Basic elements › Units of measurement

AMPLITUDE

Amplitude expresses the difference between the extreme values of a quantity (of a wave).

Amplitude in the movement

In measurements of a mechanical watch’s rate and precision, amplitude expresses the angle travelled by the balance between the dead point (equilibrium position) and its extreme position, i.e. the angle travelled during a half-vibration (Figure 1).

Figure 1

Measurement methods

It is measured using a timing machine (acoustic measurement; Figure 2) or, more rarely, by optical means.

Figure 2 – Amplitude measured on a timing machine (272°)

Values

The amplitude of the balance is directly dependent on the energy transmitted by the motor organ (power source): it decreases as this energy diminishes.

A high amplitude is a decisive quality criterion, insofar as it reduces the sensitivity of the balance-hairspring (balance spring) to external disturbances — shocks, accelerations and gravitational forces — and thus contributes to the timekeeping accuracy of the watch.

The amplitude of a balance-hairspring (balance spring) is however limited by its construction (maximum 280°-300° depending on the lift angle). Beyond these values, a knock (rebate) phenomenon causes the oscillator to run away and results in an exaggerated advance.