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Kilogram, the Last SI Unit to be Based on a Physical Quantity, Has Been Redefinedverified tick

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8 months ago
8 months ago
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The task of weighing an object does not seem a particularly daunting one to the common man in the modern world. Whip out a weighing scale, place the object under inspection on it and voila- it’s weight is displayed. This process is so wide spread that no one gives a thought to how it’s accomplished. We encounter the street side vendor with his ‘tarazu’, the kabaadi wala with his spring loaded scale, the all too glittery (and inaccurate) weighing scale at the railway station to its more accurate and understated counterpart at the airport. But all these scales have to be calibrated against a standard unit and that is where the definition of kilogram becomes important.


A Kilogram is one of the seven fundamental SI units upon which all the other units are based, namely- kilogram, second, Kelvin, Ampere, mole, candela and metre. All the other units in the world can be derived from these units. For example, speed is expressed as metres/second- therefore it is a unit of length and time. The definition of these seven units has been agreed upon by the world’s scientists and is the backbone of all engineering.

Manufacturing process depends upon accurate description of parts and their measurements. A designer’s intentions can only be understood by the manufacturer through dimensions, and these dimensions need to have a unit. And that unit has to be well understood by both the parties and should remain unchanged in all conditions. This is where the universal nature of units comes into picture. Before standardization, different regions used different units for measurement and hence collaborations between conflicting disciplines was not possible.

The General Conference on Weights and Measures which was established by the Metre Convention of 1875, brought together many international organizations to establish the definitions and standards of a new system and standardize the rules for writing and presenting measurements. This was a monumental decision and the prelude to globalization. Today, a car manufacturer can design its product in Japan, get the engine made in Korea, the seats in Brazil, the chassis in Thailand and get the whole thing assembled in China. Actually, most automobile manufacturers do utilize such an approach to keep costs to a minimum, and this is all made possible because circumventing the differences in the Japanese, Korean and Chinese languages there is the universal language of the metric system.


The units we encounter in our day to day lives were defined during the early years of the French Revolution, when the leaders of the French National Constituent Assembly decided to introduce a completely new system of measurement based on the principles of logic and natural phenomena. A metre was designated a length which served as a good quantum for macro objects, and so was kilogram. Consequently, both these quantities were fixed based on the physical features of a platinum-iridium alloy. One sample was made 1m long and another cylindrical sample’s mass was decided to be 1kg, fondly known as the Big K. These samples are kept in a glass enclosure in an underground vault in Paris. 39 identical samples were distributed around the world to serve as a reference. Many copies of these samples have been produced thereof, and the reference used for the calibration of your weighing scale was probably a copy of the copy of the copy of the original sample.

While the ensuing inaccuracy of a few milligrams may not be of significance in consumer applications, it is a reason of great worry for the scientific community. Despite their best efforts, even the ‘environmentally isolated’ sample of the Big K is apparently not isolated enough. Its weight has been shown to have reduced by approximately the weight of an eyelash in its 129 years of existence. This may, again, not seem like much, but in applications like microelectronics and drug delivery systems it may create a huge difference. The bigger problem, however, is that because of it being the reference for a kilogram itself, even after losing weight the Big K still weighs exactly 1 kilogram!

These discrepancies in values are not tolerable in science. A unit needs to remain the same at any given time and place, and hence scientists have been working hard to achieve that through ‘General Conference on Weights and Measures’ (CGPM) which take place every four years since 1891. The seventeenth CGPM in 1983 changed the definition of a metre. It was earlier defined as one ten-millionth of the distance from the equator to the North Pole, passing through Paris. The only problem was this distance in itself was not constant due to variations in the Earth’s geometry, hence neither was the metre. It was changed to the length of the path travelled by light in vacuum during a time interval of 1/299792458 second. This was a value which will remain constant at any part of the universe at any time in the future (or past) since the speed of light is a universal constant, and does not depend upon the frame of reference.

Scientists celebrating the change (Source- India Today)

This left the kilogram as the last unit that was still defined by a physical artefact. That changed on November 16 last month when the CGPM re-defined the kilogram by fixing the value of Plank’s constant, which contains mass in its unit, hence fixing the value of a kilogram as well. This value can be measured by a machine called a Kibble Balance. This re-definition is a reason to celebrate, the fact that all our units can now be derived from universal constants means that humans have completely transcended their arbitrary intuitions and now speak back to the universe in the same language as the universe speaks to us- Physics.

By Dhruv Malik

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