weighted mean based on their relative effusivities. This relationship can be demonstrated with a very simple "control volume" back-of-the-envelope calculation:

Consider the following 1D heat conduction problem. Region 1 is material 1, initially at uniform temperature , and region 2 is material 2, initially at uniform temperature . Given some period of time after being brought into contact, heat will have diffused across the boundary between the two materials. The thermal diffusivity