Kilogram is History : Long Live the Kilogram.
20th May, 2019, will remain etched in the annals of human history, not because it happens to be my 58th birthday, but for reasons more scientific and majorly significant. On this historic “World Metrology Day”, one of the most commonly used measurements, the Kilogram, has been permanently redefined and the new measurement will be more precise than what we have been using for all of 144 long years. The genesis for this change was shaped during the 16th, November, 2018, General Conference on Weights and Measures in which most participating countries including India, agreed on one of the most significant revisions to the International System of Units (SI).
Measurements are integral to our way of life and it is since time immemorial that human society has been measuring things all the time – how long, how heavy, how quickly and so on – primarily because we need units of measurements for activities such as trade and commerce and so also for advancing of our worldly knowledge. The challenge however has been getting an exactitude of standard measurements, which make sure that mine, yours and others measurements are precisely the very same. This is all the more a necessity because almost every one of us trust the standards of measurements and take what is displayed on our measuring scales for granted and consider that a second is a second, metre a metre and a kilogram a kilogram. For most of human history, however, measures of time, length and mass have been arbitrary, mostly defined according to the whims of local customs or rulers.
Metrology, or the study of units of weights and measures, has a long history and has references across civilisations including our very own Indus. The first study of some kind of a standardisation in weights during the Indus period was done by A S Hemmy (1931) on the basis of cubical objects of chert and other stones discovered from the Harappan site. Modern studies by Michel Danino, Kenoyer, Balasubramaniam and others have shown that there has been a continuing standards of length measurement (angula), which span centuries stretching from the Harappan times right until the majestic Taj Mahal construction. There however seems to have been no decrees regarding standardisation.
One of the earliest decrees that national measurements must be standardized came in 1215 AD, from the Magna Carta. Clause 35 of the Magna Carta demands standard weights and measures for grain, wine, beer and cloth. It was not until the French Revolution that necessity for standardising measurements gained momentum leading to the first standards of length and mass, with two platinum standards representing the Metre and the Kilogram. These two new standard measurements were defined on 22nd June, 1799, in Paris. Ever since, there were efforts to extend the standards of measurements across countries and it was German mathematician Carl Friedrich Gauss who advocated the idea of agreed global standards in measurements. A major breakthrough came when representatives of 17 nations came together to create the “International System of Units (SI) signing the Metre Convention treaty on the 20th of May, 1875. Subsequently over 100 countries have adopted the metric system of measurements, the SI units, which has been in practice since 1889.
In the SI units used in most parts of the globe, the base unit of mass is the kilogram. The kilogram is defined as the mass of an object, known as the International Prototype of the Kilogram (IPK), which is carefully preserved - in two safes and three glass bell jars - since 1889 at the International Bureau for Weights and Measures ( Bureau International de Poids et Mesures BIPM in French) in France. All the mass standards around the world are calibrated from time to time against this artefact object, which serves as a base standard to ensure worldwide uniformity of weight measurements in kilogram.
The IPK, also called the ‘Le Grand K’, is an alloy of 90% platinum and 10% iridium, machined into a right circular cylinder of 39.17 mm diameter and equal height. The alloy is extremely hard, of high density (21 times higher than water), resistant to oxidation and a good thermal and electrical conductor. The IPK has been preserved in an environmentally safe condition enclosed under three bell jars. Three independently controlled keys are required to take it out. The original IPK has only been weighed three times before, (1889, 1946 & 1989) against a number of near-identical copies. India too based its measurement of the kilogram on the IPK, an official copy of which came to India in 1958 and is preserved at the National Physical Laboratory, New Delhi.
The IPK was mandated to remain stable over time, but most unfortunately that was not to be, notwithstanding the highest levels of climatic controlled conditions in which the original IPK has been stored. Over the decades the scientists were surprised to see a drift in the mass of the official copies relative to the original IPK. Majority of the official copies of the IPK showed an increase in their mass with respect to the original IPK. This necessitated a rethinking among the scientists to redefine the Kilogram. Thus after more than 100 years of defining the kilogram according to a metal artefact - The IPK - metrology scientists started preparing to change the unit based on a constant, which is nature dependent and not man made.
The definition of all seven SI units, except that of the kilogram, are based on some fundamental constants of nature. The last one to be redefined was the unit of length, the Metre. Earlier, like the Kilogram, the measurement of the unit of length, the Metre, was based on a man-made artefact. It was defined as the distance between two marks on a platinum rod. In 1983, the measurement of Metre was redefined in relation to the speed of light. Accordingly, the Metre is now defined as the distance in vacuum, traveled by light in 1/ 299,792,458 seconds. Since both speed of light and also the second are based on natural universal constants, the Metre too will remain constant.
In the year 2005 the International Committee on Weights and Measures recommended that the measurement of the weight - the Kilogram - too needs to be redefined on the basis of some fundamental natures constant, which will help the measurement of the kilogram liberate its dependence on IPK. The General Conference on Weights and Measures, which met in November 2018 recommended redefining the kilogram in terms of the Planck’s constant and passed a resolution to that effect unanimously. The changes of this resolution have now come into effect from 20th May 2019.
The kilogram will now be defined by taking the fixed numerical value of the Planck constant, h, to be 6.626 07015 × 10 to the power -34, when expressed in the unit Joules, which is equal to kg metre square / second. Sounds quite complicated but not really. The Planck constant describes how the tiniest bits of matter release energy in discrete steps or chunks (called quanta). And from this fixed value of the Planck constant, scientists can derive the standard mass of a kilogram. Redefining the kilogram in terms of the Planck constant has been an immense challenge. This work required careful measurements with an incredibly complicated machine called the Kibble balance - named after Bryan Kibble from the UK’s National Physical Laboratory, who died in 2016- as well as observations of an extremely round sphere of silicon. Currently, only France, Canada and the US have Kibble balances capable of making the measurements needed to fix the Planck constant. India too - National Physical Laboratory, New Delhi - is aiming to have its own Kibble balance.
It is time for saying good bye to the IPK, which has served us well over 144 years. The kilogram of yesteryear’s, which was based on the IPK, from now now will be history but then the Kilogram will leave for eternity serving humankind in a more precise way.
I have also published a more detailed article on this subject in Science Reporter - August 2019 issue, which can be seen in this link
20th May, 2019, will remain etched in the annals of human history, not because it happens to be my 58th birthday, but for reasons more scientific and majorly significant. On this historic “World Metrology Day”, one of the most commonly used measurements, the Kilogram, has been permanently redefined and the new measurement will be more precise than what we have been using for all of 144 long years. The genesis for this change was shaped during the 16th, November, 2018, General Conference on Weights and Measures in which most participating countries including India, agreed on one of the most significant revisions to the International System of Units (SI).
Measurements are integral to our way of life and it is since time immemorial that human society has been measuring things all the time – how long, how heavy, how quickly and so on – primarily because we need units of measurements for activities such as trade and commerce and so also for advancing of our worldly knowledge. The challenge however has been getting an exactitude of standard measurements, which make sure that mine, yours and others measurements are precisely the very same. This is all the more a necessity because almost every one of us trust the standards of measurements and take what is displayed on our measuring scales for granted and consider that a second is a second, metre a metre and a kilogram a kilogram. For most of human history, however, measures of time, length and mass have been arbitrary, mostly defined according to the whims of local customs or rulers.
Metrology, or the study of units of weights and measures, has a long history and has references across civilisations including our very own Indus. The first study of some kind of a standardisation in weights during the Indus period was done by A S Hemmy (1931) on the basis of cubical objects of chert and other stones discovered from the Harappan site. Modern studies by Michel Danino, Kenoyer, Balasubramaniam and others have shown that there has been a continuing standards of length measurement (angula), which span centuries stretching from the Harappan times right until the majestic Taj Mahal construction. There however seems to have been no decrees regarding standardisation.
One of the earliest decrees that national measurements must be standardized came in 1215 AD, from the Magna Carta. Clause 35 of the Magna Carta demands standard weights and measures for grain, wine, beer and cloth. It was not until the French Revolution that necessity for standardising measurements gained momentum leading to the first standards of length and mass, with two platinum standards representing the Metre and the Kilogram. These two new standard measurements were defined on 22nd June, 1799, in Paris. Ever since, there were efforts to extend the standards of measurements across countries and it was German mathematician Carl Friedrich Gauss who advocated the idea of agreed global standards in measurements. A major breakthrough came when representatives of 17 nations came together to create the “International System of Units (SI) signing the Metre Convention treaty on the 20th of May, 1875. Subsequently over 100 countries have adopted the metric system of measurements, the SI units, which has been in practice since 1889.
In the SI units used in most parts of the globe, the base unit of mass is the kilogram. The kilogram is defined as the mass of an object, known as the International Prototype of the Kilogram (IPK), which is carefully preserved - in two safes and three glass bell jars - since 1889 at the International Bureau for Weights and Measures ( Bureau International de Poids et Mesures BIPM in French) in France. All the mass standards around the world are calibrated from time to time against this artefact object, which serves as a base standard to ensure worldwide uniformity of weight measurements in kilogram.
The IPK, also called the ‘Le Grand K’, is an alloy of 90% platinum and 10% iridium, machined into a right circular cylinder of 39.17 mm diameter and equal height. The alloy is extremely hard, of high density (21 times higher than water), resistant to oxidation and a good thermal and electrical conductor. The IPK has been preserved in an environmentally safe condition enclosed under three bell jars. Three independently controlled keys are required to take it out. The original IPK has only been weighed three times before, (1889, 1946 & 1989) against a number of near-identical copies. India too based its measurement of the kilogram on the IPK, an official copy of which came to India in 1958 and is preserved at the National Physical Laboratory, New Delhi.
The IPK was mandated to remain stable over time, but most unfortunately that was not to be, notwithstanding the highest levels of climatic controlled conditions in which the original IPK has been stored. Over the decades the scientists were surprised to see a drift in the mass of the official copies relative to the original IPK. Majority of the official copies of the IPK showed an increase in their mass with respect to the original IPK. This necessitated a rethinking among the scientists to redefine the Kilogram. Thus after more than 100 years of defining the kilogram according to a metal artefact - The IPK - metrology scientists started preparing to change the unit based on a constant, which is nature dependent and not man made.
The definition of all seven SI units, except that of the kilogram, are based on some fundamental constants of nature. The last one to be redefined was the unit of length, the Metre. Earlier, like the Kilogram, the measurement of the unit of length, the Metre, was based on a man-made artefact. It was defined as the distance between two marks on a platinum rod. In 1983, the measurement of Metre was redefined in relation to the speed of light. Accordingly, the Metre is now defined as the distance in vacuum, traveled by light in 1/ 299,792,458 seconds. Since both speed of light and also the second are based on natural universal constants, the Metre too will remain constant.
In the year 2005 the International Committee on Weights and Measures recommended that the measurement of the weight - the Kilogram - too needs to be redefined on the basis of some fundamental natures constant, which will help the measurement of the kilogram liberate its dependence on IPK. The General Conference on Weights and Measures, which met in November 2018 recommended redefining the kilogram in terms of the Planck’s constant and passed a resolution to that effect unanimously. The changes of this resolution have now come into effect from 20th May 2019.
The kilogram will now be defined by taking the fixed numerical value of the Planck constant, h, to be 6.626 07015 × 10 to the power -34, when expressed in the unit Joules, which is equal to kg metre square / second. Sounds quite complicated but not really. The Planck constant describes how the tiniest bits of matter release energy in discrete steps or chunks (called quanta). And from this fixed value of the Planck constant, scientists can derive the standard mass of a kilogram. Redefining the kilogram in terms of the Planck constant has been an immense challenge. This work required careful measurements with an incredibly complicated machine called the Kibble balance - named after Bryan Kibble from the UK’s National Physical Laboratory, who died in 2016- as well as observations of an extremely round sphere of silicon. Currently, only France, Canada and the US have Kibble balances capable of making the measurements needed to fix the Planck constant. India too - National Physical Laboratory, New Delhi - is aiming to have its own Kibble balance.
It is time for saying good bye to the IPK, which has served us well over 144 years. The kilogram of yesteryear’s, which was based on the IPK, from now now will be history but then the Kilogram will leave for eternity serving humankind in a more precise way.
I have also published a more detailed article on this subject in Science Reporter - August 2019 issue, which can be seen in this link
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