# The kilogram

The kilogram (kg) defines mass (papatipu).

Until 20 May 2019, the kilogram was defined by a physical artefact – a cylinder of platinum-iridium, known as the international prototype kilogram (IPK) – but over time, there was a mass divergence between the IPK and its official copies. The international metrology community agreed that a new definition was needed – one based on a natural constant that wouldn’t change over time.

Now, the kilogram is defined in terms of Planck’s constant (h) – a term that links the amount of energy a photon carries with the frequency of its electromagnetic wave. The value of h has been fixed to equal 6.626 070 15 x 10–34 kg m2 s–1. The metre (m) and the second (s) are already defined in terms of fundamental constants – the speed of light and the transition frequency of a caesium atom, respectively. This link allows metrologists to accurately express the kilogram in terms of h.

Discussion point: Farzana says that the IPK is drifting. What does this mean?

NOTE: This video was filmed prior to the change to SI definitions. As of 20 May 2019, the kilogram is now defined by Planck’s constant rather than the IPK.

## Transcript

FARZANA MASOULEH

Right now, 1 kilogram is the mass of the international prototype kilogram (IPK), which is kept in a vault in Paris. Now it is very unlikely that someone is going to run off with the kilogram because it is kept in the vault in Paris, but measurement historically and currently show that this international prototype kilogram is drifting in mass and scientists really don’t know why that is happening. So given that our kilogram measurement relies on an artefact, it’s better to find another physical constant, which enables us to measure the kilogram independently.

So this is the definition of kilogram right now, which is the IPK and we know exactly its mass and that’s Planck’s constant [h = 6.626 070 15 x 10-34 kg m2 s-1], which is the physical constant we are going to use to define the kilogram after 20th May [2019].

Planck’s constant is a very familiar constant for physicists, and it comes a lot in quantum physics especially with the advancement of quantum physics in the last hundred years. I just want you to look at the units that Planck’s constant has - it’s kilogram, metre squared, divided by the second. So if we know exactly how much the Planck’s constant is and if we know the metre answer and the second based on physical constants, then we are able to exactly make sure how much the kilogram is. So on 20th May, we will no longer know what is the mass of the IPK, but we will surely know how much the Planck’s constant is and that will help us to be able to judge the kilogram much accurately.

But New Zealanders still will keep the mass standards for mass for New Zealand so that, for example, trades are easy because we want to make sure that other countries rely on our measurement, that for example 1 kilogram of dried milk powder is still 1 kilogram of dried milk powder all over the world.

### Acknowledgements

This video clip is from a recording of a presentation by the Measurement Standards Laboratory of New Zealand (MSL) in celebration of the redefinition of the International System of Units (SI), which happened on 20 May 2019. The presentation by Peter Saunders and Farzana Masouleh of MSL was filmed at Unleash Space, Faculty of Engineering, Auckland University.

Filming and editing by Jonathon Potton of Chillbox Creative. MSL produced these videos to share the story of metrology development.

**Published:**15 August 2019