The laws of emission of a black body having been determined,
it is possible to calculate, with the aid of Kirchhoff's law [eqn:(48)] (48), the emissive power of any body whatever, when its absorbing power or its reflecting power is known. In the case of metals this calculation becomes especially simple for long waves, since E. Hagen and H. RubensE. Hagen und H. Rubens, Ann. d. Phys. 11, p. 873, 1903. have shown experimentally that the reflecting power and, in fact, the entire optical behavior of the metals in the spectral region mentioned is represented by the simple equations of Maxwell for an electromagnetic field with homogeneous conductors and hence depends only on the specific conductivity for steady electric currents. Accordingly, it is possible to express completely the emissive power of a metal for long waves by its electric conductivity combined with the formul for black radiation.E. Aschkinass, Ann. d. Phys. 17, p. 960, 1905.