Editing Caesium standard (section)

=== Time and frequency ===

The first set of units defined using the caesium standard were those relating to time, with the second being defined in 1967 as "the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom" meaning that:
* 1 [[second]], s, = 9,192,631,770 {{math|Δ''t''<sub>Cs</sub>}}
* 1 [[hertz]], Hz, = 1/s = {{sfrac|{{math|Δ''ν''<sub>Cs</sub>}}|9,192,631,770}}
* 1 [[becquerel]], Bq, = 1 nuclear decay/s = {{sfrac|1|9,192,631,770}} nuclear decays/{{math|Δ''t''<sub>Cs</sub>}}

This also linked the definitions of the derived units relating to force and energy (see below) and of the ampere, whose definition at the time made reference to the newton, to the caesium standard. Before 1967 the SI units of time and frequency were defined using the [[tropical year]] and before 1960 by the length of the [[solar time|mean solar day]]<ref>{{Cite web|url=https://www.bipm.org/en/history-si/second|title = Second – BIPM}}</ref>