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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…
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…
nist.gov/si-redefinitio…
It's #WorldMetrologyDay, and this is an important one: Today a revamped International System of Units (SI) goes into effect. It's the most radical set of changes to the SI since it was established in 1960 doi.org/10.1063/PT.6.2…
A lot of attention has focused on the fact that a lump of metal in Paris will no longer be used to define the kilogram. That's a big deal, because it means that there are no longer any SI units that depend on a physical artifact.
But that's just part of what makes this change so significant: Beginning today, every SI unit—not only the kilogram but also the volt, the newton, the weber, and more—will be derived by some combination of seven invariants of nature. 
Happy World Metrology Day! Today, 7 fundamental constants of nature are redefined as exact numerical values, in turn redefining base units such as the metre and the kilogram. Find out more about this reformation in Bill Philips' #MeasureForMeasure column | go.nature.com/endofartefacts
Metrology is a fascinating, and hugely important, discipline - one that underpins our fundamental understanding of science as much as the practical foundations of all international commerce throughout human history. It is worth celebrating in full: go.nature.com/2HqOs5P
You can read more about the re-defined base units in our metrology column, Measure for Measure - it is free to read and includes pieces on all the redefined base units. (1) Helen Margolis on the second go.nature.com/2BJiQ5E
