History of the International System of Units

The origin of the metric system, dates back a long way into French history. Before the metric system the old Frech measures had two fundamental problems and these were:

1. the units with the same name have varied from region to region, and
2. the subdivisions were not decimal, which increased the complexity of the commercial transaction.

The Abbe Gabriel Mouton, around 1670, proposed one-thousandth of a minute of arc (1/1000) at the meridian as a rational standard of length which is equal to 1.856 [m]. Mouton named this unit the milliare, from the Latin for one thousand, and divided it decimally into three multiple units, named respectively centuria, decuria, and virga, and three submultiples decima, centessima, and millesima. The English mathematician and architect Sir Christopher Wren, French astronomer Abbe Jean Picard, the Dutch scientist Christian Huygens, and the French geodesist Charles Marie de la Condamine independently proposed the length of the second's pendulum as a unit of length. In 1789, the idea of using the same measurement system, spread throughout France.
On March 9th, 1790, at the instigation of Charles Maurice de Talleyrand, Bishop of Autun and National Assembly Deputy, the Constituante (Name of the French Revolutionary Assembly from July 9th, 1789 until September 30th, 1791) initiated a unification project for weights and measures. After the project was adopted on May 8th, 1790, the Academy of Sciences was given the task to study the proposed project. The group of French scientists was made responsible for establishing the base unit. This group consisted of French mathematicians (Count Louis de Lagrange, Gaspard Monge, Charles de Borda, and Marie Jean Antoine de Condorcet) and one astronomer (Marquis Pierre Simon de Laplace). The group of French scientists decided on a unit of length equal to one ten-millionth of the distance between the equator and the pole, on March 19th, 1791. The defined unit was called metre, from the Greek word metron (for measure), and it is of course the origin of the name to the system itself.

The Systeme Metrique decimal was instituted in France on April 7th, 1795. The decimal metric system established a new set of units:

1. the are (Latin area) for area,
2. the litre (Greek litra, a 12-ounce weight) for volume, and
3. the gram (Greek gramma, the weight of a pea) for the mass.

In 1795 the length of the quarter meridian through Paris was determined by Jean Baptiste Delambre and Pierre Mechain, who had spent seven years measuring geodetic distances between Rodez and Barcelona, and Rodez and Dunkerque. The law established the length of the metre at 0.513074 toises de Paris, and prototype standards of dense pure platinum for the metre and the kilogram, made by Jeannetty from the agglomerated platinum sponge, were adopted in June 1799.
The old French unit system was shortly re-established by Emperor Napoleon in 1812. However, the metric system was reinstated by the Law of July 4th,1837 which declared the Decimal Metric System obligatory in France from January 1840. This law instituted penalties for the use of other weights and measures.
The metric system slowly extended its application beyond France and even adopted (not compulsory) in the United States in 1866. Internationalization can be traced back to the Commission International du Metre (International Metric commission) which was held in Paris on August 8th-13th, 1872. The treaty known as the Convention du Metre or International Metric Convention was signed by an assembly of representatives of 17 countries on May 20th, 1875. After signing the treaty the General Conference of Weights and Measures (CGPM) was established as well as the International Bureau of Weights and Measures (BIPM). The International bureau headquarters was established at the Pavillon de Breteuil, at Sevres, near Paris, considering the role of France as the birthplace of the metric system.

In 1889 the first General conference on Weights and Measures (1st CGPM) occured and the distribution of copies of the international standard prototype meter to the 21 member states of the International Metric Convention was organized. The copies of the new standard prototype (metre international) were built from platinum-iridium alloy (Pt90-Ir10) which is an outcome of the work of Sainte-Claire deville et al.

The Systeme International d'Unites (SI) is the final development of the metric system. Previous developments included:

• the cgs system (centimetre-gram-second),
• the MTS (metre-tonne-second),
• the MKS (metre-kilogram-force-second), and
• the MKSA or Giorgi system (metre-kilogram-second-ampere).

On 10th CGPM which occurred in 1954 the set of base units was adopted for the following physical quantities: length, mass, time, electric current, thermodynamic temperature, and luminous intensity. On 11th CGPM which occurred in 1960, the name System International d'Unites was adopted which is known by its international abbreviation SI. The SI system established rules for prefixes, for derived units, for supplementary units, as well as overall control of units of measurement. Since then the SI has furtherly evolved.

The SI unit system became mandatory in France in 1961. On 14th CGPM (1972) a new unit molde was defined as the amount of substance and was adopted as the seventh SI bse unit.

The SI unit system have several advantages:

• the system is both metric and decimal,
• the fractions have been eliminated,
• the multiples and submultiples are indicated by a system of standard prefixes which simplifies calculations,
• each physical qunatity is expressed by one unique unit
• the derived SI units are defined by simple equations that relates two or more base units
• the SI provides a direct relationship between mechanical, electrical, nuclear, chemical, thermodynamic, and optical units, thus forming the coherent system.