National_prototype_kilogram_K20_replica

Have you, like me, made a new year’s resolution to exercise more, eat more healthily and lose some weight? There could be an easier solution. A few years ago, the railways eased themselves towards their target for running more trains on-time by redefining ‘on-time’. Could we redefine, say, 73 kg so it includes a few extra mince pies?

The Kilogram is the only SI unit still defined by an object rather than a physical property. The definitive arbiter of mass is a British-made cylinder of platinum-iridium alloy that sits beneath three antique bell jars in a vault at the Bureau International des Poids et Mesures, just outside Paris. Fear of damage means it is rarely taken out – 1889, 1946, 1989 – so 40 replicas were created and spread around the world to do the job of practical mass definition.

The importance of knowing what a kilogram is can’t be over stated. If it changes, billions of measurements taken around the world are theoretically wrong, from the critical diameter of the holes in a fuel injector to the weight on the scales after Christmas. So imagine the alarm when it was discovered that the masses of the cylinders are slowly changing.

Now the definitive kilogram appears to weigh slightly less than the copies, possibly due to the copies having greater exposure to atmospheric hydrocarbons, allowing surface contamination to build-up more rapidly. It’s only a very tiny amount, but it does mean that our British kilogram, housed at the National Physical Laboratory, is a little heavier than it used to be.

This means that if my mass remained theoretically constant, and was measured against the British kilogram 100 years ago (I’m older than I look) and again today, the latest number on the dial would be smaller. And of course mince pies will appear to weigh less too. So based on sound scientific evidence, there is definitely room for one more before the diet begins… tomorrow.