A circuit breaker has a stationary contact and a movable contact, and an actuator device for actuating it comprises a rigid drive shaft (8) that drives the movable contact, first resilient means for driving the movable contact, and a torsion bar (2) for driving the drive shaft, the torsion bar (2) being suitable for being deformed in twisting in order to assist the resilient means during an initial period while opening the contacts of the circuit breaker. The torsion bar (2) is housed inside the drive shaft (8). A first end (4) of the torsion bar (2) is constrained to turn with a ring (10). A second end (6) of the torsion bar (2) is constrained to turn with the drive shaft (8).