Motors with solid-iron secondaries include both induction motors and, during their starting period, synchronous motors with solid poleshoes. They have peculiar characteristics arising from the fact that the secondary impedance values alter according to the slip; moreover, their relationship to slip is a complicated one owing to large variations in permeability. The object of the paper is to provide a theoretical treatment leading to the equivalent secondary impedance at any stated value of slip. The starting and running performance of the motor can then be calculated by known methods. The effects of using secondary material of higher or lower permeability and resistivity are considered, and the ideal combination of these values is established. This leads to an examination of the effects produced by adding copper to the solid iron, either as bars or end-rings, and the method of calculating the performance of such machines. The running characteristics are compared, and equivalent circuits and current locus diagrams are given.The most important application of the work is to salient-pole motors with solid pole-shoes, where equivalent pole-face impedance values must be determined for both direct and quadrature axes. These quantities are modified by the addition of copper end-rings or by the milling of grooves in the pole face, and the method of obtaining the modified values is developed.