The metric system is a decimalized system of measurement based on the metre and the gram. It exists in several variations, with different choices of base units, though these do not affect its day-to day use. Over the last two centuries, different variants have been considered the metric system. Since the 1960s the International System of Units (SI) (“System International d’ Unites’’ in French, hence “SI”) has been the internationally recognized standard metric system. Metric Units of mass, length, and electricity are widely used around the world for both everyday and scientific purposes.

The proliferation of disparate measurement systems was one of the most frequent causes of disputes amongst merchants and between citizens and tax collectors. A unified country with a single currency and a countrywide market, as most European countries were becoming by the end of the 18 th century, had a very strong economic incentive and was in a position to break with this situation and standardize on a measuring system. The inconsistency problem was not one of different units but one of differing sized units so instead of simply standardizing size of the existing units, the leaders of the French revolutionary governments decided that a completely new system should be adopted. The first official adoption of such system occurred in France in 1791 after the French Revolution of 1789. The creators of this metric system tried to choose units that were logical and practical. The revolution gave an opportunity for drastic change with an official ideology of “pure reason”. It was proposed as a considerable improvement over the inconsistent collection of customary units that existed before, and that it be based on units of ten, because scientists, engineers, and bureaucrats at the time found this more convenient for the complex unit conversion they often must do.

The adoption of the metric system in France was slow, but its desirability as an international system was advocated by geodesists and others. Since then a number of variations on the system evolved. Their use spread throughout the world, first to the non-English-speaking countries, and more recently to the English-speaking countries.

The whole system was derived from the properties of natural objects, namely the size of the Earth and the weight of water, and simple relations in between one unit and the other. In order to determine as precisely as possible the size of the Earth, several teams were sent over several years to measure the length of as long a segment of a meridian as feasible. It was decided to measure the meridian spanning Barcelona and Dunkirk which was the longest segment almost fully over land within French territory. It should be noted that even though, during the many years of the measurement, hostilities brokeout between France and Spain, the development of such a standard was considered of such value that Spanish troops escorted the French team while in Spanish territory to ensure their safety.

The whole process ended in the proclamation on June 22, 1799 of the metric system with the storage in the Archives of the Republic of the physical embodiments of the standard, the prototype metre and the prototype kilogram, both made in a platinum alloy, witnessed by representatives of the French and several foreign governments and most important natural philosophers of the time. The motto adopted for the metric system was: “for all men, for all time”.

In revolutionary France the system was not particularly well accepted, and the old units, now illegal, remained in widespread use. On February 12, 1812, Napoleon, who had other concerns than enforcement of the system, authorized the usage of Mesures usuelles, traditional French measures redefined on the base of Metric System (toise as 2 metres, livre as 500 grams, etc.) and finally in 1816 a law made these Mesures usuelles standards (this law was cancelled in 1825 and the metric system reinstated). It was also reinstated in 1820 by a somewhat unlikely person, King William I of the (United) Netherlands. Although he was generally considered more conservative, he was desperate to bring at least some form of unity to his rather disunited kingdom. His attempts were vain in that Belgium claimed its independence from the Netherlands, but the metric system survived and began a slow but steady conquest of the world. By the 1960s,the majority of nations were on the metric system and most that were not had started programmes to fully convert to the metric system (metrication). As of 2005 only three countries, the United States, Liberia, and Myanmar (Burma) had not mandated the metric system upon their populace.

Later improvements in the measurement of both the size of the Earth and the Properties of water revealed discrepancies between the metric standards and their originally intended values. The Industrial Revolution was well under way and the standardization of mechanical parts, mainly bolts and nuts, was of great importance and they relied on precise measurements. Though these discrepancies would be mostly hidden in the manufacturing tolerances of those days, changing the prototypes to conform to the new and more precise measurements would have been impractical particularly since new and improved instruments would continually change them.

It was decided to break the linkage between the prototypes and the natural properties they were derived from. The prototypes then became the basis of the system. The use of prototypes, however, is problematic for a number of reasons. There is the potential for loss, damage or destruction. There is also the problem of variance of the standard with the changes that any artifact can be expected to go through, though they be slight. Also whilst there may be copies, there must be only one official prototype which cannot be universally accessible.

The metre had been defined in terms of such a prototype and remained so until 1960. At that time, the metre was defined as a certain number of wavelengths of a particular frequency of light emitted by a certain element. Since 1983 the metre has been defined as distance light travels in a given fraction of a second in a vacuum. Thus the definition of the metre ultimately regained a linkage with a natural property, this time a property thought immutable in our universe and truly universal. The kilogram is now the only base unit still defined in terms of a prototype. Since 1899, the kilogram has been formally anchored to a single platinum- iridium cylinder in Sevres, France.

On May 20, 1875 an international treaty known as the Convention du metre (Metre Convention) was signed by 17 states. This treaty established the following organizations to conduct international activities relating to a uniform system for measurements:

• Conference general des poids et mesures (CGPM) an intergovernmental conference of official delegates of member nations and the supreme authority for all actions;
• Comite international des poids et mesures (CIPM), consisting of selected scientists and metrologists, which prepares and executes the decisions of the CGPM and is responsible for the supervision of the international Bureau of weight and Measures.
• Bureau international des poids et measures (BIPM), a permanent laboratory and world centre of scientific metrology, the activities of which include the establishment of the basic standards and scales of the principal physical quantities and maintenance of the international prototype standards.

The metric system is used widely for scientific purposes but there are some exceptions, especially at large at large and small scales, such as the parsec. It has been adopted for everyday life by most nations through a process called metrication. As of 2006, 95% of the world’s population live in metricated countries, although non-metric units are still used for some purposes in some countries. The holdouts to full metrication are the United States and, to lesser degree, the United Kingdom, where there is public attachment to the traditional units.