National Bureau of Standards scientists have made use of the invariable frequency of molecular spectrum lines to control the accuracy of a clock. The frequency of an oscillator‐initiated microwave signal is fixed exactly at the frequency of the strongest (and most easily measurable) absorption line of ammonia gas. This provides a means of controlling the oscillator, which has been used to drive a clock with such unprecedented accuracy that, according to a recent Bureau announcement, it “promises to surpass by one or two orders of magnitude the accuracy of the present primary standard, the rotating earth.”

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