Time Line for the Definition of the 

Meter 

By: William B. Penzes

 

1791 The International System (formerly called the Metric System) is the decimal 

system of weights and measures based on the meter and the kilogram. The 

essential features of the system were embodied in a report to the French 

National Assembly by the Paris Academy of Sciences. 

1799 Originally intended to be one ten-millionth part of the quadrant of the earth, the 

so called Meter of the Archives was based on a measurement of a meridian 

between Dunkirk and Barcelona. A platinum bar with a rectangular cross section 

and polished parallel ends was made to embody the meter. The meter was 

defined as the distance between the polished end faces at a specified 

temperature and it was the international standard for most of the 19th century. It 

was compared to other bars with optical comparators as a means of 

disseminating the unit. 

1859 J.C. Maxwell suggested choosing as a natural standard, the wavelength of the 

yellow spectral line of sodium. 

1866 By act of the U.S. Congress, the use of the metric system was legalized in this 

country, but was not made obligatory.The International Commission of the Meter 

made the Meter of the Archives the official definition of the meter and the 

standard of length. It was admitted that its relationship to a quadrant of the earth 

was tenuous and of little consequence anyway. The Commission had 30 

prototype meters made using the Meter of the Archives as the reference. 

1872 The International Commission of the Meter made the Meter of the Archives the 

official definition of the meter and the standard of length. It was admitted that its 

relationship to a quadrant of the earth was tenuous and of little consequence 

anyway. The Commission had 30 prototype meters made using the Meter of the 

Archives as the reference. 

1875 On May 20, the Treaty of the Meter was signed by seventeen countries, 

including the United States, at the International Metric Convention. As a result, 

the International Bureau of Weights and Measures (Bureau Intérnational des 

Poids et Mésures, BIPM) was established. 

1889 A new modified Xshaped cross-section graduated platinum-iridium line standard 

was developed and adopted as the International Prototype Meter. The meter 

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Time Line for the Definition of the Meter

William B. Penzes

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was defined as the distance between the two graduation lines at 0 °C. Each 

member country in the International Metric Convention received two copies of 

the standard with calibration reports relating them to the prototype. All meter bar 

calibrations were done by comparisons in optical comparators. 

Prototype Meter No.27 served as the U.S. primary standard from 1889 to 1960. 

Its length is known in terms of the international prototype, having been returned 

to BIPM for recomparison four times during that period. It is now on exhibit in the 

NIST Museum at Gaithersburg, Maryland. 

1890 A.A. Michelson found that the red spectral line of natural cadmium was 

exceptionally coherent. 

1892 Michelson used an interferometer that he developed to determine the length of 

the International Prototype Meter in terms of the cadmium red line wavelength. 

His measurements gave the meter a value of 1,553,164.13 times the wavelength 

of cadmium red in air, at 760 mm of atmospheric pressure at 15 °C. 

1921 A. Perard began a systematic study of the radiations of cadmium, mercury, 

helium, neon, krypton, zinc and thallium, to determine which might best serve as 

defining lengths. 

1925 The Michelson interferometer was in regular use at BIPM for measuring length. 

1950 Cadmium 114, mercury 198, and krypton 86 were candidates to be a new 

definition of the meter based on the wavelength of light. 

1960 On October 14, the Eleventh General Conference on Weights and Measures 

redefined the International Standard of Length as 1,650,763.73 vacuum 

wavelengths of light resulting from unperturbed atomic energy level transition 

2p10 5d5 of the krypton isotope having an atomic weight of 86. The wavelength is

λ = 1 m / 1,650,763.73 = 0.605,780,211 μm 

At different times some national laboratories used light sources other than 

krypton 86 as length standards. Mercury 198 and cadmium 114 were among 

these and they were accepted by the General Conference as secondary length 

standards. 

1964 Helium-Neon stabilized laser wavelengths were coming into use as length 

standards. Although the laser wavelength was generally accepted as a 

secondary standard, its widening use was mainly based on its remarkable 

coherence. Long distances could be measured by laser interferometry that 

would be impossible with atomic light sources. Line scales of length are 

measured by dynamic (fringe counting) laser interferometry at NIST. 
Time Line for the Definition of the Meter

William B. Penzes

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1980 The iodine stabilized Helium-Neon laser wavelength was accepted as a length 

standard. It had a wavelength uncertainty of 1 part in 1010 at the time. 

1983 On October 20, the meter was redefined again. The definition states that 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. The speed of light is 

c = 299,792,458 m/s 

The second is determined to an uncertainty, U = 1 part in 1014 by the Cesium 

clock. The General Conference made the iodine stabilized Helium-Neon laser a 

recommended radiation for realizing the meter at this time. The wavelength of 

this laser is 

λHeNe = 632.99139822 nm 

with an estimated relative standard uncertainty (U) of ± 2.5 x 10−11. 

In all of these changes in definition, the goal was not only to improve the 

precision of the definition, but also to change its actual length as little as 

possible. 

 

A contribution by the Precision Engineering Division of the Manufacturing 

Engineering Lab at the National Institute of Standards and Technology.