compass, navigational

The compass is a device that indicates direction on the Earth's surface.
It is the principal instrument of NAVIGATION. Without it, a pilot would have difficulty in setting
the course for a ship or airplane. Two basic types of compass exist.
A magnetic compass indicates movement relative to the Earth's geomagnetic field .
A gyroscopic compass instead incorporates a spinning wheel and responds to changes in movement
relative to the Earth's rotation. It may or may not be linked ( "slaved" ) to a magnetic compass. 

 MAGNETIC COMPASS
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There is no firm evidence identifying the inventor of the magnetic compass. The existing evidence,
however, suggests that the Chinese were aware in the very remote past that the Earth's magnetism,
through the medium of a lodestone, could be used to indicate horizontal directions.
It appears that Mediterranean seamen of the 12th century were the first to use a magnetic compass at sea. 

Two types of magnetic compass are used, the dry card and the liquid. The dry-card compass used on
ships consists of a system of magnetized needles,
suspended by silk threads from a graduated compass card about 25 cm (10 in) in diameter.
The magnetic axes of the needles are parallel to the card's north and south graduations.
At the center of the card is fitted a cap with a jewel bearing. It rests on a hard,
sharply pointed pivot. The point of support is above the system's center of gravity so that the card
always assumes a horizontal plane. Painted on the bowl that accommodates the card is a lubber line
against which the heading of the craft is read. 

In a liquid compass the card is mounted in a sealed bowl filled with a liquid of low freezing point.
The buoyancy of the card is adjusted so that it floats, thus ensuring the minimum possible
friction between the cap and the pivot. Frictional force between the cap and the pivot reduces
the sensitivity of the compass;

when the craft turns in azimuth this force tends to drag the card in the direction of the angular
motion of the ship. In a dry-card compass, friction is reduced by keeping the mass of the card and
needle system as low as possible; in a modern, magnetic dry-card compass this is a small fraction of an ounce. 

A magnetic compass mounted on a steel ship is influenced by the ship's magnetism.
Magnetic correctors, therefore, are used to neutralize this magnetism at the compass position so 
that the needle system is influenced by the Earth's magnetism only. In aircraft the magnetic
compass is often located at a position outside the cockpit where the magnetism of the craft has
the least effect. From this location, compass directions are transmitted electrically to the cockpit,
and the system is known as a transmitting compass. 

 GYROSCOPIC COMPASS
 ---------------

The magnetic compass for ships and aircraft is now being superseded by the gyroscopic compass,
or gyrocompass. When it became possible to spin a wheel electrically, serious attention was given
to the possibility of using a spinning wheel--the axis of which tends to maintain a fixed direction
in space--for directional purposes. A spinning wheel that is free to move about three axes is
known as a free GYROSCOPE. In a meridian-seeking gyroscope the Earth's force of gravity causes the
spin axis of the gyroscope to oscillate horizontally around the vertical plane of the meridian,
an oscillation that is continually reduced in amplitude by a damping device. Through the agency of
this device, the spin axis settles horizontally in the vertical plane of the meridian, thereby acting
as an efficient compass. 

William Thomson (later Lord Kelvin) made an attempt to design a gyrocompass in 1883,
but credit for inventing the first practical gyrocompass belongs to Hermann Anschutz-Kaempfe,
whose first successful compass appeared in 1907. The first American gyrocompass,
designed by Elmer A. SPERRY, was produced in 1911; the first English design, by S. G. Brown and J.
Perry, is dated 1917.