Spiral spring with improved stiffness adjustment means

A spiral spring for a watch resonator mechanism, the spiral spring comprising:

• a flexible strip wound on itself in a plurality of turns, the strip having a predefined stiffness, the spiral spring including means for adjusting its stiffness,
• the adjustment means including a flexible element arranged in series with the strip, the flexible element connecting one end of the said strip to a rigid support, so as to add additional stiffness to the strip,
• the flexible element having a stiffness greater than that of the strip,
• the adjustment means including prestressing means for applying a variable force or torque to the flexible element, so as to vary the stiffness of the flexible element, the prestressing means comprising a lever connected to the flexible element and actuatable to transmit the force or the torque to the flexible element by an actuator, the lever comprising an end movable in a first direction (D1);
• and
• retaining means arranged to prevent movement of the lever substantially in a second direction (D2).