Ryan Fitzgerald, age 23, of Harrisburg, Pa., for his question:
WHEN WAS A COMPASS FIRST USED?
A compass is an instrument that indicates direction. It is used by sailors, campers, aviators, hunters and other travelers to enable them to get from one place to another.
Two fundamental types of compass are used: the magnetic compass, which in a crude form was used as early as the 13th century, and the gyrocompass, a device developed at the beginning of the 20th century.
In the magnetic compass, directions are obtained by means of one or more magnetic needles pointing in the general direction of the magnetic North Pole under the influence of the earth's magnetic field.
The gyrocompass, which is unaffected by the magnetism of the earth, consists of a gyroscope with the spinning wheel on an axis confined to the horizontal plane so that its axle aligns itself with the north south line parallel to the axis of the rotation of the earth, thus indicating true north.
In its simplest form, the magnetic compass 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 magnetic compass, often 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.
In the liquid compass, the most stable mariner's type, the bowl is filled with a liquid, usually a mixture of alcohol and water. The liquid helps to support the graduated card.
The gyrocompass, incorporating one or more gyroscopes, is used in the navigation of all large vessels because it is unaffected by magnetism.
The ordinary mariner's compass is unreliable in aircraft because of the errors introduced by sudden turns and accelerations of the plane. To eliminate such errors, specially designed airplane compasses have magentic 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. This type of direction sensitive magnetometer consists of induction coils with appropriate windings and excitation , so that changes in direction are proportional to voltages induced by the magnetic field of the earth.
The induced voltages may be used to activate direction indicating elements at several locations on the aircraft by remote control.
In most large ocean going vessels the gyrocompass is connected electrically to a gyropilot, a device that automatically steers the ship, keeping it on course in accordance with signals from the gyrocompass.