2018 can be called a fateful year in metrology, because this is the time of a real technological revolution in the international system of units of physical quantities SI. It is about revising the definitions of the main physical quantities. Will a kilogram of potatoes in the supermarket now weigh in a new way? C potatoes will be the same. Something else will change.
Before the SI system
Common standards in weights and measures were needed in ancient times. But the general rules for measurements became especially necessary with the advent of scientific and technological progress. Scientists needed to speak in a common language: one foot is how many centimeters? And what is a centimeter in France when it's not the same as Italian?
France can be called an honorary veteran and winner of historical metrological battles. It was in France in 1791 that the measurement system was officially approved and theirunits, and the definitions of the main physical quantities were described and endorsed as state documents.
The French were the first to understand that physical quantities should be tied to natural objects. For example, one meter has been described as 1/40,000,000 of the length of a north-south meridian toward the equator. He was tied, thus, to the size of the Earth.
One gram has also been tied to natural phenomena: it was defined as the mass of water in a cubic centimeter at a temperature level close to zero (ice melting).
But, as it turned out, the Earth is not a perfect ball at all, and water in a cube can have a variety of properties if it contains impurities. Therefore, the sizes of these quantities in different parts of the planet differed slightly from each other.
At the beginning of the 19th century, the Germans, led by the mathematician Karl Gauss, entered the business. He proposed updating the centimeter-gram-second system of measures, and since then metric units have gone into the world, science and have been recognized by the international community, an international system of units of physical quantities has been formed.
It was decided to replace the length of the meridian and the mass of a cube of water with the standards that were stored in the Bureau of Weights and Measures in Paris, with distribution of copies to the countries participating in the metric convention.
Kilogram, for example, looked like a cylinder made of an alloy of platinum and iridium, which in the end also did not become an ideal solution.
The international system of units of physical quantities SI was formed in 1960. At first it included sixbasic quantities: meters and length, kilograms and mass, time in seconds, current strength in amperes, thermodynamic temperature in kelvins and luminous intensity in candela. Ten years later, another one was added to them - the amount of a substance, measured in moles.
It is important to know that all other units of measurement of the physical quantities of the international system are considered derivatives of the basic ones, that is, they can be calculated mathematically using the basic quantities of the SI system.
Away from the standards
Physical standards turned out to be not the most reliable measurement system. The kilogram standard itself and its copies by country are periodically compared with each other. Reconciliations show changes in the masses of these standards, which occurs for various reasons: dust during verification, interaction with the stand, or something else. Scientists have noticed these unpleasant nuances for a long time. The time has come to revise the parameters of the units of physical quantities of the international system in metrology.
Therefore, some definitions of quantities gradually changed: scientists tried to get away from physical standards, which in one way or another changed their parameters over time. The best way is to derive quantities in terms of immutable properties, such as the speed of light or changes in the structure of atoms.
On the eve of the revolution in the SI system
Principal technological changes in the international system of units of physical quantities are carried out through the voting of members of the International Bureau of Weights and Measures at the annual conference. If the decision is positive, the changes will take effect after a fewmonths.
All this is extremely important for scientists whose research and experiments require the utmost precision in measurements and formulations.
The new 2018 reference standards will help achieve the highest level of accuracy in any measurement at any place, time and scale. And all this without any loss in accuracy.
Redefining quantities in the SI system
It concerns four of the seven operating basic physical quantities. It was decided to redefine the following quantities with units:
- kilogram (mass) using the units of the Planck constant in the expression;
- ampere (current) with charge measurement;
- kelvin (thermodynamic temperature) with unit expression using the Boltzmann constant;
- mole through Avogadro's constant (amount of substance).
For the remaining three quantities, the wording of the definitions will be changed, but their essence will remain unchanged:
- meter (length);
- second (time);
- candela (light intensity).
Changes with Amp
What is the ampere as a unit of physical quantities in the international SI system today, was proposed back in 1946. The definition was tied to the strength of the current between two conductors in a vacuum at a distance of one meter, specifying all the nuances of this structure. Inaccuracy and cumbersome measurement are the two main characteristics of this definition from today's point of view.
In the new definition, an ampere is an electric current equal toflow of a fixed number of electric charges per second. The unit is expressed in electron charges.
To determine the updated ampere, only one tool is needed - the so-called single-electron pump, which is able to move electrons.
New mole and silicon purity 99.9998%
The old definition of a mole is related to the amount of matter equal to the number of atoms in a carbon isotope with a mass of 0.012 kg.
In the new version, this is the amount of a substance that is contained in a precisely defined number of specified structural units. These units are expressed using the Avogadro constant.
There are also many worries with Avogadro's number. To calculate it, it was decided to create a sphere of silicon-28. This isotope of silicon is distinguished by its precise crystal lattice to perfection. Therefore, the number of atoms in it can be accurately counted using a laser system that measures the diameter of a sphere.
One could, of course, argue that there is no fundamental difference between a silicon-28 sphere and the current platinum-iridium alloy. Both that, and other substance loses atoms in time. Loses, right. But silicon-28 is losing them at a predictable rate, so adjustments will be made to the reference all the time.
The purest silicon-28 for the sphere was recently obtained in the USA. Its purity is 99.9998%.
And now Kelvin
Kelvin is one of the units of physical quantities in the international system and is used to measure the level of thermodynamic temperature. "In the old way" it is equal to 1/273, 16parts of the temperature of the triple point of water. The triple point of water is an extremely interesting component. This is the level of temperature and pressure at which water is in three states at once - “steam, ice and water.”
The definition of "limped on both legs" for the following reason: the value of kelvin depends primarily on the composition of water with a theoretically known isotope ratio. But in practice, it was impossible to obtain water with such characteristics.
The new kelvin will be defined as follows: one kelvin is equal to a change in thermal energy by 1.4 × 10−23j. The units are expressed using the Boltzmann constant. Now the temperature level can be measured by fixing the speed of sound in the gas sphere.
Kilogram without standard
We already know that in Paris there is a standard of platinum with iridium, which somehow changed its weight during its use in metrology and the system of units of physical quantities.
The new definition of the kilogram is: One kilogram is expressed as Planck's constant divided by 6.63 × 10−34 m2 ·с−1.
Measurement of mass can now be done on the "watt" scales. Don't let the name fool you, these are not the usual scales, but electricity, which is enough to lift an object lying on the other side of the scale.
Changes in the principles of constructing units of physical quantities and their system as a whole are needed, first of all, in the theoretical fields of science. The main factors in the updated systemare now natural constants.
This is the logical conclusion of many years of activity of an international group of serious scientists whose efforts for a long time were aimed at finding ideal measurements and definitions of units based on the laws of fundamental physics.
