In this paper consideration is given to the applicability of the reciprocal theorem to the practice of radio direction-finding with either a rotating loop transmitter or a loop receiver. A description is given of some experiments carried out with the object of demonstrating that the permanent errors to which a rotating-loop beacon transmitter may be subject as a result of conditions local to its site are similar to those observed on a direction-finder erected on the same site. This similarity has materially assisted in locating the cause of the permanent error associated with the rotating beacon erected at Fort Monckton, near Gosport. From other experiments carried out under suitable conditions, it is shown that the occurrence of night errors is experienced on both systems of direction-finding to approximately the same extent.