168   NAVIGATIONAL COMPASSES

variations and as the magnetic condition of an iron ship is not constant, the compass indications must be checked frequently against the direction of the geographic meridian as given by the pole star or other celestial body. The directive force acting on the needle of an adjusted magnetic compass is very feeble on the deck of a war ship and is nearly zero within a submarine. It is profoundly altered when large guns and turrets are changed in position.

101. The Deviation of a Magnetic Compass Produced by a Rapid Turn. - If a hardened steel needle be balanced on a pivot, and afterwards magnetized, the north-seeking end will dip below the pivot when the needle is in northern latitudes. To use the magnetized needle as a compass it is kept horizontal by the addition of a counterpoise to the south-seeking end. The center of mass of the needle is no longer vertically below the pivot but is toward the south-seeking end of the needle. If the compass needle is on an airplane making a rapid turn, the center of mass of the needle will lag behind the north-seeking end, thereby causing the north-seeking end to move in the direction the airplane is turning. Thus, when an airplane is executing a rapid turn, the angle of turn indicated by the compass readings is less than the angle actually turned.

If an airplane while moving northward in northern latitudes executes a rapid turn eastward, the compass indicates that the airplane is pointing west of the true course. The angular speed of the needle may be greater than the angular speed of the airplane. In this case, the airplane will appear to be turning westward when really it is turning eastward.

An airplane cannot make a turn unless a force be applied toward the center of the curve. To produce this centripetal force the airplane is tilted about a fore-and-aft axis, the underside of the wings being directed away from the center of the curve. This operation is called " banking." In case the angle of bank, that is, the tilt of the wings from the horizontal, is insufficient to develop the required horizontal thrust against the wings toward the center of the curve, the airplane will not follow a circular path but will slide off along a diverging spiral. When this occurs, the northseeking end of the compass needle will be deflected in the direction opposite that in which the airplane is turning.

While an airplane is making a turn, the magnetic compass may deflect through a considerable angle either in the direction of the

THE VARIOUS TYPES   169

turn or in the opposite direction. The needle may even spin in one direction if the turn is sudden and the angle of banking is correct, or in the opposite direction if the angle of banking is much too small.

102. The Earth Inductor Compass. - A magnetic compass on the instrument board of an airplane is subject to deviations due to proximity to the large mass of magnetized steel and unmagnetized iron of the motor. It is also subject to errors when the airplane makes a turn (Art. 101). Close attention is required to observe the compass card indication - to distinguish between an indication of 43 degrees from one of 48 degrees, for example. It would be better, especially in long-distance flying, if an easily seen index would show simply whether the airplane were on the desired course, or were to the right or to the left.

The earth inductor compass was devised to avoid deviation due to the iron of the motor and also to diminish the difficulty and uncertainty in observing indications of the course. It is essentially a direct-current generator consisting of a coreless armature rotating in the magnetic field of the earth and connected to a millivoltmeter on the instrument board. The armature is placed so far astern that it is outside of the magnetic field due to the iron of the motor. It hangs pendulum-wise, with shaft vertical, from a universal joint connected to the vertical shaft of either an air-driven or an electrically driven motor. The brushes can be turned around the commutator by rotating a dial on the instrument board. The controller dial is graduated in degrees.

When the axis of commutation, that is, the line joining the points of contact of the brushes and the commutator, is perpendicular to the magnetic meridian, no electromotive force is induced in the rotating armature and the indicating millivoltmeter needle is in the zero position. When the line joining the brushes is turned about the armature shaft from this position, the indicating instrument deflects to one side or to the other depending upon the direction in which the brush holder was turned. If it be desired, for example, to fly on a course 30 degrees east of north, the pilot sets the controller disk at 30 degrees east of north and thereafter maintains the plane in the direction that will cause the indicator to remain on the zero mark.

103. The Magneto Compass. - Greater sensitivity than is possible in the earth inductor compass is obtained in Tear's magneto compass,* in which elongated pole pieces, P, and P2, of high

* General Electric Review. xxxii. 1929. D. 190