Ma Be wrote in USMA 19100: > M R wrote: ... >>According to SI >>"The second is the duration of 9,192,631,770 periods >>of radiation corresponding to the transition between 2 >>hyperfine levels of the ground state of caesium 133 >>atom. " >> >I guess the source of the problem here, Madan, is that you're using a >deprecated definition of the second. This is no longer how the s has >being defined. It's now the distance travelled by the speed of light in a >certain fraction of a second (as much as I believe that this change was a >huge mistake *conceptually*, we're stuck with it for a while! All >fundamental units should be defined in terms of *the physical entity it >tries to define/describe*. In this particular case the use of another >base unit, the second, for the definition of the meter is totally contrary >to that principle. But that's the subject for another discussion... :-) >).
On the contrary, Madan is quite right with the latest definition of the second. Marcus is thinking of the definition of the metre, which is: "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.". 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. The kilogram is the only base unit that now depends on the preservation of a prototype, "Le Kilogramme prototype" at Le Bureau International des Poids et Mesures. Note the upper case "K" as a mark of respect for the prototype. Joseph B.Reid 17 Glebe Road West Toronto M5P 1C8 TEL. 416-486-6071
