As a result of magnetic hysteresis, the propagation of electromagnetic waves through a ferromagnetic medium is accompanied by the generation of harmonic-frequency fields. An approximate theoretical method of treating the problem is developed and applied to study the third-order voltage distortion factor of an inductor as a function of frequency. An analogue method is also described which enables the study to be extended experimentally to cases where the theoretical analysis is not applicable; e.g. for large flux densities or for complex input waveforms.