James Frysinger wrote in USMA 15077
>There is a subtlety here that is often missed, I feel. Yes, the
>definitions of the metric units have changed several times. However, at
>least in virtually every case, the sizes of the metric units have not
>changed. As each new definition replaces its predecessor, drawing upon
>improved realizations, the new definition is written so as to provide
>the same size for the standard as the previous definition did, within
>the limits of the precision that technology permits at that time.
To fill out James's statement, I have dug out some details of the history
of the metre.
The metre was defined by a law of 1773 August 1 as one ten-millionth of the
distance from the pole to the equator. A provisional prototype metre was
constructed in 1795.
In 1799, after the measurement of one tenth of the quarter meridian,
between Dunkerque and Barcelona, the definitve prototype M�tre was
deposited by the Institut national des sciences et des arts in the Archives
de la R�publique. From that date there was no more mention of the quarter
of the meridian.
The "Convention du M�tre" was signed on 1875 May 29 by representatives of
17 states. It established the "Bureau International des Poids et Mesures".
The first Conf�rence g�n�rale des poids et mesures authortised the
construction of new prototype metres. These were made between 1871 and
1888. The United States received one of these prototype metres on 1890
January 1890.
In 1905 the International Congress of Solar Research defined the �ngstr�m
as 1/6 438.469 of the wave length in air of the red line of cadmium. In
1927 the Conf�rence G�n�rale recognized this value for measuring
wavelengths since it was within the limits of pecision of the official
metre to the -10.
In 1960 the Conf�rence G�n�rale defined the metre is equal to 1 650 763.73
wavelengths in vacuum of the radiation corresponding to the transition
between the 2p10 and 5d5 of the krpton 86 atom.
In 1975 the Conf�rence G�n�rale declared that the speed of light in vacuum
c = 299 792 458 metres per second
In 1983 the Conf�rence G�n�rale decided that the metre is the length of
path travelled by light in vacuum during a time interval of 1/299 792 458
of a second.
Each new definition of the metre placed it in the center of the zone of
uncertainty of the predecessor, and narrowed the uncertainty.
The effect of this chain of definitions is that any competent laboratory
anywhere in the world can redetermine with its one resources any metric
unit except the kilogram, the protoype of which is preserved with the
greatest reverence in a basement in S�vres. It is kept under three bell
jars, partially evacuated. Copies, which have been supplied to countries
around the world, have been compared with the "grand K" in 1899, 1911,
1939, and 1946.
Joseph B. Reid
17 Glebe Road West
Toronto M5P 1C8 Tel. 416 486-6071
