Context

The Definition of the Kilogram is changed by redefining the International system of units (SI) on World Metrology Day (20 May, 2019).

About

• The Kilogram will no longer derive its provenance from the weight of a block of a platinum-iridium alloy housed at the International Bureau of Weights and Measures in France.
• The Kilogram has joined other standard units of measure such as the second, metre, ampere, kelvin, mole and candela that would no longer be defined by physical objects.
• It now hinges on the definition of the Planck Constant, a constant of nature that relates to how matter releases energy.

Background

• 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, NPL maintains the National Prototype Kilogram (NPK-57), which is calibrated with IPK.
• 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.
• Last year, the International Conference of heads of Metrology Institutes held in Sevres, France had decided that the measure of kilogram will no longer be pegged to cylinder. It was decided that from 2019 onwards, it will be set by value of Planck constant in combination with definitions of meter and second.
  
  

  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 the distance travelled by light in vacuum in 1/299,792,458 of a second.

Significance

• Earlier, the change in definition of the second has helped in easing communication across the world via technologies like GPS and the Internet.
• In the same way, the change in the definition of the kilogram will led to practical advantages for scientists to make very precise measurements. This measure is now defined on the basis of unchanging universal, physics constants.
• Thus, to answer the question how much is a kilogram, we will no longer have to compare blocks of platinum or worry about scratching them. But this doesn’t mean that weights everywhere will be thrown off balance. For everyday measurements, consumers wanting to calibrate their instruments – whether it’s for high-precision drug manufacturing or retail weighing machines- will continue doing it the same way.