Ma Be quoted me in USMA 19107:
>>The advantage of these definitions of the second and the metre is that they
>>do not depend on the preservation of physical prototyypes. They can be
>>replicated by any well equipped standardizing laboratory anywhere in the
>>world.
And added:
>Perhaps so, but what was wrong with the previous one, that didn't need the
>second for its definition? We're told that this was largely to increase
>accuracy to 8 decimal places or something to that effect. Now, does this
>mean we may not have another similar alternative? Just wondering...
The metre has had several definitions, each one more precise than the
precding one.
In 1793 it one ten-millionth of the distance from the north pole to the
equator, accurate to � 0.8 mm.
In 1799 it became "l'�talon des Archives", a platinum bar, accurate to � 10 �m.
The next standard metre was constructed in 1889. It was an X-shsped bar of
90% platinum and 10% iridiium. It was accurate to � 0.1 �m.
In 1906 the angstrom was defined in terms of the wavelength of the red
spectral line of cadmium. This, in effect, defined the metre within � 10
nm.
In 1957 the CIPM declared the metre to be equal to 1 650 763.73 wavelengths
in vacuo of the radiation corresponding to the transition between the level
2p10 and 5d5 of the krypton 86 atom. This definition had an estimated
precision of � 4 nm.
In 1983 the CGPM decided that "The metre is the length of the path
travelled by light in vacuum during a time interval of 1/299 792 458 of a
second".
In view of the variety of codings for special characters, I list the ones I
have used in the above posting:
� is Greek mu
� is plus or minus
The accuracy estimates are my interpretation of those given in another
format by Maurice DANLOUX DUMESNILS in his "�tude critique du syst�me
m�trique", 1962, Gauthier-Villars & Cie.
Joseph B.Reid
17 Glebe Road West
Toronto M5P 1C8 TEL. 416-486-6071
