Small splits present in all levels due to interaction between the electron spin and the nuclear spin if it is present, i.e. the nucleus has an even number of nucleons.

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As the name suggests, this energy split is very small, since the influence of the nucleus spin on the electron spin is relatively small compared to other fine structure.

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TODO confirm: does it need quantum electrodynamics or is the Dirac equation enough?

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The most important examples:

hydrogen line useful in astronomy, and also the simplest possible case between 1s
caesium standard, which is used to define the second in the International System of Units since 1967.