The New Kilogram

Audio Version Given Below

 

 My grandfather used to ask me riddles when I was a kid. One riddle was comparing weights. He used to ask “Which is heavier among one-kilogram bale of cotton and a one-kilogram block of iron?” I used to get confused, giving different answers every time. I am sure as children, we have been asked a similar riddle and as adults sometimes even we ask the same to the kids around.

As children, we may have gotten confused at times but now we all know that though the density is different for both, cotton and iron, the mass is the same: One-kilogram.

What is a Kilogram?

We all know Kilogram is the SI Unit (International System of Units) of mass.

  

The International Bureau of Weights and Measures (BIPM), France gives definitions for SI Units. According to BIPM, Kilogram “is equal to the mass of the International Prototype of the Kilogram” (IPK). BIPM is an international organization responsible for working on matters related to measurement science and measurement standards through its member countries.

International Prototype of Kilogram

The International Prototype of Kilogram is an artefact made of an alloy of Platinum and Radium and is maintained by BIPM in Saint Cloud, Paris.

The definition of IPK keeps changing. Before the current definition of IPK was given in 1889, for the previous 90 years, the IPK was a 100% Platinum prototype and its weight was said to be equal to the weight of one-cubic-decimeter of water at 4 degrees Celcius.

The Recent Update

In the 26^th meeting of General Conference of Weights and Measures (CGPM) held from 13^th to 16^th November 2018, it was decided that Kilogram should be redefined w.e.f. 20^th May 2019.

The Kilogram is now defined by taking fixed value for Planck’s Constant ‘h’. Planck’s Constant is 6.62607015×10⁻³⁴  For the purpose of the new definition of the Kilogram, the Planck’s Constant is expressed in the unit Joule Second.

 What is Planck’s Constant?

We said that the Kilogram will be defined by taking a fixed value of the Planck’s Constant. Now the question arises, as to what is the Planck’s Constant?

The electromagnetic waves travel in the form of ‘Photons’: “the energy packets”. The energy of the photon is directly proportional to its frequency. The value of the proportion of energy and frequency of the photon is the Planck’s Constant. It is denoted by ‘h’. Since the Planck’s Constant was discovered by Max Planck, it is named after him.

For the purpose of redefined Kilogram, the value of the Planck’s Constant will be used to calculate the test mass on Kibble’s balance.

What is Kibble’s Balance?

We use different types of instruments to calculate the mass of various objects. For weighing heavy vehicles, a weighbridge is used. To weigh smaller objects, we use floor scales. In laboratories, we use the physical balance. To measure something with high accuracy we use the precision balance etc. Similarly, for still higher accuracy, Kibble Balance (also called Watt Balance) is used.

It is an electromechanical measuring device. In order to accurately know the weight of a test mass, an electromagnetic force is created on one side to balance the weight of the object on the other side.

On the basis of the value of electromagnetic force achieved at the scale’s equilibrium and the Planck’s Constant, the mass of the object is recorded.

Why New Definition of Kilogram was needed?

The new definition comes into effect from 20^th May 2019. A new definition was necessary because the Kilogram has been the only SI Unit directly defined by an artefact rather than a physical property that can be produced independently in other laboratories. Moreover, the value of three base SI units and 17 derived SI Units depends on Kilogram, so the stability of Kilogram is very important.

Though the original IPK maintained in Paris is kept in supervision, but because it is a physical artefact, it is bound to change in terms of weight due to external factors. This leads to the difference from the copies of IPK maintained worldwide.

With the new definition, this problem will be resolved, as now the Kilogram will be defined on the basis of a physical constant.

Bonus Fact

Since you have read the article till the end, here is a bonus fact to thank you.

The National Prototype of Kilogram held by certain Member Countries is a copy of the IPK located in France. India’s prototype of Kilogram is the Prototype no. 57 maintained at the National Physical Laboratory, New Delhi.

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