The International Prototype of the Kilogram (referred to by metrologists as the IPK or Le Grand K; sometimes called the ur-kilogram, or urkilogram, particularly by German-language authors writing in English:30) is an object whose mass was used to define the kilogram from 1889, when it replaced the Kilogramme des Archives, until 2019, when it was replaced by a new definition of the kilogram based entirely on physical constants. During that time, the IPK and its duplicates were used to calibrate all other kilogram mass standards on Earth.
The IPK is a roughly golfball-sized object made of a platinum–iridium alloy known as "Pt10Ir", which is 90% platinum and 10% iridium (by mass) and is machined into a right-circular cylinder with perpendicular height equal to its diameter of about 39millimetres to reduce its surface area. The addition of 10% iridium improved upon the all-platinum Kilogramme des Archives by greatly increasing hardness while still retaining platinum's many virtues: extreme resistance to oxidation, extremely high density (almost twice as dense as lead and more than 21 times as dense as water), satisfactory electrical and thermal conductivities, and low magnetic susceptibility.
By 2018, the IPK underpinned the definitions of four of the seven SI base units: the kilogram itself, plus the mole, ampere, and candela (whose definitions at the time referenced the gram, newton, and watt respectively) as well as the definitions of every named SI derived unit except the hertz, becquerel, degree Celsius, gray, sievert, farad, ohm, siemens, henry, radian, and steradian.
The IPK and its six sister copies are stored at the International Bureau of Weights and Measures (known by its French-language initials BIPM) in a controlled environment in a basement vault at the BIPM's Pavillon de Breteuil in Saint-CloudThe Pavillon's (and hence the BIPM's) postal address is in the neighbouring commune of Sèvres, so it is often reported as being located there, but the grounds are in the commune of Saint-Cloud (OpenStreetMap). on the outskirts of Paris (see External images, below, for photographs). Three independently controlled keys are required to open the vault. Official copies of the IPK were made available to other nations to serve as their national standards. These were compared to the IPK roughly every 40 years, thereby providing traceability of local measurements back to the IPK.The International Bureau of Weights and Measures official site: Verifications, retrieved August 4, 2013
Creation The Metre Convention was signed on 20 May 1875 and further formalised the metric system (a predecessor to the SI), quickly leading to the production of the IPK. The IPK is one of three cylinders made in London in 1879 by Johnson Matthey, which continued to manufacture nearly all of the national prototypes as needed until the new definition of the kilogram came into effect in 2019. F.J. Smith. "Standard Kilogram Weights: A Story of Precision Fabrication". Platinum Metals Rev. 17 (2) (1973) 66-68 Terry Quinn. From Artefacts to Atoms: The BIPM and the Search for Ultimate Measurement Standards. Oxford University Press. p. 321. In 1883, the mass of the IPK was found to be indistinguishable from that of the Kilogramme des Archives made eighty-four years prior, and was formally ratified as the kilogram by the 1st CGPM in 1889.
Copies of the IPK
The IPK and its various copies are given the following designations in the literature: The IPK itself, stored in the BIPM's vault in Saint-Cloud, France. Six sister copies: K1, 7, 8(41), 32, 43 and 47.The International Bureau of Weights and Measures official site: The international prototype of the kilogram and its six official copies Stored in the same vault at the BIPM. Ten working copies: eight (9, 31, 42′, 63, 77, 88, 91, and 650Nos. 42′, 77 and 650 are called "standards" rather than "prototypes" because they are slightly underweight, slightly too much material having been removed when they were manufactured. Other than being more than 1 mg below the nominal 1 kg mass, they are identical to the prototypes, and are used during routine calibration work.) for routine use and two (25 and 73) for special use. Kept in the BIPM's calibration laboratory in Saint-Cloud, France. National prototypes, stored inThe International Bureau of Weights and Measures official site: Some BIPM calibrations and services in mass and related quantities, retrieved August 4, 2013 Argentina (30), Australia (44 and 87), Austria (49), Belgium (28 and 37), Brazil (66), Canada (50 and 74), China (60 and 64; 75 in Hong Kong), Czech Republic (67), Denmark (48), Egypt (58), Finland (23), France (35), Germany (52, 55 and 70), Hungary (16), India (57), Indonesia (46), Israel (71), Italy (5 and 76), Japan (6, 94 and E59), Kazakhstan, Kenya (95), Mexico (21, 90 and 96), Netherlands (53), North Korea (68), Norway (36), Pakistan (93), Poland (51), Portugal (69), Romania (2), Russia (12 and 26), Serbia (11 and 29), Singapore (83), Slovakia (41 and 65), South Africa (56), South Korea (39, 72 and 84), Spain (24 and 3), Sweden (40 and 86), Switzerland (38 and 89), Taiwan (78), Thailand (80), Turkey (54), United Kingdom (18, 81 and 82), and the United States (20, 4, 79, 85 and 92). Some additional copies held by non-national organisations, such as the French Academy of Sciences in Paris (34) and the Istituto di Metrologia G. Colonnetti in Turin (62).
Stability of the IPK
Before 2019, by definition, the error in the measured value of the IPK's mass was exactly zero; the mass of the IPK was the kilogram. However, any changes in the IPK's mass over time could be deduced by comparing its mass to that of its official copies stored throughout the world, a rarely undertaken process called "periodic verification". The only three verifications occurred in 1889, 1948, and 1989. For instance, the US owns five 10%iridium (Pt10Ir) kilogram standards, two of which, K4 and K20, are from the original batch of 40 replicas distributed in 1884.The other two Pt10Ir standards owned by the US are K79, from a new series of prototypes that were diamond-turned directly to a finish mass, and K85, which is used for Kibble balance experiments. The K20 prototype was designated as the primary national standard of mass for the US. Both of these, as well as those from other nations, are periodically returned to the BIPM for verification. Great care is exercised when transporting prototypes. In 1984, the K4 and K20 prototypes were hand-carried in the passenger section of separate commercial flights.
None of the replicas has a mass precisely equal to that of the IPK; their masses are calibrated and documented as offset values. For instance, K20, the US's primary standard, originally had an official mass of (micrograms) in 1889; that is to say, K20 was 39μg less than the IPK. A verification performed in 1948 showed a mass of . The latest verification performed in 1989 shows a mass precisely identical to its original 1889 value. Quite unlike transient variations such as this, the US's check standard, K4, has persistently declined in mass relative to the IPK—and for an identifiable reason: check standards are used much more often than primary standards and are prone to scratches and other wear. K4 was originally delivered with an official mass of in 1889, but as of 1989 was officially calibrated at and ten years later was Over a period of 110 years, K4 lost relative to the IPK.
Beyond the simple wear that check standards can experience, the mass of even the carefully stored national prototypes can drift relative to the IPK for a variety of reasons, some known and some unknown. Since the IPK and its replicas are stored in air (albeit under two or more nested bell jars), they gain mass through adsorption of atmospheric contamination onto their surfaces. Accordingly, they are cleaned in a process the BIPM developed between 1939 and 1946 known as "the BIPM cleaning method" that comprises firmly rubbing with a chamois soaked in equal parts ether and ethanol, followed by steam cleaning with bi-distilled water, and allowing the prototypes to settle for days before verification. Before the BIPM's published report in 1994 detailing the relative change in mass of the prototypes, different standard bodies used different techniques to clean their prototypes. The NIST's practice before then was to soak and rinse its two prototypes first in benzene, then in ethanol, and to then clean them with a jet of bi-distilled water steam. Cleaning the prototypes removes between 5 and 60μg of contamination depending largely on the time elapsed since the last cleaning. Further, a second cleaning can remove up to 10μg more. After cleaning—even when they are stored under their bell jars—the IPK and its replicas immediately begin gaining mass again. The BIPM even developed a model of this gain and concluded that it averaged 1.11μg per month for the first 3 months after cleaning and then decreased to an average of about 1μg per year thereafter. Since check standards like K4 are not cleaned for routine calibrations of other mass standards—a precaution to minimise the potential for wear and handling damage—the BIPM's model of time-dependent mass gain has been used as an "after cleaning" correction factor.
Because the first forty official copies are made of the same alloy as the IPK and are stored under similar conditions, periodic verification using a number of replicas—especially the national primary standards, which are rarely used—can convincingly demonstrate the stability of the IPK. What has become clear after the third periodic verification performed between 1988 and 1992 is that masses of the entire worldwide ensemble of prototypes have been slowly but inexorably diverging from each other. It is also clear that the IPK lost perhaps of mass over the last century, and possibly significantly more, in comparison to its official copies. The reason for this drift has eluded physicists who have dedicated their careers to the SI unit of mass. No plausible mechanism has been proposed to explain either a steady decrease in the mass of the IPK, or an increase in that of its replicas dispersed throughout the world.Conjecture why the IPK drifts, R. Steiner, NIST, Sep 11, 2007. Moreover, there are no technical means available to determine whether or not the entire worldwide ensemble of prototypes suffers from even greater long-term trends upwards or downwards because their mass "relative to an invariant of nature is unknown at a level below 1000μg over a period of 100 or even 50 years". Given the lack of data identifying which of the world's kilogram prototypes has been most stable in absolute terms, it is equally valid to state that the first batch of replicas has, as a group, gained an average of about 25μg over one hundred years in comparison to the IPK.The mean change in mass of the first batch of replicas relative to the IPK over one hundred years is +23.5μg with a standard deviation of 30μg. Per The Third Periodic Verification of National Prototypes of the Kilogram , G.Girard, Metrologia 31 (1994) Pg.323, Table3. 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 Figure7 of G.Girard's 1994 paper.