Most vessel of any size carry at least two magnetic compasses,they are called the standard compass and the steering compass. The standard compass is located on the ships center line,and on a weather deck near the bridge, at a point where it will be least affected by magnetic influences. Headings read from this compass are termed per standard compass (psc). The steering compass in most ships is also located on the centerline, just forward of the steering wheel, where it can be seen by the helmsman. Its headings are termed per steering compass (Psc).
Magnetism is a fundamental physical phenomenon which occurs both naturally, and artificially by induction. It is the property of certain metals to attract or repel items of like material or certain other metals; it is also an effect of electrical currents. An object which exhibits the property of magnetism is called a magnet. It can be elongated, as in a bar magnet, shaped like a horseshoe, or take other forms. The space around each magnet in which its influence can be detected is called its field; this can be pictured as being composed of many lines of force. The lines concentrate at both ends, or poles, of a magnet. Each magnet always has two-no more, no less-areas of opposite polarity; one is termed north and the other south. Each magnet,regardless of size, has two POLES where the magnetic action appears to be concentrated. These poles have opposite characteristics and are termed NORTH and SOUTH. The basic law of magnetism is very simple: Opposites attract; likes repel. The N pole is attracted to an S pole, but repels another N.
The earth as a whole has magnetic properties and can be thought of as having a powerful bar magnet near its center. The magnetic properties of the earth are not uniformly distributed, and its magnetic poles are not at the locations of geographic poles. The magnetic pole in the northern hemisphere is named the NORTH MAGNETIC POLE because of its location near the North Geographic Pole. The SOUTH MAGNETIC POLE is the one located several hundred miles from the South Geographic Pole. A compass has more than one bar magnet from which to align with the earth's magnetic field. Several long, thin permanent magnets are attached, parallel to each other, to a light non-magnetic wire frame; like poles of the magnets are adjacent. A light, round non-magnetic disc is mounted on the frame; this is the CARD of the compass and is marked around its circumference with graduations from which the direction can be read.
Placed under the frame is a bearing which rides on a hard sharp point called the PIVOT; this in t is supported from the outer case, which is the BOWL of the compass. A cover of transparent material, either flat or hemispherical, is fastened to the top of the bowl with a leak-proof seal. Through a plug opening, the bowl is filled with a special non-freezing liquid. An EXPANSION BELLOWS in the lower part of the bowl allows the fluid to expand and contract with temperatures changes without bubbles being formed.
A FLOAT may be attached under the frame to suspend the card and magnets, and reduce friction and wear on the bearing and pivot. The compass liquid also helps to dampen out any rapid oscillations or overswings of the card when direction is changed or rough weather. GIMBALS when provided, either internally or externally, are designed to help the compass card remain level despite any rolling or pitching of the vessel. The best arrangement is two sets of rings, one pivoted on a fore-and-aft axis and the other on an athwartship axis.
The card of a modern compass is divided into DEGREES, 360° to a full circle; the degrees increase clockwise around the card. North is 0° and the scale continues on around through 90° (East), 180° (South), and 270° (West).
In the old days compass cards were subdivided into points - 32 points to a complete circle, one point equal to 11 1/4 degree were named, not numbered, and a seaman learned to "box the compass" by naming all the points. There were also half and quarter-points when a smaller subdivision was needed .
The CARDINAL points, North, East, South. and west-and the INTERCARDINAL points, Northeast, Southeast, Southwest, and Northwest-are still in common use as rough directions and as descriptions of wind direction.
VARIATION - Magnetic meridians indicate the direction of the earth's magnetic field; but only in a very few places do the magnetic and true meridians coincide. The difference at any location between the directions of the magnetic and true meridians is the variation, sometimes called magnetic declination. It is called easterly (E) if the compass needle, aligned with the magnetic meridian, points eastward or to the right of true north, and westerly (W), if it points to the left. Variation results from the horizontal component of the earth's magnetic field.
Variation is important to the navigator because the magnetic compass, responding to the earth's magnetic field, is in error in measuring true geographic direction by the amount of the variation. The magnetic variation and its annual change are shown on charts, so that directions indicated by the magnetic compass can be corrected to true directions. Variation is caused by the earth's magnetic field, its value changes with the geographic location of the ship, but is the same for all headings of the ship. A compass needle free to turn horizontally tends to align itself with the earth's magnetic lines of force. Unfortunately, it is not free to do so in a steel ship; such ships have marked magnetic properties of their own, and these tend to deflect the compass from the magnetic meridian. The difference caused between the north-south axis of the compass card and the magnetic meridian is called deviation. Even in a vessel made of wood or fiberglass there is enough magnetic material on board-engines, fuel and water tanks, rigging, etc.-to cause deviation.
The possibility of deviation from electrical circuits must not be overlooked. Direct currents flowing in straight wires establish magnetic fields. Care must be taken that all wiring in the vicinity of a compass is properly installed to eliminate or reduce any effect on the compass; checks must be made for deviation with the circuits turned 0n and off.
Although deviation differs from variation in is caused by the earth's magnetism, the two are designated in the same manner. If no deviation is present, the compass card lies with its axis in the magnetic meridian and its north point indicates the direction of magnetic north. If deviation is present and the north point of the compass points eastward of magnetic north, the deviation is named easterly and marked E. If it points westward of magnetic north, the deviation is named westerly and marked W.
The navigator can easily find the correct variation by referring to the chart of his location. Deviation varies not only on different ships, but on any particular ship varies with changes in the ship's heading. It changes with large changes in the ship's latitude.
The navigator must understand how to apply variation, deviation, and compass error, you might be required to use them in converting one kind of direction to another.
There are three ways which a direction can be expressed:
As true, when referred to the true (geographic) meridian as th reference of measurement.
As magnetic, when referred to the magnetic meridian as the reference of measurement.
As compass, when referred to the axis of the compass card as th reference of measurement.
Any given direction may be expressed in all three of these ways.
True differs from magnetic by variation. Magnetic differs from compass by deviation. Compass differs from true by compass error.
