A watch’s frequency is measured by the number of semi-oscillations a balance spring makes in the time period of one hour (vibrations per hour, or vph), which can also be expressed in hertz. Hertz (abbreviated as Hz), named for German physicist Heinrich Hertz, is the unit of frequency defining this periodic phenomenon and is calculated by dividing the number of oscillations by the period of time in which they are measured. A watch oscillating at a frequency of 18,000vph (9,000 full oscillations) beats at 2.5Hz (9,000 divided by 3,600 seconds equals 2.5).
As a rule, higher frequency equals higher precision. Until the last decade, the one drawback to this has been the fact that higher frequency also equals faster wear and tear to the movement. Improved materials creating less friction — naturally needing less or no lubrication — have allowed watchmakers to set escapements to higher frequencies to gain more precision. Until about the 1950s, wristwatches were mainly set to a frequency of 18,000vph; since then, frequencies have been steadily rising, with 28,800vph having become the modern norm as a good precision standard.
The number of vibrations per hour also determines how time is broken up on the dial — the “steps” that a seconds hand makes. For instance, 18,000vph equals a vibration duration of one-fifth of a second, resulting in five jumps of the seconds hand per second. Correspondingly, 21,600vph (3Hz) equals a vibration duration of one-sixth of a second; 28,800vph (4Hz), one-eighth of a second; and 36,000vph (5Hz), 1/10th of a second.
Excerpted from an article by Elizabeth Doerr in REVOLUTION’s archives.