Kilogram

• 1. Introduction

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  The kilogram spent 130 years living as a single lump of metal in France. From 1889, a cylinder of platinum and iridium served as the International Prototype of the Kilogram. Every kilogram measured anywhere on Earth ultimately answered to that one object. Then in 2019, the metal was retired. The unit that anchors mass for the International System of Units was handed instead to the Planck constant, the speed of light, and a transition frequency of the caesium-133 atom. How does a unit go from a hunk of platinum to three numbers borrowed from physics? Why did the world abandon an object it had trusted for a century? And how did a measure born in a single litre of water during a revolution end up defined by a constant of nature?

• 2. Water And Revolution

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  In 1795, during the French Revolution, the kilogram was first defined as the mass of one litre of water. The original reference temperature was 0 degrees Celsius, later changed to the temperature of water's maximum density, roughly 4 degrees Celsius. Two years earlier, in 1793, a precursor called the grave had been set as the mass of one litre of water, which was determined to equal 18841 grains. The gram entered provisionally in 1795 as the mass of one cubic centimetre of water at the melting point of ice. By 1799, this water standard gave way to a physical object: the platinum Kilogramme des Archives, manufactured as a prototype with a mass equal to one cubic decimetre of water at its densest. The modern definition still honors that watery origin. It agrees with the 1795 figure to within 30 parts per million, a deviation of 0.003 percent, with water at peak density sitting very close to one kilogram per litre.

• 3. A Greek Word In French Law

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  The word kilogramme was written into French law in 1795, in the Decree of 18 Germinal. It was a learned coinage. French scholars prefixed the Greek stem khilioi, meaning a thousand, to gramma, a Late Latin term for a small weight that itself came from Greek. The 1795 decree reshuffled an earlier provisional system: the term gramme replaced an older word called gravet, and kilogramme replaced the grave. When English first borrowed the word in 1795, it took the French spelling, while the spelling kilogram took hold in the United States. In the United Kingdom both forms appear, though kilogram has become far more common, and UK trade law permits either. The kilogram carries one quirk no other base unit shares. It is the only SI base unit whose name already contains an SI prefix, the kilo, a fact that complicates how its smaller and larger multiples are named.

• 4. The Trouble With Kilo

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  In the 19th century, the clipped French word kilo crossed into English, used loosely for both kilogram and kilometre. Its standing has been contested ever since. The Canadian government's Termium Plus system states that SI usage in scientific and technical writing does not allow kilo, and one dictionary of units calls it a common informal name. The United States Congress permitted kilo as an alternative when it gave the metric system legal status in 1866, then revoked that status in 1990. The SI Brochure, which the BIPM began publishing in 1970, is blunt about shorthand. It states that it is not permissible to use abbreviations for unit symbols or unit names. Practical confusion runs deeper at the small end of the scale. Serious medication errors have come from mixing up milligrams and micrograms when micrograms is abbreviated. The abbreviation mcg, not the symbol with the Greek letter mu, is formally mandated for US medical practitioners by the Joint Commission on Accreditation of Healthcare Organizations. In the United Kingdom, health guidance insists micrograms and nanograms be written in full and never abbreviated at all.

• 5. Why The Metal Failed

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  By the early 21st century, the International Prototype of the Kilogram had a problem: it was changing. Evidence built up over many years that the mass of the IPK and its official replicas was drifting apart. The IPK had diverged from its copies by roughly 50 micrograms since their manufacture late in the 19th century. The whole point of a prototype is that it never changes, yet here was the master standard quietly slipping against its own siblings. That instability set off competing efforts to build measurement technology precise enough to replace the artefact with a definition tied to fundamental constants. There was precedent. In 1960, the metre had itself been freed from a platinum-iridium bar marked with two lines, redefined first by the wavelength of light from krypton and later by the speed of light. That move meant any properly equipped laboratory could reproduce the standard from a written specification, with no single object to guard or to trust.

• 6. Handing Mass To Physics

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  At the 94th Meeting of the CIPM in 2005, the committee recommended doing for the kilogram what had been done for the metre. In October 2010, the CIPM voted to forward a resolution asking the General Conference on Weights and Measures to take note of an intention to define the kilogram through the Planck constant. The 24th CGPM accepted that resolution in October 2011, and the 25th conference discussed it further in 2014. Progress was real, but the data were judged not yet robust enough, so the work continued toward the 26th meeting scheduled for 2018. The constant at the heart of it carries dimensions of energy times time, which works out to mass times length divided by time. The CIPM approved the revision in November 2018, fixing the Planck constant at exactly 6.62607015, with the new definition taking effect on the 20th of May 2019. Any apparatus precise, accurate, and stable enough to express the kilogram through that constant could now serve as a standard. The Kibble balance is one such instrument. Every one of these methods converts a weight measurement into a mass, which means each depends on precisely measuring the local strength of gravity, the practice known as gravimetry.

• 7. The Family Of Masses

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  Because no SI unit may carry two prefixes, the kilogram forces an odd workaround. Prefixes attach to the gram rather than to the base unit, since the base unit already wears the kilo. One-millionth of a kilogram is therefore written as one milligram, not one microkilogram. The retail world keeps some of these multiples alive in colorful ways. The hectogram, 100 grams, known in Italian as the etto, is very commonly used in Italy's food shops. An older spelling style produced the abbreviation dkg for the dekagram, 10 grams, still used in parts of central Europe to sell foods like cheese and meat. At the large end sits the megagram, a name almost no one uses. For 1000 kilograms, people reach instead for the tonne, whose symbol t was adopted by the CIPM in 1879. The BIPM accepts the tonne for use with the SI, and notes that some English-speaking countries call it the metric ton, the everyday name for a unit the strict SI vocabulary would rather call something else.