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On Friday, October 13, 1967, the General Conference of Weights and Measures adopted the atomic definition of the unit of time, the second. The development of atomic frequency standards is still most heavily weighted toward cesium beam devices, with considerable work also on hydrogen masers [Vessot et al., 1966; McCoubrey, 1967]. Probably because of budget problems, there has been little work on thallium beams. There has been significant interest in optical frequency standards with a notable development of a methane-stabilized laser by John Hall [Barger and Hall, 1969]. |
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