All of us had a brush with scientific metrology in our high school physics curriculum. Most of us may recall that there are seven fundamental physical variables viz., mass, length, time, temperature, current, luminance and amount of substance. Their corresponding units of measurements are kilogram, meter, second, Kelvin, Ampere, candela and mole. All measurements of all types are based on one or more of these independent units. Two supplemental independent units radian and stradian are also recognized internationally, both dealing with angular measurements. All around the world metrologists are trying to represent the seven base units in terms of constants of nature such as velocity of light in vacuum and the Planck’s constant. It is important therefore to have reliable and accurate measurements which are agreed and accepted by the relevant authorities worldwide.
Every year, May 20 is celebrated as World Metrology Day tocommemorate the signing of the Meter Convention in 1875. The InternationalBureau of Weights and Measures (BIPM) at Paris was founded as an outcome ofthis treaty. It is the apex body responsible for scientific metrology. Amongother responsibilities, it is the custodian of Le Grand K or the International Prototype Kilogram (IPK) a globally valid standard of measuring massor weight. Standards are objects or ideas that are designated as beingauthoritative for some accepted reason. Whatever value they possess, is usefulfor comparison to unknowns for establishing an assigned value based on thestandard. The design of this comparison process for measurements is metrology.Metrology is an old science, which has evolved over many centuries. The earliest systems of weights and measureswere based on human morphology and naturally occurring substances. Consequently,these units of measurement varied from place to place. Metrologists aretherefore continuously involved in the development of new measurementtechniques, instrumentation and procedures, to satisfy the ever-increasingdemand for greater accuracy, increased reliability and rapidity of measurements.Although standardization of weights and measures has been a goal of social andeconomic advance since very early times, it was not until the 18th century thatthere was a unified system of measurement. The execution of measurementcomparisons for the purpose of establishing the relationship between a standardand some other measuring device is known as calibration. The ideal standard isindependently reproducible without uncertainty. Metrology is not, however, exclusively thedomain of scientists. It is something of vital importance to all of us. Ourdaily grocery and vegetable purchase is impossible without metrology. TheInternational Prototype Kilogram ensures that wherever we are in the world onekilogram actually weighs one kilogram. Every country maintains its ownmetrology system. The National Physical Laboratory, India is the custodian ofNational Prototype of the Kilogram, copy No. 57 (NPK-57), which was provided bythe BIPM in 1958 after its first calibration in 1955. The NPK-57 has been recalibratedin 1985, 1992, 2002 & 2012 so far at the BIPM. It is served as primarystandard in India. The mass value of NPK-57 is disseminated to NPL massstandards from 1 mg to 2,000 kg, solid density standards and other derivedparameters.
The kilogram was originally defined as the weight of acertain volume of water, a convenient and readily purified liquid. That washowever an inconvenient quantity to precisely replicate, so in 1799 a platinumartifact ‘Kilogram of the Archives’ was fashioned to define the kilogram. Thatartifact, and the later IPK, have been the standard of the unit of mass for themetric system ever since. The IPK is made of a platinum alloy known as”Pt10 Ir”, which is 90% Platinum and 10% Iridium and is machined into a right-circular cylinder of about39 millimeters height and same diameter to minimize its surface area. Theaddition of 10 % iridium improved upon the all-platinum Kilogram of theArchives by greatly increasing hardness while still retaining platinum’s manyvirtues: extreme resistance to oxidation, extremely high density satisfactory electrical and thermalconductivities, and low magnetic susceptibility. The IPK and its six sistercopies are stored at the BIPM in an environmentally monitored safe in the lowervault located in the basement of the BIPM’s Pavilion de Breteuil in Saint Cloud on the outskirts of Paris. TheIPK is rarely used or handled. Copies of the IPK kept by national metrologylaboratories around the world werecompared with the IPK in 1889, 1948, and 1989 to provide traceability ofmeasurements of mass anywhere in the world back to the IPK. In spite of bestefforts to maintain it, the IPK has diverged from its replicas by approximately50 micrograms since their manufacture late in the 19th century.
This led to efforts to develop measurement technologyprecise enough to allow replacing the kilogram artifact with a definition baseddirectly on physical phenomena, which is now scheduled to take place on 20 May2019. The new definition is based on the Planck’s constant, thereby fixing thevalue of the kilogram in terms of the second and the meter, and eliminating theneed for the IPK. Since the second and meter are defined completely in terms ofphysical constants, the kilogram is defined in terms of physical constantsonly. All one needs now to have a Kibble balance to find the value of onekilogram accurately. The grand K will be just an artifact henceforth. After thekilogram’s definition is changed officially- on 20th May, 2019, most peoplewill never notice the difference. It would not change baking ingredients on akitchen scale or our daily purchase of grocery. National Physical Laboratory,Director D.K. Aswal said the facility to produce ‘Kibble balance’ machines tobe used across the country will be ready in the next three-four years. It isexpected to cost Rs 60 crore.
Dr Seemin Rubab is HoD Physics, NIT Srinagar