Hertz

The hertz is one of the SI units with a special name. BIPM's SI Brochure lists it in its table of coherent derived units as "frequency hertz (d) Hz s−1" (that is, one per second) and attaches footnote (d): "The hertz shall only be used for periodic phenomena and the becquerel shall only be used for stochastic processes in activity referred to a radionuclide." The unit therefore counts repetitions of something that repeats regularly, which is exactly what an oscillator in a watch does.

The hertz also sits inside the definition of the second itself. The same brochure states: "The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency, ∆νCs, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz".

In watches the figures are small or very large depending on the oscillator. For quartz, the Seiko Museum records that the 1969 Quartz Astron used "a quartz oscillator with a vibration frequency of 8,192 Hz" and that "a frequency of 32,768 Hz became the standard for later quartz timepieces". For mechanical movements the hertz is used too (European patent application EP 2 802 944 A1, filed by LVMH Swiss Manufactures, refers in its background to a "4Hz oscillator"), while vibrations per hour is also used in the trade for a mechanical beat rate, a unit the SI Brochure does not list.