## The Last Artifact

Don’t pack away your Royal Wedding party gear just yet! Today (Sunday 20th May) is World Metrology Day 2018!

And that means there are just 5 months and 26 days until the commencement of the 26th General Conference on Weights and Measures (CGPM).

At this governmental level gathering, it will hopefully be decided to go ahead with the redefinition of four of the base units of the International System of Units, the SI.

And if matters proceed as planned, in one year’s time – World Metrology Day 2019 – we will finally make the change.

All the unit redefinitions – of the kilogram, the ampere, the kelvin and the mole – are important.

But the redefinition of the kilogram has been the hardest and is considered an event of such significance that someone is making a high-end film about it.

I was fortunate enough to meet the co-director Ed Watkins and his crew when they swung by NPL last year to film.

The film will be released on World Metrology Day 2019, but the trailer (below) certainly looks intriguing.

How mass measurement will change.

At the moment, when we weigh something we:

• compare the force of gravity on that object with the force of gravity on a standard object.
• and the force of gravity on that standard object is known by comparison against the force of gravity on a more special standard object
• Add so we proceed in many steps until eventually, we encounter a weighing against the International Prototype of the Kilogram (the IPK). This single unique ‘artifact’ currently defines what we mean by ‘one kilogram’.

This kind of repeated comparison against standards until we reach a defining artefact is completely normal in traditional metrology.

In future, when we weigh something we will:

• compare the force of gravity on that object with the force of gravity on a standard object.
• and the force of gravity on that standard object is known by comparison against the force of gravity on a more special standard object
• Add so we proceed in many steps until eventually, we encounter a weighing on a Kibble Balance or a weighing against a specially-made silicon sphere.

It is these two new options that represent the change.

• When we weigh an object on a Kibble Balance, we compare the gravitational force on an object with an electromagnetic force which can be calculated in terms of volts and amperes and related to fundamental physical constants.
• Alternatively, the special silicon spheres have their mass calculated in terms of their physical properties: size, density etc.

In either case, the final definition of what we mean by one kilogram is determined by the basic physical measurements, and is no longer simply a comparison against an arbitrary physical artifact.

That’s it. It’s a small change, but as I am sure the film will make clear, a profound one.

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