Basic elements › Units of measurement

FREQUENCY

Frequency expresses the number of periods (cycles) that occur one after another per unit of time. It applies to phenomena as varied as alternating electric current, laser pulses or, in watchmaking, the oscillations of the regulating organ. The official unit of measurement of the International System of Units (SI) is the hertz (Hz), which expresses the number of cycles per second.

Depending on the nature of the phenomenon studied, different methods can be used to measure a frequency: electrical, electronic, acoustic, optical or comparative.

Units of measurement in watchmaking

In a mechanical watch, chronometric precision relies on the regularity of the oscillations of the Balance–Hairspring (balance spring). Traditional mechanical watches operate at frequencies between 2.5 Hz and 5 Hz, meaning up to five oscillations per second.

While the hertz is the reference unit in the SI system, watchmakers commonly use another unit: the number of Vibrations per hour (A/h). The conversion is straightforward, one oscillation corresponds to two vibrations. Thus, a balance oscillating at 3 Hz produces 21,600 A/h — a typical value for traditional manufacture calibers.

Acoustic measurement using a timing machine

The Frequency of a mechanical watch is measured acoustically using a timing machine. The movement is placed on a support equipped with a microphone, which picks up the sound of the five characteristic ticks produced by the Escapement at each vibration. The interval between these impacts is compared in real time to a standard frequency; the measured deviation allows the watch’s rate variation to be read directly, expressed in seconds per day (s/d).