Carleton in USMA 19297 discussed the succwsaive defintinons of the metre. >> > 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 =B1 4 nm.
>> Note that this does _not_ require the observer to know >> how long a "second" is, while the below does. I suspect that was >> the crux of Ma Be's question, which has been ignored in the answer. >> > >Ma Be: > >> > 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". >> Actually, I have a sneaking suspicion that the transition >> above _did_ in fact alter the "defined length" by some minuscule >> amount The length of the metre has not changed since 1799, it has only been more precisely defined. Each definition narrowed the zone of uncertrainty, and placed it in the middle of the previous zone of uncertainty. The last definition, that of 1983, defines the metre as the distance travelled by light in a certain interval of time. That is because the second is the most accurately defined unit that we have. The uncertainty of the second is like an uncertainly of 1 mm in the distance from the pole to the equator. The International Astronomical Union decided in 1952 that "The second is the fraction 1/31 556 925.9747 of the troipical year for 1900 January 0 at 12 hours ephemeris time". This definition is much more precise tha previous defintions of the metre, so in 1983 the metre was defined in terms of the distance travelled by light in vacuum in a certain duration of time. Since the rotation of the earth is slowing down due to tha action of wind and tide, an extra second is added at end of the year and again, if necessary, at the middle of the year. >> You do realize that the folkloric value of 98.6 F for mean >> human body temperature is the result of a false gain in precision >> from a double conversion from Fahrenheit (in which the original >> data were collected) to Celsius (then called Centigrade) and then >> _back_, right. Right In Britain normal temerature was 98.4�F. In metric countries this was rounded up to 37�C, and then converted back to 37.6�F in the USA and other foot-pound countries. Joseph B.Reid 17 Glebe Road West Toronto M5P 1C8 TEL. 416-486-6071
