On 20th May 2019 i.e. the World Metrology Day, India along with over 100 countries, adopted a global resolution to redefine four of the seven base units – kilogram , Kelvin , mole, and ampere. A resolution to redefine four of the seven base units was passed by representatives of 60 countries at the General Conference on Weights and Measures (CGPM) of the International Bureau of Weights and Measures (BIPM), Paris on 16 November 2018. As per new definitions:
- The kilogram – will be defined by the Planck constant (h)
- The ampere – will be defined by the elementary electrical charge (e)
- The kelvin – will be defined by the Boltzmann constant (k)
- The mole – will be defined by the Avogadro constant (NA)
Seven Base Units:
The International System of Units (SI) is the modern and most widely used system of measurement. It specifies 7 base units and 22 other derived units. it was formalised in 1960 and has been updated several times to account for development in measurement technology. 7 Base units are:
- Kilogram – SI unit of weight,
- Kelvin – SI unit of temperature,
- Mole – SI unit of amount of substance,
- Ampere – SI unit of current,
- Second -SI unit of time,
- Metre – SI unit of length,
- Candela – SI unit of luminous intensity.
Global standards of Kg
- The global standards for measurement are set by the International Bureau of Weights and Measures (BIPM), of which India became a member in 1957.
- At BIPM in Sèvres, near Paris, stands a cylinder of platinum-iridium locked in a jar.
- Since 1889, the kilogram has been defined as the mass of this cylinder, called Le Grand K, or International Prototype Kilogram (IPK). It has been the world’s one true kilo, against which all others were measured.
- In India, CSIR-NPL maintains the National Prototype Kilogram (NPK-57), which is calibrated with IPK.
Why New Standardisation was required
- Kilogram was the last base unit remaining, defined physical artefact.
- The problem is, the IPK cylinder doesn’t always weigh the same. It puts on a little extra mass when tiny dust particles settled on it and sheds some of its original mass when cleaned.
- Scientists have long stressed that the fundamental units should be defined in terms of natural constants.
The new Standard of Kilogram – Kibble balance
- On November 16, 2018 representatives of 60 countries agreed that the kilogram should be defined in terms of the Planck constant.
- The new definition involves an apparatus called the Kibble balance, which makes use of the Planck constant to measure the mass of an object using a precisely measured electromagnetic force.
Benefits of recalibration
- First and foremost, it will improve the accuracy of measuring kilogram.
- It will be a boon to scientific community. The new technique will allow us to actually measure mass directly at any scale. Previously, subdivision was done to relate small masses to large masses, which built up the uncertainties.
- It will also spare nations, the need to occasionally send their kilos back to France for calibration against the Grand K.
How are Other Base Units Defined
- The second is defined as the time it takes for a certain amount of energy to be released as radiation from atoms of Caesium-133.
- A metre is defined as the distance travelled by light in vacuum in 1/299,792,458 of a second.
- Kelvin is defined in terms of a fixed value for the Boltzmann constant of 1.380649×10−23 J/K.
About World Metrology Day
- The World Metrology Day (WMD) is celebrated annually on this very day as the Metre Convention was signed by representatives of seventeen nations on May 20, 1875.
- The Convention set the framework for global collaboration in the science of measurement and in its industrial, commercial and societal applications.