For almost a century and a half, much of the world has used a highly polished, golf ball-sized cylinder of a platinum alloy as the international standard for mass measurement – a kilogram (kg). But where did this unit of measurement come from?
From Cradle to Grave
Near the end of the 1700s, King Louis XVI of France ordered a new system of measurement in an effort to crack down on rampant cheating taking place under the existing systems. The king's commission recommended what would become the decimal metric system, and for mass measurements proposed a new unit called a grave which was defined as the mass of a liter of water at the ice point.
Then came the French Revolution. The new Republic adopted the idea of the metric system with a few changes. Instead of a grave, the Republic picked a new standard – the gramme, whose definition was the absolute weight of 1 cm3 of water at 4 °C.
However, a 1-gram artifact made of water – not much bigger than a pea – was impractical for commercial use. So they chose something more grave-like instead, a solid metal artifact a thousand times more massive than a gram: a kilogram.
On June 22, 1799, the Republic adopted the new mass artifact, forged from platinum and dubbed the "kilogram of the archives" after the building in which it was stored. This new kilogram reigned as the mass standard in France for almost a hundred years.
The Meter Convention of 1875
As the global economy grew in the 1800s, so did mass measurement problems. Each country maintained its own standards which were often incompatible with those of other countries.
On May 20, 1875, seventeen different countries signed a treaty called the Meter Convention to establish new international prototypes for mass and length. The agreement defined the official unit of mass as the kilogram (kg) and embodied it in a new metal artifact whose mass was essentially the same as the "kilogram of the archives." This new standard was made from an alloy of platinum (90%) and iridium (10%) and called the International Prototype Kilogram (IPK).
Copies of the IPK, also referred to as "le grand K" ("the big K"), were sent to the countries which had signed the agreement. In 1890, the United States received the fourth and twentieth copies – known as K4 and K20 – and the latter still serves as the official national kilogram standard while the former acts as its check standard. They are kept at NIST's Gaithersburg campus in Maryland, along with three additional kilogram standards that the U.S. has since acquired.
➤ Who makes these decisions? Click here for information about the three international organizations that were created in the Meter Convention and that oversee the official standards, which are now known as the International System of Units (SI).
Beyond "The Big K"
After redefinition in 2018, the IPK will lose its official standard status. But artifacts will still be used as the most practical way to transfer the new standards to all the places it will need to be used – everywhere from world-class labs to grocery stores.
- SI brochure, 8th edition, Section 1.8: Historical Note (pp 19-22), NIST Special Publication 330, 2008 Edition, Barry N. Taylor and Ambler Thompson, Editors
- NIST Reference on Constants, Units, and Uncertainty
- BIPM (Bureau international des poids et mesures):
- The Kilogram and Measurements of Mass and Force, Section 1.1: The Unit of Mass, J. Res. Natl. Inst. Stand. Technol. 106, 25–46 (2001)