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Scientists agree to update the definition of a kilogram

A replica of Le Grand K (Photo: Japs 88)Scientists will soon be updating the definition of a kilogram. Since 1889, the exact mass of one kilogram has been based on a lump of metal known as Le Grand K, a cylindrical object made of an alloy of platinum and iridium. Le Grand K is kept under three glass bell jars in a locked vault at the Pavillon de Breteuil in Saint-Cloud, near Paris, France. Over the years, there have been minute variations in its weight caused by pollutants in the air, despite regular cleanings. This has led to the decision to change the definition of a kilogram from a physical object to one derived from the fundamental laws of physics.

A kilogram weight (Coyau)The kilogram was originally defined as the mass of a litre (a cubic decimetre) of water at 0°C (32°F). But because this quantity was difficult to replicate exactly, in 1799 an object made of platinum was used instead. This was later replaced by Le Grand K, or, to give it its official name, the International Prototype of the Kilogram (IPK). All modern measurements of mass are based on Le Grand K.

Dozens of copies of this piece of metal have been made. They are stored around the world and used to standardize the weights and measures in their host countries. Britain’s copy is kept at the National Physical Laboratory (NPL) at Teddington, near London.

A Kibble balance, for measuring Planck's constantThe device that will replace the platinum-iridium cylinder is called the Kibble balance, invented by British physicist Bryan Kibble (1938–2016). It works by measuring the electric current that is required to produce an electromagnetic force exactly equal to the gravitational force acting on a mass. An electric current can itself be measured using what is known as  the Planck Constant, which has a precise, unchanging value. The Planck Constant will, in future, replace Le Grand K for defining a kilogram.

The main reason for the replacement of Le grand K is the need to be able to carry out much more precise measurements, for example, pharmaceutical ingredients that have to be measured in terms of a few billionths of a gram. Another reason is for security. If the Pavillon de Breteuil burned down and the Le Grand K melted, there would be no longer be a reference for the world’s metric weights system.

Copy of the first metre standard (Ken Eckert)The standard definitions of other units of measurement have been updated in recent years using digital technology. For example, the metre was originally defined as one ten-millionth of the distance from the North Pole to the Equator. The measure was preserved as a metal bar, also kept in Paris. But in the 1980s, scientists re-defined the metre as the distance that light can travel through a vacuum in about one 300-millionths of a second.

Similarly, the basic unit of time, a second, was once defined as 1/86,400 of an average day. However, because we now know that the Earth’s rotation varies, it is not possible to measure time precisely this way. So a second has been re-defined as being the time taken for a atom of the alkali metal caesium to vibrate precisely 9,192,631,770 times.

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