Abstract: A heat pipe includes a heat input end including an evaporator an adiabatic section and an output end including a condenser, with the evaporator and the condenser joined by a hollow adiabatic section containing a wicking material and a coolant, a heat pipe including a plurality of heat pipe stages connected in cascade with the condenser of the preceding stage secured to the evaporator of the succeeding stage each of the stages having a larger internal cross-sectional area at the condenser than at the evaporator. The stages of heat pipes are interconnected to form an integral part of a unitary heat pipe, with the condenser and the evaporator screwed together, or individual heat pipes are interconnected by sleeves of variable lengths screwed one into the other. A heat pipe can be composed of flexible material, and more particularly the heat pipe is connected to the box containing the device as a heat sink.