The original kilogram is kept under guard in Sèvres, France. It is based on the weight of a liter of pure water.

France

Every kilo you’ve ever encountered is based on a little piece of metal in the town of Sèvres, France. In 1799, one kilogram was defined as the mass of one liter of water at 4°C, but 90 years later, traders and scientists needed a more stable standard, and the International Prototype Kilogram was born. About as big as a hen's egg, the original kilo was cast in platinum-iridium alloy, polished and honed, then locked in the basement of the Bureau international des poids et mesures in Sèvres, kept in an environmentally controlled chamber under three bell jars that can only be opened with three keys kept by three different people. Every 50 years, it is removed from its vault and compared to six sister copies, which are used as models for 34 replicas kept around the world. 

But the original kilo is shrinking. When last measured it had lost 50 micrograms – roughly the weight of a grain of sand – and no one knows why. The difference is tiny, but much of the metric system hangs in the balance. Pressure, temperature, voltage: all are measured in relation to kilograms. And if the original has changed, then every other kilogram is now slightly heavier than it should be.

 
Weights: What tips the scale

 

18 kilo-002The picul

The picul, a unit of weight used for trade in China and Southeast Asia, originally referred to the maximum load one man could carry. It varied region by region until it was standardized in Hong Kong in 1844 as equivalent to 133.33 pounds (60 kilograms).

18 kilo-003The kilogram

The original kilogram kept in Sevres, France, has changed weight, so it may soon be replaced by a more chemically stable sphere of pure silicon-28 atoms. Scientists in Germany are currently working on it, but have only achieved 99.99 percent purity so far.

18 kilo-004The grain

Due to their consistent size and mass, individual barley grains used to underpin the traditional English system of measurements. The weight of a pound (450 grams) was set in 16th-century England to be equivalent to exactly 7,000 grains of barley.

18 kilo-005The stone

When a standard weight was needed for English wool exports in 1389, a stone of wool was fixed at 14 pounds (6.35 kilograms), the same weight as the fleece of a large ram. The stone is still commonly used to measure body weight in the UK.

 



From the pages of COLORS #85 - Going to Market.