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 (b. 1548 Brugge, Flanders-now Belgium; d. 1621) published
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 (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,
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,
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
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
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
|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.
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
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,
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,
(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
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
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
1812 Napoleon temporarily
suspended the compulsory provisions of the 1795 metric system adoption
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
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
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
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
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
The second section provided an official conversion table, effectively fixing
the relationship between "metric denominations" and "denominations in use".
That 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.
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
|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.
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 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
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,
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
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
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
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
||= 0.9144 m, exactly
|1 pound avoirdupois
||= 0.453 592 37 kg, exactly
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:
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
|amount of substance
|* 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.
|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.
|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.
INTÉZET, 2000 July 04.
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]
Mint, 2000 July 04.
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.
Bureau of Weights and Measures (BIPM), 2000 July 06.
12. U.S. Metric Association,
"A chronology of the SI metric system", http://lamar.ColoState.edu/~hillger/dates.htm,
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
17. Biographical Notes, http://www.egiptologia.pl/epr_ebi.html.
18. NUNCIUS: Annali di Storia della Scienza, 1998
(Istituto e Museo di Storia della Scienza, Firenze ITALIA), http://galileo.imss.firenze.it/pubblic/e1998.html
(in Enlish), http://galileo.imss.firenze.it/pubblic/i1998.html (in Italian).
19. General information regarding History, Culture
and local Traditions: Agordino (the area of Agordo), http://www.dolomiti-altevie.it/inglese/altaVia/Agordino.htm.
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This page was launched on 2000 July 04 in celebration of Independence Day. It was last updated 2003 December 05.