EARTH INDUCTOR COMPASS, THE, is related to the ordinary magnetic compass (q.v.) only in its use of the earth's magnetic lines as the directive force. There is no physical simi larity between the two types of compass. The earth inductor com pass comprises three principal units. That which reacts with the earth's field and therefore corresponds to the magnets of an ordi nary compass is designated the inductor generator. This resem bles an ordinary electric generator, except that it has no arti ficial field. It has an armature, commutator and brushes, and produces electromotive force by reaction with the earth's mag netic field. As in the case of any electric generator, the electro motive force depends upon the angle between the brushes and the field. In the earth inductor generator the brushes may be oriented about a vertical axis so that they may be set in any angular rela tion to the fore-and-aft line of the aeroplane. They may therefore be turned in relation to the earth's field, either inde pendently or with the aeroplane. As in any generator, turning the brushes in relation to the field discloses two positions of maximum potential and two positions of zero potential. In the earth inductor compass, use is made of one of these posi tions of no potential. The sec ond unit is known as the di rection controller. This carries a dial similar to an ordinary compass card and has a crank by which this dial may be turned to any position. There is a mechanical connection between the controller dial and the brushes of the generator, so that the angular position of the brushes is indicated upon the controller dial. The third unit of the compass, the steering indicator constitutes a sensitive zero-centre gal vanometer, the dial of which is marked left and right. The indi cator is electrically connected to the brushes. (See fig. I . ) When the compass is installed, the connection between the con troller and the generator is made such that when the aeroplane is headed in the direction indicated on the controller, the brushes will be in a position of zero potential. The steering indicator hand will therefore remain in the centre of the dial. If the aero plane is turned to the right, the generator brushes will be turned in relation to the earth's field and the generator will produce an electric current in such a direction as to cause the hand of the steering indicator to move to the right, showing the pilot that the aeroplane has turned to the right; turning the aeroplane to the left will put the generator brushes into such position that current in the opposite direction is generated, causing the steering indi cator hand to move to the left. As the compass is ordinarily used, the desired "bearing" or direction is set upon the dial of the con troller and the aeroplane is then turned until the steering-indicator hand comes to zero. Turns to the right or left away from this heading will cause the hand to move accordingly. When it is de sired to change the direction, the controller dial is cranked to a new position and the aeroplane again turned to bring the steering indicator hand to zero. (See fig. 2 and fig. 3.) The earth inductor compass of fers many advantages over the ordinary magnetic compass, par ticularly for use on aircraft (see AERIAL NAVIGATION) . It iS much easier to follow the hand of the steering indicator than it is to keep a certain mark on a swaying compass card oppo site the lubber-line (see Coat PASS). The outstanding advan tage, however, is the dissociation of the direction-responsive element and the direction-indicating units. In the ordinary mag netic compass these cannot be separated, and in order that the pilot may see his compass it is frequently necessary to place it in a position where it is subject to strong local mag netic fields. With the inductor compass, on the other hand, the generator may be placed in a position which is practically free from magnetic disturbance, while the steering indicator is placed in the best position for observation by the pilot. The controller may be operated by the pilot or by a navigator. One form of the earth induction compass, the "Pioneer," is the invention of M. M. Titterington, and was used by the United States army fliers on their round-the-world and Pan-American flights, on Commander Rodgers's Hawaiian flight, Commander Byrd's North Pole flight, on the trans-Atlantic flights of Lindbergh, Chamberlin and Byrd, and on the flight of Brock and Schlee from New York to Tokyo.
(C. H. C.)