Thermal conductivity heat flow may not be exactly the same as the direction of the thermal gradient. In metals, thermal conductivity approximately tracks electrical conductivity according to the Wiedemann–Franz law, as freely moving valence electrons transfer not only electric current but also heat energy. However, the general correlation between electrical and thermal conductance does not hold for other materials, due to the increased importance of phonon carriers for heat in non-metals. Highly electrically conductive silver is less thermally conductive than diamond, which is an electrical insulator, but due to its orderly array of atoms it is conductive of heat via phonons. The
Thermal conductivity measurement a heater or temperature sensor element. By having strong thermal insulation surrounding the diode, heat escapes only through an exposed cooling tab, which is used as the probe for the thermal interface material measurements. This method shares similarities with the ASTM D5470 standard, such as the measurement of the thermal resistance at different material thickness levels. The system is designed to measure high thermal conductivity thermal interface materials. Its applicability for the measurement of insulators is more limited. A thermal conductance tester, one of the instruments of gemology, determines if gems are genuine diamonds using diamond's uniquely high thermal conductivity.