The field of ferro-inductance has been much neglected for many years. This investigation was instituted in an attempt to develop methods which would make ferro-inductive circuits as susceptible of analysis as circuits with constant parameters;The work has led to the development of an accurate empirical equation for the magnetization curve of a steel. This equation has been applied in predicting the inductance of a ferro-inductor at any value of current, and methods have been developed for the measurement of this inductance, these methods checking the theory closely;The equation for ferro-inductance has been generalized into a relation for effective ferro-reactance by an assumption, and experimental evidence has been obtained to support the validity of the assumption. Methods were also developed for calculating the effective resistance of an iron-cored reactor. Application was then made of both of these developments in calculating the impedance of circuits containing ferro-reactors, and the results were again well supported by experimental evidence. The latter calculations have apparently never been possible before;Another application of ferro-reactance was made to the ferro-resonant circuit, and simple relations for resonant voltage, capacity required, and the value of the critical resistance developed. These were all checked by experiment to a satisfactory degree of accuracy. Analytic methods for these solutions were not previously available in simple form.