José Martins asked on Friday 2001-01-26-20:22 > I know that the explanations must stay simple, but (there's always >a "but", >what's a second? > > With my full respects, > > José A. C. Martins The length of the day varies throughout the year, and the years themselves vary in length, so the Comité International des Poids et Mesures decided in 1956 to define the second in terms of the length of the year 1900. This second could not be determined with sufficient accuracy for the present needs of metrology, so in 1968 the Conférence Générale des Poid et Mesures changed the definition of the secend to "the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom". Please don't ask me to explain that definition, because I can't. The metre was redefined in 1983 by the Conférence Générale des Poids et Mesures as "the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second. Note that the second is defined by 10 significant digits and the metre by only 9 significant digits, so the second is the most precisely defined unit in physics. The kilogram is now the only unit that is defined by a physical object: the prototype Kilogram preserved by the Bureau International des Poids et Mesures at Sèvres, France, under the most strict protection. All the other units, except those involving mass, can be reconstructed anywhere from their definitions. A way to reconstruct the kilogram in case of catastrophe has not yet been found. Joseph B. Reid 17 Glebe Road West Toronto M5P 1C8 Tel. 416 486-6071
