Abstract: A thermal switch having relatively high gain for stabilizing the temperature of a heat source or a heat sink, especially useful in vacuum applications. A strip of heat conducting material is disposed to conduct heat from one heat station to another heat station. The contact made between the strip of heat conducting material and a heat station is subjected to a variable contact pressure. This contact pressure varies in response to changes in the temperature of a quantity of a heat expandable fluid material confined to a chamber thermally connected to whichever heat station is to be subjected to temperature control. This fluid preferably has a high bulk modulus. The fluid actuates a piston disposed to bear against the strip of heat conducting material whereby the pressure at the interface between the strip and the contacted heat station is varied. The heat conducting materials in contact with each other at this interface preferably have a low yield strength.