It is shown that the heat flow in a single homogeneous medium of thermal conductivity &sgr;(T), between boundaries at temperaturesT2andT1is the same as that for the same geometry in a medium of unit thermal conductivity and boundary ``thermal potentials'' &Sgr;(T2) and &Sgr;(T1), where&Sgr;(T)=∫0T&sgr;(T′)dT′. Some applications are given, as well as a short table of &Sgr;(T) for common cryogenic temperatures and materials.