SCIENCE & TECHNOLOGY
REDEFINITION Ian Robinson, a fellow in electrical metrology at the U.K.'s National Physical Laboratory, uses the watt balance to determine values for the Planck constant. (NPL photographs: © Crown copyright 1999. Reproduced by permission of the Controller of HMSO and the Queen's Printer for Scotland)
REDEFINING THE BASE UNIT OF MASS Experts propose that the standard for the kilogram should be based on a property of nature MICHAEL FREEMANTLE, C&EN LOND Ν
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NEW DEFINITION OF THE KILO-
gram is required, an international team of scientists contends. T h e current one is imprecise, they say, because it is not linked to an unvarying property of nature. The imprecision injects uncertainties in measurements and physical constants used in calculations and calibrations. They urge a redéfinition based on a specific value of either the Planck constant or the Avogadro constant. The kilogram is the base unit of mass in the International System of Units (SI). At present, the unit is defined as the mass of the international prototype of the kilogram—a plum-sized cylindrical artifact WWW.CEN-0NLINE.ORG
made from an alloy of platinum and iridi um byJohnson Matthey in 1885. The pro totype is kept in a safe, with six official copies, in a vault at the International Bureau ofWeights & Measures (BIPM) at Sèvres, on the outskirts of Paris. It was designated as the unit of mass in the metric system in 1889 and has been weighed against its copies three times in the past 100 years— in 1890,1948, and 1992. "The previous definition of the kilogram was the mass of the cubic decimeter ofwater," notes Ian M. Mills, emeritus professor of chemistry at the University of Reading, in England. 'The standard was difficult to reproduce accurately because of impurities and air dissolved in water and the
variation of density with temperature. BIPM therefore established the kilogram in terms of the prototype." Mills points out that copies of the artifact calibrated against the prototype at BIPM are available to countries that have signed a diplomatic treaty known as the Convention of the Metre. The convention gives authority to the General Conference on Weights & Measures (CGPM), the International Committee for Weights & Measures, and BIPM "to act in matters of world metrology, particularly concerning the demand for measurement standards of ever-increasing accuracy, range, and diversity and the need to demonstrate equivalences between national measurement standards."The treaty has now been signed by 51 countries. Because the international prototype at BIPM is a material artifact, it has one important limitation: It is not linked to an invariant of nature, note Mills and coauthors TerryJ. Quinn, emeritus director at BIPM, and scientists PeterJ. Mohr, Barry N . Taylor, and Edwin R. Williams at the National Institute of Standards & Technology (NISTiinarecentpaperiA^iroA^w 2005, 42,71}. They point out that the prototype is accessible only at BIPM and could be damaged or destroyed. "It collects dirt from the ambient atmosphere and must be carefully washed in a prescribed way prior to use," they observe. "It cannot be used routinely for fear ofwear, and its mass maybe changing with time" perhaps by 50 μg or significantly more per century "Unfortunately, as we have not got a more stable mass reference, any drift in mass cannot be measured direcdy " saysJeff Flowers, a scientist at the National Physi cal Laboratory (NPL), Middlesex, England. THE KILOGRAM is one of seven SI base units. The others are the meter (base unit of length), second (time), ampere (electric current), kelvin (thermodynamic tempera ture), mole (amount of substance), and candela (luminous intensity). Three of these units—the ampere, the mole, and the candela—rely on the definition of the kilo gram. Any uncertainty in the definition of the kilogram, therefore, propagates into these three base units. All other SI units of measurement are derived from the seven base units. Derived units such as the newton (force), the joule (energy), and the pas cal (pressure) are related to the kilogram. The kilogram is the only one of the SI base units defined by a physical artifact rather than an invariable property of na ture. The meter, for example, was redefined by CGPM in 1983 in terms of a value for C & E N / JULY 18, 2005
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SCIENCE & TECHNOLOGY the speed of light, which is equal to its constant (h) and mass (m) is given by hv = perfect roundness. The result is an experi wavelength multiplied by its frequency mc2, where ν is frequency and c is the speed mental determination of the mass of the "The meter has now been redefined as of light. Fixing the value of h wouldfixthe crystal based on a specific value of the Avo the distance light travels in a vacuum dur definition of the kilogram because the SI gadro constant, Mills and coauthors note. ing one-299,792,458th of a second, and base units of length and time are already To date, the most accurate experimental the second is defined in terms of natural vi fixed to invariants of nature. value of the Avogadro constant, based on brations ofthe cesium atom," Williams says. Currently, the most advanced experi the fixed mass of the kilogram prototype 'The time has come for the definition ofthe mental approach for realizing the defini in Paris, is 6.022141527 Χ 1023 per mole of kilogram to also be based on an unchang tion of the kilogram based on afixedval particles (such as atoms or molecules). Mills ing natural phenomenon, either a quantity ue ofh is the moving-coil watt balance. The and coauthors suggest that, ifthe definition of light or the mass of afixednumber of apparatus has a balance plate, on which an ofthe kilogram isfixedto the Avogadro con atoms." artifact to be weighed is placed, surround stant, the constant should befixedat this Mills and coauthors note that scientists ed by coils of copper wire. When electric- value. The team suggests that the kilogram prototype should be retained as a sec and engineers use physical constants to ondary standard. "Its mass, which make numerous types of calculations. would be determined in terms of the The constants are also used to design new definition by either the watt bal and calibrate quantum-based measure ance or the XRCD experiment, could ment systems that are becoming im then be used to calibrate mass mea portant in the development of various surements, as is done at present," Mills technologies and in trade that relies in says. "Eventually, when the watt bal creasingly on electronic testing, envi ance and the XRCD experiments are ronmental monitoring, and quality con sufficiently accurate, the kilogram ar trol. A new definition of the kilogram tifact will no longer be needed." fixed to an invariant of nature would, unlike the Paris prototype, be available Flowers observes, however, that the to anyone at any time and in anyplace. silicon sphere used in the XRCD The group proposes that the kilo experiment is an artifact. "It will be gram should be redefined so as tofixits difficult to ensure that it remains un value for all time to a specific value of changed and to compare it with sec either the Planck constant or the Avoondary standards," he notes in a re gadro constant, both of which are in view of atomic and quantum standards variants of nature. The uncertainties {Science 2004,306,1324). 'Although of many of the fundamental constants it could be reproduced ifdamaged, this would then immediately be reduced by would require major and time-con more than a factor of 10, Mills says. suming effort." PR OTOTYPE The mass of the international Redefining the kilogram in this way kilogram artifact in Paris may be changing. Mills and coauthors note that the could have immediate benefits, Quinn ~~ ~~ Planck constant could befixedby any says. "For instance, it would improve the ity is passed through the coils, magnetic physical experiment that links electrical to precision ofcertain electrical measurements fields that are produced offset the weight mechanical quantities and the Avogadro 50-fold and would enable physicists to make of the artifact. Because the electromag constant by any experiment that counts mi more precise calculations when studying netic forces acting on the balance are re croscopic entities. Such experiments need the fundamental quantum properties of lated to the force generated by the mass, to be carried out with the required accura the experiment links mechanical power- cy however, and there lies a problem. matter." Scientists use the fundamental con in terms oflength, mass, and time—to elec According to Flowers, the target accu stants as part of their language to connect trical power, in terms of voltage and re racy for measurements to define the kilo experiment and theory and to connect past sistance. The result is an experimental gram is one part in 108. The moving-coil and present data, Williams points out. "Re determination of the mass of an unknown watt balance andXRCD experiments have ducing changes in the constants makes this standard, Mills and coauthors explain. yet to relate the mass ofthe kilogram to the communication easier," he says. 'Artifi Planck and Avogadro constants, respec cially large uncertainties caused by the THE MOST ADVANCED method for deter- tively, at accuracies much better than one defining of the kilogram as a macroscopic mining the Avogadro constant uses an X- part in 107. Furthermore, the difference in artifact rather than an atomic mass or a ray crystal density (XRCD) method to mea the results from the two approaches is al fundamental constant could hide impor sure the lattice spacing and macroscopic most one part in 106. tant future discrepancies." mass density of a single crystal of ultrapure 'There is some support for a definition The relationship between the Planck silicon shaped to form a sphere of almost based on the atomic mass unit," Flowers
The kilogram is the only one of the SI base units defined by a physical artifact rather than an invariable property of nature. 30
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tells C&EN. 'Then the kilogram would be, for example, the mass of a certain number of unbound carbon-12 atoms in their ground state." By définition, the mass of 1 mole ofcarbon-12 atoms at rest and in their ground state is 12 g or 0.012 kg. The kilogram could then be defined as the mass of exactly 6.022141527 Χ 1023 such carbon atoms divided by 0.012. The kilogram would thus become an invariant of nature, because the mass of a carbon-12 atom at rest and in its ground state and, therefore, the mass of the Avogadro number (6.022141527 Χ 1023) of these atoms are invariants of nature. "This definition has the advantage of being easily understandable: the large mass is the sum of the microscopic masses," Flowers says. "In practice, however, the watt balance is likely to prove more accu rate in overall mass measurement." THE ADVANTAGE of the watt balancePlanck constant definition is that it is based on electrical units that are exactly based on fundamental constants and on the second, which is also a fundamental con stant. "This argument may well sway the
metrological community," Flowers says. Flowers proposes an interim measure in which constants such as the Avogadro con stant, the atomic mass unit, the electron volt, and the Planck constant are given agreed values. "These would be additional to the kilogram artifact and not replace it," he says. "Lat er, when there is good ex perimental evidence on the situation, a redefinition could be made." Mills and coauthors argue that redefining the kilogram should proceed without de lay and that, for the time be ing, the artifact in Paris should be retained as a work ing reference for highly pre cise comparisons to indi vidual countries' national Mills kilogram standards. Anew definitionfixedto the Planck con stant or the Avogadro constant would be immediately beneficial, they say "By re taining the prototype kilogram as a sec ondary standard, the present excellent worldwide uniformity of 1-kg Pt-Ir mass
standards would be maintained, while at the same time the many benefits ofhaving either the Planck or Avogadro constants exacdy known would be realized," they sug gest. "Moreover, each SI base unit would then be defined in terms of invariants."
Flowers
The group hopes that the 23rd CGPM, when it next convenes in Paris in October 2007, will adopt a new definition of the kilogram based on the best available nu merical values of the Planck or Avogadro constant at the time. •
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