The Future of the Kilo: a Weighty Matter

A lump of metal in a building near Paris has long served as the global standard for the kilogram. That's about to change. From a report: Later this month, at the international General Conference on Weights and Measures, to be held in France, delegates are expected to vote to get rid of this single physical specimen and instead plump to use a fundamental measurement -- to be defined in terms of an electric current -- in order to define the mass of an object. The king of kilograms is about to be dethroned. And crucially much of the key work that has led to the toppling of the Paris kilogram has been carried out at the National Physical Laboratory where the late Bryan Kibble invented the basic concepts of the device that will replace that ingot in the Pavillon de Breteuil. The Kibble balance works by measuring the electric current that is required to produce an electromagnetic force equal to the gravitational force acting on a mass. A second stage allows the electromagnetic force to be determined in terms of a fundamental constant known as the Planck constant which will, in future, be used to define a kilogram. These machines will provide the standard for weighing objects -- and that means no more dusting of old lumps of alloy to ensure they stay pure and accurate. 'One key reason for doing this work is to provide international security,' says Robinson. 'If the Pavillon de Breteuil burned down tomorrow and the kilogram in its vaults melted, we would have no reference left for the world's metric weights system. There would be chaos. The current definition of the kilogram is the weight of that cylinder in Paris, after all.' Another major motivation for the replacement of le grand K is the need to be able to carry out increasingly more and more precise measurements. 'Pharmaceutical companies will soon be wanting to use ingredients that will have to be measured in terms of a few millionths or even billionths of a gram,' says Prior. 'We need to be prepared to weigh substances with that kind of accuracy.' Suggested reading: A thread on Twitter which discusses SI units and the redefinition of the kilogram.

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