Second (source code)
= Second
{title2=$\mathrm{s}$}
The <second>, symbol $\mathrm s$, is the <SI> base unit of <time>. Its definition fixes the unperturbed <frequency> of the ground-state <atomic transition> between levels split by <hyperfine structure> in <caesium> with mass number $133$ to exactly $9192631770\,\mathrm{Hz}$. One second therefore corresponds to that many cycles of the specified <electromagnetic radiation>.<The International System of Units (SI), ninth edition, SI Brochure>[§2.3.1: definition of the second through the caesium transition frequency]{ref}
A practical <atomic clock> realizes the reference by interrogating <atoms> and controlling a reference <oscillation>. <Magnetic fields>, motion and environmental effects require corrections: an exact unit definition does not make every physical realization free of <measurement uncertainty>.
Back to article page
