In a sandwich consisting of a thin Au film and a thin Bi film insulated by a 500 Å SiO layer the electrons of one film are heated above the lattice temperature while in the other film they are kept in thermal equilibrium temperature while in the other film they are kept in thermal equilibrium with the phonons. By measuring the magnetoresistance of both films we obtain the temperature difference between electrons and phonons from which we imply the energy-relaxation time τε from 0.3 K to 2 K. We find that electron-phonon scattering determines τε and, above 5 K, also the inelastic collision time τi obtained from weak localization theory in thermal equilibrium.