[chris (OP)]

What we call a kilo corresponds to a lump of metal in a case in Paris; other countries have their local clones of the hallowed "gold standard" too. But scientists are changing this.

How, and why?

[Janus]

What we call a kilo corresponds to a lump of metal in a case in Paris; other countries have their local clones of the hallowed "gold standard" too. But scientists are changing this.

How, and why?

The why is because every time you need to use your standard lump of metal to make a comparison, you have to handle it. And no matter how carefully you do so, you risk altering it.   Other effects can also creep in. ( for example some of those clones now weigh less than the original. So did they lose mass or did the original gain it? How would you tell if your standard has changed.

The how is by using a Kibble balance which measures the mass based on fundamental constants which do not change.  No longer would you have to physically compare your text mass to the one in Paris or to one of its clones. You just need a Kibble balance.
The Kg is just the last of the basic units to undergo this redefinition based on fundamental constants rather than some physical reference object.

[evan_au]

This 2012 article gives a good overview of the reasons they are changing to a non-physical definition of the kilogram. It outlines the Harry Potteresque traditions around accessing the official kilogram.
https://spectrum.ieee.org/consumer-electronics/standards/the-kilogram-reinvented

In 2012, there was a contest between the two leading contenders for the new kilogram standard:
- The Kibble balance, using electrical current to generate a magnetic force,
- and ultrapure silicon spheres where you would count the atoms.
- The international standards committee is very conservative, so it has taken this long to agree to the change

As we know, the Kibble balance won the contest. The name of this British scientist was applied to the Watt Balance in 2016.

And now we know Avogadro's number to much better precision, thanks to the runner-up in this contest.

Soon, the official platinum-iridium kilogram can make its final journey from the safe to the museum, adjacent to the official platinum iridium meter, which was obsoleted in 1960.

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

[alancalverd]

The real reason is to steal the kilogram from the untrustworthy French and have it defined by a British invention, along with the cesium clock.

[Bored chemist]

The real reason is to steal the kilogram from the untrustworthy French

The French don't have it.
BIPM does.

[alancalverd]

BIPM is situated in the Parc de Saint-Cloud, at Sèvres, in the suburbs south-west of Paris.
Postal address: Pavillon de Breteuil, F-92312 Sèvres Cedex

Which was in France the last time I looked.

[Bored chemist]

BIPM is situated in the Parc de Saint-Cloud, at Sèvres, in the suburbs south-west of Paris.
Postal address: Pavillon de Breteuil, F-92312 Sèvres Cedex

Which was in France the last time I looked.

The british embassy in Paris is in France, but it isn't French.
BIPM enjoys a similar status.

I didn't say it wasn't in France, I said the French didn't have it.
BIPM is not France and BIPM is not French.

[chris (OP)]

I don't completely understand the basis of the Kibble balance. Various reports I've read go on about the Planck constant, but I am struggling to understand how all this ties together - can someone explain for me?

[Bored chemist]

I can't explain it nearly as well as this does.
https://en.wikipedia.org/wiki/Kibble_balance
But, if you want the minimalist version...
You set up a coil hanging from a balance in a magnetic field and let it fall under gravity. It generates a voltage and you measure that. From that you get a very accurate measurement of the (effective) number of turns on the coil, how strong the field is and what the local value of g is.
Then you put the "test mass" on the coil, and see how much current you need to put through the coil to hold up the test mass.
That current depends on the things you measured in the first experiment, and the mass of the test mass.
So you can calculate the mass in terms of a current and a voltage and a few other fundamental constants.

(It's more complicated than that- see the wiki page for a better explanation.

[chris (OP)]

Thanks @Bored chemist - I'd got as far as working out how the electromagnetic definition of the kilo would work, but it was references to the Planck constant that I didn't understand; I now see that it's also a way to measure the Planck constant; I don't know enough physics to appreciate why this matters or what the relationships are, but at least I now follow why reference was made to it.

[evan_au]

Various reports I've read go on about the Planck constant, but I am struggling to understand how all this ties together

The Kibble balance depends on the product of voltage and current, which is measured in Watts.

Voltage can be measured very accurately by means of a Josephson junction, which is a quantum effect where a known voltage is produced when the junction is irradiated by a microwave signal of known frequency. Frequency and Time are the fundamental constants we can measure with the most accuracy - the accuracy of some experimental clocks is quoted as seconds in the age of the universe.

Current (and resistance) can be measured very accurately by means of the quantum Hall effect. This is produced by electrons in a magnetic field. By selecting a suitable material, the current is almost independent of the applied magnetic field, so the current can be measured very accurately.

Both the voltage in the Josephson effect, and the current in the quantum Hall effect are measured in terms of Planck's constant. So if future measurements are able to measure the Planck constant with greater accuracy, the definition of the kilogram will get more accurate.

The new kilogram definition is based on the assumption that the Planck constant is really constant (just like our definition of the meter assumes that the speed of light is really constant).