What is SI

In the last post, we talked about the history metric system. In this post we will talk about SI - the system of units as we are using now.

SI is the short form of the French name "Système International d'Unités", in English it is called "International System of Units". SI is the modern form of the metric system and is the world's most widely used system of measurement, used in both commerce and science. It comprises a coherent system of units of measurement built on seven base units. It defines twenty-two named units, and includes many more unnamed coherent derived units. The system also establishes a set of twenty prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units.

SI was published in 1960 as the result of an initiative that started in 1948. In 1948 an overhaul of the metric system was set in motion which resulted in the development of the "Practical system of units" which, on its publication in 1960, was given the name "The International System of Units". In 1954 the 10th CGPM identified electric current as the fourth base quantity in the practical system of units and added two more base quantities—temperature and luminous intensity-making six base quantities in all. The units associated with these quantities were the metre (m), kilogram (kg), second (s), ampere (A), kelvin (K) and candela (cd). In 1971 a seventh base quantity, amount of substance represented by the mole (mol), was added to the definition of SI.

The motivation for the development of SI was the diversity of units that had sprung up within the CGS systems and the lack of coordination between the various disciplines that used them. The CGPM, which was established by the Metre Convention of 1875, brought together many international organizations to not only agree on the definitions and standards of the new system but also agree rules on writing and presenting measurements in a standardized manner around the world.

SI consists of a set of base units, a set of derived units with special names, and a set of decimal-based multipliers that are used as prefixes.

Base units:

The SI base units are the building blocks of the system and all other units are derived from them. There are 7 base units in SI:

Length: m
Mass: kg
Time: s
Electric current: A
Thermodynamic temperature: K
Amount of substance: mol
Luminous intensity: cd

Derived units:

The derived units in the SI are formed by powers, products or quotients of the base units and are unlimited in number.  Derived units are associated with derived quantities. The dimensions of derived units can be expressed in terms of the dimensions of the base units. For the sake of convenience, there are 22 derived units have special names and symbols.  Such units may themselves be used in combination with the names and symbols for base units and for other derived units to express the units of other derived quantities.

Prefixes:

Prefixes are added to unit names to produce multiple and sub-multiples of the original unit. All multiples are integer powers of ten and above a hundred or below a hundredth all are integer powers of a thousand. The prefixes are never combined in SI.

Here is an example: If the displacement of an object is 10 cm, and it takes 10 s to move the object, the velocity of this object is 10 cm/s. 

In this sentence, "m" is a base unit, and the prefix "centi-" stands for 0.01, so "cm" means 0.01m, "s" is a base unit as well, then "m/s" is a derived unit, derived by length and time.

In the next post, we will talk about SI units thoroughly. We will talk about the definitions of base units, 22 derived units which have special names and symbols, SI prefixes, and other non-SI units adopted by SI, etc.