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mariner's compass and simple magnetic compass

An instrument for measuring direction parallel to the Earth's surface. Most compasses make use of the Earth's magnetic field; if a bar magnet (see magnetism) is pivoted at its center so that it is free to rotate horizontally, it will seek to align itself with the horizontal component in its locality of the Earth's magnetic field. A simple compass consists of a magnet so arranged and a compass card marked with the four cardinal points and graduated in degrees.

In a mariner's (ship's) compass, to compensate for rolling, the card is attached to the magnet and floated or suspended in a liquid, usually alcohol. Aircraft compasses often incorporate a gyroscope to keep the compass horizontal.

The two main errors in all magnetic compasses are variation (the angle between lines of geographic longitude and the local horizontal component of the Earth's magnetic field) and deviation (local, artificial magnetic effects, such as nearby electrical equipment). Both vary with the siting of the compass, and may be with more or less difficulty compensated for.

A radiocompass, used widely in aircraft, is an automatic radio direction finder, calibrated with respect to the station in which it is tuned.

History of the compass

It is said that the Chinese emperor Hwang-ti had an instrument on his chariot which indicated south. If so, this must have been one of the very first magnetic compasses, for he lived around 250 BC. The introduction of the compass to Europe, via the Arabs, brought about a revolution in the art of navigation and played a vital role in the great voyages of discovery.

Science of the compass

The magnetic compass consists essentially of a small magnet in the shape of a needle which is allowed to swing freely on a pivot. Wherever it may be, the pivoted magnet will turn so that its ends point north and south. This is because the Earth itself behaves like a giant bar magnet whose axis almost coincides with the axis upon which the Earth spins. Since unlike magnetic poles attract each other (and like poles repel) the pivoted magnet in the compass will turn so that its north and south poles face the Earth's south and north magnetic poles respectively. Since the compass needle indicates the direction of the north and south it enables other directions to be determined.

The main snag with the magnetic compass lies in the fact that the north and south magnetic poles do not quite correspond to the north and south geographic poles (i.e., the points where the Earth's rotational axis emerge from the planet's surface). Thus a compass needle does not point towards true north and the error is known as variation. Fortunately this is a known error which can be allowed for in a compass reading.

Another error in the reading is caused by the presence of iron-bearing metal in the vicinity of the compass. The Earth's lines of magnetic force find it easier to pass through iron-bearing metal than they do through air. Hence they converge in the metal and produce a local magnetic field of greater strength than normal which may deflect the compass needle from its north-south position considerably. Such an error, known as deviation, is especially marked in a ship. It is reduced as much as possible by fitting compensating magnets around the compass and checking it at regular intervals.

Mariner's compass

In a normal small compass the pivoted magnetized needle swings about a fixed compass card with the cardinal points (north, south, east, and west) and further sub-divisions. But in a mariner's compass the card itself is pivoted, and attached to the underside of it are several strips of magnetized metal to increase the magnetic effect. In this case a reference mark is drawn on the inside of the compass box exactly in line with the keel of the ship and the reading on the compass card which falls opposite it indicates the ship's course.

One of the difficulties involved in a mariner's compass is the swinging of the compass card due to the movement of the ship. This is overcome by filling the compass bowl with alcohol so that the card nearly floats. Thin wires attached to the card hand down into the liquid and stop it swinging too much.

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