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. |
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 twenty 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 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. |
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. |