Essen, L.; Parry, J. V. L. (1955). "An Atomic Standard of Frequency and Time Interval: A Cæsium Resonator". Nature. 176 (4476). s. 280. Bibcode:1955Natur.176..280E. doi:10.1038/176280a0. p.280.
Essen, L.; Parry, J. V. L. (1955). "An Atomic Standard of Frequency and Time Interval: A Cæsium Resonator". Nature. 176 (4476). s. 280. Bibcode:1955Natur.176..280E. doi:10.1038/176280a0. p.280.