Abstract: An actuator mechanism comprising a bimetal element secured at one end and having a heating element mounted thereon, the bimetal element being deflectable by energising the heating element to effect thermal transfer to the bimetal element, the degree of deflection being related substantially to the power applied to energise the heating element, the deflection causing an actuator armature to move along a linear path against a biasing means adapted to return the armature to its non-deflected position. The bimetal element includes a deflector limb adapted to engage the actuator armature to effect movement thereof. The deflector limb is or includes a compensator limb for compensating for the effects of ambient temperature fluctuation. Heating element, the deflection causing an actuator armature to move along a linear path against a biasing means adapted to return the armature to its non-deflected position.