2009/6/12 Ian Wright <watchm...@fastmail.fm>: > this illustrates one of the > sillinesses of our use of the metric system - 1 inch is a > good approximation but 2.54mm is not an approximation, its a > direct conversion from an inch and accurate to 4 thou.
FWIW an inch is _exactly_ 25.4mm as it was officially redefined that way, so it can be traced back to the same standards. As pointed out elsewhere, the metre is defined as a decimal fraction of the quarter-circumference of the earth, measured badly. As for the metre being defined in terms of the distance travelled by light, this is to relate the metre back to the second and as part of a movement away from physical prototypes. It means that someone can calibrate their equipment without borrowing the international prototype meter. The second is defined in terms of oscillation counts of an atomic clock. Given such a clock and a laser, you can calibrate length. In effect you are using the assumption that all Rubidium (I think) atoms are identical to define both. Physical protypes are troublesome. The Wikipedia page on the International Prototype Kilogram goes into this in some detail, as it seems that either the master kilogram mass is getting lighter, or all the national standards are getting heavier. Hence the idea of defining a kilogram as "exactly 2.1507303E22 atoms of 28Silicon" Sorry, you have touched on one of my geekier interests here :-) -- atp ------------------------------------------------------------------------------ Crystal Reports - New Free Runtime and 30 Day Trial Check out the new simplified licensing option that enables unlimited royalty-free distribution of the report engine for externally facing server and web deployment. http://p.sf.net/sfu/businessobjects _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
