Tremendous Weight

India’s most important kilogram

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

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.

That would allow a more universal and stable definition of the kilogram. Though the main protoype is isolated and protected, it still interacts with its physical surroundings, and over time its weight has varied by an average of fifty micrograms—fifty millionths of a gram—compared to the weights of its official copies. It could also be destroyed or damaged. The Bureau International des Poids et Mesures, or BIPM, the body that defines SI units, considered alternative methods of definition at its twenty-fifth Congress on Weights and Measures last November, but decided against a switch for now. (Another proposed method does away with watt balances, and relies on X-ray crystal density and the Avogadro constant).

But though watt balance experiments are promising, they are notoriously difficult. “You have to measure six or seven fundamental quantities, and you have to measure them all precisely,” Stephan Schlamminger, a physicist working on a cutting-edge watt balance at the US National Institute of Standards and Technology, told me over Skype. Scientists are still striving to achieve the required exactness. “What is the point in having a standard which is worse than your artefact?” Budhani said.

Like an exquisite dish, the Indian artefact is stored under two glass bell jars to prevent contamination. Access is jealously guarded, and I was not allowed to see it. The NPL director keeps the key to a vault where No 57 is housed. Budhani clammed up when asked if the vault was in the NPL building. “I cannot tell you all that,” he said. “This is a confidential matter.”

To maintain the kilogram standard in the face of inevitable fluctuations in the masses of the artefacts, all official prototypes must be regularly calibrated against the original. No 57 was last in Sevres, for this purpose, in 2012. The trip required special government permission, customs clearances, and an appointment with the BIPM.

Budhani described a “traceability chain.” As No 57 traces back to the original prototype, so all official Indian measures of weight trace back to No 57. To not constantly disturb the protoype, the NPL issues “transfer standards” calibrated against it. These are then used in such work as determining the exact masses of payloads for the Indian Space Research Organisation—vital for calculating satellite orbits, fuel needs for rocket launches, and much else.

Schlamminger told me there’s a good chance the prototype will be supplanted by 2018. “I find it philosophically more satisfying that you have a definition that relies on the fundamental constants of nature,” he said. He posed a hypothetical: we contact a civilisation on another planet, and want to talk about physics. “The first thing you want to do is establish a set of units.” But using our current method, “how do you tell them what our kilogram is? You can’t, because they cannot come to Paris and pick up the kilogram, right?”