In this paper it is shown that, owing to the effect of the ship on the incoming wave, a vertical magnetic field may, through re-radiation, cause errors in the observed bearing taken with a wireless direction-finder. Experiements show that in the case of calibrations carried out with a roof aerial, if the receiving ship is not in line with the roof, errors of 1° or 2° may arise if the roof is not symmetrical. The error, however, decreases rapdily with increasing distance from the transmitter, and also with increasing frequency.The expressions for these errors are then derived mathematically and it is shown that they are closely allied to the ordinary deviation due to “ ship effect”. The error should be most marked in the direction of the electric axis of the ship (that in which the deviation is zero, usually the fore-and-aft line) and non-exitent at right angles to this.The work was carried out in a light cruiser with one of H.M. destroyers as transmitting ship, and this paper has been prepared in H.M. Signal School.