Caesium hyperfine frequency

Δν_Cs = 9 192 631 770 Hz

Unit: Hz
Value: 9 192 631 770
Exact: yes
Note: Fixes the second: the tick of the caesium atomic clock every other unit is built on.
Symbol: Δν_Cs
Category: Defining (SI)
Numeric: 9192631770
Uncertainty: exact

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From the Constants database in Notion, last edited 2026-08-15. This page is the row, rendered by the build; nothing on it is written by hand.