**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.

**It’s all about the kilogram**

**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.