Period of existence or persistence. The metric system unit for this quantity is "s" (second). The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency ∆νCs, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s⁻¹. This definition implies the exact relation ∆νCs = 9 192 631 770 Hz. Inverting this relation gives an expression for the unit second in terms of the defining constant ∆νCs: 1 Hz = ∆νCs / 9 192 631 770 or 1 s = 9 192 631 770 / ∆νCs

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SI Base Unit