The Cylinder in the Vault
Written by Studio AM.
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From 1889 until 2019, the kilogram was defined by the mass of a platinum-iridium cylinder kept near Paris. Known as the international prototype, it provided the reference against which national copies were calibrated. The unit of mass remained tied to a particular physical object.
Repeated comparisons revealed small changes in the differences between the prototype and its copies. These differences were measured in micrograms, millionths of a gram. The comparisons alone could show that the objects were drifting relative to one another, but could not establish an independently fixed mass for the prototype itself. Under the old definition, its assigned mass remained exactly one kilogram.
On May 20, 2019, a new definition took effect. It fixed the numerical value of the Planck constant in SI units, replacing the artifact as the foundation of the kilogram. Laboratories can connect practical mass measurements to that definition through methods including Kibble balances and carefully measured silicon spheres.
The definition is exact; its experimental realization still has uncertainty. That distinction is useful. It separates the agreed basis of a unit from the performance of a particular instrument. The old cylinder can now be measured against the new definition and assigned a result with uncertainty. Its mass is no longer what determines the kilogram.
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Questions
Choose an answer. The explanation appears after you answer.
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Question 1 of 4
What is the main idea of the passage?
The answer is A: An object-based kilogram hid changes in the standard, so a constant of nature replaced it.
The passage explains the limit of an artifact reference and the move to a definition based on a fixed constant.
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Question 2 of 4
Why could comparisons with the copies alone not establish a change in the prototype's own defined mass?
The answer is C: The prototype itself defined the kilogram rather than being measured against an independent fixed mass.
The comparisons showed relative differences, while the definition assigned the prototype exactly one kilogram.
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Question 3 of 4
What does “prototype” mean in this passage?
The answer is B: the original object that serves as the official standard
Here prototype is the official reference object, not merely an early experimental model.
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Question 4 of 4
What took effect on May 20, 2019?
The answer is D: A kilogram definition based on a fixed numerical value of the Planck constant.
The third paragraph states the date and the new basis of the definition.
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