# Primary time standard

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In [telecommunications](/source/Telecommunications), a **primary time standard** is a [time standard](/source/Time_standard) that does not require [calibration](/source/Calibration) against another time standard.

Examples of primary time, (*i.e.*, frequency standards) are [caesium standards](/source/Caesium_standard) and [hydrogen](/source/Hydrogen) [masers](/source/Maser).

The international [second](/source/Second) is based on the [microwave](/source/Microwave) [frequency](/source/Frequency) (9,192,631,770 Hz) associated with the atomic resonance of the [hyperfine](/source/Hyperfine_level) [ground state](/source/Ground_state) levels of the [caesium-133](/source/Caesium) atom in a magnetically neutral environment. Realizable caesium frequency standards use a strong electromagnet to deliberately introduce a [magnetic field](/source/Magnetic_field) which overwhelms that of the Earth. The presence of this strong magnetic field introduces a slight, but known, increase in the atomic [resonance](/source/Resonance) frequency. However, very small variations in the calibration of the electric current in the electromagnet introduce minuscule [frequency](/source/Frequency) variations among different caesium oscillators.

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