You may have seen something in the press about the kilogram no longer being defined in terms of the platinum-iridium International Prototype kept in a vault in France.

It is over 125 years old and discrepancies have developed between it and several others made at the same time. But the change is much deeper than just getting a new prototype. The kg was the last basic unit in the SI system that was not defined in terms of a physical constant. The second is defined in terms of a cesium atom oscillation, the metre in terms of the second and the speed of light, and so on. Now, by fixing the value of Planck's Constant and using a Kibble balance, objects can be weighed in terms of an electric current.

Most countries in the world, apart from the USA, now adhere to the SI system. Another exception is Myanmar where I travelled recently. They use a mix of units. Miles serve for distances on roads where vehicles drive on the right but most are right hand drive second hand from Japan. Weights may be in kg or pounds or viss (1 viss = 1.63 kg approx). To someone interested in measurement and units, sighting such an endangered species of unit is exciting. Back in the day, the King would own prototype units, usually bronze castings in the shape of mythical beasts, and merchants would source their own and have them certified by comparison with the standard. The British brought with them more prosaic cylindrical weights, but the old weights are very decorative and collectible, generally going under the name of 'opium weights'.  I could not resist collecting a few, some of which might even be genuine. In fact, Myanmar has announced that it will embrace the SI system soon, although whether it will root out the old units as ruthlessly as the Australian government did back when we 'went metric' remains to be seen.

I suppose the International Prototype Kilogram is now equally superfluous, but I expect I would have to join a long queue to get my hands on it. The platinum must be worth over $25000. At nearly $1500 per troy ounce (another unit in my crosshairs!)  iridium is more expensive than platinum. The National Measurement Laboratory in Sydney holds copy Number 44, which could come in handy next time the government cuts their funding.