Frequency” is the speed at which a watch ticks; it is measured by the number of oscillations (or semi-oscillations) that the balance spring – the “heart” of the watch movement – makes in the time period of one hour (VPH, or vibrations per hour), which can also be expressed in hertz.
In a watch movement with a balance wheel, one vibration is a single swing of the balance wheel. A vibration is one-half of an oscillation, or a semi-oscillation. So, one swing of the balance wheel in either direction, for example clockwise, is one vibration or semi-oscillation, and two swings, for example clockwise then counter-clockwise, is an oscillation.
Presented differently, to determine the VPH of a watch, you can multiply its Hz by 3600 (the number of seconds in an hour).
The most common frequency for modern mechanical wristwatch movements is 4 Hz, or 28,800 vph. A watch ticking at 4Hz makes 4 oscillations per second, or 8 semi-oscillations (or vibrations) per second.
To illustrate the contrast, a very high beat rate would be 36000 VPH (think Zenith or Seiko Hi-Beat).
A very slow beat rate would be 14400 VPH (probably an old vintage mechanical wrist watch or pocket watch).
As an real life example that most will be familiar with – a newer Rolex Submariner would have a 28,800
The Rolex Submariner that Sean Connery (James Bond) wore in 1962 had an18,000 vph. So you can see how technology advances.
So what does this all mean?
Higher beat rate means greater precision but also means the watch has to work harder meaning possible shorter service intervals. The sweep of the second hand will be smoother with high VPH and will appear slight more jittery on a low VPH.
36,000 bhp (10 beats per second)
28,800 beats per hour (about 8 ticks a second),
21,600 bhp (six beats per second)
18,000 bhp (five beats per second).