Metrication is the process of introducing what is now the International System of Units, also known as SI units, to replace a jurisdiction's traditional measuring units. U.S. customary units have been defined in terms of metric units since the 19th century, and, according to United States law, the SI has been the "preferred system of weights and measures for United States trade and commerce" since 1988. The U.S. has a national policy to adopt the metric system. Under Executive Order 12770, signed in 1991 by President George H.W. Bush, all U.S. agencies were required to adopt the metric system. However, conversion was made not mandatory, so U.S. customary units remain in common use in government (for example, speed limits are still posted in miles per hour) and in many industries. There is government policy and metric (SI) program to implement and assist with metrication, but there is strong social resistance to further metrication. In cooking, baking, distance measurement and temperature measurement, customary units are generally used rather than metric. However, the SI is used extensively in fields such as science, medicine, electronics, the military, automobile production and repair, and international affairs. The U.S. uses metric measures in photography (35 mm film, 50 mm lens), medicine (1 mL of medication), nutrition labels (grams of fat), bottles of soft drink (liters), and volume displacement in engines (liters). The scientific and medical communities use metric units almost exclusively, as does NASA. All aircraft and air traffic control use only Celsius temperature at US airports and while in flight. Post-1994 federal law also mandates most packaged consumer goods be labeled in both customary and metric units. The U.S. has fully adopted the second, the SI unit for time.
History
18th century
Immediately after gaining independence from Great Britain, the United States used a variety of units of measure, including Dutch units and English units. The 1789 Constitution grants Congress the authority to determine standards of measure, though it did not immediately use this authority to impose a uniform system. The United States was one of the first nations to adopt a decimal currency, under the Coinage Act of 1792. In 1793, Thomas Jefferson requested equipment from France that could be used to evaluate the metric system within the United States. Joseph Dombey returned from France with a standard kilogram. Before reaching the United States, Dombey's ship was blown off course by a storm and captured by pirates, or strictly (British) privateers in the Caribbean, he died in captivity on Montserrat.
19th century
In 1832, the customary system of units was formalized. In the early 19th century, the U.S. Coast and Geodetic Survey, the government's surveying and map-making agency, used a physical meter standard, the "Committee Meter", brought from Switzerland. Shortly after the American Civil War, the 39th United States Congress protected the use of the metric system in commerce with the Metric Act of 1866 and supplied each state with a set of standard metric weights and measures. In 1875, the United States solidified its commitment to the development of the internationally recognized metric system by becoming one of the original seventeen signatory nations to the Metre Convention, also known as the Treaty of the Metre. The signing of this international agreement concluded five years of meetings in which the metric system was reformulated, refining the accuracy of its standards. The Metre Convention established the International Bureau of Weights and Measures (Bureau international des poids et mesures, BIPM) in Sèvres, France, to provide standards of measurement for worldwide use. Under the Mendenhall Order of 1893, metric standards, developed through international cooperation under the auspices of BIPM, were officially adopted as the fundamental standards for length and mass in the United States, though some metric standards were used in practice before then. The definitions of United States customary units, such as the foot and pound, have been based on metric units since then. The 1895 Constitution of Utah, in Article X, Section 11, originally mandated that: "The Metric System shall be taught in the public schools of the State." This section was repealed, effective July 1, 1987. On July 4, 1876, Melvil Dewey (known for his Dewey Decimal Classification) incorporated the American Metric Bureau in Boston to sell rulers and other metric measuring tools. Dewey had hoped to make his fortune selling metric supplies.
20th century
The General Conference on Weights and Measures is the governing body for the modern metric system and comprises the signing nations of the Treaty of the Metre. The General Conference on Weights and Measures approved an updated version of the metric system in 1960 named Le Système international d'unités (International System of Units) and abbreviated SI. On February 10, 1964, the National Bureau of Standards (now known as the National Institute of Standards and Technology) issued a statement that it would use the metric system, except where this would have an obvious detrimental effect.
In 1968, Congress authorized the U.S. Metric Study, a three-year study of systems of measurement in the United States, with emphasis on the feasibility of metrication. The United States Department of Commerce conducted the study. A 45-member advisory panel consulted and took testimony from hundreds of consumers, business organizations, labor groups, manufacturers, and state and local officials. The final report of the study concluded that the U.S. would eventually join the rest of the world in the use of the metric system of measurement. The study found that metric units were already implemented in many areas and that their use was increasing. The majority of study participants believed that conversion to the metric system was in the best interests of the United States, particularly in view of the importance of foreign trade and the increasing influence of technology in the United States.
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