> a cylinder of a platinum alloy stored at the BIPM in France, will now be retired. It will be replaced by the Planck constant – the fundamental constant of quantum physics.
As I understand it the problem was not so much to find a mathematical definition of the kg based on other units but rather to find a definition suitable for experiments. If you can't practically use the definition to reproduce the prototype then it's not a very good definition indeed.
It seems that they settled on this definition because a Kibble balance[1] has shown to be precise and practical enough:
> Accuracy criteria were agreed upon in 2013 by the General Conference on Weights and Measures (CGPM) for replacing the current definition of the kilogram [...] with one based on the use of a Kibble balance. These criteria have since been met, and the definition of the kilogram and several other units will change on May 20, 2019, World Metrology Day, which celebrates the establishment of the SI, or metric system, in 1875, following the final vote by the CGPM on November 16, 2018.
I recently ordered a picture frame from Amazon. It came in its packaging box, which was placed in a box by the manufacturer. That box was put in another box by Amazon and shrink wrapped to a large piece of cardboard, which was then put inside yet another box.
On the surface, it seems crazy, but Amazon did manage to get it to me undamaged.
My wife recently ordered five storage boxes. They arrived in two packages on the same day. One package neatly fit four of the boxes. The other package was large enough for five, and contained one storage box and a excessive amount of packing material. Not an Amazon purchase, so it must be an industry issue.
I have already sent an email to the Norwegian bureau of standards to inquire about buying one of their secondary kilogram standards - presumably they'll want a couple for displays and museums, but it would make for a great letter weight. (Just hoping the secondary ones are NOT also made from platinum. That is a tad too extravagant, even for a measurement nut.)
I do happen to have the previous generation time standard in my garage, though, a HP 5061A Cs clock.
Also the tertiary ones? (I was -poorly- trying to indicate I was after a secondary /national/ standard - the ones calibrated against the national reference which is in turn calibrated against the Paris one.
Considering they have measurement history on that particular object going back over a century, I guess they will continue to regularly measure it to see what happens.
Last I heard, the original Big K, along with (some of) the secondary copies, will not be sold. Instead, they will be observed/studied in an on-going basis, in order to better understand why their masses appear to have drifted apart over the years (which is partially the reason for this re-definition).
I thought they did that long time ago.