The Atomichron: An atomic frequency standard

The Atomichron is the first practical device to refer time to an atomic standard. The four most basic quantites involved in physical measurements are length, mass, charge and time and it is essential that a standard unit be defined for each of these entities - a standard unit which is unvarying and is readily accessible for comparison at any desired time. It is an accepted fact that the elementary particles, such as protons and electrons, and individual atoms never change their character. Thus, if comparison to these elementary building blocks can be achieved with simplicity and high accuracy, they constitute an ideal set of primary reference standards. Length, mass, and charge have all been referenced to such atomic standards. The building block used as a reference standard in the Atomichron is the cesium isotope with atomic mass 133, or more specifically a particular frequency which is associated with a transition in the hyperfine levels of this isotope. The two states which are used in cesium by the Atomichron are called hyperfine levels. The difference in energy between the levels arises from the magnetic interaction between the nuclear and electronic magnetic moments, and the frequency of the photon associated with the transition is 9192.631830 ± 0.000010 mc. This frequency is defined in terms of astronomical time, which has been the conventional standard of time. It is now equally possible to define time with respect to the cesium resonance.