Le Grand K

Ever wondered how did we all agree as to what a kilogram actually is?

How does a scale ‘know’ what this unit does it equal to? The world might have not been able to agree on a lot of stuff, but they somehow managed to settle on the weight of the kilogram, which has been based on the International Prototype of the Kilogram (IPK), also known as Le Grand K, a global standard weight for a kilogram since 1889. Le Grand K is a cylinder made of a platinum-iridium alloy, which was manufactured in 1879 in London and since then stored in three locked bell jars in environmentally monitored safe and kept in a vault in the International Bureau of Weights and Measures Pavillon de Breteuil, Saint-Cloud in Paris. But now the IPK is about to be retired this November. 

The reason for its retirement is mainly lack of precision – in order to ‘realise’ a kilogram, scientists all over the world are still using over 100-years old physical artefact. Every 40 years or so, the original cylinder was removed from its vault to compare it to the 67+ copies stored across the globe. Over the years, there have been discrepancies which caused a stir in the scientific community – some believe that it’s the Le Grand K itself losing weight but others blame the copies, which could be gaining weight due to atmospheric pollutants. No one really knows and since the only point of reference, the Le Grand K, may be changing itself there is no way to check.

Over the years, metrologists have been searching for an upgrade in kilogram’s definition. The method which would provide the most precision is based on constants, already used by physicists to describe the natural world. As a matter of fact, the kilogram is not the first measure to be upgraded using constants – it follows the meter, which used to be measured against a piece of metal stored in the same atmosphere controlled vault as the Le Grand K. In 1983, the measure of the meter was redefined as the distance light travels in a vacuum over 1/299,792,458 of a second. Since the speed of light is unchanging and remains a constant, this means the meter will also never change.

Kilogram’s replacement presented to be a tad more tricky than that of a meter. A different constant has been chosen by metrologists to replace the IPK, called the Planck’s constant (the ‘h’ in Planck’s equation of E=hv), the quantum-mechanical number that relates how a particle’s energy is related to its frequency, and through Einstein’s E=mc², to its mass. This with the use of the Kibble scale, a complex device which measures mass precisely through the use of electrical measurements, the combination would allow to make the kilogram measure unchanging and keeping it constant for the years to come.

With the Le Grand K retiring this November, comes a sentimental realisation that the weight of all kilograms in the world, whether a kilogram of sugar in Nigeria, kilogram of rice in Japan or a kilogram of flour in the UK, will no longer be based on the same tiny cylinder stored in controlled atmosphere in Paris. All of the sudden, an item of the utmost international importance for over 100 years will become useless, belonging in a museum rather than a highly secured vault.

Farewell!