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SCIENCE – How many kilograms weigh? It is important here that you do not answer 1000 grams, the problem would be wrong in the wrong way. It can also be said "the weight of one liter of water". This answer was good in 1793. In addition, since 1875, we should move on to the weight of a cylinder made of alloy titanium and iridium, chosen by a set of scientists.

And this Friday, November 16, the General Conference on Weights and Measures (GFCM) will definitely change this definition. When applied next May, one kilogram will be hard … Well, something a little weird. A very important constant value of quantum physics that can measure atomic scale interactions.

Before we learn this new system, we need to clarify why the General Conference on Weights and Measures (GFCM), which lasts from Tuesday, November 13-16, in Versailles, wanted to change the definition of kilograms.

**The need for unity**

Centuries weight is measured in different ways, with different values, often related to parts of the body or everyday objects. Which is not practical for trade. Then, in the 18th century, Louis XIVI called on universal measurement units.

In 1793, after the French Revolution, a decree laying the foundations of the metric system, which became the International System of Units, was published. At the time we are not talking about a kilogram, but about "serious". This corresponds to the weight of a cubic decimeter of water, i.e. 2 kilos, 5 wool or 49 grains.

Parallel inventions are grams, which should be used to measure much easier things, according to the International Bureau for Weights and Measures (BIPM). "However, the standard of a gram that is difficult to use to establish, he decided to represent a unit of mass with a standard of one kilogram," the official site said. Finally, the name of the kilogram will remain in history as a unique measure.

**The unbearable lightness of the "big K"**

The problem is that water does not always do the same thing, depending on its composition, its temperature. "That's why we tried to find a more stable definition," he explains. *It's HuffPost *Francois Nez, CNRS researcher in metrology (science of measurement). "We used the best available technology, metallurgy at the end of the 19th century."

Kilogram is then a small cylinder that logically weighs 1 kg. To be precise and its weight does not change with time, the researchers used very stable material in 1875: platinum and iridium alloys. A few years later, in 1889, the first General Conference on Weights and Measures set an international standard for a kilogram: a platinum platinum of iridium, 3.9 centimeters long and 3.9 centimeters in diameter, called the "International Prototype Kilogram" (IPK). Or simply, "big K".

To ensure that it does not move over time, this cylinder is a Sevres guard by BIPM, in the suburbs of Paris, under three hermetic glass bells. A few copies have been created in parallel and scattered, so that they can define the true weight of the whole world.

Everything seemed to go the best in the best world. In addition, over time, we realized that the account is no longer there. In 1946, the large K was compared to his official copies. But none of them had the same weight. Since then, things have deteriorated: the original kilogram is lighter than the rest.

Not much: 35 micrograms or 35 million grams. It's nothing to think about. "To compare, if a kilogram of 300 billion grains of sand for over 100 years would be lost, we would lose 10 grains," says Francois Nez. Bride.

But this can be a problem, reminiscent of *Washington Post*. A few years ago, the American metrology agency had to redefine its kilogram because it no longer weighed the same weight as the big K. Companies based on these budgets also made changes and tried to accuse the US agency of incompetence.

**Intangible weight**

To end this problem, it was decided a few years ago to redefine the kilogram. "Metrologists have invented devices that allow the comparison of kilograms with elementary quantities", at quantum level, explains Francois Nez. Why? Because "the smaller the thing, it's all universal: for example, the electron is the same across the universe."

In order to measure the weight of the kilogram, the researchers therefore used a basic constant, a physical quantity whose numerical value is a priori fixed and the same throughout the universe. His name is Planck's constant. It manages many interactions, especially on the connection between energy and frequency. "We can say that this is the unity of exchanges that take place in quantum physics," says Francois Nez.

Relation with a kilo? To simply say this, researchers have developed strange measuring instruments, called "wick balances" or Kibble. "Imagine that the scale of the conventional scale is inclined to one side, because there is a large K on it, the aim is to push the other board up with an electrical device to restore balance," explains Francois. nos.

What the researchers measure, thanks to Planck's constant, is the force that makes the apparatus. And the goal is to be equal to the exact weight of a large K. Anyway, it's very close to her. Several experiments were performed, with precision that makes the Planck constant value uncertainty 0.0000013%.

To simplify even more, "as if a standard kilogram had been allowed to measure the weight of apple pounds, then we decided that the shepherd is now an apple," notes Francois Nez. Apart from here, apple is the basic constant of physics that will never move in time and it is everywhere. Therefore, the new definition of kilograms at this time is not much more precise than universal.

At the same time, the General Conference on Weights and Measures (GFCM) will also redefine the definition of other units: amperes (electrical current units), moles (material units) and kelvin (temperature units) will also be associated with fundamental constants. The only unified measure that will not be based on a fundamental, immaterial statement is the other. "It's based on the cesium atom, but it's also the most accurate measurement, far," says Francois Nez.

The amperes will be connected with elementary charge, electric charge flow. The krt, the unit of quantity of matter, which is mainly used in chemistry, will be defined through the Avogadro constant. Kelvin, measured from water, will be redefined from Boltzmann's constant (k), which refers to the measurement of thermal agitation of the basic constituents of the body.

That's why the world of measurement is even safer and universal than before. In any case, until we find that the constant is not so constant.

**Let me see, too HuffPost :**

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