Thermal Resistance (RT)

By direct analogy with electrical resistance $V/I$ (potential difference divided by current), the ratio of temperature difference to heat-flow rate is called the thermal resistance $R_T$ of a conducting slab or rod:

Its SI unit is °C·s/J (or, in older units, °C·s/kcal), with dimensional formula $[M^{-1}L^{-2}T^3K^1]$. Since $R_T$ is inversely related to $k$, a LOWER thermal conductivity means a HIGHER thermal resistance, and materials with a high $R_T$ are good thermal insulators. It is worth distinguishing thermal RESISTIVITY $\rho_T = 1/k$ (a property purely of the MATERIAL) from thermal RESISTANCE $R_T$ (a property of a particular piece -- a specific slab or rod -- which depends also on its thickness or length, not the material alone). …