In 1952, Harold Lyons of the National Bureau of Standards put into brief operation an atomic clock of the ammonia‐absorption type, and thus reflected the intense interest in devising atomic standards for measuring time intervals. This interest has grown rather than diminished. As progress continued with the advent of cesium‐beam frequency controls—first with L. Essen and J. V. L. Parry at the National Physical Laboratory in England, and later with the models investigated at the Boulder Laboratories of the NBS, and by Bonanomi in Neuchatel, Switzerland, by Kalra in Canada, and by McCoubrey and Holloway of the National Company in the United States—the standard for time measurement of highest possible accuracy was removed in effect from the astronomic realm to the atomic. In collaborating with Essen, W. Markowitz of the US Naval Obsevatory determined in 1958 the frequency characteristic of the cesium standard in terms of the accepted international unit of time, the ephemeris second. Since the international adoption of a temporary atomic standard to realize the unit of time, the figure they gave with a considerable uncertainty, 9 192 631 770 Hz, must be regarded not as a measured value, but rather as an exactly defined one, accurate to any number of significant figures. The uncertainty of 2 or 3 parts in 109 that originally attached to the measurement was evidently due to the uncertainties and inconvenience in the methods used to realize the ephemeris second; that is, the limitation lay in the astronomical observations, not in the atomic device.

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