:: The evolution of the metre's definitions ::
The metre was defined as the ten-millonth part of a quarter of a terrestrian
meridian. The definition of the unit of weight was linked to it, since it is
the weight of a cubic decimetre of water. These two definitions are the "natural
and universal" basis of the metric system.
These definitions were theoretically kept, but the law added: "The platinum
metre and kilogramme deposited in the National Archives are the final
standards." These material standards, representatives of theoretical
definitions, became actually the practical and legal basis of the metric
It was decided producing prototype metres by using the original metre
preserved in the French Archives as a reference. The original metre and
kilogramme, called Legal Metre and Legal Kilogramme are the 1799 ones.
The Metre Convention was signed.
Thirty prototype metres and fourty prototype kilogrammes were created and
measured. It required comparing - with an accuracy never reached - the new
line standards with the new x-section ones, and comparing them with the
Legal Metre too, which is an end standard. It involved developping a special
measuring apparatus and a definite and reproducible temperature scale.
metre prototype and a kilogramme prototype were chosen, so that they could
become the International Prototypes. The national prototypes were
distributed. The International Prototypes were deposited in the IBWM on 28
September 1889 where they are still kept nowadays.
Michelson suggested using optical interferometres to measure lengths.
Afterwards, he received the Nobel prize in physics in 1907, mainly for his
work in metrology.
Michelson and Benoît used the Michelson's interferometre in the IBWM to
determine the metre value in wavelengths of the red line of cadmium.
The above-mentionned measurement was confirmed by Benoît, Fabry and Pérot by
means of Pérot and Fabry's interferometre.
The national prototypes were checked for the first time by comparing them
with each other and with the International Prototype. There were new
improved determinations of the prototypes coefficient of expansion.
There was the International Agreement that defines the angstrom, based on
the wavelength of the red line of cadmium determined in 1893 and 1906. The
angstrom as so defined will be used as a unit of length in spectroscopy and
in atomic physics until it is abandonned in 1960.
The ICWM decided to go into the possibility of redefining the metre
according to a light's wavelength and created for that purpose the Advisory
Committe on the Metre Definition (that is nowadays the Advisory Committe on
The GCWM adopted a definition of the metre based on the wavelength in vacuum
of the radiation corresponding to the transition between specified levels of
the krypton 86 atom.
the IBWM, measuring the line measures according to this wavelength replaced
comparing line measures. New facilities were installed to do these measures
by optical interferometry.
The GCWM recommended for the speed of light in vacuum a value that resulted
from the measurement of the wavelength and the frequency of one laser.
The GCWM redefined the meter as the length of the distance covered by the
speed of light in a vacuum during a precise fraction of second. It invited
the ICWM to establish instructions for this new definition implementation.
The ICWM, because it had anticipated this invitation, indicated general
methods to link directly lengths to the metre as defined. There is among
these methods the use of the wavelength of one of the recommended five
radiations of lasers or spectroscopic lamps. The wavelengths and frequencies
values, as well as their associated uncertainties, of these radiations are
specified in the metre's definition implementation.
In the IBWM, the comparison of laser frequencies by optical beatings
completes the measurement of line standards according to the wavelengths of
1987: The IBWM began a new series of international comparisons of
wavelengths of laser radiations - by optical interferometry - and of
frequencies - by beating, in order to check the exactness of the practical
realizations based on the new definition. The latter included comparisons of
laser components, especially absorption cells that contain the atoms or
molecules to which laser is a slave, as well as overall comparisons (optics,
discharge tube, absorption and electronic cell).
The ICWM decided to reduce uncertainties because of the works done in the
national laboratories and in the IBWM. These uncertainties are the ones on
the recommended radiations emitted by some lasers and that appear in the
implementation. The ICWM decided to increase the number of these radiations
from five to eight.
The ICWM modified the 1992 implementation, by adding four new laser
radiations, which increased their number to twelve, and by reducing again
the uncertainties associated to the radiations of some lasers.
Works were going on in the IBWM and elsewhere to identify the factors that
limit the reproducibility of lasers, wavelength and frequency standards.