# SI Units Defined In Simple Words

SI short for Système International d’Unités(French Words) is a set of 7 base units and 22 derived units which are agreed upon at 26th meeting of the Conférence Générale des Poids et Mesures (CGPM).

7 base SI units are defined as constants which then are used for deriving other 22 units which can be used for doing different types of measurements in Sciences.

Let’s see what are these 7 base units in SI.

## SI Unit of Time – The Second (Explained)

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