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CONTRIBUTIONS BY CONTRIBUTIONS BY STUDENTSSTUDENTS
CONTRIBUTIONS BY ONTRIBUTIONS BY STUDENTSSTUDENTS
C

AU REVOIR “LE GRAND K”AU REVOIR “LE GRAND K”
A
AU REVOIR “LE GRAND K”U REVOIR “LE GRAND K”
(REDEFINING THE SI UNIT OF MASS)REDEFINING THE SI UNIT OF MASS)
( (REDEFINING THE SI UNIT OF MASS)(REDEFINING THE SI UNIT OF MASS)

What are SI Units? It is the modern metric units which are widely used for Measurement. Almost
everyone around the world, from a Great Scientist to a Primary School kid uses these Units every day.

In November 2018, many people were baffled & scared when they heard the words, “Redefinition of
Mass’ SI Unit Kilogram” as all the works & activities which relied on this unit (Kilogram) went in vain.
But no need to worry, because only its definition is going to be changed and not every conversions or
the way in which we used to apply it every day.
It is very important for everyone to be updated with Science as the discoveries are likely to increase
exponentially in the coming years. The Chief body behind this work is the General Conference of
Weight & Measures.

Brevity in explanation is a forte to be adapted to make this clear for everyone. Here are the key
takeaways from this updating.

Since 1799, SI Unit of Mass ‘Kilogram’ is defined as the Mass of a cylinder made with Platinum-Iridium
Alloy in vacuum condition. It is called as “LE GRAND K”.

It is kept in a vacuum vault in an observatory in Paris, France. Many replicas of it are kept in many
other places.

The mass of this cylinder and its National Standards was found to be diverging over the years.

Having a definition for an SI unit which is not constant is unacceptable& Kilogram is the last of 7 base
SI units to still be defined by a Physical object & not a Universal constant.

For example, SI unit of Length (meter) is defined as ‘1 meter is equal to the distance traveled by light
in 1/299,792,458 second in vacuum’. Here, the distance traveled by light in vacuum is a constant.

Therefore a quest for defining ‘Kilogram’ with a Universal constant arose.

The National Institute of Standards and Technology with the guidance of General Conference of
Weights and Measures decided to define the ‘Kilogram’ based on the Planck’s constant (h).

An Absolute value for the Planck’s constant is obtained from the Kibble Balance experiment
conducted in two modes, the Weight mode and the Velocity mode.

From this experiment, the Planck’s constant was found to be h= 6.62607015 x 10 -34 Js
But Kilogram cannot be redefined in isolation because other SI Units depend on it.

For example, MOLE is defined as the amount of substance that contains the same number of particles
as there are atoms in 12 grams of Carbon-12. This is called as the Avogadro’s constant N A=
23
-1
6.022140857 x 10 mol

Avogadro’s constant also depends on the old ‘Kilogram’ whose prototype’s characteristic was found to
be varying. Hence, the Avogadro’s constant is also set to be redefined using the newly found Planck’s
constant as obtained from the Kibble Balance experiment.

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Likewise, Ampere will be redefined based on the charge of an electron (e=1.602176634x10 C )&
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Kelvin is redefined based on the Boltzmann Constant (k= 1.380649x10 J/K).

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