France, May 25 -- France - and the majority of the rest of the world - is marking 150 years since the Metre Convention first united them in a shared language of measurement, laying the foundations for international scientific cooperation.

There aren't many 136-year-old metal cylinders tucked away in Paris basements that can claim global fame.

Yet "Prototype 35"- a shimmering iridium-platinum artefact- quietly changed the course of modern life.

At just 39 millimetres high and wide, this unassuming 1 kilogram weight helped anchor the world's understanding of mass-and with it, the uniformity of measurement that underpins everything from baking a cake to building a bridge.

This week marked the 150th anniversary of the Metre Convention, si...