Thermally responsive switches

Claim 1

A thermally responsive electric switch including first and second snap-over bimetallic substantially part-spherical thermally responsive actuating elements which reverse curvature with a snap-action in operation, the two elements being disposed at one end of a housing so that their centres move along a common axis in operation coaxial with the housing axis; a pair of electrical contacts disposed at the other end of said housing; a common force transmitting member arranged to transmit the movement of either plate to the contacts, the contacts being in a first condition only when both plates have a first curvature, the contacts otherwise being in a second condition, the first element reversing from its first curvature to its second curvature when its temperature rises above a first predetermined temperature, and returning to its first curvature if its temperature subsequently falls below a second, lower, predetermined temperature, and said second element reversing from its first curvature to its second curvature when its temperature rises above a third predetermined temperature higher than said first predetermined temperature and thereafter retaining this curvature regardless of temperature, at least over a range extending appreciabl below said second predetermined temperature, thus maintaining the switch contacts in their second condition including the improvement that said first and second elements are dimensionally substantially identical and are freely supported between the force transmitting member and the adjacent end of the housing, so as to lie in nesting contacting relationship, concave towards said member when both elements have their first curvature, the element nearer the force transmitting member bearing directly on the end of said member and the other element bearing on said end of said member via said nearer element.