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Hi Eli! I think there are lots of great resources on national Metrology Institute (NMI) websites (eg @NPL,? @NIST) and YouTube channels and of course @BipmMedia. But I’ll have a go at explaining it in a thread from my perspective... https://twitter.com/ethonraptor/status/1355648657312395266
To understand the SI, you need to understand start with the French Revolution and how they wanted to rationalise measurements for scientific purity and practically to take power from local Lords who controlled local units. They wanted a system that belonged to the people.
(Aside)Britain already had only one set of units from the Magna Carta, so the drive wasn’t as strong (they were also scared of revolution). The fledgling USA argued with the French about whether the metre should be based on the Meridian through Paris or a pendulum in Washington.
So the French wanted a metre based on the Earth and sent Mechain and Delambre off to measure the Meridian, from Dunkerque to Barcelona through Paris. They would also measure the height of stars in those locations to extrapolate the distance from the North Pole to the equator.
The metre would be 1 ten millionth of that distance and therefore belong to the people. The kg would be the mass of 10cmx10cmx10cm of water and so on ... the story of that original metre is an adventure. See “The measure of all things” by Ken Alder.
One of the scientists went mad because he didn’t understand the difference between systematic and random effects in uncertainty analysis. It’s a cautionary tale! And one I read with great understanding during problems with my international key comparison.
I can still quote from memory “the heroism that [he] had to display was different - the ability to work for countless days and nights on a repetitive task while working towards an ever receding goal”. I certainly recommend getting this story in!
Anyway, eventually they defined their metre - and made a bar of metal that represented their best estimate of one ten millionth of the distance from the NorthPole to the Equator through Paris. That - the Metre of the Archive - was the beginning of metrology.
The really key point is that it belonged to the people. When the British Imperial yard burnt in the House of Commons fire, they had to reestablish the yard from copies. In theory, had the metre burnt, they could have remeasured the Earth. That was the point.
The history of the SI is a history of definitions that are stable over centuries and identical worldwide and coherent - you can combine units by multiplying by one and an electrical watt is the same as a thermal watt is the same as a mechanical watt is the same as an optical watt
(Aside) that’s why the candela is still a base unit - you have to multiply watts by 1/683 not 1.
The stability, consistency and coherence is created by a robust definition and by traceability (links back to those definitions). That traceability relies on uncertainty analysis, auditing and comparisons. And in Metrology there are formal approaches for those.
(Aside): my research is in bringing traceability through uncertainty analysis, “auditing” (less formal) and comparisons to satellite Earth Observation. My best paper on the subject is here: https://doi.org/10.1088/1681-7575/ab1705
The definition has to “belong to the people” (eg dimension of Earth not local Lord’s favourite bar) and it has to be consistent with the previous definition (no step changes). The metre started practically as the length of the bar - but linked to the Earth.
When lasers came, they realised they had a better method - so they redefined the metre to be a certain number of wavelengths of a particular laser. That was more robust than comparing bars. And it belonged to the people - anyone could realise the metre.
But then lasers got better. And they needed to change the definition as it was a limiting uncertainty. They debated which laser should replace the old one. But then someone made the inspired suggestion that instead of defining a certain number of wavelengths of any laser...
... they could define the speed of light and they would never have to redefine the metre again. That was a huge step change and one that really needs deep understanding. You no longer measure the speed of light, you realise the metre.
The speed of light becomes a scaling constant from the fundamental physical world (“God’s units”? - interpret metaphorically if desired) to human units - a length convenient to us as about the length of an arm.
It took a long time for almost all of the other units to catch up (the second still has a definition based on one atom). And that’s because metrology works slowly and exceedingly cautiously. It’s so important that we have international consistency and permanent stability.
The SI units are now all (but two - s and cd) defined relative to fundamental physical constants. The Boltzmann constant scales from vibrational energy to our human temperature scales, the electron charge scales from electrons to our human amps and volts, etc.
This is the important message. The details of how a Kibble balance works are interesting, but less fundamental to the reason why it was developed.
And the candela - it is how bright a lightbulb looks to the “standard human eye” at almost 555 nm wavelength (a slightly turquoisy green and the peak of the human eye response curve). The constant isn’t “fundamental” but it is a scaling from watts to human units even more deeply!
It’s mostly there because the NMIs were established in the 1890s and electrical light was the big revolution of the time. We’re now thinking about “the digital SI” and bringing traceability to “big data” and data science for similar reasons!
