New weight measure

When we buy a kilogram of groceries at the friendly neighbourhood store, we keep faith with the weights used. A finicky customer wondering whether the weight “really weighs 1 kg” would soon learn how every sort of weighing device is calibrated with standard weights distributed around the world. In turn, all standard 1 kg weights are defined by the ‘international prototype kilogram (IPK)’, a platinum-iridium alloy block locked in an underground vault in Paris. To ensure its mass remains constant, the IPK block is kept within three vacuum-sealed bell jars. However, this block too has been unstable — leaking atoms — all adding to a loss equal to an eyelash in weight. This may be insignificant when buying groceries, but not in laboratory calculations of masses. And so the international prototype kilogram will be consigned to history next week, just as the international prototype metre (also made of platinum-iridium alloy) had been in 1960. Since then, the meter has been calculated by the length of path travelled by light in vacuum during an exceedingly minute but precise time interval. The International Bureau of Weights and Measures (which regulates the SI or metric system) will henceforth calibrate the kilogram by a device called Kibble balance using Planck’s constant. Instead of comparing the weight of one mass against another, this balance weighs a mass against an electromagnetic force which can be measured with great accuracy. Along with the kilogram, 3 other basic units — the ampere, the Kelvin and the mole — will also be given updated calibrations to be effective from next year. The completed project will link each of the 7 basic units — second, metre, kilogram, ampere, Kelvin, mole and candela — to a fundamental constant of nature. Arbitrariness will end, accuracy will be paramount.