial expectations of the ISO Technical Advisory Group involved in the preparation of the GUM, and in which Kuyatt and Taylor played a major role.
• New ANSI Standard: SI-10. The FCDC played a key role in the four-person writing committee (two from ASTM, two from IEEE) that prepared the ANSI approved, joint IEEE/ ASTM SI 10-1997 standard, entitled Standard for Use of the International System of Units (SI): The Modern Metric System, which was published in July 1997. This single, joint standard replaces ANSI/IEEE Std 268-1992, American National Standard for metric Practice; and ASTM E380-93, Standard Practice for Use of the International System of Units (SI): The Modernized Metric System. The new standard culminates a 4-year effort by ASTM Committee E-43, Metric Practice, and IEEE Standards Coordinating Committee 14, Quantities, Units, and Letter Symbols, to combine their two separate standards into a single national standard. As a consequence, for the first time in over 20 years, the United States has but one standard on metric practice. The new standard is expected to find wide use among the engineers who participate in the work of ASTM and IEEE, especially those involved in writing applications standards.
• Precision Measurement Grants. The FCDC awarded, on behalf of NIST, new Precision Measurement Grants to Prof. J.H. Gundlach of the University of Washington, and Prof. D.C. Shiner of the University of North Texas. The grants are in the amount of $50,000 per year, renewable for two additional years. NIST sponsors these grants to promote fundamental research in measurement science in U.S. colleges and universities and to foster contacts between NIST scientists and researchers in the academic community actively engaged in such work.

  The aim of Gundlach’s project, "Measurement of Newton’s Constant G Using a New Method," is to determine the Newtonian constant of gravitation G with a relative standard uncertainty of less than 10^-5. Such a result would represent the most accurate value of this important but poorly-known constant ever achieved, and would illuminate the cause of the discrepancies among current values. The measurement will be carried out using a rotating torsion balance in an acceleration feedback mode, which promises to overcome all of the major sources of systematic error in previous experiments. For example, because the torsion fiber of the balance does not twist, the torsion constant of the fiber does not have to be known nor remain constant during the measurements; and by using a 2-dimensional vertical pendulum, the mass distribution and dimensions of the pendulum do not have to be known to high accuracy.

  The aim of Shiner’s project, "Laser Spectroscopy of the Helium Atom for a Determination of the Fine-Structure Constant," is to determine the 32 GHz fine-structure interval in the helium atom with a relative standard uncertainty of 10^-8, which has never before been achieved. This result, when combined with an improved theoretical expression for the interval currently being calculated by a number of researchers (including Prof. J. Sapirstein, University of Notre Dame, an earlier NIST Precision Measurement Grant recipient), will provide a value of the fine-structure constant α with a relative standard uncertainty of 5 × 10^-9. Such a value of α will allow scientists to critically test one of the most accurate theories in physics, quantum electrodynamics (QED), by comparing the theoretical expression for the magnetic moment anomaly of the electron a_e with the experimental value. To eliminate or reduce significant sources of error, an electro-optic laser modulation technique will be used to directly measure the 32 GHz fine-structure interval in Hz.