Heat-pipe thermostats of high precision

In a gas controlled heat-pipe thermostat of high precision having a temperature-controlled chamber arranged at least partly within the evaporation and condensation cycle and a gas reservoir connected to said heat-pipe, the improvement comprising in that in the heat-pipe a cooling surface is arranged for the production of condensate and for contacting and scavenging the surface of said temperature-controlled chamber, said cooling surface being in a condensation zone outside the direct effect of the control gas and separate from the chamber and connected to the outer wall of said chamber by liquid conducting capillary structures.