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Old 11-11-2010, 03:52 PM
glowacks glowacks is offline
Join Date: Aug 2008
Posts: 1,669
Originally Posted by beowulff View Post
Except a clock...
Yes, but you know your clock to be accurate because you've used the definition of a second, 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, to build it.