Dr Peter Saunders notes that, from 1879–2019, the kilogram and the metre were defined by physical objects known as international prototypes.
Discussion point: Over time, the IPK has changed and is a tiny bit different from the replicas that were distributed in 1875 – but only by 50 micrograms. Does this matter?
Shortly after the Metric Treaty was signed in 1875, they decided to make some new artefact masses and length standards. Instead of being based on platinum, these are based on platinum-iridium – so it was an alloy – and the iridium makes the metal a lot more stable and less porous. So they made 30 of the length bars and 40 of the standard masses, and they picked one of each at random to become the international prototype kilogram and the international prototype metre, and those were deposited into a vault in the basement of the BIPM – in fact, they’re still there today. So the remaining standards that they made, so the other 30 and the other 40 were distributed amongst the member states of the Metric Treaty.
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.