Tremendous Weight

India’s most important kilogram

01 February 2015
No 57, India’s standard kilogram, arrived in the country in 1958
Courtesy NPL
No 57, India’s standard kilogram, arrived in the country in 1958
Courtesy NPL

IN HIS HIGH-CEILINGED OFFICE, dressed in a brown corduroy coat, RC Budhani held a pencil up with a thumb and a finger at each end. Behind him hung a photograph of Jawaharlal Nehru, without his characteristic topi,meeting Albert Einstein at Princeton University. A window opened to a garden under a December sun. “Because of the force, the coil will be pulled downwards, like this here,” Budhani said, tilting the pencil one way. “So now you counter with a weight there.” He brought it back level.

Budhani, the director of the National Physical Laboratory, was explaining the workings of a watt balance, an instrument that measures the weight of an object against the force exerted by a coil in a magnetic field. From behind the pink facade of its main building in central Delhi, the NPL standardises measures of the seven base units of the SI system—the metre, kilogram, second, ampere, kelvin, candela and mole—for all of India. Of these, the kilogram is the last still defined by a physical artefact. The metre, for instance, was defined by a metal rod of a set length until 1960, but is now held to be the distance light travels over an exact period.

The original kilogram, adopted as such in 1889, is a cylinder of an alloy of platinum and iridium housed in a vault in Sevres, outside Paris. The NPL is entrusted with the care of India’s copy of it—protoype No 57, which arrived in the country in 1958. But as technology advances and watt balances become increasingly precise, they threaten to depose the protoype by defining the standard unit of mass in terms of the Planck constant.

Virat Markandeya Virat Markandeya is a freelance journalist based in Delhi, who covers science and technology. He has written for IEEE Spectrum and the Nature Publishing Group among others.

Keywords: watt balance National Physical Laboratory India’s standard kilogram Bureau International des Poids et Mesures
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