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Title: Kilogram  
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Subject: Comparison of netbooks, International System of Units, Proposed redefinition of SI base units, Metric system, Litre
Collection: Si Base Units, Units of Mass
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took possession in 1949 is an especially high 78 µg per century while that of Germany’s prototype has been even greater at 126 µg/century ever since it took possession of K55 in 1954. However, still other data for other replicas isn’t supportive of this theory. This mercury absorption theory is just one of many advanced by the specialists to account for the relative change in mass. To date, each theory has either proven implausible, or there are insufficient data or technical means to either prove or disprove it.
  • ^ Even well respected organizations incorrectly represent the relative nature of the mass divergence as being one of mass loss, as exemplified by this site at Science Daily, and this site at, and this site at Sandia National Laboratories. The root of the problem is often the reporters' failure to correctly interpret or paraphrase nuanced scientific concepts, as exemplified by September 2007 story this 12 by the A kilogram just isn't what it used to be. The 118-year-old cylinder that is the international prototype for the metric mass, kept tightly under lock and key outside Paris, is mysteriously losing weight — if ever so slightly". Like many of the above-linked sites, the AP also misreported the age of the IPK, using the date of its adoption as the mass prototype, not the date of the cylinder’s manufacture. This is a mistake even Scientific American fell victim to in a print edition. 
  • ^ The mean change in mass of the first batch of replicas relative to the IPK over one hundred years is +23.5 µg with a standard deviation of 30 µg. Per The Third Periodic Verification of National Prototypes of the Kilogram (1988–1992), G. Girard, Metrologia 31 (1994) Pg. 323, Table 3. Data is for prototypes K1, K5, K6, K7, K8(41), K12, K16, K18, K20, K21, K24, K32, K34, K35, K36, K37, K38, and K40; and excludes K2, K23, and K39, which are treated as outliers. This is a larger data set than is shown in the chart at the top of this section, which corresponds to Figure 7 of G. Girard’s paper. 
  • ^ Assuming the past trend continues, whereby the mean change in mass of the first batch of replicas relative to the IPK over one hundred years was +23.5 σ30 µg. 
  • ^ The combined relative standard uncertainty (CRSU) of these measurements, as with all other tolerances and uncertainties in this article unless otherwise noted, have a 1σ standard deviation, which equates to a confidence level of about 68%; that is to say, 68% of the measurements fall within the stated tolerance.
  • ^ The Planck constant's unit of measure, the joule-second (J·s), may perhaps be more easily understood when expressed as a joule per hertz (J/Hz). Universally, an individual photon has an energy that is proportional to its frequency. This relationship is 6.62606957(29)×10−34 J/Hz.
  • ^ The proposal originally was to redefine the kilogram as the mass of 84,446,8863 carbon-12 atoms.[64] The value 84,446,886 had been chosen because it has a special property; its cube (the proposed new value for the Avogadro constant) is evenly divisible by twelve. Thus with that definition of the kilogram, there would have been an integer number of atoms in one gram of 12C: 50,184,508,190,229,061,679,538 atoms. The uncertainty in the Avogadro constant narrowed since this proposal was first submitted to American Scientist for publication. The 2010 CODATA value for the Avogadro constant (6.02214150(27)×1023) has a relative standard uncertainty of 50 parts per billion and the only cube root values within this uncertainty must fall within the range of 84,446,888.0±1.2; that is, there are only two integer cube roots (…87 and …88) in that range and the value 84,446,886 falls outside of it. Neither of the two integer values within that range possess the property of their cubes being divisible by twelve; one gram of 12C could not comprise an integer number of atoms.
  • ^ The sphere shown in the photograph has an out-of-roundness value (peak to valley on the radius) of 50 nm. According to ACPO, they improved on that with an out-of-roundness of 35 nm. On the 93.6 mm diameter sphere, an out-of-roundness of 35 nm (undulations of ±17.5 nm) is a fractional roundness (∆r /r ) = 3.7×10−7. Scaled to the size of Earth, this is equivalent to a maximum deviation from sea level of only 2.4 m. The roundness of that ACPO sphere is exceeded only by two of the four fused-quartz gyroscope rotors flown on Gravity Probe B, which were manufactured in the late 1990s and given their final figure at the W.W. Hansen Experimental Physics Lab at Stanford University. Particularly, “Gyro 4” is recorded in the Guinness database of world records (their database, not in their book) as the world’s roundest man-made object. According to a published report (kB PDF, here 221) and the GP‑B public affairs coordinator at Stanford University, of the four gyroscopes onboard the probe, Gyro 4 has a maximum surface undulation from a perfect sphere of 3.4 ±0.4 nm on the 38.1 mm diameter sphere, which is a r /r = 1.8×10−7. Scaled to the size of Earth, this is equivalent to an undulation the size of North America rising slowly up out of the sea (in molecular-layer terraces 11.9 cm high), reaching a maximum elevation of 1.14 ±0.13 m in Nebraska, and then gradually sloping back down to sea level on the other side of the continent.
  • ^ In 2003, the same year the first gold-deposition experiments were conducted, physicists found that the only naturally occurring isotope of bismuth, 209Bi, is actually very slightly radioactive, with the longest known radioactive half-life of any naturally occurring element that decays via alpha radiation—a half-life of (19±2)×1018 years. As this is 1.4 billion times the age of the universe, 209Bi is considered a stable isotope for most practical applications (those unrelated to such disciplines as nucleocosmochronology and geochronology). In other terms, 10000000000% of the bismuth that existed on Earth 4.567 billion years ago still exists today. Only two mononuclidic elements are heavier than bismuth and only one approaches its stability: thorium. Long considered a possible replacement for uranium in nuclear reactors, thorium can cause cancer when inhaled because it is over 1.2 billion times more radioactive than bismuth. It also has such a strong tendency to oxidize that its powders are pyrophoric. These characteristics make thorium unsuitable in ion-deposition experiments. See also Isotopes of bismuth, Isotopes of gold and Isotopes of thorium.
  • ^ Criterion: A combined total of at least five occurrences on the British National Corpus and the Corpus of Contemporary American English, including both the singular and the plural for both the -gram and the -gramme spelling. 
  • ^ The practice of using the abbreviation "mcg" rather than the SI symbol "µg" was formally mandated in the US for medical practitioners in 2004 by the  
  • References

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    6. ^ "Kilogram". Oxford Dictionaries. Retrieved November 3, 2011. 
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    17. ^ a b "Decree on weights and measures". April 7, 1795. Gramme, le poids absolu d'un volume d'eau pure égal au cube de la centième partie du mètre , et à la température de la glace fondante. 
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    26. ^ Terry Quinn. From Artefacts to Atoms: The BIPM and the Search for Ultimate Measurement Standards. Oxford University Press. p. 321.
    27. ^ Isotopic composition and temperature per London South Bank University’s “List of physicochemical data concerning water”, density and uncertainty per NIST Standard Reference Database Number 69 (Retrieved April 5, 2010)
    28. ^ The International Bureau of Weights and Measures official site: The international prototype of the kilogram and its six official copies
    29. ^ Davis, R.S.; Coarasa, J. (January 2005). "Director’s Report on the Activity and Management of the International Bureau of Weights and Measures". Bureau International des Poids et Mesures. pp. 206–207. Retrieved August 3, 2013. 
    30. ^ a b c d G. Girard (1994). "The Third Periodic Verification of National Prototypes of the Kilogram (1988–1992)". Metrologia 31 (4): 317–336.  
    31. ^ The International Bureau of Weights and Measures official site: Calibration and characterization certificates: Mass, retrieved August 4, 2013
    32. ^ The International Bureau of Weights and Measures official site: Some BIPM calibrations and services in mass and related quantities, retrieved August 4, 2013
    33. ^ Picard, A. (February 2012). "Director’s Report on the Activity and Management of the International Bureau of Weights and Measures; Supplement: scientific Departments". Bureau International des Poids et Mesures. Retrieved August 3, 2013. 
    34. ^ ]KilogramКилограмм [ (in Русский), retrieved December 28, 2013, Из 40 изготовленных копий прототипа две (№12 и №26) были переданы России. Эталон №12 принят в СССР в качестве государственного первичного эталона единицы массы, а №26 — в качестве эталона-копии. 
    35. ^ TÜBİTAK National Metrology Institute official site: [1], retrieved June 16, 2014
    36. ^ National Physical Laboratory official site: Making the first international kilograms and metres, retrieved August 4, 2013
    37. ^ Z. J. Jabbour; S. L. Yaniv (2001). "The Kilogram and Measurements of Mass and Force". Journal of Research of the National Institute of Standards and Technology 106: 26. Retrieved August 4, 2013. 
    38. ^ Z.J. Jabbour; S.L. Yaniv (Jan–Feb 2001). 3.5 "The Kilogram and Measurements of Mass and Force". J. Res. Natl. Inst. Stand. Technol. 106 (1): 25–46.  
    39. ^ Girard, G. (1990), The washing and cleaning of kilogram prototypes at the BIPM, BIPM 
    40. ^ a b Mills, Ian M.; Mohr, Peter J; Quinn, Terry J; Taylor, Barry N; Williams, Edwin R (April 2005). "Redefinition of the kilogram: a decision whose time has come". Metrologia 42 (2): 71–80.  
    41. ^ Davis, Richard (December 2003). "The SI unit of mass". Metrologia 40 (6): 299–305.  
    42. ^ R. S. Davis (July–August 1985). "Recalibration of the U.S. National Prototype Kilogram". Journal of Research of the National Bureau of Standards 90 (4). 
    43. ^ a b Conjecture why the IPK drifts, R. Steiner, NIST, Sep 11, 2007. 
    44. ^ Report to the CGPM, 14th meeting of the Consultative Committee for Units (CCU), April 2001, 2. (ii); General Conference on Weights and Measures, 22nd Meeting, October 2003, which stated “The kilogram is in need of a new definition because the mass of the prototype is known to vary by several parts in 108 over periods of time of the order of a month…” (MB ZIP file, here 3.2). 
    45. ^ BBC, Getting the measure of a kilogram. 
    46. ^ "FAQs". BIPM. Retrieved April 3, 2011. 
    47. ^ Cumpson, Peter (October 2013). "Stability of reference masses: VI. Mercury and carbonaceous contamination on platinum weights manufactured at a similar time as the international and national prototype kilograms". Metrologia 50 (5): 518–531.  
    48. ^ General section citations: Recalibration of the U.S. National Prototype Kilogram, R. S. Davis, Journal of Research of the National Bureau of Standards, 90, No. 4, July–August 1985 (MB PDF, here 5.5); and The Kilogram and Measurements of Mass and Force, Z. J. Jabbour et al., J. Res. Natl. Inst. Stand. Technol. 106, 2001, 25–46 (MB PDF, here 3.5) 
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      "Frequently Asked Questions". National Research Council Canada. 
      "Converting Measurements to Metric—NIST FAQs". NIST. 
      "Metric Conversions". UK National Measurement Office. 
      "Fed-Std-376B, Preferred Metric Units for General Use By the Federal Government" (294 KB PDF). NIST. 
    52. ^  
    53. ^ "Recommendation 1: Preparative steps towards new definitions of the kilogram, the ampere, the kelvin and the mole in terms of fundamental constants". 94th meeting of the International Committee for Weights and Measures. October 2005. p. 233. Retrieved November 3, 2011. 
    54. ^ "NIST Backs Proposal for a Revamped System of Measurement Units". Retrieved April 3, 2011. 
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    56. ^ "Resolution 1 - On the possible future revision of the International System of Units, the SI". 24th meeting of the General Conference on Weights and Measures. Sèvres, France. October 17–21, 2011. Retrieved October 25, 2011. 
    57. ^ "General Conference on Weights and Measures approves possible changes to the International System of Units, including redefinition of the kilogram." (Press release). Sèvres, France:  
    58. ^ "An initial measurement of Planck's constant using the NPL Mark II watt balance", I.A. Robinson et al., Metrologia 44 (2007), 427–440;
      NPL: NPL Watt Balance
    59. ^ "CODATA Value: Planck constant". The NIST Reference on Constants, Units, and Uncertainty. US  
    60. ^ R. Steiner, Watts in the watt balance, NIST, Oct 16, 2009.
    61. ^ R. Steiner, No FG-5?, NIST, Nov 30, 2007. “We rotate between about 4 resistance standards, transferring from the calibration lab to my lab every 2–6 weeks. Resistors do not transfer well, and sometimes shift at each transfer by 10 ppb or more.”
    62. ^ "CODATA Value: Avogadro constant". The NIST Reference on Constants, Units, and Uncertainty. US  
    63. ^ Hill, Theodore P; Miller, Jack, Censullo, Albert C (June 1, 2011). "Towards a better definition of the kilogram". Metrologia 48 (3): 83–86.  
    64. ^ Georgia Tech, “A Better Definition for the Kilogram?” September 21, 2007 (press release).
    65. ^ Brumfiel, Geoff (October 21, 2010). "Elemental shift for kilo". Nature 467: 892.  
    66. ^ NPL: Avogadro Project; Australian National Measurement Institute: [ Redefining the kilogram through the Avogadro constant]; and Australian Centre for Precision Optics: The Avogadro Project
    67. ^ a b The German national metrology institute, known as the Physikalisch-Technische Bundesanstalt (PTB): Working group 1.24, Ion Accumulation
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    72. ^
    73. ^ BIPM: SI Brochure: Section 3.2, The kilogram 
    74. ^ BIPM: SI Brochure: Section 4.1, Non-SI units accepted for use with the SI, and units based on fundamental constants: Table 6 

    External links

    External images
    BIPM: The IPK in three nested bell jars
    NIST: K20, the US National Prototype Kilogram resting on an egg crate fluorescent light panel
    BIPM: Steam cleaning a 1 kg prototype before a mass comparison
    BIPM: The IPK and its six sister copies in their vault
    The Age: Silicon sphere for the Avogadro Project
    NPL: The NPL’s Watt Balance project
    NIST: This particular Rueprecht Balance, an Austrian-made precision balance, was used by the NIST from 1945 until 1960
    BIPM: The FB‑2 flexure-strip balance, the BIPM’s modern precision balance featuring a standard deviation of one ten-billionth of a kilogram (0.1 µg)
    BIPM: Mettler HK1000 balance, featuring 1 µg resolution and a 4 kg maximum mass. Also used by NIST and Sandia National Laboratories’ Primary Standards Laboratory
    Micro-g LaCoste: FG‑5 absolute gravimeter, (diagram), used in national laboratories to measure gravity to 2 µGal accuracy
    • NIST Improves Accuracy of ‘Watt Balance’ Method for Defining the Kilogram
    • The UK’s National Physical Laboratory (NPL): Are any problems caused by having the kilogram defined in terms of a physical artefact? (FAQ - Mass & Density)
    • NPL: NPL watt balance
    • Metrology in France: Watt balance
    • Australian National Measurement Institute: Redefining the kilogram through the Avogadro constant
    • International Bureau of Weights and Measures (BIPM): Home page
    • NZZ Folio: What a kilogram really weighs
    • NPL: What are the differences between mass, weight, force and load?
    • BBC: Getting the measure of a kilogram
    • NPR: This Kilogram Has A Weight-Loss Problem, an interview with National Institute of Standards and Technology physicist Richard Steiner
    • Avogadro and molar Planck constants for the redefinition of the kilogram
    • Realization of the awaited definition of the kilogram
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