Explanation

In steady state, the amount of heat flowing from one face to the other face in time dt is given by $dH = kA(T_{1} - T_{2})dt/\ell$ $\Rightarrow dH/dt = (kA/\ell)\Delta T$ (k = coefficient of thermal conductivity) $\therefore k = \ell dH/(Adt\Delta T)$ Unit of $k = Wm^{-1}K^{-1}$