From UPSC perspective, the following things are important :
Prelims level : Units and Measurements
Mains level : Read the attached story
- The General Conference on Weights and Measures (CGPM) at BIPM held on 16 November 2018 has unanimously adopted the resolution to redefine four of the seven base units.
- These included kilogram (SI unit of weight), Kelvin (SI unit of temperature), mole (SI unit of amount of substance), and ampere (SI unit of current).
- The new SI is being implemented worldwide from 20th May 2019 i.e. the World Metrology Day.
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).
- In India, CSIR-NPL maintains the National Prototype Kilogram (NPK-57), which is calibrated with IPK.
- The IPK was the last physical artifact used to define any of the fundamental units.
- IPK would put on a little extra mass when tiny dust particles settled on it; when cleaned, it would shed some of its original mass.
- Scientists have long stressed that the fundamental units should be defined in terms of natural constants.
- On November 16, 2018 representatives of 60 countries agreed that the kilogram should be defined in terms of the Planck constant.
- The Planck constant is a quantity that relates a light particle’s energy to its frequency.
- Using a machine called a Kibble balance, in which the weight of a test mass is offset by an electromagnetic force, the value of the Planck constant was fixed, the kilogram was redefined.
How was this achieved?
- The new definition for kilogram fits in with the modern definitions for the units of time (second) and distances (metre).
- Today, 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.
- By its modern definition, a metre is the distance travelled by light in vacuum in 1/299,792,458 of a second (which is already defined).
- This is where the Planck constant comes in.
- It has been measured precisely at 6.626069… × 10^(-34) kilograms per second per square metre.
- With the second and the metre already defined, a very precise definition for the kilogram
Benefits of recalibration
- What was 1 kg earlier is still 1 kg today. An updated kilogram doesn’t mean that weights everywhere will be thrown off balance.
- All that has changed is the definition, for the sake of accuracy.
- A mass measured as 1 kg earlier would have meant 1 kg, plus or minus 15-20 micrograms.
- Using the new definition, a mass measured as 1 kg will mean 1 kg, plus or minus 1 or 2 nanograms.
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.
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