The Metric System

The modern metric system of units and standards of measure is rooted in 17th-and-18th century efforts to establish a simple, easily used system of weights and measures universally acceptable to the countries of the world. These efforts were motivated by two guiding principles. In the first place, there were many who hoped for the definition of a single unit of measure that could serve as the basis for the logical construction of a complete and consistent system of units of measure; in the second place, there was also a growing number of people favoring decimal relationships for the necessary units of the same quantity; that is, multiples by factors of ten or submultiples by factors of one-tenth were considered to be the desirable means of obtaining systematic units of measure that would be a convenient size for all needs.

The forces driving toward a change from diverse and essentially unrelated customary systems of measure included rapidly growing international commerce and the changing political structure of Europe and its colonial dependencies. Within the new national structures it became necessary to accommodate many incompatible ways of doing business. Moreover, the growth of scientific investigation not only created new demands for accuracy and uniformity in measurements, it also provided the vision for a universally acceptable scientific basis for a system of measurements. The customary systems, handed down mainly from the Babylonians, Egyptians, Greeks, and Romans, were based on unrelated objects and phenomena, including human anatomy, with no practical hope for uniformity within integrated communities, states, or aggregated nations.

Origins of the Metric System

The birth of the metric system occurred in the climate of bold reform and scientific rationalization that prevailed in France during the latter part of the 18th century. In April 1790, Charles Maurice de Talleyrand, then Bishop of Autun, placed before the National Assembly of France a plan based on a unit of length equal to the length of a pendulum that would make one full swing per second. The French Academy of Sciences organized special committees to study the related issues. While many scientists favored the concept of a unit of length derived from a pendulum, there were many recognized practical difficulties. These included variations with temperature and the different values of gravitational force at different places on the surface of the Earth. After scientific consideration of the alternatives, the committee recommended a new unit of length equal to one ten-millionth of the length of the arc from the equator to the North Pole, or a quadrant of the Earth's meridian circle. In May 1793 this unit was given the name metre, derived from the Greek word metron, meaning "a measure." From the same word came the name of the new system. The unit of mass, the kilogram, was defined as the mass of water contained by a cube whose sides are one-tenth the unit of length. The unit of volume, the liter, was defined in the same way; thus the unit of length became the basis for the system. At that time the units of length, mass, area, volume, and time satisfied the needs of commerce. The new Republic of France adopted the recommendations of the French Academy in 1795.

Development of the System

The French Academy of Sciences also recommended, for practical reasons, that the primary reference standard for the unit of length be realized from the definition of the unit by a very precise measurement of the arc of meridian between Dunkirk, France, and Barcelona, Spain. The length of the arc from the equator to the North Pole was then to be inferred from astronomical measurements of angle. The survey was completed in November 1798, and platinum artifact reference standards for the meter and the kilogram were constructed in June 1799. These two standards, deposited in the French National Archives in Paris, later came to be known as the Meter of the Archives and the Kilogram of the Archives.

The introduction of the metric system in France met with the usual resistance to change. In 1812 the old units of measure were restored by Napoleon, Emperor of France. In 1840 the metric system again became mandatory in France, and it has remained so ever since. Meanwhile, the use of the metric system spread slowly to other European countries and even to the United States, where it became legal, but not mandatory, in 1866. The international acceptance of the metric system was implemented by the Diplomatic Conference of the Meter, convened by the French government on Mar. 1, 1875, and attended by delegates from 20 countries. This conference produced the Treaty of the Meter, signed on May 20, 1875, by the delegates of 17 countries--including the United States, the only English-speaking country to sign.

The metric treaty provided the institutional machinery needed to promote the refinement, diffusion, and use of the metric system. The International Committee for Weights and Measures, widely known as CIPM (Committee International des Poids et Mesures), was established under the broad supervision of the General Conference on Weights and Measures, CGPM (Conference General des Poids et Mesures), consisting of delegates from member countries. The first General Conference met in September 1889 to approve new international metric prototype reference standards to redefine length and mass. These prototypes were based on the Archives standards. The First CGPM also ratified the equality (within known uncertainties) of a number of national prototype standards for length and mass and distributed these standards to the member nations. This was the beginning of the diffusion of a uniform metric system throughout the world. The Metric Convention also established the INTERNATIONAL BUREAU OF WEIGHTS AND MEASURES, BIPM (Bureau International des Poids et Mesures), to carry out the scientific work of the International System under the supervision of CIPM.

Metric expansion throughout the world

Following the reinstitution of the metric system in France in 1840, the use of the system expanded slowly into parts of Germany, Italy, Greece, the Netherlands, and Spain. After 1850 the growing interest in large international commercial exhibitions accelerated the expansion of the use of the metric system as a common language of measurements, and by 1880 the major European countries and most of South America had adopted it. At the beginning of the 20th century the metric system was officially in use in 35 countries, and the only large industrialized countries not included in that number were the British Commonwealth countries and the United States. Both the United States (in 1875) and Great Britain (in 1884) had become signatory nations of the Treaty of the Meter, though, thereby recognizing the importance of a common international basis for their national systems of measurement.

The metric displacement of customary measurement systems in major English-speaking countries of the world has developed much more slowly. Changes in the patterns of international trade and the importance of new markets in developing--as well as developed-- countries has nevertheless brought about a practical regard for the necessity of uniform units of measure on an international scale. The shift toward metric conversion was well established in English- speaking countries by the middle of the 20th century. Official action to adopt the system for nationwide everyday use was finally taken, after the establishment of the International System of Units in 1960, by Great Britain (1965), South Africa (1968), New Zealand (1969), Canada (1970), and Australia (1970). As the final quarter of the 20th century approaches, only the United States, Liberia, and Burma remain uncommitted to the mandatory use of the metric system in daily life.

The Metric System in the United States

In the United States, there had been much official and scientific interest in the development of the metric system during the earliest days of the nation. President Washington urged Congress to take action toward uniform measurements throughout the land. Thomas Jefferson and John Quincy Adams, during their terms as secretary of state, carried out comprehensive studies that included consideration of the merits of the metric system developments in France. Following an additional special study by the newly organized National Academy of Sciences in January 1866, Congress enacted legislation authorizing (but not mandating) the use of the metric system in the United States. This legislation was signed into law by President Andrew Johnson on July 20, 1866.

The Act of 1866 was an important turning point in the history of measurements in the United States. By making it lawful to employ weights and measures of the metric system, the Act made a first step toward eventually harmonizing the U.S. measurement system with those of other nations. The Act also defined by law the relationships to be used in calculating the values of customary units of measurement used in the United States from the corresponding metric units. Moreover, in that same year a joint resolution authorized and directed the secretary of the treasury to furnish each state with one set of standard metric weights and measures.

The United States was an important participant in the Convention of the meter held in Paris in 1875. Following its signing of the Metric Convention on May 10, 1875, the nation received its prototypes of the standard meter bar and standard kilogram in 1893. These became the nation's official fundamental standards for length and mass. In 1901 the U.S. NATIONAL BUREAU OF STANDARDS was established for the purpose of serving the worlds of science and technology. Despite its efforts, little progress was made toward a wider U.S. acceptance of metric units.

Following World War II, however, and particularly following the USSR's successful launching (1957) of the first space satellite, Sputnik--which opened the age of space exploration--a renewed interest in the metric system developed in the United States. By 1968 the spread of metric measurements throughout the world was nearly complete. Arguments for conversion based on expanding foreign markets were becoming increasingly persuasive. Recognizing these trends, Congress, on Aug. 9, 1968, authorized the secretary of commerce to undertake an intensive study to determine the advantages and disadvantages of increased U. S. use of the metric system. The resulting report, The U. S. Metric Study (1970-71), concluded that the nation eventually would join the rest of the world in using the metric system and urged a carefully planned transition to this use. On the recommendation of the study, Congress enacted the Metric Conversion Act of 1975 and established the U. S. Metric Board "to coordinate the voluntary conversion to the metric system." The Office of Metric Programs then replaced the Metric Board in 1982.

Despite such efforts by the federal government, no states have as yet enacted legislation mandating the adoption of International Units. Thus, popular use of the metric system was still almost nonexistent by the early 1990s. The kind of pressure to adopt the system that has a greater likelihood of success is instead coming from the business community. Such pressure is being exerted in the cause of international competition and trade. Organizations such as the European Economic Community, for example, have threatened to restrict U.S. imports that do not conform to metric standards, and some nations on occasion have already rejected shipments outright for such reasons. Rather than trying to maintain dual inventories for domestic and foreign markets, a number of U.S. corporations have chosen to go metric. (For example, motor vehicles, farm machinery, and computer equipment are manufactured to metric specifications.) As business goes, so probably will go the nation. The Omnibus Trade Bill, passed in 1988, has already required almost all federal agencies to use metric units in their procurements, grants, and business activities by 1992.

Base units and derived units

When the metric system was first conceived, one of the goals was the definition of a single unit from which the essential system of measurements could be constructed. Indeed, it was thought that the unit of length, the meter, should be regarded in this way, and much scientific effort went into the careful selection of an acceptable definition. It was also necessary to rely on the properties of pure water in order to define a unit of mass, the kilogram. Thus, the measurement system required for trade and commerce in the 18th century rested on the definitions of two units; units for other necessary quantities, such as area and volume, were derived from them. The ultimate goal of a complete system of measurements logically derived from the definition of a single unit was not realizable when the metric system was first established, and it is not realizable today. Nevertheless, the fundamental idea persisted, and a modern metric system has been founded on six base units and designated by the 11th CGPM (1960) as the International System of Units with the international abbreviation SI (see UNITS OF MEASUREMENT). The SI base units--expanded to seven in 1971--are independent by convention, and are the meter, kilogram, second, ampere, kelvin, mole, and candela. It is possible, in principle, for industrial nations to maintain complete systems of measurement that are equivalent within acceptable limits of uncertainty by comparing national standards for the SI base units to those maintained by the International Bureau of Weights and Measures, BIPM (Bureau International des Poids et Mesures), in Sevres, France.

Future trends

The seven SI base units constitute a complete set in the sense that all the other necessary units of measure can be logically derived from them. In a practical sense, these seven also constitute an irreducible set within which no member can be derived from any combination of the others. It is, however, possible that advances in science and technology will result in a reduction of the number of SI base units. Since 1967 the SI unit for time, the second, has been defined as exactly 9,192,631,770 periods of radio radiation emitted as a result of gyroscopic precession of the outermost electron in undisturbed cesium atoms. From 1960 to 1983 the SI unit for length, the meter, was defined as exactly 1,650,763.73 wavelengths of one of the spectral lines of krypton-86. By 1983, however, even this laser-generated wavelength came to be considered insufficiently accurate in reproducibility, and the meter was redefined as the length of the path traveled by light in a vacuum during a time interval of 1/299,792,458 of a second. That is, the standard unit of length is defined in terms of the speed of light.

Modern methods for the measurement of luminous energy provide another example of advances that may, in principle, reduce the number of necessary SI base units. The unit of luminous intensity, the candela, is defined in terms of the radiation from a defined small area of a platinum body at a specified high temperature. It has become possible to measure such radiation by direct comparison to equivalent small amounts of electrical power. Therefore, electrical units--watts--are, in principle, sufficient for the measurement of optical radiation flux, as well as of electrical power.

Recent advances using X rays to sense the positions of atoms in pure samples of perfect crystalline structures have made it possible to determine the number of atoms in a known amount of substance with great accuracy. On this basis, it may also become practical to derive the SI mole directly from the kilogram, contributing thereby to further simplification of the SI base units. Such advances even point the way to a possible redefinition of the kilogram in terms of the mass of a selected universally available atom. The present kilogram is the only SI base unit that is still defined in terms of an artifact kept at Sevres.

In the case of special units in different disciplinary fields, it is clearly desirable to encourage a trend toward uniform practice. For example, the units used to measure the physiological effects of optical radiation include a factor for the average efficiency of the human eye, while the corresponding units used in physics and engineering for the same quantity do not. Similar examples exist in the case of other units that are used for physiological responses, including acoustic power and energy, and ionizing radiation dose. Those who are concerned with the refinement of the modern metric system seek ways to harmonize such diverse measurement practices while at the same time avoiding any tendency to make the system less useful to those who have special needs. The objective is to reduce the potential for confusion and error arising from the limitations of measurement language used in widely different fields of scientific and technological specialization.

Arthur O. McCoubrey
Biblio:   Bibliography:  Batchelder, J.  W., Metric Madness:  150 Reasons for
Not Converting to the Metric System (1981);  Chisholm, L. J., Units
of Weights and Measures (1967;  repr.  1975); Griffin, H.  J., ed.,
Comparative History of Metrology (1983); Hewitt, P.  L., Modern
Techniques in Metrology (1984);  Lytle, R.  J.  American Metric
Handbook (1981);  Mechtly, E.  H., The International System of Units
(1977);  Nelson, R.  A., The International System of Units, 2d ed. 
(1983);  Watson, F.  D., Going Metric (1991).

                         Table 1
Prefix            Symbol            Multiplier
exa                 E               1,000,000,000,000,000,000
peta                P               1,000,000,000,000,000
tera                T               1,000,000,000,000
giga                G               1,000,000,000
mega                M               1,000,000
kilo                k               1,000
hecto               h               100
deka                da              10
deci                d               0.1
centi               c               0.01
milli               m               0.001
micro               u               0.000 001
nano                n               0.000 000 001
pico                p               0.000 000 000 001
femto               f               0.000 000 000 000 001
atto                a               0.000 000 000 000 000 001

                    Table 2
Unit                Metric                   English
acre                0.405 hectares           4,840 sq yards
barrel (oil, US)    159 liters               42 gallons (US)
carat               200 milligrams           .007 ounce avdp
degrees, C          water boils at 100       multiply by 1.8
 (Celsius)            deg. C, freezes at       and add 32 to
                      0 deg. C                 obtain degrees
degrees, F          subtract 32 and divide   water boils at 21
   (Fahrenheit)       by 1.8 to obtain         deg. F, freezes
                      degrees C                at 32 deg. F
foot                0.3048 meters            0.333 yards
gallon (US)         3.79 liters              4 quarts, liquid
hectare             0.1 sq kilometer         2.47 acres; 10,00
                                                sq meters;
                                                11,960 sq yard
hectoliter          100 liters               26.42 gallons (US
hundredweight       45.36 kilograms          100 pounds, avdp
inch                2.54 centimeters         0.0278 yards
kilogram            0.001 tons, metric       2.2 pounds, avdp
kilometer           1,000 meters             0.621 miles
kilometer, square   100 hectares             247 acres; 0.386
                                               sq miles
liter               0.01 hectoliter          1.06 quarts,
meter               100 centimeters          1.09 yards
meter, square       1.196 sq yards
meter, cubic        1,000 liters             1.308 cu yards; 4
                                                board feet
mile, nautical      1.852 kilometers         1.151 miles
mile (statute)      1.61 kilometers          5,280 feet
mile,square         259 hectares             640 acres; 2.59 s
pound, avdp         0.454 kilograms          16 ounces, avdp
quart (US)          0.946 liters             0.25 gallons (US)
ton, deadweight     1.016 metric tons        2,240 pounds, avd
ton, long           1.016 metric tons        2,240 pounds, avd
ton, metric         1,000 kilograms          2,205 pounds, avd
ton, register       2.83 cu meters           100 cu feet
ton, short          0.907 metric tons        2,000 pounds, avd
yard                0.914 meters             3 feet
yard, square        0.836 sq meters          9 sq feet
yard, cubic         0.765 cu meters          27 cu feet

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