Pat Naughtin asked in USMA 18401
>Dear Jim,
>
>I have a feeling that, some time ago, you referred to SI as the sixth metric
>system. Is this so, and if so how do you count the various historical metric
>systems.
>
>My counting goes:
>
>1 Original metric system 1795
>2 cgs metric system of about 1872
>3 metric system of the 'Treaty du Metre' in 1885
>4 Giorgi's mksA metric system of 1901
>5 Modern metric system known as SI of 1960 with developments from 1948.
I suspect the six metric systems referred to were six definitions of the metre.
These were:
1793; 1/10 000 000 of the meridian from the North Pole to the Equator
1799: etalon des Archives
1889: M�tre international
1906: �ngstr�m defined in terms of wavelength of red cadmium light
1960: 1 650 763.73 wavelengths of orange light of krpton 86
1975: speed of light in vacuo = 299 792 458 m/s
1983: metre = distance travelled by light in vacuo in 1/299 792 458 s
Each definition of the metre narrowed its zone of uncertrainty and placed
it at the middle of the zone of uncertainty of the preceding definition.
Joseph B.Reid
17 Glebe Road West
Toronto M5P 1C8 TEL. 416-486-6071
