Vibrations per hour

A mechanical watch's beat rate is usually published as a number of vibrations per hour (18,000, 21,600, 28,800, 36,000) rather than in hertz. A vibration is a half-swing: the balance travelling one way and unlocking the escapement once. Two vibrations make one complete oscillation, so a figure in vibrations per hour is 7,200 times the figure in hertz. No metrology institute, standards body or museum consulted for this entry defines the unit or states that conversion, and BIPM's SI Brochure does not list vibrations per hour among the non-SI units it tabulates.

The Swiss industry does standardise the subject. The Federation of the Swiss Watch Industry FH publishes a list of available NIHS standards, valid from April 2026, which includes "NIHSG_30-10_fr: Frequences - Appareils horaires a balancier-spiral" (1968, modified 2022): frequencies for balance-and-spring timekeepers. The list gives titles only, so that document's definitional clauses have not been read here, and no standard in the list is titled for the unit itself.

What can be shown is how primary documents use the count. US patent 3,465,515, granted to Girard-Perregaux in 1969, describes "the balance wheel to oscillate 18,000 times per hour whereby the escape wheel is freed and advances one-half its tooth pitch, five times per second": 18,000 an hour and five a second being the same count, each event one unlocking. EP 2 802 944 A1 (LVMH Swiss Manufactures, 2014) speaks of "a first number of alternations per hour, for example 21'600 vibrations per hour or 28,800 vibrations per hour", treating alternations and vibrations as one thing.

What this Codex does not do: attribute the factor of two to an authority. Dividing those trade figures by 7,200 gives 2.5, 3 and 4 Hz, which is this Codex's own arithmetic from two vibrations to the cycle rather than a conversion any source cited here states. It is recorded here as trade usage, not as a defined unit.