Vibrations per hour

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The balance of a watch oscillates at a predictable rate, typically referenced as vibrations or alterations per hour (A/h) or hertz (Hz).

Overview

One vibration ("semi-cycle" or "beat") is a movement limited by two end positions. So the balance wheel of a watch may makes eight semi-cycles per second, which is 28,000 per hour. This is often divided by two and counted in hertz (Hz). Thus, a 28,800 A/h movement moves at 8 semi-cycles per second or 4 Hz.

The number (frequency) of the vibrations of the balance wheel is determined by the diameter and the weight of the balance wheel and hair spring. The higher the frequency, the greater the reduction to the gear train must be.

Typical watch oscillation rates

A/h Hz V/s Notes
18,000 2.5 5 Typical pocket watch
19,200 2.6 5.2 Faster pocket watch
21,600 3 6 Classic watch
28,800 4 8 Classic "high-beat"
36,000 5 10 Modern "high-beat"

See Also