April 2019

Some weight off your shoulders: The Kilogram and its SI redefinition

Whether holding a newborn baby, buying vegetables at the farmer’s market, pushing yourself to lift a heavier weight at the gym or considering whether to further stuff your suitcase before your flight, the kilogram has definitely crossed all of our minds at some point. The kilogram is a heavy topic in 2019 and understanding why scientists have decided to redefine the kilogram is vital in the world of measurement.

How did the Kilogram come to be?

The kilogram has a long history and can be traced back to the French Revolution. The measuring standard at the time (the grave) was the mass of a litre of water once it was at its ice point. The new French republic however, desired a new standard: the gram. From its English definition, the gram is the absolute mass of one cubic centimetre of water at four degrees Celsius. Shortly after, the people realised that such a small unit was very impractical and called for something greater; one thousand times greater.  This is how in June 1799, the French republic adopted its newest official measurement artefact, “the kilogram of the archives”.

In 1875, 17 countries realised the need for an international agreement on measuring mass and length in order to improve trade relations and thus held the Metre Convention, creating official prototypes for mass and length measurement. Known as the International Prototypes of the Kilogram (IPK), the prototypes are based on the weight of the official kilogram (a cylinder made from platinum and iridium alloy that sits in a vault near Paris). These became the international standard and were given to countries participating in the convention. 

Why exactly should we redefine the kilogram?

Since the late 19th century, the kilogram has been defined by the official artefact. However, despite being undeniably valuable, this artefact has changed over its century-long life. This is why scientists have been searching for a way to define the kilogram using a fixed value based in nature: Planck’s constant (h).

Planck’s constant, as the name suggests, remains constant. By adopting this redefinition, we no longer need to worry about an artefact being contaminated, damaged or lost, affecting our capability to measure accurately.

This redefinition also has the additional benefit of improved accuracy at the extreme ends of the scale: weighing objects that are either significantly smaller or larger than your actual standard decreases accuracy. 

How has it been done?

Two approaches to define the kilogram using Planck’s constant have been investigated over the last thirty years.

One of the ways involves counting the number of atoms in a kilogram of a ultra-pure sphere of silicon-28. (the NML was involved in developing methods that could be used to show this sphere was actually as isotopically pure as expected).

The other option involves using a Kibble balance, a device named after Bryan Kibble, a British physicist from the National Physical Laboratory (NPL). The Kibble balance, used in reverse, can measure an unknown mass if it has a set value of h. After much debate, the General Conference on Weights and Measures (CPGM) agreed that the “electric kilogram” could be calculated this way if a minimum of three laboratories meet the requirement of calculating the value of Planck’s constant with an uncertainty value no greater than 50 parts per billion.

Coming together, many experts in metrology (measurement science) from around the world combined their efforts with their Kibble balances and pure silicon-28 spheres to determine the value of h, with a remarkable uncertainty of only 10 parts per billion. This is the value that will redefine the kilogram.

The moment we have all been waiting for, the SI redefinition, is taking place on World Metrology Day, 20 May 2019, with more than 80 countries celebrating! Planck’s constant will finally define our kilogram. There is no need to panic! Newborn babies, bags of vegetables, gym weights and suitcases will still weigh what they did; the only difference in the mass measurement however is that the value is certain and will not change with the aging of an archived artefact. How cool is it to say that your bag of potatoes is defined by the quantum of electromagnetic action!

Want to find out more about the redefinition?

Our blog has monthly updates on the SI, the BIPM download area is a good source of information, and the NIST and NPL websites have really interesting content. Watch a recording of the vote in this webcast.