The phonon drag resistivity for potassium is calculated by solving the Boltzman equations for both the electrons and phonons as opposed to the conventional method of Ziman where the phonon equation is not considered. By an application of the Schwartz inequality we can show that the drag resistivity in the present formalism is larger than that obtained by the conventional method. We substantiate this result by numerical calculation for potassium at very low temperatures, using a realistic phonon spectrum obtained from inelastic neutron scattering data.
