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Ever-shrinking kilo 22nd February 2005 In your item (News & Curiosities, March) about the standard for the unit of mass, you mentioned in an aside that the speed of light is used to define the second. This is not true: it is the metre that is defined in terms of the speed of light, being the distance travelled by light in a vacuum in 1/299792458 seconds. Since 1968 the second has been defined by caesium atomic clocks, based on the interaction between quantum transitions and electromagnetic radiation. The frequency of the radio waves that stimulates a particular transition in the…

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