Why Did We Change the Kilogram?

India uses the Metric System to measure everything.

The United States uses the Imperial System.

Although even after two hundred years of the invention of the metric system (Which is considered to be better for mathematical calculations) we haven’t come on the same page.

We are all familiar with the Kilogram as it is the world standard for measurement, except in the United States, and also because it’s definition was recently changed.

What is the definition of a Kilogram?

It’s quite easy to answer. In the 18th century when the Kilogram was introduced, it was defined as “the absolute weight of a volume of pure water equal to the cube of the hundredth part of the meter.”

In simple words, one cubic decimeter or one litre.

The French scientists that came up with this definition, soon realised it isn’t reliable. As the weight of water depends on a lot of factors like purity and atmospheric pressure.

So they replaced the definition with a metal artefact. It remains defined by a platinum alloy cylinder, the International Prototype Kilogram manufactured in 1889, and carefully stored in Saint-Cloud, a suburb of Paris.

The prototype Kilogram has been essential. If there was no standard, How would scientific research be shared? How would you measure proportions in a recipe? How would countries trade? How would you trust your weighing scale?

That little artefact of metal lays the foundation of the modern world.

But scientists around the world have voted to replace it with something far different!

International Bureau of Weights and Measures, the body that takes care of the metric system, invited scientists from 60 different countries to take an open vote, and it ended with doing away with the current definition of the Kilogram.

With this end, came the culmination of two centuries worth of scientific research and discoveries.

Now you can fit all the International System of Units (SI) onto a singular card.

The prototype kilogram and its artefact successor have defined the standards of weight across the globe.

Now we use the “Working Standard” to define weights, and calibrate our machines according to it.

After the announcement that came two months ago, now we don’t use the metal artefact, instead, we use a nature’s constant to define the Kilogram.

The kilogram is the last unit in the metric system to go through this change.

The metre was originally defined in 1793 as one ten-millionth of the distance from the equator to the North Pole – as a result the Earth’s circumference is approximately 40,000 km today. In 1799, it was redefined in terms of a prototype metre bar (the actual bar used was changed in 1889). In 1960, the metre was redefined in terms of a certain number of wavelengths of a certain emission line of krypton-86. In 1983, the current definition was adopted.

“The metre is defined as the length of the path travelled by light in a vacuum in 1/29,97,92,458 second.”

The New Definition of the Kilogram

Today the Kilogram is tied to the smallest action that can be made by a photon. Which essentially is the smallest possible physical action that is possible. The Plank’s Constant.

In order to measure the kilogram with the Plank’s Constant, scientist used the “Kibble balance.”

The Kibble balance (known as a “watt balance” before 2016) is essentially a single-pan weighing scale that measures the electric power necessary to oppose the weight of a kilogram test mass as it is pulled by Earth’s gravity.

So, Now the new definition of the Kilogram will be “The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.62607015×10−34 when expressed in the unit J⋅s, which is equal to kg⋅m2⋅s−1, where the metre and the second are defined in terms of c and ΔνCs.

A consequence of this change is that the new definition of the kilogram is dependent on the definitions of the second and the metre.

But, the Big question still remains. Why go through this trouble?

Scientists have given us two reasons.

First, Accuracy.

The problem with using physical standards remains that they change with time. The original prototype kilogram has lost about fifty micrograms or about the weight of a human eyelash. Which makes all the measurements inaccurate by fifty micrograms.

And with nanotechnology growing, this difference could make a huge impact.

The Second and more important reason being ideology, the original prototype Kilogram was based in France, because it was created during the French revolution.

This was the time when revolutionaries were tearing down and reconstructing many of societies norms and that included weights and measurements.

They wanted to make units that were consistent and fair to the working men, and that was shared between countries.

They wanted to unite the world!

Their moto was “Pour tous les temps pour toutes les personnes” (For all time for all people).

But, if we define mass by a single prototype kilogram locked away in a basement in France, then it can’t be for all men and all women, and if it changes when we measure it, then it can’t be for all time.

So the scientist say by tying the kilogram to a constant of nature we are freeing it from its dependence on physical factors. We are achieving the dream of the creators of the metric system. A journey that’s been more than two centuries in the making.


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Vishrant Khanna

I'm an excellent communicator, have thirst for knowledge and desire to share it with others. I tend to be highly enthusiastic, impulsive and objective. Follow Me: Facebook | Twitter | Instagram | Google+ | LinkedIn | YouTube | Pinterest

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