Atomic Second

From 1965, the following is a scan of Metrologia, volume 1, number 1, page 28 where the Twelfth General Conference of Weights and Measures postpones for a few years the decision to define the second in terms of an atomic time (9,192,631,770 Hz) and continues to define it in terms of the ephemeris time (1 / 31,556,925.9747 of the tropical year for 1900).