II.

Magnetic Compass

In its simplest form this type consists of a magnetized needle mounted on a pivot at the center of a fixed graduated card so as to permit the needle to swing freely in the horizontal plane. The mariner’s compass, a large magnetic type used aboard ships, has bundles of parallel magnetic needles attached to the underside of the compass card, which pivots about its center in a glass-covered bronze bowl. The bowl is hung in gimbals, and hence the card retains a horizontal position despite the pitching and rolling of the ship (see Navigation).

In the liquid compass, which is the most stable type of mariner’s compass, the bowl is filled with a liquid, usually a mixture of alcohol and water. The liquid helps to support the graduated card, which, in this type of compass, pivots about its center and floats in the liquid, thereby reducing pivot friction and lessening the vibrations of the card caused by the motion of the vessel. Because of these advantages the liquid compass is used more often than the dry compass. In both types a black vertical line, known as the lubber’s line, is drawn on the inner surface of the bowl. The course of the ship may be obtained by reading the number of degrees on the card opposite the lubber’s line. The magnetic compass points toward the magnetic north only if the ship is free of magnetism and if no iron or steel objects are nearby. If the ship is magnetic and iron and steel objects deflect the magnetic needle, the error known as deviation occurs. To correct deviation, the compass is installed in a stand called a compensating binnacle, which is equipped with a system of magnets arranged to compensate for the disturbing influences.

In order to obtain readings of true north on the magnetic compass, it must be corrected also for variation, the angle between the magnetic and true meridians. This angle, also called the magnetic declination, varies in amount, in direction from east to west of the true meridian principally with geographical position, and to some extent with time. The amount, direction, and annual change of the variation for most localities on the surface of the earth have been determined, and these data are recorded on all charts. Transient, unpredictable changes in variation occur, mainly in the higher latitudes, as a result of magnetic storms (see Geophysics).

The ordinary mariner’s compass is unreliable in aircraft because of the errors introduced by sudden turns and acceleration of the plane. To eliminate such errors, specially designed airplane compasses have magnetic directional units stabilized against the motion of the craft by pendulums or gyroscopes. An important type of gyrostabilized magnetic compass, developed for aircraft and known as the Gyro Flux Gate, operates on the principle of magnetic induction. In this type the direction-sensitive magnetometer consists of induction coils with appropriate windings and excitation so that changes in direction are proportional to voltage induced by the magnetic field of the earth. The induced voltage may be used to activate direction-indicating elements at several locations on the aircraft by remote control.