Movement frequency
A movement frequency is how often a mechanical watch's movement ticks, providing a steady stream of discrete increments for the clockwork mechanics to gear down and turn into a display of the time.
This frequency is created by the escapement, and then used by the rest of the movement. It's measured in terms of BPH (beats per hour), otherwise known as VPH (vibrations per hour), or in Hertz (full cycles per second).
Measuring a watch's frequency in BPH involves counting the balance wheel's back and forth movement separately, as each produces a forward movement in the escape wheel. In contrast, measuring its frequency in Hertz involves counting each back and forth movement together as a single cycle, counting pairs of forward movement in the escape wheel. The frequency in Hertz can therefore be multiplied by 7,200 (2 ticks × 60 seconds × 60 minutes) to determine the frequency in BPH.
Common movement frequencies include 2.5 Hz (18,000 BPH), 3 Hz (21,600 BPH), and 4 Hz (28,800 BPH). Rare ones include 1 Hz (7,200 BPH) and 5 Hz (36,000 BPH).
The frequency can be finely tuned using a regulator lever, preferably with a swan neck.
As a 3 Hz watch has 6 ticks per second, it progresses its second hand one degree of arc per tick. (6 ticks per second × 60 seconds = 360 ticks per minute.) Such a solution, in the style of Babylonian mathematics, seems fitting for measuring time.
Mechanical watches: 1963 | Movement frequency
