The International System of Units (SI) is the set of measurement units adopted as standard in most countries of the world.
Among many conventional units, some are called the standard unit because they do not derive from any other unit; these are:
Subway - unit of length;
Second - unit of time;
Kilogram - unit of mass;
Ampere - Unit of electric current;
Kelvin - temperature unit;
Mol - unit of quantification of matter;
Candela - Unit of light intensity.
Because they are not derived from any other unit, they are standardized by measures that may seem somewhat curious, as you can see below:
What is metro?
The meter is defined as the length of the path traveled by light in a vacuum over a time interval of 1 / 299,792,458 of a second.
What is second?
The second is defined as the duration of 9 192 631 770 radiation periods corresponding to the transition between two hyperfine levels of the cesium atom ground state 133.
What is kilogram?
The kilogram is defined on the basis of a standard unit, which has been stored in the International Bureau of Weights and Measures in Sevres, France, since 1889. This standard unit is a 39 mm high by 39 mm diameter equilibrium cylinder. , composed of Iridium and Platinum.
What is amp?
Amp is defined as current that produces an attractive force of Newton per meter in length between two straight, parallel, infinite length conductors and negligible circular cross-section, placed one meter apart in free space.
What is kelvin?
Kelvin is defined as the fraction 1 / 273.16 of the triple point thermodynamic temperature of water.
What is mol?
Mole is the amount of matter in a system that contains as many elemental entities as there are atoms contained in 0.012 kilograms of carbon-12.
What is candela?
The candela is defined as the luminous intensity emitted by a source in a given direction of monochrome light of 540 x 1012 Hertz frequency, and whose radiation intensity in that direction is 1/683 watts per spheradian.