‘Weightless’ makeover for the kilogram

The physical kilogram standard should not depend on climatic conditions. This core idea was mentioned even in the Arthasastra

Published: 12th July 2019 04:00 AM  |   Last Updated: 12th July 2019 01:55 AM   |  A+A-

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In 2013, an unusual case came up before the Karnataka High Court. The Controller of Legal Metrology had slapped a fine of `10,000 on Cadbury India Limited for using the Kannada word ‘angula’ (meaning ‘an inch’) in a humourous television advertisement for promoting their popular chocolate brand. The case was that ‘inch’ is a non-­metric standard for length measurements and its usage is prohibited by the provisions of Legal Metrology Act (2009). The chocolate maker’s submission before the court was to quash the fine. Though this case might sound amusing and even frivolous to most people, it also highlights how enforcement of uniform measurement standards and nomenclature is crucial to ensure fairness in trade and commercial transactions. 

When India joined the rest of the world in adopting the new definition of kilogram on May 20 this year, it was entirely drowned in the backdrop of the general elections. For nearly 130 years, the standard kilogram prototype was a block of metal maintained in a climate­-controlled vault in Paris. Governments across the world, through their legal metrology departments, ensure that the weighing devices in the markets are calibrated against the weight of the block kept in Paris or its replicas.

For consumers and traders, this guarantees that one kilogram of any item traded in Chennai or Cherrapunji or online markets weighs exactly the same. This iconic block was finally retired and replaced by a new standard that defines a kilogram in terms of fundamental constants of nature such as the speed of light and Planck’s constant. Collectively, these unchanging parameters characterise the fabric of our universe as we know it today. 

It might seem that a physical block provides a better sense of a kilogram than abstruse constant parameters that do not seem to exist anywhere. But the block kept in Paris had its share of problems. Its weight was changing over time. As a scientific principle it was not a great idea. Ideally, the physical kilogram standard should not depend on climatic conditions or place, in our neighbourhood or even in another galaxy. This core idea can be traced back to almost antiquity.

About 2,200 years ago, Kautilya recognised the importance of measurement standards for trade and commerce and wrote in Arthasastra that “weights shall be made of iron or of stones .... or of such things as will neither contract when wetted, nor expand under the influence of heat”. By the 1500s, lakhs of local standards for weights existed across Asia and Europe. This was both a source of corruption and confusion. Even the Magna Carta, principally a charter of political rights granted by the British monarch in 1215, stated that “there shall be standard measures .... throughout the Kingdom. Weights are to be standardised similarly”. 

It was in 1795, as the French revolution was still underway, that the kilogram was defined as part of the new metric system in France based on a scientifically reproducible technique for the first time; in terms of weight of a litre of pure water at 4ºC under standard atmospheric pressure. Within a few years, as practical problems with this definition mounted, it was replaced by a physical prototype made of Platinum.

The prototype in Paris, made of Platinum-Iridium alloy, that was rested two months ago had its origins in the Treaty of the Metre signed in 1875 by several countries including the US. Since then, many copies of this prototype were made and distributed across the world. India received a replica in 1958, which is kept at the National Physical Laboratory in New Delhi and serves as our official standard for a kilogram. 

Over the decades, the weights of the international prototype at Paris and its copies throughout the world did not agree with one another. Weight differences of about 50 micrograms was noticed in the last audit done in 1989. Originally, the metre, as a standard for length, was also defined using a prototype metallic rod. However, by the 1980s as speed of light was measured with sufficient accuracy, the metre rod was replaced by a definition in terms of speed of light, one of the fundamental physical constants. This gave further impetus to standardise kilogram along similar lines. 

The biggest breakthrough came with the discovery in 1980 that under certain conditions of low temperature and strong magnetic fields, passage of electrons through effectively two­-dimensional conductors is related to the Planck’s constant and charge of an electron. Quantum Hall effect, as it is called, opened the doors for defining a kilogram in terms of these constants which are unaffected by factors such as environmental conditions and, remarkably, are valid anywhere in the universe. The practical devices such as Kibble balance will soon make the “exact” kilogram available for calibrating other replicas in the laboratories. 

A pioneer of quantum theory, Max Planck believed that if the basic units for mass, length and time are defined in terms of fundamental physical constants, then they will “necessarily retain their significance for all cultures, even unearthly and nonhuman ones”. With the kilogram’s makeover, this cherished goal has been achieved.


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