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International Prototype of the Kilogram

International Prototype of the Kilogram is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand International Prototype of the Kilogram rather than just read about it. In short: 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…

International Prototype of the Kilogram — main illustration
International Prototype of the Kilogram — illustration

Key takeaways

  • International Prototype of the Kilogram belongs to science; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect International Prototype of the Kilogram to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of International Prototype of the Kilogram from memory before moving on to harder problems.

Reference excerpt

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 "Pt‑10Ir", 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 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 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-Cloud 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.

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. 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. Stored in the same vault at the BIPM. Ten working copies: eight (9, 31, 42′, 63, 77, 88, 91, and 650) 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 in 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 90% platinum / 10% iridium (Pt‑10Ir) kilogram standards, two of which, K4 and K20, are from the original batch of 40 replicas distributed in 1884. 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 1 kg − 39 μg (micrograms) in 1889; that is to say, K20 was 39 μg less than the IPK. A verification performed in 1948 showed a mass of 1 kg − 19 μg. 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 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 a period of 110 years, K4 lost 41 μg relative to the IPK.

… excerpt ends here. Continue reading the full article.

Illustrations

International Prototype of the Kilogram: The International Prototype Kilogram, stored in a vault in Paris, was replaced in 2019 by a formula that uses the Planck constant, since the IPK’s mass is unstable over time.
The International Prototype Kilogram, stored in a vault in Paris, was replaced in 2019 by a formula that uses the Planck constant, since the IPK’s mass is unstable over time.
International Prototype of the Kilogram: National prototype kilogram K20, one of two prototypes stored at the US National Institute of Standards and Technology in Gaithersburg, Maryland, which served as primary standards for defining all units of mass and weight in the United States. This is a replica for public display, shown as it is normally stored, under two bell jars.
National prototype kilogram K20, one of two prototypes stored at the US National Institute of Standards and Technology in Gaithersburg, Maryland, which served as primary standards for defining all units of mass and weight in the United States. This is a replica for public display, shown as it is normally stored, under two bell jars.
International Prototype of the Kilogram: Mass drift over time of national prototypes K21–K40, plus two of the IPK's sister copies: K32 and K8(41).[Note 2] All mass changes are relative to the IPK. The initial 1889 starting-value offsets relative to the IPK have been nulled.[17] The above are all relative measurements; no historical mass-measurement data is available to determine which of the prototypes has been most stable relative to an invariant of nature. There is the distinct possibility that all the prototypes gained mass over 100 years and that K21, K35, K40, and the IPK simply gained less than the others.
Mass drift over time of national prototypes K21–K40, plus two of the IPK's sister copies: K32 and K8(41).[Note 2] All mass changes are relative to the IPK. The initial 1889 starting-value offsets relative to the IPK have been nulled.[17] The above are all relative measurements; no historical mass-measurement data is available to determine which of the prototypes has been most stable relative to an invariant of nature. There is the distinct possibility that all the prototypes gained mass over 100 years and that K21, K35, K40, and the IPK simply gained less than the others.
International Prototype of the Kilogram: Until May 2019, the magnitude of many of the units composing the SI system of measurement, including most of those used in the measurement of electricity and light, were highly dependent upon the stability of IPK.
Until May 2019, the magnitude of many of the units composing the SI system of measurement, including most of those used in the measurement of electricity and light, were highly dependent upon the stability of IPK.
International Prototype of the Kilogram: A replica of the prototype kilogram on display at Cité des Sciences et de l'Industrie in Paris, featuring the protective double glass bell.
A replica of the prototype kilogram on display at Cité des Sciences et de l'Industrie in Paris, featuring the protective double glass bell.

Worked examples

Example 1 — a first encounter with International Prototype of the Kilogram

Start with the simplest possible case. Write down what International Prototype of the Kilogram claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to International Prototype of the Kilogram before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about International Prototype of the Kilogram ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of International Prototype of the Kilogram

In research
International Prototype of the Kilogram appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses International Prototype of the Kilogram in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
International Prototype of the Kilogram is common in secondary-school and first-year university syllabi. It links to neighbouring topics Metrology, Standards (metrology), Units of mass, so understanding it makes those chapters shorter.
In everyday life
Look for International Prototype of the Kilogram outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.
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How to study International Prototype of the Kilogram in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what International Prototype of the Kilogram means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain International Prototype of the Kilogram out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is International Prototype of the Kilogram in simple terms?

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…

Why does International Prototype of the Kilogram matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study International Prototype of the Kilogram?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on International Prototype of the Kilogram.

Tags

  • Metrology
  • Standards (metrology)
  • Units of mass

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