Defining SI of Time – The Second

The Second denoted by the symbol s is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency ΔνCs 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 ΔνCs = 9 192 631 770 Hz. Inverting this relation gives an expression for the unit second in terms of the value of the defining constant ΔνCs.

1 \mathrm{~Hz}=\frac{\Delta v_{\mathrm{Cs}}}{9192631770}

Or in terms of The Second above equation can be written as follows –

1 \mathrm{~s}=\frac{9192631770}{\Delta v_{\mathrm{Cs}}}

The Second is thus equal to the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the unperturbed ground state of 133Cs atom.

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