ABSTRACT
There are three mechanisms whereby energy can be transported from one region of space to another under the influence of a temperature difference. One is by transmission in the form of electromagnetic waves (radiation); the second is the process of convection, in which a bulk or local motion of the material effects the transport; and the final process is that of thermal conduction, when energy is transported through a medium. In most practical situations, energy transport is accomplished by all three processes to some extent, but the relative importance of each contribution varies markedly. For example, within an evacuated region, radiation is the sole mechanism of transport; whereas, in an opaque solid, conduction is the only mechanism possible. The fact that in most practical situations all three heat transfer mechanisms are present gready complicates the process of measurement of the thermal conductivity.
