Crude systems of weights and measures probably date from prehistoric times. Early units were commonly based on body measurements and on plant seeds or other objects from agriculture. As civilization progressed, technological and commercial requirements led to increased standardization. For example, because the length of the human foot or the width of the palm varies from individual to individual, it probably became necessary first to specify a particular individual (e.g., the king) and later to reproduce standards based on this commonly accepted unit of length. Units were usually fixed by edict of local or national rulers and were subdivided and multiplied or otherwise arranged into systems of measurement.
Standards varied greatly in different localities, although conquest and trade stimulated some correspondence between systems, e.g., between the systems of Egypt, Babylon, and Phoenicia. A high degree of standardization was achieved in the Roman Empire, but after its fall considerable diversity returned. The foot, which was one of the earliest units, is believed to have had as many as 280 variants in Europe as late as the 18th cent. Today the chief systems are the English units of measurement and the metric system.
The United States is one of the few countries still using the English system; all other major nations have either converted to the metric system or committed themselves to conversion. The English system is much older and less practical than the metric system, and in the United States there has been considerable discussion in favor of adopting the metric system as the principal system. However, attempts to legislate such a change in the U.S. Congress have failed. The basic units of the English system, the yard of length and the pound of mass, are now defined in terms of the metric standards, the meter of length and the kilogram of mass.
Before 1960 the meter was defined as the distance between two scratches on a prototype bar kept at the International Bureau of Weights and Measures (est. 1875) at Sèvres, France, near Paris. In 1960 it was redefined in terms of an atomic standard. This new standard is more stable than the old meter bar, is indestructible, and is easily reproduced, eliminating the need for periodic comparison with a single standard. The kilogram is defined in terms of a prototype cylinder kept at the bureau.
In the United States, Congress has the constitutional right to fix standards, but except for purposes of customs and internal revenue, weights and measures legislation has been, for the most part, permissive. Sets of official weights and measures were sent to the states in 1856, but legislation and enforcement are largely state prerogatives. The federal government permitted the use of the metric system in 1866 and established a conversion table based on the yard and the pound; in 1893 the yard and the pound were redefined in terms of the metric prototypes of the meter and the kilogram. The major arguments against total conversion to the metric system in the United States are that it would involve great expense in industry and would cause widespread confusion among the general public.
See the table entitled Common Weights and Measures.
See M. Blocksma, Reading the Numbers (1989).
The other organisations which maintain the SI system are the General Conference on Weights and Measures (CGPM, Conférence générale des poids et mesures) and the International Committee for Weights and Measures (CIPM, Comité international des poids et mesures).
The BIPM helps to ensure uniformity of SI weights and measures around the world. It does this with the authority of the Convention du Mètre, a diplomatic treaty between fifty-one nations and it operates through a series of Consultative Committees, whose members are the national metrology laboratories of the Member States of the Convention, and through its own laboratory work.
The BIPM carries out measurement-related research. It takes part in, and organises, international comparisons of national measurement standards, and it carries out calibrations for member states.
The BIPM has an important role in maintaining accurate worldwide time of day. It combines, analyzes, and averages the official atomic time standards of member nations around the world to create a single, official Coordinated Universal Time (UTC).