Maintenance of Standards: Since \(1889,\) the kilogram was defined as the mass of a platinum-iridium cylinder dubbed “The Big K” or “Le Grand K“ kept under three glass domes in a high-security environment in Paris. Recently, in \(2019,\) the definition of the kilogram was changed to one based on Planck’s constant, becoming the last fundamental unit to be no longer defined by a physical artefact/object. Why did it need to change? Despite such stringent measures to maintain the prototype of the kilogram, the Big K had been steadily losing weight since its adoption. By \(2018\) it had lost \(50\) micrograms! As a result, every weight measurement that relied on the Big K was affected, however slightly.

Fundamental constants of nature, however, are absolute. They do not change. And so, the definition of the kilogram changed to one based on a fundamental constant of nature, Planck’s constant. A kilogram is a standard unit of measurement against which all weight is measured. Other physical quantities also have their own standard units.  The “Le Grand K” was a prototype of the kilogram, which was supposed to weigh exactly one kilogram. But it lost weight over time in spite of every effort to maintain it. It is of utmost importance to define a unit in such a way that it doesn’t change, and it is also important to maintain its prototypes.

The science of measurement is known as metrology. How are standards of measurement or units selected? Once selected, how are standards of measurement maintained? What is the importance of maintaining the standard units? Which bodies are responsible for maintaining our measurement standards that are in use in all of modern science? We will endeavour to answer such questions.

Why Do We Need Standards of Measurements?

Ever since humans began to live in communities, a set of standards for weights and measures became necessary to carry out daily activities. If you go to a grocery store and ask for \({\rm{2}}\,{\rm{kg}}\) of vegetables, the grocer, using his weighing scale, can provide you with exactly or close to \({\rm{2}}\,{\rm{kg}}\) of vegetables.

The \({\rm{kg,}}\) which is the standard unit for weight, keeps both you and the grocer on the same page. Without a common set of standards for weights and measures, understood by both you and the grocer, you wouldn’t be able to carry out the exchange with ease. If you tell your friend over the phone that you’ll be at his house in \(15\) minutes, your friend perfectly understands how long he has to wait!

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Evolution of Standards for Weights and Measures

Early communities were isolated, so each community had its own set of standards for weights and measures. For example, the leader of one community would arbitrarily declare the length of his hand from the base of his palm to the tip of his middle finger as one standard unit. A physical object such as a rod would then be constructed having the same length as his hand, and all measurements would be made using that rod as one standard unit. As trade and commerce flourished, it became important to devise a common set of standards agreeable to all communities. You wouldn’t be able to conduct any meaningful exchange with another community if you didn’t speak a common language. So it goes with weights and measures. 

With the dawn of modern science, the limitations of arbitrarily chosen and differing standards for weights and measures became apparent. For example, a cornerstone of modern science is the reproducibility of controlled experiments. Scientific laws or theories are developed based on repeated experiments or observations. Let’s say a controlled experiment is performed in a particular lab that produces a certain result. The only way that an experiment can translate to a workable scientific theory is if another scientist in another lab in another country can produce the same result under identical conditions. The only way for the two scientists to compare notes is if they used a common and robust set of standards for weights and measures. 

Modern standards for weights and measures are a lot stricter and must satisfy several conditions. 

1. They must be invariant with space and time; the definition should not change with location and time. 
2. They should be reproducible; it should be easy to make accurate and faithful copies of the original unit. 
3. They should be of convenient size; they should be on a human scale. 
4. They should be defined without any ambiguity.

Modern Standards for Weights and Measures

To bring some rationality to systems of measurement, the French National Assembly established a committee in \(1790\) to propose a new system of measurement, with new units and new standards. That system came to be known as the metric system and is now the sole system of measurement used by all scientists and in nearly every country of the world except the United States, Liberia, and Myanmar. The units of measurement in the metric system were the gram \({\rm{g}}\) for mass, the liter \({\rm{l}}\) for volume, the meter \({\rm{g}}\) for length, and the second \({\rm{s}}\) for time. 

In \(1960,\) the metric system was modified somewhat with the adoption of new units of measurement. The modification was given the name of Le Système International d’Uniteś, or the International System of Units—the SI system.

Seven fundamental units make up the SI system with an additional two dimensionless fundamental units. These are the meter (abbreviated \({\rm{m}}\) for length, the kilogram \({\rm{kg}}\) for mass, the second \({\rm{s}}\) for time, the ampere \({\rm{A}}\) for electric current, the Kelvin \({\rm{K}}\) for temperature, the candela \({\rm{cd}}\) for luminous intensity, the mole \({\rm{mol}}\) for the quantity of a substance, the radian \({\rm{rad}}\) for plane angles, and the steradian \({\rm{sr}}\) for solid anglesAll other units can be derived from these fundamental units. For example, speed can be written as length/time or \({\rm{m/s}}{\rm{.}}\) The Coulomb \({\rm{C}}\) or the unit for charge can be written as \({\rm{current \times time}}\) or \({\rm{A}}{\rm{.s}},\) and so on.

The Seven Fundamental Units

The meter \(\left( {\rm{m}} \right){\rm{:}}\) The meter is defined as the distance traveled by light in a vacuum in \(\frac{1}{{299792458}}\) second. But this wasn’t always its definition. The earliest definition of the meter is one ten-millionth of the distance from the equator to the north pole along a meridian.

The kilogram \(\left( {\rm{kg}} \right){\rm{:}}\) The “Big K” was retired in \(2019.\) The kilogram, since then, is defined in terms of the Planck constant, h set as exactly \(6.62607015 \times {10^{ – 34}}\;{\rm{kg}}\;{{\rm{m}}^2}\;{{\rm{s}}^{ – 1}}\) with researchers able to make a precise mass measurement using equipment such as the Kibble balance. 

The second \(\left( {\rm{s}} \right){\rm{:}}\) The duration of \(9,192,631,770\) periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium\(-133\) atom. Historically, the second used to be \(\frac{1}{{86400}}\) of a day. But the earth’s rotation varies and is slowing ever so slightly. So the historical definition had to go, as the standard for measures needs to be invariant, remember?

The ampere \(\left( {\rm{A}} \right){\rm{:}}\) The flow of exactly \(1/1.602176634 \times {10^{ – 19}}\) times the elementary charge \(e\) per second. Equalling approximately \(6.2415090744 \times {10^{18}}\) elementary charges \(e\) per second. 

The kelvin \(\left( {\rm{K}} \right){\rm{:}}\) The kelvin is defined by setting the value of the Boltzmann constant k to exactly \(1.380649 \times {10^{ – 23}}{\rm{J \times }}{{\rm{K}}^{{\rm{ – 1}}}}{\rm{,}}\left( {{\rm{J = kg \times }}{{\rm{m}}^{\rm{2}}}{\rm{ \times }}{{\rm{s}}^{{\rm{ – 2}}}}} \right),\) given the definition of the kilogram, the meter, and the second. 

The mole \(\left( {\rm{mol}} \right){\rm{:}}\) The amount of substance having exactly \(6.02214076 \times {10^{23}}\) elementary entities (atoms, molecules, etc.)

The candela \(\left( {\rm{cd}} \right){\rm{:}}\) The luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency \(5.4 \times {10^{14}}\,{\rm{hertz}}\) and that has a radiant intensity in that direction of \(1/683\) watt per steradian.

Who Decides and Maintains These Standards?

The supreme or final deciding authority that not only created the SI system but ensures the propagation and improvement of the International System of Units (SI), is the General Conference of Weights and Measures which was established in \(1875\) under the terms of the Metre Convention. There are a couple of more organizations in the mix. The International Bureau of Weights and Measures does all the legwork under the supervision of the International Committee of Weights and Measures, but it’s the General Conference of Weights and Measures that has the final say!

At a national level, each country has a national body that maintains standards.  In India, National Physical Laboratory (NPL) at New Delhi is responsible for the maintenance of standards. In addition to NPL at New Delhi, many regional and state-level laboratories help in the maintenance of the national standards. 

How are the Measurement Standards Maintained?

How is the ruler you use for measuring length made? It is manufactured in a factory but cutting out an exact length of steel or wood. The factory maintains one or several prototypes of a precise \({\rm{30}}\,{\rm{cm}}\) ruler that is tested and certified by NPL or other affiliated regional or state level authorities responsible for maintaining weights and measures. Its cutting machinery is also designed based on the length of this prototype. NPL also designs and tests standard unit prototypes for other physical quantities.

How do watches keep time? Today, in the digital age we needn’t worry about our clocks being correct. Our phones and laptops simply sync up with the internet time which keeps time accurately. If you live in India, your smartphone or laptop will automatically sync up with the NPL India time server. However, a generation ago, people would sync up their clocks or watches using the radio or television or the clocktower in the city! If you still wear a mechanical wristwatch, you still have to sync up your watch with the internet time.

Summary

Industries cover a wide array of scientific applications, and many of them have attained a high degree of perfection. To thrive in a competitive world, they must constantly improve their products. For this purpose, they might need high-precision tools. Today, Intel produces microprocessors that have transistors that measure a staggering \(45\) nanometers wide. A nanometer is one-billionth of a meter! Such a high degree of precision requires high precision manufacturing capability! 

Gene splicing technology that deals with cutting DNA strands also requires high precision. A strand of DNA is about \(2\) nanometers wide! In a world where precision is becoming increasingly important, we need a robust framework for the maintenance of standards.

FAQs on Maintenance of Standards

Q.1. Why do we need standards for weights and measures?
Ans: A common set of standards for weights and measures facilitates easy to trade and commerce, scientific reproducibility, and high industrial precision.

Q.2. Which standards of weights and measures are currently in use?
Ans: The SI system is adopted by nearly every country in the world, barring a few. Some countries, such as the US, still use Imperial units such as foot \(\left( {{\rm{ft}}} \right)\) and pound \(\left( {{\rm{lb}}} \right)\) for some measurements.

Q.3. Who ensures the maintenance of standards for weights and measures?
Ans: At the international level, the General Conference of Weights and Measures is responsible for all maintenance and updates in metrology (the science of measurement). National bodies, labs, and state-level labs help maintain standards at the national level.

Q.4. Why are all standards defined in terms of universal constants?
Ans: Standards defined based on physical artifacts are obsolete. As universal constants are invariant, fundamental units are defined based on these constants.

Q.5. Name the seven fundamental units.
Ans: The seven fundamental units are the meter \(\left( {{\rm{m}}} \right)\) for length, the kilogram \(\left( {{\rm{kg}}} \right)\) for mass, the second \(\left( {{\rm{s}}} \right)\) for time, the ampere \(\left( {{\rm{A}}} \right)\) for electric current, the Kelvin \(\left( {{\rm{K}}} \right)\) for temperature, the candela \(\left( {{\rm{cd}}} \right)\) for luminous intensity, the mole \(\left( {{\rm{mol}}} \right)\) for the quantity of a substance.

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