Abstract: An improved thermal switch permitting large temperature excursions beyond the design closure temperature without stressing the switch material beyond its elastic limits. It combines a bimetal portion with a spring portion through which movement is transmitted, the former providing sufficient deformation for closure at design temperature, and the latter accommodating excess deformation beyond closure caused by high temperature. It is particularly useful in metal halide lamps having gas filled outer envelopes and requiring shorting of an auxiliary starting electrode to the adjacent main electrode during operation.