The end of the kilogram

Published: 20 November, 2018 07:30 IST | C Y Gopinath | Mumbai

It started with the crooked vegetable vendors of Matunga. It ended at a startling little conference in Versailles this week

Out with the Internal Prototype Kilogram and in with the Kibble Balance
Out with the Internal Prototype Kilogram and in with the Kibble Balance

C Y GopinathCertain vegetables are simply unavailable in Bangkok, so I buy them in Mumbai whenever I pass through. Specifically, these are arbi, zamikand or suran, tondli, methi, tinda and long karelas. My market of choice is crowded east Matunga, where all the south Indian groceries are, so I can pick up fresh, home-made Coimbatore ghee and sun-dried salted lemons as well.

My vendor of choice is Lakshmi Prasad from district Rae Bareli, UP. Like nearly every other vendor in Matunga, Lakshmi Prasad uses the rusty old beam balance called tarazu, the kind that has two pans and has to be held aloft manually. For counterweights, he uses cast-iron blocks with masses of a kilogram, and 500, 250 and 100 grams. If your vegetables don't equal some combination of those weights, he'll throw in some pebbles and wing it. Of course, he'll also subtly nudge the pans as he holds them to skew the weight and shortchange you. Summary: you'll pay more for less.

Out with the Internal Prototype Kilogram
Out with the Internal Prototype Kilogram

No one seems to notice or mind that in this exact and exacting digital world, a vendor is selling you vegetables whose precise weight he cannot measure. And thus are we diddled every morning. Lakshmi Prasad tells you it's a kilogram and you take him at his word.

But you know me - a walking devil's workshop. I began to wonder about kilograms. How do we know that something that is supposed to weigh a kilo is exactly a kilo? By measuring it against some other thing that weighed a kilo, presumably. But who weighed that other thing? And where does the buck stop? Where in the world is the kilogram to end all kilograms?

In a securely locked underground vault in Paris, there sits a block made of a platinum-iridium alloy, weighing exactly one kilogram. Called the International Prototype Kilogram (IPK), this mother of all kilograms has defined how much a kilogram should weigh since 1889. The French call it Le Grand K. Copies of the IPK are distributed throughout the world and used by countries to create their own reference weights as close as possible to the original. These become the benchmarks for every other weighing device in society, from cooking scales and weighing scales to lab instruments.

The original measure of length, the foot, came from pied a roi, or the King's foot, supposedly the foundation of all lengths. But the metric system wiped the slate clean. The metre had already been pegged as a fraction of the distance between the North Pole and the Equator. A section of this imaginary line running through Europe was measured meticulously by hand, inch by inch, in a seven-year journey across the continent. In 1798, the metre was officially redefined as 1/10,000,000th of half the Earth's meridian.

Unknown to you and me, this week in Versailles, the venerable kilogram, after 129 years of service to mankind, was quietly scrapped by the International Bureau of Weights and Measures. And why? Because material things change: metals expand, contract, corrode. Le Grand K, though protected in three vacuum-sealed jars, mysteriously lost weight over a century - 50 micrograms, roughly the weight of an eyelash. No one could explain it.

Clearly, our units of measurement had to be pegged to something that is guaranteed never to change, like a mathematical constant or a force of nature. Over the last few decades, six of the seven metric units of measurement - the metre, the second, the ampere, the Kelvin, the mole and the candela - have been redefined. Only the kilogram remained.

The gruelling, unremarkable work of redefining the kilogram has been going on quietly for about 20 years in several labs, with insights from two Nobel prizes in quantum physics. The result: a cosmically accurate machine called the Kibble Balance, whose sole function is to define the kilogram. It has been called one of the most intricate devices ever built.

The funny thing is that it works pretty much like the beam balance of Lakshmi Prasad in Matunga. While the pans of his tarazu weigh mass against mass, the Kibble Balance weighs mass against an electromagnetic force which can be measured extremely accurately. To make the switch, each country would have to build its own Kibble Balance. Only two, the USA and Canada, have succeeded so far.

But back to our hornswoggling vegetable vendor who owns two smartphones and is definitely no digital naif. In a country that can now digitally track its billions of citizens through their Aadhaar cards, and is pushing to move from paper money to online transactions, Lakshmi Prasad deliberately chooses the taraju because he'd rather define the kilogram himself than have a Kibble Balance do it for him. Everyone happily colludes in helping him get away with it.
As Confucius didn't say, imprecision is good for business.

Here, viewed from there. C Y Gopinath, in Bangkok, throws unique light and shadows on Mumbai, the city that raised him. You can reach him at Send your feedback to

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