A time standard that does not require calibration against another time standard. Note 1: Primary time standards are realized in practice as primary frequency standards, time and frequency being the mathematical inverses of one another. Examples of primary frequency standards are cesium standards and hydrogen masers. Note 2: By international agreement, the unit of time (the second) is defined in terms of the interval that transpires during a specified number of cycles (9,192,631,770 cycles) of the electromagnetic radiation associated with atomic resonance transitions between the two hyperfine ground-state levels of the cesium-133 atom in a magnetically neutral environment. Note 3: Realizable cesium frequency standards use a strong electromagnet to deliberately introduce a magnetic field of known strength, and which overwhelms that of the Earth. The presence of this strong magnetic field causes a slight, but known, increase in the atomic resonance frequency. Despite this precaution, minuscule frequency variations still exist among individual realizable cesium frequency standards. These variations are caused, among other factors, by slight variations in calibrating the currents in their respective electromagnets.