# International System of Units (SI)

Throughout history, countless units of measurement have emerged according to people’s daily needs in relation to measures. Due to the large number of units of measurement, it was necessary to create a set of units for universal use and accessible to all.

This set of defined units is the so-called International System of Units (SI), which was established by the CGPM ( General conference on Weights and Measures ). The units established by SI are recognized by all scientists, engineers, technicians, etc., and appear in all scientific manuals and publications.

The International System of Units works with two types of quantities: the so-called base quantities , which are considered independent, and the derived quantities , which are determined according to the basic quantities. As an example of derived quantities, we can mention the unit of measurement of acceleration (m / s 2 ), which depends on the units of length and time.

Regarding the basic quantities, the table below shows those that are considered by the International System of Units.

Countries establish laws that provide rules for the use of units of measure in all fields of the national plan, such as trade, health, etc. In most countries, legislation is guided by the SI, and the body that takes care of harmonization in relation to the use of the SI is the International Organization of Legal Metrology (OIML), created in 1955.

Below are the definitions of the units of measurement of the basic quantities according to the CGPM.

Don’t stop now … There’s more after the publicity;)

• Length unit – Meter: The meter is the length of the path taken by the light in a vacuum during a time interval of 1 / 299,792,458 seconds;
• Unit of mass – Kilogram:The kilogram is equal to the mass of the international kilogram prototype;
• Time unit – Second:The second is the duration of 9,192,631,770 periods of radiation corresponding to the transition between two levels of the ground state of the cesium atom 133 at rest and at a temperature of 0 K;
• Electric current unit – ampere (A):Ampere is the intensity of constant electric current that, if maintained between two parallel, rectilinear conductors, of infinite length, of negligible circular section and located at a distance of 1 m from each other, in a vacuum, produces between these conductors a force equal to 2 x 10 – 7 N per meter of wire length;
• Temperature unit – kelvin (K):The kelvin is the fraction of 1 / 273.15 of the thermodynamic temperature of the triple point of water;
• Unit of substance – mol:Corresponds to the amount of substance in a system that contains as many elementary entities as atoms in 0.012 Kg of carbon 12;
• Unit of luminous intensity – candela (cd):It is the luminous intensity in a given direction that emits a monochromatic radiation of 5.40 x 10 14 Hz frequency and that has a radiant intensity in that direction of 1/683 watt per sphere.