Abstract: A device for maintaining constant the amplitude of the oscillation of an electro-magnetically driven balance wheel of a timepiece, whereby a first voltage is generated of an amplitude proportional to the amplitude of oscillation of the wheel. This is compared with a reference voltage and a signal is derived corresponding in amplitude and sign with the difference. This signal is then used to control the application of driving pulses to the balance wheel. The said driving pulses are preferably much shorter than one half-period of the balance wheel.