SI Background

A shorter page, giving only the section headings of this page and with no citations is available at "SI History Headlines".

1585 Decimal fractions and unit relationships proposed by Simon Stevin.

Simon Stevin (b. 1548 Brugge, Flanders-now Belgium; d. 1621) published De Thiende ("Of Tenths") in which he extolled the use of decimal fractions. Stevin was a lawyer, mathematician, surveyor, and engineer who made significant contributions to engineering, fortification, geography, mathematics, mechanics, astronomy, navigation, cartography, and physics. Though decimal fractions had been used for centuries by other cultures (e.g., Arabic and Chinese) this launched their more common use in Europe. His work included several articles on units of measurement that he proposed to decimalize, including monetary, surveying, celestial (angles), cloth,  and liquor (capacity) units. He predicted that the evolution of measurement systems to a decimal basis would be only a matter of time.15

1670 Key concepts for the metric system were proposed by Gabriel Mouton.

Gabriel Mouton (b. 1618, d. 1694-09-28), a mathematician, astronomer, and vicaire perpetuel of St. Paul's Church in Lyons, France, is often credited with devising the concepts that led to the metric system. Measurements used in the Middle Ages were a mess,  consisting of a hodgepodge of units, which were locally standardized and interrelated by inconvenient multiples (12, 14, 16, etc.) if at all. Neighboring jurisdictions had units of the same name but of unequal sizes. Many people had suggested various remedies but no one idea seemed to take root until Mouton made his contribution. He proposed that a portion of a longitudinal meridian (i.e., an arc minute of latitude-just under 2 km) on the Earth's surface be used as the basis for length and that it be divided decimally. (Some sources1,2 state that he proposed using a minute of longitude, rather than latitude, as the reference, but an astronomer would have known better.) His ideas included the key concepts of decimalization, rational prefixes, and the use of the Earth as basic and universal standard for length.  His proposal was debated and amended by others for 120 years before an attempt was made to implement it.1, 2, 3

1675 Universal unit of length, the metro catholico, proposed by Tito Livio Burattini.

Tito Livio Burattini (b. 1617 in Agordo, moved to Poland in 1647, d. 1681 in Krakow) was an Italian-Polish inventor, architect, Egyptologist, scientist, instrument-maker, and traveler. In 1675 he proposed a universal unit of length, the metro catholico (literally, "universal measure"), based on the length of a pendulum which beats out seconds of time in his manuscript, Misura Universale, published in Vienna.16, 17, 18, 19 This was proposed again and again over the next few centuries, but the variations in the local acceleration due to gravity in various parts of the world would have made this a rather imprecise standard. Subsequent makers of pendulum clocks learned that they had to provide for adjustments to the pendulum's length because of this. Ironically, 308 years later in 1983, the meter was defined in terms of the second of time, but in a manner that depends on the speed of light in free space and not on the length of a pendulum. Since the speed of light in free space is a universal constant, the precision problem presented by pendula was overcome.

1781 Coinage and standards of weights and measures were reserved to the United States in Congress by the Articles of Confederation.

The Articles of Confederation, precursor to the U.S. Constitution, stated4:
The United States in Congress assembled shall also have the sole and exclusive right and power of regulating the alloy and value of coin struck by their own authority, or by that of the respective States-fixing the standard of weights and measures throughout the United States- . . .

1786 The Continental Congress approved decimal coinage.

After adoption of the dollar as the monetary unit on 1785 July 06, Congress approved a decimal monetary system on 1786 August 08.10

1789 The power to control coinage and standards of weights and measures was transferred to Congress by the U.S. Constitution.

Article 1, section 8 of the Constitution of the United States gives Congress power to control the coinage and standards of weights and measures throughout the United States. It states4:
The Congress shall have Power . . . To coin Money, regulate the Value thereof, and of foreign Coin, and fix the Standard of Weights and Measures."

1790 Secretary of State Thomas Jefferson proposed two plans to Congress to simplify the existing weights and measures then in use by uniting and decimalizing them.

In his first annual message to Congress (1790 January), President George Washington pressed for uniformity in currency, weights, and measures. The House of Representatives selected a committee to prepare a bill to this effect and asked the Secretary of State to make a report to them. Secretary of State Thomas Jefferson proposed two distinct plans. The first urged Congress to "define and render uniform and stable the existing system" and to reduce the existing separate systems of dry and liquid measures to one set of measures.4 We still have separate systems, though. For example, our dry quart (about 1.1 L) is larger than our liquid quart (about 0.95 L) so flour and milk should be measured with separate measuring devices-which differ in size!

Jefferson's second plan put before Congress was

to reduce every branch [of measurement] to the same decimal ratio already established for coin, and thus bring the calculations of the principal affairs of life within the arithmetic of every man who can multiply and divide plain numbers.
Congress took no action.  Washington urged action again in his second annual message to Congress (1790 December 08) and the House referred the matter to the Senate, whose committee in this matter declined (1791 March 01) due to negotiations in progress with the French and British Governments to obtain an international standards of measurements. Washington urged action yet again in his third annual message to Congress (1791 October 25) and this time the Senate committee referred the matter to the Senate, recommending acceptance of Jefferson's plan, including decimalization of weights and measures. Nothing further of any consequence on this issue occurred for several years.4

1791 Following the French Revolution, the French National Assembly commissioned the National Academy of Sciences to reform French weights and measures; this study lead to the first metric system put in use.

The French found themselves faced with an incredible array of units used in measurement. Just prior to the French Revolution, Louis XVI initiated action to modernize measurements in France. This work was carried on by the provisional government and then by the Republic of France. To simplify matters, the National Academy of Science (in France) decided to settle on a standard length based on measurement of the Earth. They chose to call this the "metre" (from the Greek, "metron", meaning "measure") and set it equal to 1 part in 10 000 000 of the length of a meridian from the equator to the North Pole. This of course required a survey and the method selected was to survey a portion of the meridian and then to use extrapolation (based on latitudes determined by celestial measurements) to derive its size. The  survey was conducted by Jean-Baptiste Joseph Delambre and Pierre François André Méchain, starting in 1792 June. Final results were obtained in 1799.6

1792 Congress established the U.S. Mint and authorized building it in Philadelphia.

This was the first federal building project.5

1793 The U.S. Mint produced the first coins, decimally based on the dollar.

The mint delivered its first batch of coins-11 178 copper "cents" (i.e., "hundredths of a dollar")-in March.5

1795 France adopted the metric system. An attempt to carry a copy of the earliest meter standard to the United States ended in tragic failure.

Not wishing to wait for the survey to be completed, France enacted a law requiring the use of the metric system, based on provisional standards. A provisional meter standard was devised based on an earlier survey of France, carried out from 1739 to 1744. Copies of this meter, graduated rulers, and provisional kilogram standards were manufactured and distributed to the 559 districts of the French Republic. A botanist named Dambey embarked on a trip to the United States with a copy of the provisional meter and kilogram standards. However, the ship he was on was attacked by pirates in the Caribbean and he was either captured or killed. The standards he carried were lost and have never been found.7, 8, 9, 10

1799 Delambre and Méchain presented their survey results, which were accepted at an international conference. Metric standards were fabricated and certified.

Delambre and Méchain completed their work in surveying a meridian using triangulation methods from Dunkerque to Barcelone and their work was accepted at an international conference (conseil des Cinq Cents et au Conseil des Anciens) after compromises were made regarding the ellipticity of the Earth. The Earth's ellipticity actually causes an arc minute of latitude to represent a gradually changing distance when proceeding north or south. A mean value had to be derived and the amount of ellipticity used in that calculation has a very small effect, but one that precise metrologists care about. Delambre and Méchain deduced an ellipticity of 1/150 and this was combined with the results of a survey conducted in Peru between 1736 and 1740 to settle on an accepted value of 1/334. Ever since this date the meter has remained the same size. Different methods of defining the standard have evolved; these have served only to make the standard for the meter more precise and have not changed its length.4 , 9 This series of new definitions has merely freed the definition of the meter from the results of new surveys of the Earth, so the old definition (10 Mm from equator to pole) is no longer valid. Only those opposed to using the metric system seem to be fond of quoting this out-of-date definition.

Volumes (capacities) were defined in terms of this meter, with a liter defined as equaling a cube with sides of 0.1 m. The mass of water, when at the temperature of ice (0 °C) was used as the model for the mass standard and this was designated as the kilogram.6, 9  (But see the entry for 1901.)

A new standard meter bar ("le mètre étalon" or "the archive meter") was made of platinum, based on this survey, to represent ("realize", in metrology terms) the standard meter.  This and the new standard kilogram were fabricated by Etienne Lenoir (a maker of astronomical instruments and also the provisional meter and kilogram standards mentioned above) and, after certification, deposited in the Archives of the Republic (Archives de la République) on 1799 June 22. Copies of these would later be given to the United States.6

1799 Congress enacted a law requiring the surveyors of the ports to periodically check weights and measures used in import and export tax collection and to report discrepancies to the tax collector.

This was the first law passed by Congress regarding weights and measures but no real action on this would occur for 35 years due to a lack of workable standards in the hands of the inspectors.4

1805 The United States was given an iron copy of the standard meter bar and a brass copy of the standard kilogram mass.

Twelve copies of the archive meter were made of iron under the direction of J.G. Trallés, of the Helvetic Republic and a member of the committee responsible for the archive meter (le mètre étalon) and archive kilogram (le kilogramme étalon). Copies in brass were also made of the archive kilogram.. A copy of each of these was given to Ferdinand R. Hassler, who had been selected by President Thomas Jefferson to conduct the first survey of the east coast of the United States. The iron copy of the archive meter became known in the U.S. as "the committee meter" and the brass copy of the archive kilogram became known as "the committee kilogram". Hassler sold these to a member of the American Philosophical Society who stored them with the Society. Later, Hassler borrowed these as needed and used the committee meter as the reference for the surveys conducted by the U.S. Coast and Geodetic Survey. It and the committee kilogram were used by the Survey for other scientific work as well from 1807 to 1893.

1812  Napoleon temporarily suspended the compulsory provisions of the 1795 metric system adoption in France.

This action by Napoleon seems to have unsettled the sentiments of the early Americans. Congress never followed through on Jefferson's recommendations and Jefferson himself may have eased his pressure on them.

1821 Secretary of State John Quincy Adams reported to Congress on the weights and measures issue.

Adam's report submitted on 1821 February 22 reviewed the history of weights and measures in England, Europe, and the United States. The report included a detailed description of the metric system, giving its strengths and weaknesses. He viewed the metric system as something the United States should adopt but his report painted a picture that made it appear not have matured sufficiently for use. The situation in England with its weights and measures was not much better, as they headed toward replacing their ale gallon (282 cubic inches) and wine gallon (231 cubic inches) from the time of Queen Anne with a new, imperial gallon and bushel in 1824. Ironically, the gallon that remained in common use in the United States was the older "Queen Anne wine gallon". As a result, and as usual, Congress took no action, trusting neither the British nor the metric weights and measures to remain stable and therefore worthy of adoption. Left to their own resources the states adopted their own standards of measurement, usually from England, but varying from each other. In other words, there still were no American standards of weights and measures.4 , 10

1821 The United States bought platinum copies of the archive meter and archive kilogram.

Albert Gallatin, Minister of the United States to France, bought platinum copies of the archive meter and archive kilogram that had been certified by the eminent French physicist Arago. This "Arago Meter", strangely, seems never to have seen any use in the U.S. as a standard. However, the "Arago Kilogram" was used as one of the standards of mass by the United States. Attached to the case containing the Arago Kilogram is a silver plate that states, "Kilogramme comparé pour son Poids à l'Etalon Prototype des Archives de France, et verifié par M. Arago. Fortin fecit." The accompanying certificate states that it compares within 1 mg of the archive kilogram (kilogramme étalon), but buoyancy effects may not have been taken into account. In 1884 the Arago Kilogram was taken to the International Bureau of Weights and Measures (Bureau international des poids et mesures, or BIPM) at Paris and compared to two auxiliary kilograms of known relationship to the archive kilogram. The Arago Kilogram was found to have a mass of 1000 g - 4.63 mg. 4

1828 Congress adopted the troy pound standard for the minting of coinage.

Commerce could tolerate some inconsistencies in measurement but currency was a different matter. Congress finally settled on on standardizing American coinage on a particular troy pound mass bought by the United States in 1827. This brass copy was certified to be identical to the imperial troy pound of Great Britain and was purchased by Albert Gallatin, minister of the United States.4

1832 The United States Department of the Treasury adopted yard, avoirdupois pound, gallon, and bushel standards.

In 1830 the Senate directed that a survey of the situation be made and this task was passed to F.R. Hassler, Superintendent of the Coast Survey. As expected, he found that the weights and measures used in various ports differed from each other significantly, though their average values appeared to agree well with the values used in England. He filed a preliminary report in 1831, followed by a final report in 1832, and Congress finally directed that something be done by the Secretary of the Treasury.4

The Secretary of the Treasury (under whom the Coast Survey office worked) settled upon standardizing on the yard, pound, gallon, and bushel in use in England at the time of the American Revolution. In 1815, Hassler had bought a yard scale made by Troughton of London in 1814. This Troughton scale would later serve as a U.S. standard yard, from 1832 to 1857, when it was superseded by the Bronze Yard. The pound selected was the pound avoirdupois and was scaled up from the troy pound used by the mint by the factor of 7000/5760, those two numbers being in terms of the grain, which remains the unit of mass common to both pounds. The gallon selected was the Queen Anne wine gallon, most commonly used in the United States at that time and used in Great Britain until 1824. The bushel selected was the Winchester bushel, which appears to date back in England to the time of Henry VII. The size attributed to this bushel in the United States was 2150.42 cubic inches based on the average of the American port survey results. As it turns out, the Winchester bushel used in England had been set at 2148.28 cubic inches, so right from the start the American bushel differed from even the old English bushel. At the time they were selected, the gallon and bushel selected were no longer used in Great Britain, though they had been at the time of the American Revolution. Thus, the nonmetric standards in the U.S at this time were not "Imperial" as many Americans today tend to believe. Imperial measures in Great Britain were not established until 1824, after the American Revolution.4, 14

1832 The physicist Gauss suggested using the second as the base unit for time.

Gauss, a strong supporter of the metric system, suggested using the second, as astronomically determined, to be the base unit for time. Later, he became the first to measure the magnetic field of the Earth in units of length, mass, and time.11

1836 Congress directed that standards be distributed to the states.

Apparently pleased that somebody was taking concrete action, Congress passed a resolution on 1836 June 14 directing that standards for weights and measures be distributed to the states, thus unifying the units in use within the country. This report did not specify whether those should be metric standards or standards of the yard, pound, gallon, and bushel-however the latter were the ones distributed by the Treasury. Thus they became the de facto units of commonly used measures and rapid adoption by the states in their laws and regulations made them effectively the de jure standards. Two years later Congress directed that the Treasury Department distribute balances to the states to use with those standards.4

1840 The metric system was reinstated as the compulsory system in France.

1857 The United States received a new yard standard.

The British imperial yard and troy pound, dating from 1824, were destroyed in a fire in parliament in 1834. Those were the measures copied for the mint's troy pound and the Troughton yard, used as standards in the United States starting in 1828 and 1832, respectively. In 1834, Great Britain manufactured new standards, but by 1855 it was beyond dispute that the new yard standard was not stable in length. Great Britain manufactured new standards in 1855 and in 1856 presented two copies of their new standard yard and a copy of their new standard avoirdupois pound to the United States. One of those yard measures, known as bronze yard number 11, became the standard yard in the United States from 1857 (replacing the Troughton yard) until 1893 when the United States shifted to metric standards.4

1866. The use of the metric system was made legal (but not mandatory) in the United States by the Metric Act of 1866 (Public Law 39-183).

Congress made the metric system legal for use in the United States on 1866 July 28 by passing the Metric Act (Public Law 39-183). This law is so seminal that direct quotes are warranted. The first part made the metric system legal for use in the United States, but fell short of making it the only legal system. The first section states
Be it enacted by the Senate and House of Representatives of the United States of America in Congress assembled, That from and after the passage of this act it shall be lawful throughout the United States of America to employ the weights and measures of the metric system; and no contract or dealing, or pleading in court, shall be deemed invalid or liable to objection because the weights or measures expressed or referred to therein are weights or measures of the metric system.
The second section provided an official conversion table, effectively fixing the relationship between "metric denominations" and "denominations in use". That section states
And be it further enacted, That the tables in the schedule hereto annexed shall be recognized in the construction of contracts, and in all legal proceedings, as establishing, in terms of the weights and measures now in use in the United States, the equivalents of the weights and measures expressed therein in terms of the metric system; and said tables may be lawfully used for computing, determining, and expressing in customary weights and measures the weights and measures of the metric system.
One day before enacting this legislation, Congress resolved that "the Secretary of the Treasury be, and he is hereby, authorized and directed" to deliver to the Governor of each state "one set of standard weights and measures of the metric system for the use of the States, respectively". The Office of Weights and Measures followed international practice and accepted the meter and kilogram of the Archives of France as the standards. When faced with the need to select the particular artifacts to copy, the Office of Weights and Measures selected their "Committee Meter" (see 1805) and "Arago Kilogram" (see 1821) as the models. By 1880 nearly all the states had been supplied with metric standards of length (a brass line meter and a steel end meter), capacity (a liter and a dekaliter), and mass (brass and silver weights of 10 kg and less).4

1874 The cgs system of units was developed by the British.

The British Association for the Advancement of Science (BAAS) put forth the cgs system of units, using the centimeter, gram, and second as the base units along with a set of prefixes from micro to mega. In the 1880s, the ohm, volt, and ampere were added by the BAAS, working in conjunction with the International Electrical Congress (which is now called the International Electrotechnical Commission, or IEC), because the corresponding cgs derived units were too small to be practical.11

1875 Treaty of the Metre signed in Paris by 17 nations, including the United States.

A preliminary conference of 15 nations was held in Paris in 1870 and another conference of 26 nations was held in 1872; the United States took part in both conferences, convened for the sake of forming a consensus on the construction of new metric standards. This resulted in a full, diplomatic conference in 1875 that included 20 countries and 17 of those countries, including the United States, signed a document called the "Metric Convention" ("La Convention du Mètre") or the "Treaty of the Metre". (Great Britain, Greece, and the Netherlands declined; ironically the Netherlands went metric in 1832 while the French returned to it only in 1840.) The Treaty provided for improved metric weights and measures and established the General Conference on Weights and Measures (Conférence Générale des Poids et Mesures, or CGPM), devoted to international agreement on matters of weights and measures. Under the CGPM, a working group, the International Conference on Weights and Measures (Comité International des Poids et Mesures, or CIPM) was established and the BIPM was designated to be the working laboratory and archiving facility. Several Consultative Committees report to the CIPM on various issues The CIPM, which meets annually, in turn reports to the CGPM, which meets every 4 years, for approval on major issues. Following ratification by the U.S. Senate and exchange of ratifications with the other nations, President Rutherford B. Hayes "proclaimed" the Metric Convention on 1878 September 27. By 1921, when the Treaty was amended to modify some working procedures, 37 nations had signatory status.4, 10, 11

One of those major issues approved by the CGPM was a large-scale modernization of the metric system in 1960, resulting in what is called the International System of Units (SI-the international symbol for Système international d'unités).

1889 As a result of the Treaty of the Metre, the U.S. received a prototype meter and kilogram to be used as measurement standards.

As directed by the Metric Convention, prototypes of the meter and kilogram were carefully made and selected to be the base units of length and mass. The second was chosen as the base unit of time. These three units were the basis for what became known as the MKS system of units in contrast to the BAAS's cgs system of units. In metrology (the science of measurement), "prototype" does not at all imply anything temporary; it means something more like "original specimen" or "primary standard" to which all copies must be compared. By 1889, 31 meter bars and 40 kilograms were fabricated and calibrated against the archive meter and the archive kilogram. The best of each were declared to be the international meter and the international kilogram and were deposited in a vault below ground in Sèvres. The others were delivered, with their calibration data, by lot to the signatory nations and the United States received meters 21 and 27 and kilograms 4 and 20. Meter 27 and kilogram 20 were carried under seal to the United States by George Davidson of the Coast and Geodetic Survey. The seals were broken in a ceremony at the White House on 1890 January 02, and the standards were officially accepted by President Harrison upon his inspection and certification of their authenticity. The other two standards arrived with equivalent care the next July.4, 11

1893 The United States standardized its measurements on the metric system, parting ways with Great Britain.

Throughout the replacement of yard and troy (and then avoirdupois) pound standards in Great Britain (see 1857), the United States had tried to keep its length and mass standards in equivalence with the British standards. On 1893 April 05, T.C. Mendenhall, then the Superintendent of Weights and Measures, decided that the metric standards were superior in quality to the British standards, in addition to having international standing and recognition. On that date, the Coast and Geodetic Survey published Bulletin 26, "Fundamental Standards of Length and Mass", which then became Appendix 6 to their 1893 annual report. This bulletin made the metric standards, received as a result of the 1875 Metric Convention, the official standards of weights and measures in the United States. The old units such as the yard, pound, gallon, and bushel were redefined in terms of those metric standards (which changed the sizes of the old units by a small amount important only to metrologists).  The United States ceased trying to continue matching the British standards and by 1959 this led to an even greater and more noticeable divergence. Ever since 1893, the United States has used metric standards for its weights and measures and all nonmetric units have been defined in metric terms.4

In other words, "inch", "foot", "gallon", "pound", and so forth have merely become non-decimal multiples of metric units in the United States. Of course, those are much harder to use than the metric prefixes. For example, 1 cm is defined as 0.01 m but 1 inch is defined as 0.0254 m. The pound has a metric definition in terms of kilograms that is several decimals long, which is even less convenient to multiply or divide by. And of course, the ratios between the various nonmetric units for each quantity are nondecimal also. Thus most Americans find metric units easier to use in their studies and work, once they learn the SI and get beyond emotional attachments to the old units.

1901 The liter was redefined, for a period of about 63 years.

In 1901 the 3rd CGPM used the mass of water at its densest state (at about 4 °C) as the reference point, with the liter determined by the mass of a kilogram of water, thus reversing the relationship! This would eventually evolve into what became a divergence between the cubic decimeter and the liter that amounted to about 28 parts per million.13 This was later abrogated but traces of the confusion it created can be found living on today. (See the entry for 1964.)

Another action taken by the 3rd CGPM was to distinguish between mass and weight. "Weight" was declared to be a quantity of the same nature as "force". The weight of a body was defined as the product of the mass of a body and the acceleration due to gravity at that location. Particularly, the standard weight of a body is its mass multiplied by the standard acceleration due to gravity. The standard value for the acceration due to gravity, gn, was defined as 9.806 65 m/s2 exactly, and this is the value used for uniformity in many scientific and engineering calculations. It is typical of the mid-latitude, freefall acceleration values measured near the Earth's surface.13

1901 The National Bureau of Standards was established.

Increasing industrialization of the United States led to the perceived need for a dedicated standards facility at the national level. This need was magnified greatly by the electrification taking place at the turn of the century. In response to the plethora of requests by scientists and industrialists, Congress passed an act renaming the Office of Weights and Measures as the National Bureau of Standards (NBS) on 1901 March 03. It was reorganized and given a mandate by Congress to fulfill certain functions, modeled after similar organizations in other countries. The NBS fully endorsed the action take by the Office of Weights and Measures in 1893 and accepted the meter and kilogram standards as the nation's fundamental standards. The assigned functions were expanded in 1950.4

1905 The National Conference on Weights and Measures was established.

One of the first undertakings by the National Bureau of Standards (NBS), by the direction of its first Chief of the Division of Weights and Measures, Louis A. Fischer, was a survey by inspectors of the weights and measures in use in several locations within New York state. It was found that there were many instances of inaccurate (perhaps even fraudulent) weights and measures being used; complaints from other states supported the concept that this was a national problem. Invitations went out to all states and a conference was held in Washington, DC in 1905. Although only 11 people (from 8 states, the District of Columbia, and NBS) attended this first meeting, it rapidly grew into what is now an incorporated National Conference on Weights and Measures (NCWM) and has met every year except during certain war years. The meetings are attended by state weights and measures officers, industry representatives, consumer representatives, and other interested parties.4 One of the functions provided by the NCWM has been to draft model regulations and laws that the various states may either cite directly or copy (whether verbatim or with amendment) as their individual state regulations and laws. Staff and printing support for the NCWM have been provided by NBS (later, NIST). The need for the NCWM and state weights and measures regulations and laws obviously arose because the federal government has not fully exercised its powers under Article 1, section 8 of the U.S. Constitution. (See the entry for 1789.)

1916 The U.S. Metric Association was launched.

"The Metric Association formed as a non-profit organization advocating adoption of the metric system in U.S. commerce and education. The organizational name was later changed to the U.S. Metric Association (USMA)."12

1918 The American Army went metric temporarily.

While fighting in Europe, during the First World War, the U.S. Army temporarily adopted and used the metric system.10

1948 The CGPM directed a study to simplify the metric system then in use.

Things had started getting confusing, what with various groups using either cgs or MKS units in their work, leaving others to convert back and forth. The metric system needed to be simplified and given structure, based on principles of metrology (the science of measurement). In 1948, the 9th CGPM noted that the International Union of Physics (later called the International Union of Pure and Applied Physics, IUPAP) recommended that the system be reorganized around the MKS units (meter, kilogram, and second) plus one "practical" (i.e., electrical) unit. Oddly, the recommendation did not recommend that the cgs system be abandoned by physicists! The French Government made a similar proposal and the CIPM was directed by the CGPM, to investigate the matter. One other thing that was decided here is that the unit "degree centigrade" was renamed the "degree Celsius".13

1954 The first 6 of 7 SI base units were settled upon.

The 10th CGPM chose as base units, the meter, kilogram, second, ampere, degree Kelvin (now, just "kelvin"), and candela.13

1959 The yard and pound were redefined, but still in terms of metric units.

By this time, the nonmetric units used in various countries were irritatingly discrepant. Since 1883 the U.S. had been using the definition of the pound based on a comparison with British standards but in 1889 Great Britain rounded off the number of grams used to define the pound, creating a small discrepancy. In 1893 the U.S. had redefined its yard as being exactly 3600/3937 meters while Great Britain retained its older Imperial standard yard. In both countries, the respective gallons and bushels (differing between the two countries) were defined in terms of cubic inches (36 inches = 1 yard) and the discrepancy between the yards made it difficult for each country to use the other country's gallon and bushel definitions. The Second World War (WWII) brought many of these problems to the forefront during Allied joint operations and their sharing of manufacturing resources. As a result an agreement was reached among the national standards laboratories of Australia, Canada, New Zealand, South Africa, the United Kingdom, and the United States. This agreement went into effect on 1959 July 01 and provided the following joint definitions:
1 yard = 0.9144 m, exactly
1 pound avoirdupois = 0.453 592 37 kg, exactly
As a result, a new unit, the International Mile, was defined as being exactly 5280 feet (3 feet = 1 yard) and the inch became a more or less convenient 2.54 cm exactly. This shift in the size of the yard and hence the foot and the inch raised a complaint among those concerned about land surveys in the United States. The old definition of the yard was then used to create two new U.S. units, the "survey foot" (= 1200/3937 m, exactly) and "survey mile" (= 5280 feet) to maintain the scale used in previous surveys. (U.S. surveyors do not use inches; they measure to hundredths of a foot!) Along with an international agreement in 1954, creating an International Nautical Mile of exactly 1852 m, this gives the United States three different miles by which to measure things! Fortunately, there is only one meter and all three miles are defined in terms of it.4

1960 The SI was born.

Based on the 1954 CGPM and the 1958 CIPM recommendation, the six base units named above and the prefixes pico through tera were approved by the 11th CGPM and the name "Système International d'Unités" was given to it. The symbol "SI" was selected to represent that name. Additionally, the CGPM accepted two "supplementary units" (later redesignated as non-SI units accepted for use with the SI) and 27 specially named, derived units.13

Another important concept was born at this time. The movement away from manmade artifacts as standards was begun, replacing them with universally available and measurable properties of nature. The second had been defined astronomically by the CIPM in 1956 based on a specified fraction of the tropical year at its rate during the first 12 h of 1900. The 11th CGPM ratified this definition in 1960 and redefined the meter in terms of the wavelength of a certain light emitted by krypton 86. This superseded a decision made by the 7th CGPM in 1927 that defined the meter in terms of the manmade standard meter in Sèvres. Later, other base units would likewise be redefined in terms of properties of nature, leaving only the kilogram mass (affectionately known sometimes as "L'Grande K") in Sèvres as a manmade SI standard. Expectations are that this too shall be replaced, making the standards of measurement universally available.13

This rebirth of the metric system as the SI, in effect, did away with the old cgs units such as erg, dyne, and calorie. Later revisions to the SI made it even more explicit that these units should not be used unless absolutely necessary and then only if defined in terms of SI units. Nonetheless, scientists are humans also and habits die hard. Their use is gradually diminishing, though as younger, more modern scientists come onto the scene. Perhaps unit preference could be used as a scientist dating scheme.

1964 The liter was restored to its original definition.

The decision of the 3rd CGPM (1901) regarding the liter  was abrogated by the 12th CGPM in 1964-once again defining "liter" as a special name for 1 dm3.13 No longer is there a difference between the milliliter and the cubic centimeter. Despite the insistence of some uninformed people, who rely on old practices and references, 1 mL = 1 cm3 exactly.

1968 The "degree Kelvin" was changed to just "kelvin", the second was redefined, and atomic clocks became the universal time standards.

The 13th CGPM changed the unit name "degree Kelvin" (°K) to just "kelvin" (K) and redefined its basis as the triple point of water. It also changed the definition of the second to be the amount of time it takes a cesium 133 atom to emit 9 192 631 770 waves of a certain light under specified conditions. This enabled scientists to create their own, valid time standards (atomic clocks) independently of any artifacts or historical records.13

1971 The 7th SI base unit was accepted.

The 14th CGPM added a seventh base unit, the mole. This completed the set of SI base units that we use today and combinations of only these 7 SI base units can be used to measure every physical quantity now imagined13:
quantity unit name symbol defined*
length meter m 1795
mass kilogram kg 1795
time second s 1832**
electric current ampere A 1881
thermodynamic temperature kelvin K 1954
amount of substance mole mol 1971
luminous intensity candela cd 1946
* The dates given, except for the second, are the original dates of the definitions. Some unit definitions were later revised-not to change the size of the units-but to improve the precision with which they could be measured. Always, the new definitions were calibrated to the previously defined units.
** Date of proprosal by Gauss. The current definition dates from 1956, which differs from the previous definition by an amount measureable only in an excellent physics lab.
The SI is only a bit over 40 years old (counting 1960 as its birth year) but it actually represents work that has been carried out for over 130 years (since 1870), with precedents that go back another 80 years (to 1790). The United States has been an active participant in that work since 1870 (and encouraged its development even before that). Therefore, the United States has had both "voice and vote" at every step of the way. SI is neither French nor American-it is indeed international. The ground at Sèvres, France, upon which the BIPM now stands, was ceded by the French to the world; it is international territory. Today, SI is the international language of measurement and is used in all fields from commerce and industry to science and technology. If the metric system may be considered to have been born in the 1790s, it came of age in 1875 and became a mature adult in 1960.
SI measurements are now the only ones used by over 95 % of the people of the world in their everyday lives, as well as at work-from supper on the table to satellites in space.

1971 The United States Metric Study, initiated in 1968, was completed.

As reported by the USMA,12
The U.S. Metric Study resulted in a Report to the Congress: A Metric America, A Decision Whose Time Has Come. The report concluded that the U.S. should, indeed, "go metric" deliberately and carefully through a coordinated national program, and establish a target date 10 years ahead, by which time the U.S. would be predominately metric.

1974 The federal government asked education systems to teach the metric system.

As reported by the USMA,12
The Education Amendments of 1974 (Public Law 92-380) encouraged educational agencies and institutions to prepare students to use the metric system of measurement as part of the regular educational program.

1975 Congress took action to metricate the United States, via a newly created U.S. Metric Board, on an industry-led, voluntary basis.

The Metric Conversion Act of 1975 (Public Law 94-168) passed by Congress established the U.S. Metric Board. This board was tasked to coordinate and plan the increasing use of the metric system and to devise a plan for voluntary conversion. However, the Act was devoid of any target dates for metric conversion. President Jimmy Carter, in an effort to have the board represent as many interests as possible, appointed members ("stakeholders") so diverse in their interests that the board was not very effective.

1979/1980 Wine and liquor bottling in the U.S. went metric.

As reported by the USMA,12
The Treasury Department's Bureau of Alcohol, Tobacco, and Firearms (BATF) required wine producers and importers to switch to metric bottles in seven standard [liter and milliliter] sizes and they required distilled spirits (hard liquor) bottles to conform to the volume of one of six standard metric [liter and milliliter] sizes.

1982 The U.S. Metric Board was disbanded by President Reagan.

As reported by the USMA,12
President Ronald Reagan disbanded the U.S. Metric Board and canceled its funding.

Responsibility for metric coordination was transferred to the Office of Metric Programs in the Department of Commerce. Within the Department of Commerce, the National Institute of Standads and Technology (NIST), was charged with this tasking.

1983 The meter was redefined, based on the speed of light in vacuum.

By 1975 it was recognized that any difference between the speed of light in vacuum and the world's best estimated value of 299 792 458 m/s was less than the uncertainty in our knowledge of the length of the meter. In other words, our reliance on the old definition of the meter was the leading cause of uncertainty in measuring the speed of light whereas time measurements could be more precisely made than length measurements. Since nature is deemed more stable than man-made meter bars or the ability to measure wavelengths, improvement in precision was to be had by assuming the number above as the speed of light and using that as a natural standard meter bar. The 17th CGPM then defined the meter as the length of the path light takes in a vacuum in 1/299 792 458 s.13 This tends to confuse people, but essentially all that happened is a rewording that allowed the use of more precise measurements in using the standards. Therefore, there was no change in the length of the meter and it is the same length it has been since 1799.

1988 The SI was designated as the preferred system of measurement in the U.S. and the Metric Conversion Act of 1975 was strengthened.

As reported by the USMA,12
The Omnibus Trade and Competitiveness Act of 1988 (Public Law 100-418) amended and strengthened the Metric Conversion Act of 1975, designating the SI metric system as the preferred measurement system, and requiring each federal agency to be metric by the end of fiscal year 1992.

1991 President George Bush directed federal agencies to metricate.

As reported by the USMA,12
President George Bush signed Executive Order 12770, Metric Usage in Federal Government Programs directing all executive departments and federal agencies implement the use of the metric system. The Executive Order is also available as an appendix to Interpretation of the SI for the United States and Federal Government Metric Conversion Policy, NIST Special Publication 811.

This EO 12770 directed that the Secretary of the Department of Commerce submit an annual report to the President on the status of metrication in the Executive Branch. As a result of this support of the Omnibus Trade and Competitiveness Act, most federal agencies are now metricated. All federal building (under the General Services Adnministration) is now metric. NASA missions which were "born" after 1988 are metric, except when exempted for one of a small set of allowable reasons; this exemption requires a waiver to allow the use of non-metric units. Nonetheless, the NASA Inspector General found, after the Mars Climate Orbiter disaster, that NASA was not adhering to its own requirements at all times and that tighter controls were needed.

1994 Inclusion of metric units on labels was required for all retail goods under federal auspices.

As reported by the USMA,12
The Fair Packaging and Labeling Act (FPLA) was amended by the Food and Drug and Administration (FDA) to require the use of dual units (inch-pound AND metric) on all consumer products.

1996 The National Weather Service's temperatures went metric.

As reported by the USMA,12
As of July 1996 all surface temperature observations in National Weather Service (NWS) METAR/TAF reports are now transmitted in degrees Celsius.

However, tasked by Congress to meet the needs of their customers, the NWS continues to provide temperatures in degrees Fahrenheit, when requested. Most other NWS measurements and records (e.g., pressures at various altitudes) have been metric for a very, very long time.

1999 The Uniform Packaging and Labeling Regulation (UPLR) of the NCWM was amended to allow metric-only labeling.

The National Conference of Weights and Measures (NCWM), a voluntary professional organization of the Weights and Measures Officers from all of the states amended its model Uniform Packaging and Labeling Regulation (UPLR) to allow metric-only labeling. Fourteen states implement their laws and regulations by citing this model UPLR as being legal in their jurisdictions, so in those fourteen states, metric-only labeling was allowed for retail goods under the auspices of the states. Most of the other states have rapidly followed suit, making metric-only labeling a widespread option available for many retail goods.

2000 September 30 Congress backed down from requiring metrication of highways

As reported by the USMA,12
The deadline by which all agreements, contracts, and plans processed by individual states for federally-funded highway construction were to be in metric units was canceled by Congressional action, leaving metric conversion as voluntary but still recommended to comply with the Omnibus Trade and Competitiveness Act of 1988.

Several State Departments of Transportation continue to use the metric system despite the deadline being rescinded. The Federal Highway Administration (FHWA) continues to work in metric and the American Association of State Highway Officials (AASHTO) continues to support and encourage the use of metric units.

2002 November 07 NIST convened a public forum on allowing metric-only labeling in the U.S.

NIST convened a public forum, "Efforts to Update Federal and State Packaging and Labeling Laws and Regulations to Give Manufacturers the Option to Voluntarily Label Packages with Only Metric Units", in Washington, DC at the Department of Commerce's Herbert C. Hoover Building. The forum was attended by a diverse audience who strongly, and nearly unanimously, supported a proposed amendment to the Fair Packaging and Labeling Act (FPLA) in order to allow metric-only labeling on goods in the U.S. This was seen as an essential cost-savings step for U.S. businesses, as well as a means of making labels less cluttered and easier to read. At the forum, the NCWM reported that approximately 40 states had already revised their laws and regulations to allow metric-only labeling on goods regulated by them. (See the 1999 entry on the UPLR.)

2003 November 06 NIST convened a second public forum on allowing metric-only labeling in the U.S.

NIST convened a second public forum on permissible metric-only labeling in Washington, DC at the Department of Commerce's Herbert C. Hoover Building. The forum was again attended by a diverse audience, larger than the previous year's, who strongly, and nearly unanimously, supported a proposed amendment to the Fair Packaging and Labeling Act (FPLA) in order to allow metric-only labeling on goods in the U.S. The reasons given were the same as before but the tone has shifted from one of hope to one of expectation. At the forum, the NCWM reported that approximately 45 states now allow metric-only labeling on goods which are regulated under their jurisdictions.

The future....

NIST plans to submit an amendment to the Fair Packaging and Labeling Act (FPLA), possibly as soon as 2005 January and thus after the 2004 national elections as described in the entry for 2002, and many in industry hope that it will pass. However, this is only permissive and does not declare that only SI units are legal in the United States. It would be fitting and proper for the U.S. to take this last step and fully metricate, as we signed up to do in 1875. One hundred and thirty years is long enough for us to have dawdled. Americans are ready to metricate and need to convince their government of that fact. When that happens, "The future...." above will be replaced by a final date and this web page will be complete.


1. Encyclopedia Brittanica, key word: Gabriel Mouton, 2000 July 04.
2. MATEMATIKA INTÉZET, 2000 July 04.
3. "Mathematicians in Richard S. Westfall's archive", 2000 July 04.
4. "Weights and Measures Standards of  the United States: a brief history", U.S. Department of Commerce, National Bureau of Standards [now, National Institute of Standards and Technology, NIST], Miscellaneous Publication 247, 1963 October. [out of print]
5. U.S. Mint, 2000 July 04.
6. La Méridienne verte projet, 2000 July 04
7. personal communication, Michel LeFebvre, 2000 June 30.
8. personal communication, Louis Jourdan, 2000 June 30.
9. personal communication, Joseph Reid, 2000 July 01; reporting on a book, Le Mètre du monde by Denis Guedj, and drawing on his other resources and notes.
10. personal communication, Roger Pello (BIPM), 2000 July 01.
11. International Bureau of Weights and Measures (BIPM), 2000 July 06.
12. U.S. Metric Association, "A chronology of the SI metric system",, 2000 July 04.
13. The SI brochure, officially known as Le Système international d'unites (The International System of Units), BIPM, 7th edition, 1998.
14. personal communication, Chris Keenan, 2000 July 09 et seq., reporting on a book, Weights & Measures of England by Robin Connor, and drawing on his other resources and notes.
15. personal communication, Pat Naughtin, CAMS, 2000 August 11, from his personal research notes.
16. personal communication, Louis Jourdan, 2002 July 04.
17. Biographical Notes,
18. NUNCIUS: Annali di Storia della Scienza, 1998 (Istituto e Museo di Storia della Scienza, Firenze ITALIA), (in Enlish), (in Italian).
19. General information regarding History, Culture and local Traditions: Agordino (the area of Agordo),

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This page was launched on 2000 July 04 in celebration of Independence Day. It was last updated 2003 December 05.