Measurement, Metric System of

The metric system of measurement, more correctly called the International System of Units, is a system of weights and measures agreed through a network of international agreements. Using the first two initials of its French name Système International d'Unités, the International System is called SI. The foundation of the system was laid out in the Treaty of the Meter (Convention du Mètre), signed in Paris on May 20, 1875. The United States was a founding member of this metric club, having signed the original document in 1875. Forty-eight nations have signed this treaty, including all of the major industrialized countries.

The Bureau International des Poids et Mesures (BIPM) is a small agency in Paris that supervises the SI units. The units are updated every few years by the Conférence Générale des Poids et Mesures (CGPM), which is attended by representatives of all the industrial countries and international scientific and engineering organizations.

Gabriel Mouton of Lyon originally proposed the metric system as early as 1670. His proposals arose out of attempts to measure nautical distance and to find a suitable unit of length that could be related to the degrees of the arc on lines of longitude and other meridians . Mouton's system was incomplete, but French scientists continued work on his ideas throughout the next century.

In 1790, Talleyrand introduced the subject of standard measurements to the French National Assembly. The result of the ensuing debate was a directive to the French Academy of Sciences to make recommendations to the government. Talleyrand invited both England and the United States to collaborate on the new system of measures, but both declined. England declined because it thought that importing a revolutionary new system of measurement would bring with it the social aspects of the French Revolution. The United States, upon hearing of the English response, also declined to join the French initiative. In addition to substantial opposition in Congress, the United States did not want to damage the fragile relationship between the two countries so soon after American independence.

Birth of the Meter

After much deliberation, the academy recommended that the length of the line of longitude passing through Paris be determined from the North Pole to the equator. This distance was to be divided by one million and formed the length now known as the meter. This single unit of measure was used to determine all other units, both subdivisions and multiples. In an attempt to unify length and weight, the standard weight was defined as the amount of water in a cube whose side was one-hundredth of the new unit of length.

The experimental determination of the new unit of length was deemed too impractical. As a result, the meridian that ran through Dunkirk, France, and Barcelona, Spain was used as the basis of determining the standard meter. Initially, there was debate over whether to use a decimal or duodecimal system of subdivisions, with the decimal system being eventually adopted. There was also a recommendation for the decimalization of time with 10-hour days, 100-minute hours, and 100-second minutes, but this last proposal never left the committee rooms.

The final law of 1795 organized the basic standards and their prefixes. For multiples of the standard unit, Greek prefixes were used: kilo for thousand, hecto for hundred, and deca for ten. For subdivisions of the standard, Latin prefixes were assigned: milli for one-thousandth, centi for one-hundredth, and deci for one-tenth.

In 1798, three platinum standards, together with several iron copies, were manufactured. Just one year later the French Republic collapsed, and the Consulate and Empire that ruled after the Republic had little interest in popularizing the new system of measurement. When the monarchy was restored in 1814, the new system had made little impact on everyday life, as there had been a total failure to distribute copies of the standards throughout France. The new units of measurement were met with great skepticism from the public and the timetable for change was continually extended. There was also a second system, closely aligned to the traditional units of France, enacted to give the populace a feeling of continuity. It was not until 1840 that the metric system became the official single method of measurement in France.

Although the metric system took a long time to gain popular support, it did achieve rapid acceptance within the scientific community. No sooner had the meter been adopted as the official unit of measure than a problem arose. In 1844 the German scientist Bessel conducted experiments on the shape of Earth and determined that it was not spherical, which was what the construction of the meter was based on. Bessel's finding was confirmed

by George Everest of the British Army, von Schubert of the Russian army, and Clarke of the British Ordinance Survey. These findings did not affect the metal standards constructed in 1798, but they did remove the original connections between Earth and the unit of length.

Call for a New Standard

In 1867, the International Geodetic Association met in Berlin and recommended the construction of a new standard to reflect the scientific improvements since the construction of the original meter. To ensure that there was continuity, the convention recommended that the new physical standard should be as close as possible to the original standard meter held in the French Archives. This first recommendation was followed by other scientific organizations petitioning the French Academy to construct a new standard for the meter.

In 1870, the delegates of twenty-four nations met in Paris to begin work on a set of identical physical standards that would be distributed to each country. The conference was cut short by the Franco-Prussian War, but reconvened in 1872, with thirty nations taking part.

The results of these meetings included the construction of standard meter bars of 90 percent platinum and 10 percent iridium created from a single ingot produced at a single casting. The temperature at comparison to the standard meter was to be 0° C. The original attempt to connect the unit of weight to the volume of a cubic centimeter of water was abandoned. Instead, the kilogram standard that had been in the Archives was made the basis of the kilogram, which was copied for the new standard. This new kilogram was to be made of the same alloy as the standard meter and was to be compared to the original kilogram in a vacuum. This is the kilogram still in use in the twenty-first century, with the standard held in Paris being considered the primary standard. The participating nations signed the Treaty of the Meter in Paris on May 20, 1875, and each nation was presented with a standard meter and kilogram. The treaty also established and maintained a permanent international bureau to help with the maintenance and propagation of the metric system.

Recent History

In 1960, the International System of units was adopted by most countries as the basis for all measurement. The unit of length, the meter, was redefined as 1,650,763.73 wavelengths in a vacuum of the orange-red line of the spectrum of krypton-86. This somewhat awkward definition had two advantages: First, it did not necessitate any change in the length of legally certified meter standards already in existence, and second, it removed the need to keep a physical standard in a vault under a particular climatic condition.

As science progressed through the twentieth century, however, this light wavelength definition became inadequate. At the 17^th CGPM in 1983, the meter was again redefined to its current definition:

The meter is the length of the path traveled by light in vacuum during a time interval of 1/299,792,458 of a second.

This 1983 definition depends on the definition of a second, which was also refined in the twentieth century. In 1955, at the National Physical Laboratory in Teddington, England, a clock was made that used a stream of atoms from the element cesium. The clocks could be so accurately tuned that they were accurate to one part in a million million for commercially produced instruments and one part in fifty million million for instruments in laboratories dedicated to making physical measurements for standards.

From 1955 to 1958 a joint experiment between the U.S. Naval Observatory and the National Physical Laboratory established that a particular transition of a cesium atom whose atomic weight was designated as 133 was associated with a photon whose frequency was 9,192,631,770 cycles per second. In 1960, the General Conference on Weights and Measures redefined the second in terms of the transition of the cesium-133 atom.

To prevent any problems that might become associated with the new second, it was set as equal to 9,192,631,770 periods of radiation emitted or absorbed in the transition of a cesium atom. The measurement was taken at sea-level under clearly defined conditions. The second thus defined corresponds with the second that had been in use before that date and had been calculated by making observations of the Moon's movement, which had been known as the Ephemeris second.

see also Measurement, English System of.

Phillip Nissen

Bibliography

Conner, R. D. The Weights and Measures of England. London: Her Majesty's Stationery Office, 1987.

Donovan, F. Prepare Now for a Metric Future. New York: Weybright and Talley, 1970.

Zupko, R. E. Revolution in Measurement: Western European Weights and Measures Since the Age of Science. Philadelphia: The American Philosophical Society, 1990.