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 , 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 this unit was given the name metre, derived from the Greek word metron, meaning "a measure. 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 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 , and platinum artifact reference standards for the meter and the kilogram were constructed in June 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 the old units of measure were restored by Napoleon, Emperor of France. In 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 The international acceptance of the metric system was implemented by the Diplomatic Conference of the Meter, convened by the French government on Mar.
This conference produced the Treaty of the Meter, signed on May 20, , 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 first General Conference met in September 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. Metric expansion throughout the world Following the reinstitution of the metric system in France in , the use of the system expanded slowly into parts of Germany, Italy, Greece, the Netherlands, and Spain. After 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 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 and Great Britain in 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 , by Great Britain , South Africa , New Zealand , Canada , and Australia 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 , 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, The Act of 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.
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 Following its signing of the Metric Convention on May 10, , the nation received its prototypes of the standard meter bar and standard kilogram in These became the nation's official fundamental standards for length and mass.
In the U. Despite its efforts, little progress was made toward a wider U. Following World War II, however, and particularly following the USSR's successful launching 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 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. The resulting report, The U. Metric Study , 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 and established the U.
Metric Board "to coordinate the voluntary conversion to the metric system. 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 s. 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. Rather than trying to maintain dual inventories for domestic and foreign markets, a number of U. 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 , has already required almost all federal agencies to use metric units in their procurements, grants, and business activities by 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. The SI base units--expanded to seven in 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 the SI unit for time, the second, has been defined as exactly 9,,, periods of radio radiation emitted as a result of gyroscopic precession of the outermost electron in undisturbed cesium atoms. From to the SI unit for length, the meter, was defined as exactly 1,, 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. Copyright by Grolier Electronic Publishing, Inc.