1. Used by Physikalisch-Technische Bundesanstalt as a slogan for a clock exhibition in 2005, on the occasion of the 50th birthday of the atomic clock
2. J. Vanier, C. Audoin, The Quantum Physics of Atomic Frequency Standards (Adam Hilger, Bristol, 1989)
3. F. Riehle, Frequency Standards: Basics and Applications (Wiley-VCH Verlag, Weinheim, 2004)
4. A. Scheibe, U. Adelsberger, Ann. Phys. 5. Folge 18, 1 (1933)
5. A. Scheibe, U. Adelsberger, Physik. Zeitschr. XXXVII 37, 185 (1936)