Fifty years of commercial caesium clocks

The first commercial caesium-beam clock was the Atomichron, developed by National Company. The first unit was delivered in 1956. This paper first presents a brief history of commercial caesium-beam clocks. It then covers many of the sources of frequency error and frequency instability. Some details of the technical design of the caesium-tube and electronics are presented. Finally, we present some possible directions for future commercial caesium clocks.