SI units, basis of measurement

Basic quantities are measured very often and similarly to history.
A sophisticated system of interconnected units is used.

SI system

The International System of Units (from the French original Le Système International d‘Unités) is a system of units of physical quantities that are interconnected. The SI system contains seven basic units: meter (length), kilogram (weight), second (time), ampere (electric current), mole (mass amount), kelvin (temperature) and candela (luminosity). Readers will look at the exact new (year 2018) definitions of units on Wikipedia.

Basic units of the SI system

Meter
In 1790, the French Constituent Assembly commissioned a scientific commission to establish a new system of units. The Commission has proposed as a basis for a new system of ten millionths of the Earth's quadrant reduced to sea level. The Peruvian sah, which was then a valid unit of length in France, was used for the measurement. Thus, the final length of the meter was determined: 0.51307 Peruvian sah (toise de Pérou) at 13 ° Ré (Réaumura), which is 16.25 ° C. A platinum rod was formed in this length. The meter was a length (final scale) of 1 m at 0 ° C (métre primitif). Simultaneously with the definition of the meter, multiple and partial length units were created using decimal multiplication and division, and this system was called the metric system. Later, a new meter standard was created as a dash scale to achieve an accuracy of up to 0.0001 mm. This is the well-known meter standard with an asymmetrical X profile made of an alloy of platinum and iridium. Austria received copies No. 15 and 19, Hungary copy No. 14. The metric system in our territory came into force on January 1, 1876. However, even this accuracy was not enough, moreover, it was not possible to reproduce the length of the meter without access to the standard. The physicist Michelson constructed an interferometer and used it to define a meter as a multiple of the length of the light waves of a cadmium lamp. It was therefore possible to reconstruct the length of the meter very accurately and independently of the standard. Since 1960, the meter has depended on the wavelength of krypton radiation. In 1983, the definition was refined as a fraction of the speed of light.

Kilogram
This unit of mass was defined as the mass of 1 liter of air-free water atthe temperature at which water has a maximum density (3.98 ° C), at normal atmospheric pressure (760 mm Hg). A small error is the circular dependence of the definition, where the unit of mass is defined by the pressure, which is defined by the mass, which is defined by the pressure (etc., etc. et infinitum). Therefore, since 1889 the kilogram has been defined by the weight of a prototype in the shape of a cylinder, which is made of an alloy of platinum and iridium. It would seem that the problems are over. However, the opposite is true, it is an almost detective story. About 50 micrograms of weight have been lost since the production of the standard, no one knows how, where and why. So what weighed 1,000 kg a hundred years ago now weighs 1,000,000 05 kg. Hydrogen atoms that entered the alloy during production and later escaped into space are suspected. Employees who could have abraded those 50 micrograms while cleaning the prototype are also suspicious. Because the prototype can only be weighed by comparison with the weight of other prototypes, other prototypes that could have gained 50 micrograms in some way also belong to the circle of suspects. Therefore, the International Committee on Weights and Measures has proposed a completely new definition of the kilogram, depending on the Planck constant. It is based on energy equations, where the energy of a photon is defined through its frequency and Planck's constant (E = hf) and at the same time from the relationship between energy and mass (E = mc²). Simple, isn't it?

Second
The first definition of a second is based on the sixtieth division of hours and minutes. In prehistory, the hour was defined as the unit of time - the Egyptians defined the hour as the twelfth of the day or night. In ancient Greece, they were a little more accurate - an hour was defined as one-twenty-four on a mid-sunny day. With the development of the clock pendulum, it was possible to measure the second. The Royal Society of London proposed to use the swing of the second pendulum as a unit of time as early as 1660. The time of the Earth's orbit around the Sun was taken as the basis for the definition of the second. Therefore, the second was defined in 1960 as part of the tropical year. But there was a problem. The earth does not move regularly. Due to the development of the atomic clock, the second was defined by the frequency of the electromagnetic radiation of the cesium isotope Cs 133. This definition has survived to the present day and seems to continue to last because it relates to an unchanging natural phenomenon.

Ampere
This unit describes the electric current. This is a relatively new unit. He also suffered an ampereseveral changes. The first definition from 1909 was related to electrolysis and the time taken for a certain amount of silver to be removed from a silver nitrate solution. With the introduction of the SI system, the definition was completely changed and is based on the force generated between the conductors by the passage of electric current. This definition already sounds complex enough, but uses force that is based on the kilogram. Therefore, in 2018, a brand new definition was proposed, based on the elementary charge - the smallest indivisible electric charge of one electron.

Kelvin
Kelvin is a unit of thermodynamic temperature and also a unit of temperature difference. It is the same size as a degree Celsius, but is defined by other phenomena. The degree Celsius is defined by the boiling point of water (100 ° C) and the melting point of ice (0 ° C). The degree of Kelvin is based on the temperature of absolute zero and the temperature of the triple point of water. The triple point of water is a state where water boils at the same time, and contains both ice and water vapor. It is a big problem to ensure completely clean water, moreover, in its definition, the pressure is used, which is linked to the already mentioned problematic kilogram. Therefore, a new definition was proposed, based on the Boltzman constant, depending on the speed of light, the Planck constant and the decay of the isotope Cesium.

Luminosity
Luminosity is one of the units that does not have such an interesting and long history as a meter or a kilogram. In 2018, a new definition was proposed. It would seem that the difference is only in the wording. But this is not the case, the original definition was the problematic kilogram, so with the new definition, the candela unit will not be exactly the same as in the past (let's not forget the loss of 50 micrograms of the weight of the kilogram standard).

Substance quantity
Substance quantity is also one of the new units without a tumultuous past. Nevertheless, the wording of the definition of the pier was changed. It follows, inter alia, that the mole unit is completely independent of the other units, since it is defined as the number of elementary particles (in one mole). A new formulation was chosen for the new definition (2018), which is based on the Avogadro's constant (total number of atoms in 12 g of carbon nuclide 12C). It should be noted that the number of atoms is relatively difficult to measure, so even the Avogadro's constant is not measured exactly and its current experimentally determined value and accuracy is (6,022 140,857 ± 0.000 000 074) × 1023 mol − 1.