A highly complex timekeeping instrument, such as a cesium fountain clock, that directly realizes the SI definition of the second. It does not require calibration against another clock because its uncertainty is evaluated entirely from first principles and fundamental physics. These standards sit at the absolute top of the traceability chain.
The NMI uses a cesium fountain as their primary time standard to steer the national time scale.
Related terms
Primary standard
A standard that gets its value straight from the unit definition, not from calibration against another standard.
SI
International System of Units, the coherent set of seven base units on which all traceable measurement rests.
NMI
National metrology institute, the body that realizes a country's measurement units at the top of its traceability chain.
Traceability chain
The documented sequence of calibrations linking a working instrument back to national or SI-level standards.