Cesium was already a popular element for atomic clock research, and by 1968, some cesium clocks were even commercially available.
铯已经是原子钟研究的热门元素， 到 1968 年，一些铯钟甚至可以商用。
英语百科
Caesium standard
（重定向自Cesium clock）
A caesium standard or caesium atomic clock is a primary frequency standard in which electronic transitions between the two hyperfine ground states of caesium-133 atoms are used to control the output frequency. The first caesium clock was built by Louis Essen in 1955 at the National Physical Laboratory in the UK.