Almost Educational: What’s The Deal – The Rise and Fall of The Kilogram

The new show on URY, Almost Educational, a pool of science facts and chat – Wednesdays @ 4pm on ury.org.uk

In a bell-jar, in a bell-jar, surrounded by its siblings, in a triple-locked vault, in a Parisian basement is a lump of platinum-iridium alloy. It’s cylindrical, shiny, and weighs exactly one kilogram. To what accuracy, well it doesn’t just weigh a kilogram. This is the kilogram. Or at least it was until 2018. 


Under these bell jars sat the very item that defined a mass, used across the world as a standard in scientific research. 40 copies of this International Prototype Kilogram, K1, were made originally. Distributed to different countries with their masses measured in relation to K1, so that everyone can calibrate their weighing equipment.

K4 was one of the masses distributed to the United States when in the US it was measured as weighing 1-kilogram minus 39 micrograms in 1889. No masses were delivered with the exact same weight as K1, but all have deviated significantly from their original mass, K1 lost a total of 41 micrograms over the 110 years it was in place. 

The thing that can be hard to wrap your head around is that this lump of Metal always weighed exactly one kilogram. But if the duplicates of this object seemed to change in mass, we can be sure the original is unstable too.

Weight loss or even gain is a huge problem for the kilogram. Any change in mass will literally change the definition of the kilogram. On top of that, the masses each country, or even two sides of the same country, drift away from one another. Of course, that’s only a problem if you can even get a hold of the thing to calibrate your equipment in the first place.

This is specifically problematic in the likes of pharmaceuticals were a very small amount of active ingredient may be present. Of course with any percentage uncertainty the smaller the amount you want to measure the more of an issue uncertainty becomes.

In 2018 the scientific community had had enough. It was time to oust the kilogram. A lot of discussion came about the best way to define the kilogram in a way that was both accessible and stable. 

The kilogram has come a long way since its early days. 

The SI base units – or Système International d’unités came from the need to define scientific methods. And if you hadn’t noticed, they were French.

The grave was the original kilogram – equal to the mass of a litre of cold water. This was decided upon by, among others, Antoine Lavoisier. But in revolutionary France the grave sounded an awful lot like graff – a title of nobility; well this simply wouldn’t do. This, among other things, took Lavoisier’s head to the business end of the Guillotine. 

The new governing body of France introduced the gramme – this, however, was far too small. They went back to the original grave but changed the name – the kilogram was born.

First being defined in 1795 the kilogram was the mass of one litre of water, this ended up being kind of tricky to use as a calibration in practice. Just 4 years later they gave up with the water and a platinum cylinder was produced to define the kilogram. Platinum was chosen for its longevity and stability but suffered from being a bit on the fragile side. In 1879, we switched to a cylinder of a platinum-iridium alloy, and it was used all the way up until 2018 when it was time to move on.

A number of ideas were discussed that attempted to shift to universal constants rather than a single changing physical object. One of these ideas resulted in the generation of the roundest objects ever made. A sphere of silicon-28 atoms. The proposed definition was to be the mass of an exact number of silicon atoms.

They settled on relation to Planck’s constant – the smallest unit of distance possible. Frankly, the exact definition is kind of complicated and uses all sorts of interesting physics. 

From the battlefield of Eqalite to cylinders to spheres. The kilogram has had quite the life, and as one of the 7 SI standards, it still seems like it’s got plenty of life left in it yet. 

Colin Roitt, Almost Educational, URY

You can listen to Almost Educational on Wednesdays at 4pm at ury.org.uk

Sources and reading;

https://nvlpubs.nist.gov/nistpubs/jres/106/1/j61jab.pdf

https://www.vox.com/science-and-health/2019/5/17/18627757/kilogram-redefined-world-metrology-day-explained

https://en.wikipedia.org/wiki/SI_base_unit

https://www.nist.gov/si-redefinition/kilogram-silicon-spheres-and-international-avogadro-project

The new method relies on the principle of a Watt Balance;

https://en.wikipedia.org/wiki/Kibble_balance

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