Inside, the roof was lined with copper screening so that the body of the operator would not influence the inductance and affect the compass. The radio compass itself, a great wrapping of wire on a rectangular frame, like the four sides of a rectangular box, was mounted on a vertical metal rod, so it could be twirled round in a circle. Encircling the revolving vertical shaft was a circular plate, not unlike the steering wheel of a motor-car, upon which were marked the three hundred and sixty degrees of a circle. The compass was at zero when its windings or wire-wrapped sides were parallel with the ship. As the compass was revolved, the listening operator would hear, with varying degrees of loudness, the signal he was watching for. Now he heard the sound with maximum distinctness. Again it grew faint, and, as he twisted the compass farther around the circle, the signal once more reached its loudest pitch. The two maximum sound points the operator noted on the degree-marked circular plate. Halfway between these two maximum points, or at the point of minimum distinctness, was the desired bearing, the point whence came the desired signal. A zero bearing meant that the signal came from either dead ahead or astern.
Now young Belford carefully closed the door of the compass shack, adjusted the head-phones,