Measurements of resistivities of dilute iron based alloys show strong deviations from Matthiessen's rule. These deviations can be explained by a model in which spin up and spin down electrons conduct in parallel. The results are consistent with the theory of impurity shielding in these alloys. [This 1967 paper provides the first experimental demonstration of two current conduction at low temperatures in a ferromagnetic metal. One direct consequence of this property is the Giant Magnetoresistance discovered in 1988 by the groups of Albert Fert and of Peter Gr\"unberg].