International Prototype of the Kilogram

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Lore & Background

The Metre Convention was signed on 20 May 1875, further formalising the metric system and quickly leading to the production of the IPK. The IPK is one of three cylinders made in London in 1879 by Johnson Matthey. 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. The IPK and its six sister copies (K1, 7, 8(41), 32, 43 and 47) are stored in a controlled environment in a basement vault at the BIPM's Pavillon de Breteuil in Saint-Cloud on the outskirts of Paris. Three independently controlled keys are required to open the vault. Official copies were made available to other nations to serve as their national standards, compared to the IPK roughly every 40 years. The only three verifications occurred in 1889, 1948, and 1989. For instance, the US owns five Pt‑10Ir kilogram standards, two of which, K4 and K20, are from the original batch of 40 replicas distributed in 1884. In 1984, the K4 and K20 prototypes were hand-carried in the passenger section of separate commercial flights.

Reader's Guide

By 2018, the IPK underpinned the definitions of four of the seven SI base units: the kilogram itself, plus the mole, ampere, and candela, 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 may have lost perhaps 50 μg of mass over the last century, and possibly significantly more, in comparison to its official copies. The reason for this drift has eluded physicists. 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. The IPK exhibits a short-term instability of about 30 μg over a period of about a month in its after-cleaned mass. The precise reason is not understood but is thought to entail surface effects. The BIPM developed a cleaning method between 1939 and 1946 comprising 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 7–10 days before verification. Cleaning removes between 5 and 60 μg of contamination. After cleaning, the IPK and its replicas immediately begin gaining mass again, averaging 1.11 μg per month for the first 3 months and then decreasing to about 1 μg per year thereafter.

Did You Know?

• The IPK is one of three cylinders made in London in 1879 by Johnson Matthey.
• The IPK and its replicas are stored in air under two or more nested bell jars.
• The BIPM cleaning method, developed between 1939 and 1946, uses a chamois soaked in equal parts ether and ethanol, followed by steam cleaning with bi-distilled water.
• After cleaning, the IPK and its replicas immediately begin gaining mass again, averaging 1.11 μg per month for the first 3 months.
• In 1984, the US prototypes K4 and K20 were hand-carried in the passenger section of separate commercial flights.

Material and Form: Engineering a Perfect Standard

The International Prototype of the Kilogram is a roughly golfball-sized object made of a platinum–iridium alloy known as Pt‑10Ir, which is 90% platinum and 10% iridium (by mass). It is machined into a right-circular cylinder with perpendicular height equal to its diameter of about 39 millimetres 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 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.

From Convention to Ratification

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. 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.

A Global Web of Standards

The IPK and its six sister copies (K1, 7, 8(41), 32, 43 and 47) are stored in a controlled environment in a basement vault at the BIPM's Pavillon de Breteuil in Saint-Cloud on the outskirts of Paris. Three independently controlled keys are required to open the vault. Ten working copies—eight (9, 31, 42′, 63, 77, 88, 91, and 650) for routine use and two (25 and 73) for special use—are kept in the BIPM's calibration laboratory in Saint-Cloud. National prototypes are stored in dozens of countries including 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 are 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). These were compared to the IPK roughly every 40 years, thereby providing traceability of local measurements back to the IPK.

Drift, Decline, and the End of a Physical Standard

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. K20, the US's primary standard, originally had an official mass of 1 kg − 39 μg in 1889; a verification in 1948 showed a mass of 1 kg − 19 μg; the latest verification in 1989 shows a mass precisely identical to its original 1889 value. K4 was originally delivered with an official mass of 1 kg − 75 μg in 1889, but as of 1989 was officially calibrated at 1 kg − 106 μg and ten years later was 1 kg − 116 μg. Over 110 years, K4 lost 41 μg relative to the IPK. What has become clear after the third periodic verification 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 50 μg of mass over the last century, and possibly significantly more, in comparison to its official copies. The reason for this drift has eluded physicists. No plausible mechanism has been proposed. There are no technical means available to determine whether or not the entire worldwide ensemble of prototypes suffers from even greater long-term trends. Given the lack of data, it is equally valid to state that the first batch of replicas has, as a group, gained an average of about 25 μg over one hundred years in comparison to the IPK. The IPK exhibits a short-term instability of about 30 μg over a period of about a month in its after-cleaned mass. The precise reason is not understood but is thought to entail surface effects. In 2019, the kilogram was redefined based entirely on physical constants, replacing the IPK.