An electronic device is described by which the rest position of a mechanical oscillating system can be locked onto a given point or angle. This is obtained by generating a square wave that develops a dc component every time the rest position of the system does not coincide with the required one, and by making a motor automatically displace the oscillation center until the dc component of the square wave is zero. The device has been tested in connection with a torsion pendulum for internal friction measurements: it can correct large displacements of the rest position of the oscillating pendulum within few a periods, provided the angular velocity of the compensation motor is suitably chosen in relation to pendulum oscillation amplitude and period.