Thread by @ChemistryWorld

In celebration of #WorldMetrologyDay, who wants to know 10 interesting facts about SI units?

1. A ball made of ultra-pure silicon – created to measure Avogadro’s constant for the mole’s new definition – is likely to be the most perfectly round object in existence.

chemistryworld.com/news/purest-si…

2. The Planck-constant-defining Kibble balance is an instrument big enough to take up an entire room. But if you still want one in your living room – there’s a Lego version:

nist.gov/si-redefinitio…

3. The most accurate measurement of the Planck constant (for defining the kilogram) has an uncertainty of just 9.2 parts per billion

4. 102 countries are members or associates of the metre convention treaty. The latest to join were Ukraine, Montenegro, Uzbekistan, Tanzania, Kuwait and Ethiopia

5. In early 2018, Yemen was thrown out of the metre convention club, joining the Dominican Republic, North Korea and Cameroon. The countries either failed to show up at meetings for a number of years or didn’t pay their fees 🇾🇪

6. Although the Big K has been carefully guarded (only a few select people were ever allowed to touch it, and only with gloves), the reason its mass has changed is likely due to air pollutants, mainly mercury and carbon

7. The Big K has six sister copies stored in the same vault. Another ten working copies are stored in a calibration lab for regular use. There are also 70 national prototypes

8. In one of the most unintuitive experiments, scientists have measured the Boltzmann constant (for the Kelvin’s definition) by measuring how sound resonates inside a box filled with helium gas

9. In practice, pretty much no one will notice today's change. Unlike a physical object – the Big K – natural constants can’t break and are, as far as we know, the most stable thing in the universe

And finally, 10. The next unit that will likely be redefined is the second, in 2026. For this, researchers are working on atomic clocks that are a thousand times more accurate than the caesium clocks we have now