What is the kilogram?
Since 1889 a platinum-iridium cylinder was made, which was to serve as a global standard for weighing: the kilogram.
Now, scientists have voted to change the way a kilogram is defined - from Le Grand K (as it is known), to a measure based on electric currents.
The kilogram was stored in a strong room in Sevres, near Paris, France. Every forty years it is withdrawn - but only so it can be compared to replica kilos, situated in every country which has adopted the metric system.
Why the kilogram needed to change
In 2018, the kilogram was the only standard measurement unit, known as a SI, to be based on a physical artefact. However, the master kilogram and its copies have been seen to change - ever so slightly over the years - as they deteriorated.
Air pollution is set to be the major threat to the current kilogram - despite it being stored in filtered laboratory air at constant temperature.
Accurate weight measurement is critical in many areas, from drug development to nanotechnology - so a more robust definition was needed.
Dr Stuart Davidson, head of mass metrology at NPL said “We know from comparing the kilogram in Paris with all the copies of the kilogram that are all around the world and we know that there are discrepancies between them and Le Grand K itself.”
“This is not acceptable from a scientific point of view. So even though Le Grand K is fit for purpose at the moment, it won’t be in 100 years’ time.”
The new kilogram
It has taken scientists time to come up with a solution - as no one has been able to measure measure Planck’s constant precisely enough to improve on the sacred platinum-indium.
This month all is changing as multiple teams submit their results for computer analysis.
The new kilogram will use a measurement based on electric currents to define it.
The vote was cast at the General Conference on Weights and Measures. Representatives from 60 countries gathered in Versailles, France to witness it.
“The vote is unanimous; I hope that such votes will be possible for many other issues in the world,” said Professor Sebastien Candel, president of the French Academy of Sciences.
The new process works as follows. Electromagnets generate a force. The pull of the electromagnet, the force it exerts, is directly related to the amount of electrical current going through its coils. This shows there is a direct relationship between electricity and weight. Scientists therefore can define a kilogram in terms of the amount of electricity needed to counteract the weight.
Planck’s constant relates weight to electrical current, denoted by the symbol h. To measure h you need a super-accurate set of scales, known as the Kibble balance. An electromagnet pulls down on one side of the scales, and a weight is on the other. The electrical current is increased through the electromagnet until it is perfectly balanced with the weight.
“The SI redefinition is a landmark moment in scientific measurement,” said Dr Jan-Theodoor Janssen, director of research at the UK’s National Physical Laboratory.
“This will pave the way for far more accurate measurements and lays a more stable foundation for science.”
For a full history of the kilogram as a unit of measure, click here to read the story from the National Institute of Standards and Technology.