Switch units for electric immersion heaters

A switch unit for an electric immersion heater for mounting on the wall of a liquid heating vessel, the switch unit including thermally responsive means sensitive to boiling liquid on a body of the switch unit in a position for exposure, in use, to vapour emerging from the vessel, said thermally responsive means comprising a snap-acting bimetallic actuator comprising a member of sheet bimetal having an aperture with an outer perimeter and an inner perimeter defining a tongue free at one end intermediate two lobe portions of said aperture, said inner perimeter and arcuate portions of said outer perimeter smoothly merging at rounded ends of the aperture adjacent the tongue root, the tongue being, at least in part, generally centrally disposed with respect to the member, the member having been deformed in a die pressing operation to conform in shape to a die of domed configuration, said domed area being such as to reverse its curvature with a snap action with change in temperature, and the width of the domed area being greatest in the region of the tongue root, an overcentre spring mechanism coupling said bimetallic actuator to a movable contact of an electrical switch for opening and closing said switch, said overcentre spring mechanism being movable between a first position in which said switch is closed and power can be supplied to the immersion heater and a second position in which said switch is open for interrupting the supply of power to the immersion heater, a lever of the overcentre mechanism being pivotally mounted on said body and a spring of the overcentre mechanism being mounted between fulcrums provided by said lever and said body so that the spring biasses said lever into direct or indirect engagement with said bimetallic actuator in the first position of the overcentre mechanism.