As of November, the kilogram and three other units of measure (Kelvin, Mole and Ampere) will change definition. But rest assured, a kilo of anything will always contain the same amount. Indeed, this change is particularly important for the scientific community.

These 4 measurement units included in the international system will be redefined to obtain an extreme precision. Currently, the definitions for these 4 units vary from one reference to another, thus biasing the accuracy of the measurements. In order to solve this problem, the scientific community decided to define these units by introducing physical constants.

These changes will take place during the General Conference of Weights and Measures to be held in Versailles, France from November 13th to 16th.

Originally, 1 meter was ten millionth of the distance from the equator to the North Pole. A piece of standard metal was the international reference of the meter. It was in 1983 that the definition of the current meter was given according to a constant, thus giving it a remarkable precision. The meter today corresponds to the distance traveled in the vacuum by light for a duration of 1/299 792 458 of a second.

Since the 19th century, the kilogram is defined as the mass of one liter of water at zero degrees Celsius. In order to represent a precise standard of this mass, a weight consisting of iridium and platinum was used. It is this stallion – called the big K – that serves as a reference for all the other “copies” of this kilo. This reference kilo has only been released twice in history. When compared with other copies that were also used as a reference, the scientists noticed that there were 50 micrograms of difference. By definition, the big K always weighs a kilo. Thus, even when it loses a tiny part of its weight, it is estimated that it is still a kilo.

It is to solve this significant problem of precision that the redefinition of the kilo will take place.

The kilogram will therefore be defined by the Planck constant, by relating the meter and the second, which are fixed values. The kilo will therefore be deduced from the Planck constant, with a unit expressed in m2 · kg / s.

Kelvin is a unit of temperature measurement. The point of origin of the Kelvin was the absolute zero, corresponding to the lowest possible temperature (-273 ° C). But, the kelvin was defined in relation to Celsius, itself defined by the melting point of water. Thus, the Boltzmann constant will define the kelvin with a unit depending this time on the kelvin, the meter, the kilogram and the second.

Ampere is the unit of measure of the intensity of an electric current. Its definition is theoretical and is given by the International Committee of Weights and Measures of 1948: “An ampere is the intensity of a constant current which, if it is maintained in two linear and parallel conductors, of infinite lengths, of sections negligible and distant by one meter in the vacuum, produced between these two conductors a linear force equal to 2 × 10-7 newton per meter “. This time, the ampere will be defined by the elementary charge of the proton and the second.

The mole is the unit that designates a quantity of matter. It is the number of atoms or molecules that can be found in a given volume of gas, solid or liquid. For the moment, one mole corresponds to the number of atoms present in 12 grams of carbon 12. This unit will be specified by the use of the number of Avogadro.