Science · Level 5 · 213 words

The Cylinder in the Vault

Original passage © Studio AM, written for Fluency.

Until 2019, the kilogram was an object. A platinum-iridium cylinder, cast in 1889, sat under three nested bell jars in a vault near Paris, and a kilogram was, by definition, whatever that cylinder weighed. Mass was the last base unit still tied to a single physical thing.

The arrangement contained a trap. Official copies, sent to national laboratories, were carried back every few decades and compared with the prototype. The comparisons showed the family of cylinders slowly drifting apart, until by the late twentieth century the copies disagreed with the original by roughly fifty micrograms, about the mass of a fingerprint. What no measurement could reveal was which cylinder had changed. The prototype could not be wrong, because being right was its only job: if atoms rubbed off its surface, every kilogram on Earth grew lighter with it, and every scale remained, officially, correct.

The escape was to stop defining mass with an object. In 2019 the kilogram was fixed instead through the Planck constant, a number built into nature and identical in every laboratory, realized by an instrument called a Kibble balance. The cylinder is still in its vault, and it still weighs about a kilogram. But for the first time in 130 years, that sentence is a measurement, not a definition.

Comprehension questions

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4 questions
1. What is the main idea of the passage?

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A. Because an object-based kilogram made drift undetectable in principle, mass was redefined through a constant of nature.
The passage is built as problem and escape: a standard that cannot be checked against anything outside itself, resolved by moving the definition from the cylinder to the Planck constant.

2. Why could no measurement reveal which cylinder had changed?

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C. Every comparison used the prototype as the standard, so the standard's own drift had nothing outside itself to be measured against.
Comparisons only show relative disagreement. Since the prototype defined the unit, a change in it changed every kilogram at once, which is exactly why every scale stayed 'officially' correct.

3. In 'compared with the prototype,' prototype means:

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B. the original object that serves as the official standard
In engineering a prototype is a first draft, but here the word keeps its older sense: the original against which all copies are judged. Its role in the vault fixes the meaning.

4. By how much did the copies disagree with the original by the late twentieth century?

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D. About fifty micrograms
The passage states roughly fifty micrograms and glosses it as about the mass of a fingerprint. The wrong options change the unit or the digit, which the gloss rules out.

Source: Written for Fluency. Original passage © Studio AM, written for Fluency.