17th November 2018

Weighty decision: Kilo to be redefined

The decision has been taken to redefine the kilo – and all SI units – in terms of constants


By JTS Staff
Journal of Trading Standards' in-house team
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[This is] about as excited as you're going to see metrologists get. I can’t believe we’re finally getting it done

The kilo is to have a new definition, following a decision made by researchers meeting at the recent 26th General Conference on Weights and Measure held in Versailles in November.

The current definition is based on ‘Le Grand K’, a metal cylinder cast in the late 1800s, and held in a vault outside Paris, which until now has been the prototype underpinning every measurement of mass made anywhere in the world. Instead the kilo will now be defined in terms of electrical current.

It’s part of a move by metrologists to redefine international units of measurements on scientific constants – such as the speed of light, the charge of an electron and the Planck constant – rather than prototypes, which no matter how carefully created and protected, can never be perfectly stable.

‘Le Grand K’ will be consigned to the same graveyard as the platinum-iridium bar which previously underpinned the metre before it was abandoned in 1983 – these days it’s defined in terms of the speed of light. The Ampere, Kelvin and Mole will also be redefined.

This kind of big change doesn’t happen very often in metrology – a field that is defined by its commitment to keeping things uniform.

David Newell of the National Institute of Standards and Technology was reported as saying the news would make measurement experts “about as excited as you’re going to see metrologists get” adding “I can’t believe we’re finally getting it done”

The change is unlikely to have practical ramifications for enforcement. It’s anticipated Le Grand K will prove to be about 50 micrograms per billion out – less than the weight of a single eyelash – but the shift from objects to constants is a profound change in perspective for measurement specialists everywhere.