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 reciprocal of thermal conductivity is called thermal resistivity. The general equation for thermal conductivity is formula_1, where formula_2 is the heat flux, formula_3 is the thermal conductivity and formula_4 is the temperature gradient. In the International System of Units (SI), thermal conductivity is measured in watts per meter-kelvin (W/(m⋅K)). Some papers report in watts per centimeter-kelvin (W/(cm⋅K)). In imperial units, thermal conductivity is measured in BTU/(hr⋅ft⋅°F). The dimension of thermal conductivity is MLTΘ, expressed in terms of the dimensions mass (M), length (L), time (T), and temperature (Θ). Other units which are closely related to the thermal conductivity are in